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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Tektronix plug-in parts substitution
Background: I'm repairing a non-working Tek FG501 function generator plug
in. So far I've determined that the voltage in the regulated +20V circuit is zero. I'm assuming -- for the moment -- that this is caused by a failure in the regulator section which consists of a reference diode, an Op Amp and a transistor booster (with a second transistor for current limiting) (and a suitable number of passive components). All of this is very simple circuitry and should be easy to debug. Using my Tek cross-reference manual, I've determined that the two transistors are basic 2N3904 (current limit) and MJE340 (pass). Great, I've got dozens of those in the drawer. BUT ... The Op Amp is specified as a "741" selected for +-20V supply! In fact, it's operated with a single supply with V- at ground and V+ at the capacitor-filtered, rectified 25 VAC raw input. A quick calculation reducing the AC voltage by 2 diode drops (full-wave bridge), gives roughly at 34V DC. I'm sure that the 25V voltage level is for a nominal AC input of (probably 110V) and I've got to expect that it could be as much as 10% higher so that the Op Amp would see at least 37V power differential which is, in any event, greater than the normal absolute limit for a commercial grade 741 of +-18V. In the NS data book, they give the absolute supply voltage limits of +-22V for the A/E/- grades and +-18V for the C grade. I don't think I even have any better-than-commercial new 741s on the shelf. Digi-Key only carries the C-grade! AFAICT, so does Mouser. SO ... I'm stuck it seems with several alternatives: 1. Finding a supplier of the higher grade 741 Op Amp. 2. Trying to select from my own stock one or two ICs that meet the criterion 3. Finding an acceptable substitute. Even though I have literally dozens of RFE 741s in a box -- and I'm sure several were probably better than commercial grade when originally used -- any testing for survival of the ICs with a 40V supply differential would certainly be destructive and not exactly what I'd really like to spend my time doing. It's likely that I can find a modern "high voltage" Op Amp which meets or beats 741 specifications in a "commercial" grade and quite probable, I think, that it'll be cheaper than purchasing some new MIL-grade 741s. I only have suspicions and assumptions at this point and I'm soliciting recommendations on a good course of action for getting a replacement for the Op Amp. TIA Norm |
#2
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Norm Dresner wrote:
Background: I'm repairing a non-working Tek FG501 function generator plug in. So far I've determined that the voltage in the regulated +20V circuit is zero. I'm assuming -- for the moment -- that this is caused by a failure in the regulator section which consists of a reference diode, an Op Amp and a transistor booster (with a second transistor for current limiting) (and a suitable number of passive components). All of this is very simple circuitry and should be easy to debug. Using my Tek cross-reference manual, I've determined that the two transistors are basic 2N3904 (current limit) and MJE340 (pass). Great, I've got dozens of those in the drawer. BUT ... The Op Amp is specified as a "741" selected for +-20V supply! In fact, it's operated with a single supply with V- at ground and V+ at the capacitor-filtered, rectified 25 VAC raw input. A quick calculation reducing the AC voltage by 2 diode drops (full-wave bridge), gives roughly at 34V DC. I'm sure that the 25V voltage level is for a nominal AC input of (probably 110V) and I've got to expect that it could be as much as 10% higher so that the Op Amp would see at least 37V power differential which is, in any event, greater than the normal absolute limit for a commercial grade 741 of +-18V. In the NS data book, they give the absolute supply voltage limits of +-22V for the A/E/- grades and +-18V for the C grade. I don't think I even have any better-than-commercial new 741s on the shelf. Digi-Key only carries the C-grade! AFAICT, so does Mouser. SO ... I'm stuck it seems with several alternatives: 1. Finding a supplier of the higher grade 741 Op Amp. 2. Trying to select from my own stock one or two ICs that meet the criterion 3. Finding an acceptable substitute. Even though I have literally dozens of RFE 741s in a box -- and I'm sure several were probably better than commercial grade when originally used -- any testing for survival of the ICs with a 40V supply differential would certainly be destructive and not exactly what I'd really like to spend my time doing. It's likely that I can find a modern "high voltage" Op Amp which meets or beats 741 specifications in a "commercial" grade and quite probable, I think, that it'll be cheaper than purchasing some new MIL-grade 741s. I only have suspicions and assumptions at this point and I'm soliciting recommendations on a good course of action for getting a replacement for the Op Amp. TIA Norm There may have been design changes in production. All I can tell you is that in my manual, that power supply design is flawed on many levels. It depends on the fact that the op-amp can't supply much current. A faster/different op-amp may well oscillate. First, are you sure it's the op-amp? If it is, can you swap it for another that's running on 20V and use the commercial grade replacement at 20V? Even tho they're selected differently, they may have in fact put the same part in all places. Check what's written on each part. Have you tried ordering one from TEK factory service? Can I assume you've tried it in a different hole to rule out the possibility of a mainframe transistor failure or failure in a solder joint on one of the transformer windings? Since you didn't mention any of the other supplies being down, one might rightly or wrongly assume that they're ok and the reference is working. Look for shorted tantalum caps. Sometimes they short so bad they don't even get warm. I'd worry about the op amp specs after you determine it's bad. mike -- Return address is VALID. 500MHz Tek DSOscilloscope TDS520 Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/ |
#3
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SEE ANSWERS INLINE
"mike" wrote in message ... Norm Dresner wrote: Background: I'm repairing a non-working Tek FG501 function generator plug in. So far I've determined that the voltage in the regulated +20V circuit is zero. I'm assuming -- for the moment -- that this is caused by a failure in the regulator section which consists of a reference diode, an Op Amp and a transistor booster (with a second transistor for current limiting) (and a suitable number of passive components). All of this is very simple circuitry and should be easy to debug. First, are you sure it's the op-amp? Well, I've measured the voltages at just about every point in that part of the circuit. I can explain every one except that the non-inverting input to the Op Amp is 6.2 volts, the inverting input is ~8.2 and the output is ~27.2 volts. Since the voltages on the pass transistor Q410 are consistent with a normal diode drop from collector to base and a reasonably saturated transistor between collector and emitter, I've ruled out failure of this transistor. Similarly, the voltages on the current limiting transistor Q415 also indicate a good unit. Based on the conclusions that the transistors and the resistors in the circuit are okay, that leaves only the Op Amp which is clearly misbehaving. BTW, I've checked with a 'scope and the Op Amp is definitely not oscillating; the output shows pretty much the same ripple as the input power. If it is, can you swap it for another that's running on 20V and use the commercial grade replacement at 20V? Even tho they're selected differently, they may have in fact put the same part in all places. I think what I might do is to lift the lower leg of the pass transistor's emitter resistor (also the current limit sensing resistor) and inject +20V from a known good supply at this point. If the unit still doesn't work, I'd be tempted to scrap it because there may be who-know-how-many consequent failures in the remainder of the circuit. What's not totally obvious to me from a very, very cursory inspection of the circuit diagram is what would not work with the higher "regulated" voltage -- unless something else has failed from that higher voltage, perhaps a tantalum capacitor as you've suggested -- though I don't "see" anything that's broken or burned. Check what's written on each part. It's clearly different than the other "741" Op Amps in the circuit. Have you tried ordering one from TEK factory service? No, I've just started working on this unit. Can I assume you've tried it in a different hole to rule out the possibility of a mainframe transistor failure or failure in a solder joint on one of the transformer windings? Yes. I've also looked at and measured the AC input and its rectified and filtered version and they look fine. Since you didn't mention any of the other supplies being down, one might rightly or wrongly assume that they're ok and the reference is working. Look for shorted tantalum caps. Sometimes they short so bad they don't even get warm. I haven't even tried to measure the other voltages -- I think that they're all set "proportional" to this one. I'd worry about the op amp specs after you determine it's bad. I'm pretty sure it's bad. mike -- Return address is VALID. 500MHz Tek DSOscilloscope TDS520 Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/ |
#4
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As described in the Tek manual for the FG501, the +20V supply is the
reference supply for the remainder of the power supplies. AFAICT every other supply is running "perfectly" proportional to the too-high voltage on this line. I fully intend to swap out the Op Amp, but given that all of the supplies are running "perfectly" at 1.35 x normal voltage, why isn't the unit operational? I suppose it could be there's a resistor somewhere whose value is chosen to be just below a diode-drop with the correct voltage and with the higher voltage the base is running too high and always turned on -- or because some other component failed. Has anyone had a similar experience with this type of unit? TIA Norm "Norm Dresner" wrote in message ... SEE ANSWERS INLINE "mike" wrote in message ... Norm Dresner wrote: Background: I'm repairing a non-working Tek FG501 function generator plug in. So far I've determined that the voltage in the regulated +20V circuit is zero. I'm assuming -- for the moment -- that this is caused by a failure in the regulator section which consists of a reference diode, an Op Amp and a transistor booster (with a second transistor for current limiting) (and a suitable number of passive components). All of this is very simple circuitry and should be easy to debug. First, are you sure it's the op-amp? Well, I've measured the voltages at just about every point in that part of the circuit. I can explain every one except that the non-inverting input to the Op Amp is 6.2 volts, the inverting input is ~8.2 and the output is ~27.2 volts. Since the voltages on the pass transistor Q410 are consistent with a normal diode drop from collector to base and a reasonably saturated transistor between collector and emitter, I've ruled out failure of this transistor. Similarly, the voltages on the current limiting transistor Q415 also indicate a good unit. Based on the conclusions that the transistors and the resistors in the circuit are okay, that leaves only the Op Amp which is clearly misbehaving. BTW, I've checked with a 'scope and the Op Amp is definitely not oscillating; the output shows pretty much the same ripple as the input power. If it is, can you swap it for another that's running on 20V and use the commercial grade replacement at 20V? Even tho they're selected differently, they may have in fact put the same part in all places. I think what I might do is to lift the lower leg of the pass transistor's emitter resistor (also the current limit sensing resistor) and inject +20V from a known good supply at this point. If the unit still doesn't work, I'd be tempted to scrap it because there may be who-know-how-many consequent failures in the remainder of the circuit. What's not totally obvious to me from a very, very cursory inspection of the circuit diagram is what would not work with the higher "regulated" voltage -- unless something else has failed from that higher voltage, perhaps a tantalum capacitor as you've suggested -- though I don't "see" anything that's broken or burned. Check what's written on each part. It's clearly different than the other "741" Op Amps in the circuit. Have you tried ordering one from TEK factory service? No, I've just started working on this unit. Can I assume you've tried it in a different hole to rule out the possibility of a mainframe transistor failure or failure in a solder joint on one of the transformer windings? Yes. I've also looked at and measured the AC input and its rectified and filtered version and they look fine. Since you didn't mention any of the other supplies being down, one might rightly or wrongly assume that they're ok and the reference is working. Look for shorted tantalum caps. Sometimes they short so bad they don't even get warm. I haven't even tried to measure the other voltages -- I think that they're all set "proportional" to this one. I'd worry about the op amp specs after you determine it's bad. I'm pretty sure it's bad. mike -- Return address is VALID. 500MHz Tek DSOscilloscope TDS520 Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/ |
#5
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Norm Dresner wrote:
SEE ANSWERS INLINE "mike" wrote in message ... Norm Dresner wrote: Background: I'm repairing a non-working Tek FG501 function generator plug in. So far I've determined that the voltage in the regulated +20V circuit is zero. I'm assuming -- for the moment -- that this is caused by a failure in the regulator section which consists of a reference diode, an Op Amp and a transistor booster (with a second transistor for current limiting) (and a suitable number of passive components). All of this is very simple circuitry and should be easy to debug. First, are you sure it's the op-amp? Well, I've measured the voltages at just about every point in that part of the circuit. I can explain every one except that the non-inverting input to the Op Amp is 6.2 volts, the inverting input is ~8.2 and the output is ~27.2 volts. Since the voltages on the pass transistor Q410 are consistent with a normal diode drop from collector to base and a reasonably saturated transistor between collector and emitter, I've ruled out failure of this transistor. Similarly, the voltages on the current limiting transistor Q415 also indicate a good unit. Based on the conclusions that the transistors and the resistors in the circuit are okay, that leaves only the Op Amp which is clearly misbehaving. BTW, I've checked with a 'scope and the Op Amp is definitely not oscillating The oscillation was a caution about using a different op-amp type. ; the output shows pretty much the same ripple as the input power. OK, now I think I understand what you're talking about. The main 20V supply is controlled by U420. The supply you're talking about is the 20V reference supply. Your first post said, "Background: I'm repairing a non-working Tek FG501 function generator plug in. So far I've determined that the voltage in the regulated +20V circuit is zero" This (and the next) post says that the supplies are all high by 35%...not zero???? Don't know what to make of that discrepancy. First thing I'd do is measure the voltage across R411 to make sure the op-amp is sourcing current. Shorted CB junction in Q410 will cause your symptom. Again, all this is based on my schematic, which may be different from yours depending on serial number. For a general solution, you don't have a lot of choices about what to do. Get the high voltage op-amp. (see discussion of swapping opamps around further down) If you're willing to accept restricted operating parameters, you can put a LM317 in the +supply of the op-amp. The generator won't work at low line voltage any more, but that may be an acceptable tradeoff. FYI, the FG501 probably doesn't work at low line worst case mainframe load conditions anyway. It was designed before people started worrying about such things. Worst case condition is extremely rare. If it is, can you swap it for another that's running on 20V and use the commercial grade replacement at 20V? Even tho they're selected differently, they may have in fact put the same part in all places. I think what I might do is to lift the lower leg of the pass transistor's emitter resistor (also the current limit sensing resistor) and inject +20V from a known good supply at this point. If the unit still doesn't work, I'd be tempted to scrap it because there may be who-know-how-many consequent failures in the remainder of the circuit. What's not totally obvious to me from a very, very cursory inspection of the circuit diagram is what would not work with the higher "regulated" voltage -- unless something else has failed from that higher voltage, perhaps a tantalum capacitor as you've suggested -- though I don't "see" anything that's broken or burned. What do you mean by regulated? If the op-amp input voltages are not equal, the output cannot be "regulated"??? You will blow up tantalum caps if they haven't already. The sine shaper is VERY sensitive to power supply voltage...just to name one. Check what's written on each part. It's clearly different than the other "741" Op Amps in the circuit. According to my manual, ALL the opamps were switched to -06 starting with B010204. Maybe yours got replaced with a low-voltage unit and failed again. If that's the case, put U40 in the U410 slot and put a low-voltage amp in the u40 slot. Have you tried ordering one from TEK factory service? No, I've just started working on this unit. Can I assume you've tried it in a different hole to rule out the possibility of a mainframe transistor failure or failure in a solder joint on one of the transformer windings? Yes. I've also looked at and measured the AC input and its rectified and filtered version and they look fine. Since you didn't mention any of the other supplies being down, one might rightly or wrongly assume that they're ok and the reference is working. Look for shorted tantalum caps. Sometimes they short so bad they don't even get warm. I haven't even tried to measure the other voltages -- I think that they're all set "proportional" to this one. I'd worry about the op amp specs after you determine it's bad. I'm pretty sure it's bad. mike -- Return address is VALID. 500MHz Tek DSOscilloscope TDS520 Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/ -- Return address is VALID. 500MHz Tek DSOscilloscope TDS520 Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/ |
#6
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My OP contained an error in that the voltage on the +20 regulated supply
line wasn't zero but in fact was 27V. Or maybe it was zero the first time I powered up the unit measured it and then something "snapped" and it went to 27V. [Maybe I measured the wrong thing the first time -- we'll never know.] In any event, the last 3 times I've powered the unit the voltage is a clearly unregulated ~27V which results from the output of U420 (the error amp) being stuck as high as a 741 will go in spite of the imbalance in the inputs with the voltage at the inverting input almost 2 volts higher than the voltage at the non-inverting input. The voltages at the pass transistor Q420 are completely consistent with it being perfectly sound and just being driven into (near) saturation. There are no shorts in this transistor. Neither is there any problem with the current-limiting transistor either. All voltages in the +20V regulation circuit are consistent with a failed Op Amp and with no other hypothesis I can propose. As far as I can determine, all of the other voltage regulating circuits are functioning "properly" -- that is their outputs are all perfectly proportional to the 27V on the 20V line as they're supposed to be. Based on the serial number this is an early unit. The Op Amp at U420 has completely different markings from the other 741's in the unit which are all of a single type. The soldering on U420 looks exactly like the others and there are absolutely no signs of any hand-work in replacing it; I have to believe that it's the original as it came from the factory. The voltage on the +5V line is ~6.7 volts. There is a single TTL IC (7402) and an MC1711CL which IIRC is some kind of ECL gate. Neither of these would be very happy with almost 7V on Vcc. Everything else on the board is either discrete transistors or 741's (or similar) so I don't think that any of the other ICs would have been damaged. While the sine shaper might not put out a good waveform with the higher voltage, none of the other outputs are working either so I suspect there's a fundamental problem with the main oscillator. That's why I'd like to simply substitute a a good +20V supply for the output of this circuit and see if the rest of the unit is intact. It may in fact not be and it may simply not be worth the time to debug it. I've been thinking that I could easily rig up an LM317 (as you suggest) or an LM7818 to substitue for this circuit. My point in checking for oscillations was that sometimes an oscillating Op Amp can give some very strange readings on a simply DMM. Norm |
#7
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Norm Dresner wrote:
My point in checking for oscillations was that sometimes an oscillating Op Amp can give some very strange readings on a simply DMM. Norm It is probably just a blown opamp, but I have also seen this behavior on some manufacturers 741's when any input is forced to be higher than 1.2V from the + rail. As a simple check, short the inv and non inv inputs of the opamp together, and then: a) check their voltage relative to the +supply. and b) check the output voltage. -Chuck Harris |
#8
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"Chuck Harris" wrote in message
... Norm Dresner wrote: My point in checking for oscillations was that sometimes an oscillating Op Amp can give some very strange readings on a simply DMM. Norm It is probably just a blown opamp, but I have also seen this behavior on some manufacturers 741's when any input is forced to be higher than 1.2V from the + rail. As a simple check, short the inv and non inv inputs of the opamp together, and then: a) check their voltage relative to the +supply. and b) check the output voltage. -Chuck Harris Chuck I really don't think it'll do much good to short the inputs together. The non-inverting input was 6.2V (the voltage from a zener) and the non-inverting input was about 8.0-8.2V while the output was stuck at ~28V. (It's being operated single supply with a ~30V filtered, unregulated output from the diode bridge and capacitor). If that much unbalance doesn't swing the output off the positive rail, then I don't feel there's much hope for it at all. This 741 is the error amplifier with an MJE340 pass transistor which is supposed to output a regulated +20V based on the zener input. This is an ancient Tektronix FG501 plug-in function generator which I bought used knowing that the pilot light came on and that it was otherwise untested. Norm |
#9
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Norm Dresner wrote:
This 741 is the error amplifier with an MJE340 pass transistor which is supposed to output a regulated +20V based on the zener input. This is an ancient Tektronix FG501 plug-in function generator which I bought used knowing that the pilot light came on and that it was otherwise untested. Norm Ok, I have the schematic for a SG505 in front of me, which ought to be similar to your FG501 supply, and what I see is a simple application for any 741 opamp. The 741 is rated for use to a maximum of 44V in a single supply circuit, and your circuit has only 30V. Am I missing something here? Why aren't you simply replacing the opamp? -Chuck Harris |
#10
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Norm Dresner wrote:
My OP contained an error in that the voltage on the +20 regulated supply line wasn't zero but in fact was 27V. Or maybe it was zero the first time I powered up the unit measured it and then something "snapped" and it went to 27V. [Maybe I measured the wrong thing the first time -- we'll never know.] In any event, the last 3 times I've powered the unit the voltage is a clearly unregulated ~27V which results from the output of U420 (the error amp) being stuck as high as a 741 will go in spite of the imbalance in the inputs with the voltage at the inverting input almost 2 volts higher than the voltage at the non-inverting input. The voltages at the pass transistor Q420 are completely consistent with it being perfectly sound and just being driven into (near) saturation. There are no shorts in this transistor. Neither is there any problem with the current-limiting transistor either. All voltages in the +20V regulation circuit are consistent with a failed Op Amp and with no other hypothesis I can propose. As far as I can determine, all of the other voltage regulating circuits are functioning "properly" -- that is their outputs are all perfectly proportional to the 27V on the 20V line as they're supposed to be. Based on the serial number this is an early unit. The Op Amp at U420 has completely different markings from the other 741's in the unit which are all of a single type. The soldering on U420 looks exactly like the others and there are absolutely no signs of any hand-work in replacing it; I have to believe that it's the original as it came from the factory. The voltage on the +5V line is ~6.7 volts. There is a single TTL IC (7402) and an MC1711CL which IIRC is some kind of ECL gate. Neither of these would be very happy with almost 7V on Vcc. Everything else on the board is either discrete transistors or 741's (or similar) so I don't think that any of the other ICs would have been damaged. While the sine shaper might not put out a good waveform with the higher voltage, none of the other outputs are working either so I suspect there's a fundamental problem with the main oscillator. That's why I'd like to simply substitute a a good +20V supply for the output of this circuit and see if the rest of the unit is intact. It may in fact not be and it may simply not be worth the time to debug it. I've been thinking that I could easily rig up an LM317 (as you suggest) or an LM7818 to substitue for this circuit. Just to be clear, I recommended you use the lm317 to power the new 741. The sine distortion is gonna be critically dependent on the stability of this reference voltage. I never used a 317 anywhere I cared about voltage stability and have no direct experience. May be just fine. Also, if you use it to replace the supply, you also need to worry about voltage differential when you short the 20V supply. Don't remember what the max differential for a lm317 might be. While you're at it, put some resistance in pin 3 of U410 so C406 won't blast the input on power down. mike My point in checking for oscillations was that sometimes an oscillating Op Amp can give some very strange readings on a simply DMM. Norm -- Return address is VALID. 500MHz Tek DSOscilloscope TDS520 Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/ |
#11
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"mike" wrote in message ...
Norm Dresner wrote: Just to be clear, I recommended you use the lm317 to power the new 741. The sine distortion is gonna be critically dependent on the stability of this reference voltage. I never used a 317 anywhere I cared about voltage stability and have no direct experience. May be just fine. Also, if you use it to replace the supply, you also need to worry about voltage differential when you short the 20V supply. Don't remember what the max differential for a lm317 might be. While you're at it, put some resistance in pin 3 of U410 so C406 won't blast the input on power down. mike My point in checking for oscillations was that sometimes an oscillating Op Amp can give some very strange readings on a simply DMM. Norm Aha! I understand -- use the IC regulator to drop the voltage to the Op Amp to a level where I could use a commercial grade safely. Good point. Regarding max voltage inputs, I think I can safely use an LM7824 to drop whatever the input is down to 24 volts to drive a jellybean 741 and I've got one or two in some drawer somewhere around here. As for C406, my circuit diagram shows a diode CR410 whose anode is connected to the + side of the capacitor and the cathode to the V+ input to the Op Amp to keep its input voltage no more than a diode drop above the supply voltage. I couldn't find the diode on the circuit board anywhere or in the parts location diagram but it's clearly on the schematic. If I can't find it, I'll put a 1N4148 in as a "jumper" Thanks Norm |
#12
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"Chuck Harris" wrote in message
... Norm Dresner wrote: This 741 is the error amplifier with an MJE340 pass transistor which is supposed to output a regulated +20V based on the zener input. This is an ancient Tektronix FG501 plug-in function generator which I bought used knowing that the pilot light came on and that it was otherwise untested. Norm Ok, I have the schematic for a SG505 in front of me, which ought to be similar to your FG501 supply, and what I see is a simple application for any 741 opamp. The 741 is rated for use to a maximum of 44V in a single supply circuit, and your circuit has only 30V. Am I missing something here? Why aren't you simply replacing the opamp? The circuit's input comes from the 25V AC on the backplane. Through the diode bridge and the filter capacitors, we'd expect ~34 volts peak on the input. Allowing for some high line voltage puts this at 37 or so. A commercial grade 741 is only "guaranteed" to +-18V or 36V single supply. The Tektronix schematics and parts lists indicate that U410 should be "selected for +-20V supply" which more or less agrees with my calculation and says that I'll need something better than a commercial grade Op Amp. I don't have any MIL-grade 741's in 8-pin DIPs at the moment. I'm pretty sure that some of the 14-pin ceramic DIPs I have are full range and could be used if I make the space by replacing the 8-pin package by 8 pins of a wire-wrap socket to raise the 14-pin body above the other components. I may also have one or two 741's in 8-pin TO-5 cans that are better than commercial but I'm not certain. The original question was: SO ... I'm stuck it seems with several alternatives: 1. Finding a supplier of the higher grade 741 Op Amp. 2. Trying to select from my own stock one or two ICs that meet the criterion 3. Finding an acceptable substitute. And I commented that option 1 was expensive and time-consuming, that option 2 was likely to involve destructive testing and that I had no real knowledge of any "acceptable" substitutes where the substitute was a commercial grade Op Amp that I was likely to have easy access to. Also, neither of my two main suppliers, DigiKey and Mouser, had any MIL-grade 741's in stock according to their websites. It's been suggested that I use a 3-terminal IC regulator as a pre-regulator for the Op Amp to drop it's supply voltage down to where I could use a commercial part and I think that also makes a lot of sense. Thanks Norm |
#13
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Chuck Harris wrote in
: Ok, I have the schematic for a SG505 in front of me, which ought to be similar to your FG501 supply, SG505 PS is not going to be close to the FG501's PS circuitry. -- Jim Yanik jyanik-at-kua.net |
#14
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Norm Dresner wrote:
The circuit's input comes from the 25V AC on the backplane. Through the diode bridge and the filter capacitors, we'd expect ~34 volts peak on the input. Allowing for some high line voltage puts this at 37 or so. A commercial grade 741 is only "guaranteed" to +-18V or 36V single supply. The Tektronix schematics and parts lists indicate that U410 should be "selected for +-20V supply" which more or less agrees with my calculation and says that I'll need something better than a commercial grade Op Amp. I don't have any MIL-grade 741's in 8-pin DIPs at the moment. I'm pretty sure that some of the 14-pin ceramic DIPs I have are full range and could be used if I make the space by replacing the 8-pin package by 8 pins of a wire-wrap socket to raise the 14-pin body above the other components. I may also have one or two 741's in 8-pin TO-5 cans that are better than commercial but I'm not certain. Go to mouser and order: 511-UA741CN or 511-UA741IN They are made by ST, and sport a +/-22V abs max supply voltage. The CN is a 0-70C DIP, and the IN is a -55-+125C DIP -Chuck Harris |
#15
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Jim Yanik wrote:
Ok, I have the schematic for a SG505 in front of me, which ought to be similar to your FG501 supply, SG505 PS is not going to be close to the FG501's PS circuitry. Really? It is drawn just as the OP describes. The only real difference from his description is they use a 1458 in the SG505, and a 741 in the FG501. -Chuck Harris |
#16
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Norm Dresner wrote:
"mike" wrote in message ... Norm Dresner wrote: Just to be clear, I recommended you use the lm317 to power the new 741. The sine distortion is gonna be critically dependent on the stability of this reference voltage. I never used a 317 anywhere I cared about voltage stability and have no direct experience. May be just fine. Also, if you use it to replace the supply, you also need to worry about voltage differential when you short the 20V supply. Don't remember what the max differential for a lm317 might be. While you're at it, put some resistance in pin 3 of U410 so C406 won't blast the input on power down. mike My point in checking for oscillations was that sometimes an oscillating Op Amp can give some very strange readings on a simply DMM. Norm Aha! I understand -- use the IC regulator to drop the voltage to the Op Amp to a level where I could use a commercial grade safely. Good point. Regarding max voltage inputs, I think I can safely use an LM7824 to drop whatever the input is down to 24 volts to drive a jellybean 741 and I've got one or two in some drawer somewhere around here. Here's your quandry. You need high enough volts to supply 20V + 1 diode drop + 2 resistor drops + the headroom the 741 needs to supply that current. 24V might not be enough??? I haven't looked up the specs on a 741 in 25 years. I always designed to a minimum trough voltage of 22V. It may be higher depending on the max load on C400. You should measure the trough voltage at maximum output and low line. That'll give you how much headroom you have for the preregulator. It's probably gonna be a negative number. That's why you'll have to decide how low you can let the line go and how fine you can cut the 741 headroom. As for C406, my circuit diagram shows a diode CR410 whose anode is connected to the + side of the capacitor and the cathode to the V+ input to the Op Amp to keep its input voltage no more than a diode drop above the supply voltage. I couldn't find the diode on the circuit board anywhere or in the parts location diagram but it's clearly on the schematic. If I can't find it, I'll put a 1N4148 in as a "jumper" The diode is not on my early schematic. The FG501 was one of the first TM500 units designed. Over time the more glaring design mistakes got fixed. Thanks Norm -- Return address is VALID. 500MHz Tek DSOscilloscope TDS540 Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121 Bunch of stuff For Sale and Wanted at the link below. http://www.geocities.com/SiliconValley/Monitor/4710/ |
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"Chuck Harris" wrote in message
... Norm Dresner wrote: The circuit's input comes from the 25V AC on the backplane. Through the diode bridge and the filter capacitors, we'd expect ~34 volts peak on the input. Allowing for some high line voltage puts this at 37 or so. A commercial grade 741 is only "guaranteed" to +-18V or 36V single supply. The Tektronix schematics and parts lists indicate that U410 should be "selected for +-20V supply" which more or less agrees with my calculation and says that I'll need something better than a commercial grade Op Amp. I don't have any MIL-grade 741's in 8-pin DIPs at the moment. I'm pretty sure that some of the 14-pin ceramic DIPs I have are full range and could be used if I make the space by replacing the 8-pin package by 8 pins of a wire-wrap socket to raise the 14-pin body above the other components. I may also have one or two 741's in 8-pin TO-5 cans that are better than commercial but I'm not certain. Go to mouser and order: 511-UA741CN or 511-UA741IN They are made by ST, and sport a +/-22V abs max supply voltage. The CN is a 0-70C DIP, and the IN is a -55-+125C DIP -Chuck Harris Aha! I was fooled by the part number. At DigiKey they have the National Semi part which also has the CN suffix but only operates at +-18V. Thanks for the info Norm |
#18
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"Norm Dresner" wrote in message
... "Chuck Harris" wrote in message ... Norm Dresner wrote: Go to mouser and order: 511-UA741CN or 511-UA741IN They are made by ST, and sport a +/-22V abs max supply voltage. The CN is a 0-70C DIP, and the IN is a -55-+125C DIP -Chuck Harris Aha! I was fooled by the part number. At DigiKey they have the National Semi part which also has the CN suffix but only operates at +-18V. Thanks for the info Norm Browsing through some papers, I just discovered that the "Signetics" NE5534N Op Amp has an absolute maximum voltage spec of +-22V. I've got a bunch of them and it sounds like this should solve my problem. Well, I might have to add some additional compensation to slow it down to 741-speed, but I think that's the way to go. Norm |
#19
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Norm Dresner wrote:
Browsing through some papers, I just discovered that the "Signetics" NE5534N Op Amp has an absolute maximum voltage spec of +-22V. I've got a bunch of them and it sounds like this should solve my problem. Well, I might have to add some additional compensation to slow it down to 741-speed, but I think that's the way to go. Norm The ST part is a grand total of $0.88 each. If you buy nothing else, then that will be coupled with a $5 shipping charge... still not too bad for the real thing. -Chuck |
#20
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"Chuck Harris" wrote in message
... Norm Dresner wrote: Browsing through some papers, I just discovered that the "Signetics" NE5534N Op Amp has an absolute maximum voltage spec of +-22V. I've got a bunch of them and it sounds like this should solve my problem. Well, I might have to add some additional compensation to slow it down to 741-speed, but I think that's the way to go. Norm The ST part is a grand total of $0.88 each. If you buy nothing else, then that will be coupled with a $5 shipping charge... still not too bad for the real thing. You're right -- there's nothing like the "real thing", unless there's something better. One of the reasons that was given for not using a simple 3-terminal regulator here was that the OpAmp-based regulator would provide much better regulation in general and better ripple rejection in particular. If that's really true, then how could a better Op Amp hurt? I can't believe that the designers of the circuit chose the 741 as the optimum device to use over dozens of Op Amps that hadn't been designed yet. BTW, I've also found out that the OP-27 has +-22V specs too and I've got those as well. It's got lower noise and offset voltage, higher slew rate, and wider bandwidth than the 741. Which of those do you think make it less suitable? Norm |
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Norm Dresner wrote:
You're right -- there's nothing like the "real thing", unless there's something better. One of the reasons that was given for not using a simple 3-terminal regulator here was that the OpAmp-based regulator would provide much better regulation in general and better ripple rejection in particular. If that's really true, then how could a better Op Amp hurt? I can't believe that the designers of the circuit chose the 741 as the optimum device to use over dozens of Op Amps that hadn't been designed yet. BTW, I've also found out that the OP-27 has +-22V specs too and I've got those as well. It's got lower noise and offset voltage, higher slew rate, and wider bandwidth than the 741. Which of those do you think make it less suitable? Norm The higher slew rate and wider bandwidth will likely screw your circuit. When the circuit was designed, and the board was layed out, it was done with a 741. The things are very slow, and as such perfectly suited to power supply use. With a faster opamp, you may find that your ciruit has a tendency to oscillate, and to be bothered by powerline transients. Or... it might work perfectly well. It is easy enough to try. Do your circuit board a favor, and clip the pins off of the old IC flush with its body, and remove them one at a time... it is always easier on the board than solder wick, or those hand held vacuum thingies. I even do that with my PACE desoldering station if I don't need to save the chip. -Chuck Harris |
#22
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"Chuck Harris" wrote in message
... Norm Dresner wrote: You're right -- there's nothing like the "real thing", unless there's something better. One of the reasons that was given for not using a simple 3-terminal regulator here was that the OpAmp-based regulator would provide much better regulation in general and better ripple rejection in particular. If that's really true, then how could a better Op Amp hurt? I can't believe that the designers of the circuit chose the 741 as the optimum device to use over dozens of Op Amps that hadn't been designed yet. BTW, I've also found out that the OP-27 has +-22V specs too and I've got those as well. It's got lower noise and offset voltage, higher slew rate, and wider bandwidth than the 741. Which of those do you think make it less suitable? Norm The higher slew rate and wider bandwidth will likely screw your circuit. When the circuit was designed, and the board was layed out, it was done with a 741. The things are very slow, and as such perfectly suited to power supply use. With a faster opamp, you may find that your ciruit has a tendency to oscillate, and to be bothered by powerline transients. Or... it might work perfectly well. It is easy enough to try. Do your circuit board a favor, and clip the pins off of the old IC flush with its body, and remove them one at a time... it is always easier on the board than solder wick, or those hand held vacuum thingies. I even do that with my PACE desoldering station if I don't need to save the chip. -Chuck Harris As soon as my eye is healed enough (from surgery) to be able to bend down and see well enough to solder I'll give it a try. Thanks for the desoldering tip. Norm |
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Norm Dresner wrote:
body, and remove them one at a time... it is always easier on the board than solder wick, or those hand held vacuum thingies. I even do that with my PACE desoldering station if I don't need to save the chip. -Chuck Harris As soon as my eye is healed enough (from surgery) to be able to bend down and see well enough to solder I'll give it a try. Thanks for the desoldering tip. Norm Hi Norm, If you are just recovering from eye surgery, I would like to suggest that you do two things before soldering: 1) wear a pair of eyeglasses, or safety glasses, and 2) use a small muffin fan to draw the fumes away from your face. The last thing you want after eye surgery is to get a solder burn on your cornea, or the irritation can come from flux fumes. -Chuck |
#24
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"Chuck Harris" wrote in message
... Norm Dresner wrote: body, and remove them one at a time... it is always easier on the board than solder wick, or those hand held vacuum thingies. I even do that with my PACE desoldering station if I don't need to save the chip. -Chuck Harris As soon as my eye is healed enough (from surgery) to be able to bend down and see well enough to solder I'll give it a try. Thanks for the desoldering tip. Norm Hi Norm, If you are just recovering from eye surgery, I would like to suggest that you do two things before soldering: 1) wear a pair of eyeglasses, or safety glasses, and 2) use a small muffin fan to draw the fumes away from your face. The last thing you want after eye surgery is to get a solder burn on your cornea, or the irritation can come from flux fumes. -Chuck I've worn glasses to "see" ever since I was in the 2nd grade but my wife (a former industrial chemist) and I had this discussion only yesterday evening and concluded that I should wear the same protective goggles that I wear in the workshop -- their advantage is that they're closed on the sides as well. She (and I) are also well aware of the problem from the fumes. Thanks for taking the time to make these suggestions. Norm |
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