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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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Problem with Tektronix 475 "ADD" function
I own a Tektronix 475 that works well, but requires some realignment.
So far, I've been able to recalibrate the channel preamps and adjust the input attenuators. Individually, each channel now produces an accurate trace; however, I have a troublesome problem that I can't quite figure out. If I feed the same signal to both channels, invert Channel 2 (press "INVERT") and then add it to Channel 1 (press "ADD"), I get a flat trace that's offset by nearly -6 volts. The flat trace is good, but no matter what I try, I can't get rid of this offset. I've traced through the block diagrams and schematics and at this point have pretty much ruled out any problem with the preamp board. I fear the problem is down stream, probably in the signal combination circuitry prior to the final vertical amplifier. Is integrated circuit U370 the source of the problem? Is there an adjustment I've missed? My eyes have grown weary tracing schematics, so I could really use some sage wisdom here to save my eyesight. -Dave Drumheller |
#2
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Problem with Tektronix 475 "ADD" function
This has got to be a DC offset in the inverter stage itself, that is
even though it night not be a seperate stage, whatever stage does it. This offset is being compensated for by the front panel position control, so after to get that straightened out you'll have to reset it's range of course. If the problem is indeed in a vertical amp IC, you probably can't get it, but you may be able to swap it with the channel that doesn't invert. You would actually be adjusting out a problem, but if it works it works. There is a guy in this NG who can probably give you loads of more specific info named Jim Yanik. Watch the thread, if you get a reply from him it will probably be quite enlightening. You might also search the group for his name and find something useful. Another piece of information that would be helpful is whether the 6 volt offset is at a certain scale, and reads 6 volts at different gain settings, or is it simply at a certain position on the screen, i.e., if you change the range does indicated offset or the trace position remain constant ? This will indicate whether it's before or after the attenuator. Also does the trace move when you use the variable control, that is bringing it out of the calibrated stop ? If you can't solve this problem without parts and can't live with it there are surplus places that might have one with serious screen burn or a bad CRT or something. One such place is Electronic Surplus in Cleveland Ohio. If they were open 24/7 guys like us could spend days there looking at all their cool stuff. If you talk to the right person there they will probably know if they have what you need. They also used to have quite a selection of Tek plugins, but IIRC the 475 doesn't use them. If it did it would simply be a matter of sliding in another vertical amp. I don't have their number handy and at the moment I dunno if they got a website. You may be able to get some NLA parts via mail from them but it's nowhere near as much fun as being there. If you're near Ohio it might be worth a visit if you have the time. If near Florida you might want to check out a place called Skycraft in the Orlando area. I would definitely call first, but if they have what you need a visit would be interesting if it is feasible for you. In either place I suggest you take your gold card, you will find some goodies believe me. JURB |
#3
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Problem with Tektronix 475 "ADD" function
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#4
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Problem with Tektronix 475 "ADD" function
Jim and JURB,
Ok, I've checked the value of the offset that appears when using the "ADD" function and it is a constant one division (5 subdivisions) regardless of the vertical scale deflection setting. (Of course, both channels are always set to the same vertical deflection). So does this suggest that the problem is in the channel preamps or a following stage? Also, I poked around the vertical switching circuit on the input side to U370 (monolithic switch) and found all DC voltages to be consistent with the schematic, thus suggesting that there's nothing wrong with it. Suggestions? Thanks. This is very helpful. Best, -Dave |
#6
Posted to sci.electronics.repair
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Problem with Tektronix 475 "ADD" function
D; regarding your statement:
Also, I poked around the vertical switching circuit on the input side to U370 (monolithic switch) and found all DC voltages to be consistent with the schematic, thus suggesting that there's nothing wrong with it. If current setting or limiting resistors go out of spec, rather then open, you may well see no change in voltage readings. Even if they go completely open you still might not see any significant change. What's more any change you see might well be less than the device to device tolerance. A voltmeter is not the solution to everything. Even it's drop from a power supply isn't an accurte indicator whether the porper current is flowing or not. There is a definitive way to check this without ridiculous test equipment, but it involves math and alot of thinking. Thus I'll go into it another time. It's late and I gotta work. JURB |
#7
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Problem with Tektronix 475 "ADD" function
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