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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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Replacement meter has fsd current about three times old damagedmeter.
Repairing a used VSWR meter; which has a badly damaged meter. Meter
front broken off, most of the meter needle missing etc. If had another similar meter would have a go at fixing the meter and making a scale for it but nothing of similar manufacture! Unit is Bendix Microwave Devices: Micro Match Model 70N? (Nameplate slightly scarred so maybe it is model 7 ON?) Has 3 ranges Fwd and Rev. 30, 75, 300 Watts. 'N' type Load and Transmitter connectors. Have replacement meter (said to be from a Heathkit) approx same dimensions which mechanically fits perfectly; but has an fsd of about 100 microamps (I think) whereas the old damaged one appears to be about 30 microamps fsd. Idea is to fit a small DC amplifier mounted on a piece of vero-board etc. inside to amplify the DC signal that comes through the range and FWD/REV switch from the probes. The DC amp. Will require small battery (probably a 9 volt attached with one of those snap on pigtail connectors) that can perhaps be activated by either a push switch or maybe a two position toggle switch (Momentary/On). To avoid running down the battery might add an LED "ON" warning as well? It will also be necessary to adjust the DC gain to 'calibrate' the meter deflection. There is sufficient space inside IMO to easily accommodate all this. Where, as an old 'tube' man, could I find a suitable circuit or buy a suitable little module to fit module? Help/advice most appreciated. terry |
#2
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Replacement meter has fsd current about three times old damaged meter.
terry writes:
Repairing a used VSWR meter; which has a badly damaged meter. Meter front broken off, most of the meter needle missing etc. If had another similar meter would have a go at fixing the meter and making a scale for it but nothing of similar manufacture! Unit is Bendix Microwave Devices: Micro Match Model 70N? (Nameplate slightly scarred so maybe it is model 7 ON?) Has 3 ranges Fwd and Rev. 30, 75, 300 Watts. 'N' type Load and Transmitter connectors. Have replacement meter (said to be from a Heathkit) approx same dimensions which mechanically fits perfectly; but has an fsd of about 100 microamps (I think) whereas the old damaged one appears to be about 30 microamps fsd. Idea is to fit a small DC amplifier mounted on a piece of vero-board etc. inside to amplify the DC signal that comes through the range and FWD/REV switch from the probes. The DC amp. Will require small battery (probably a 9 volt attached with one of those snap on pigtail connectors) that can perhaps be activated by either a push switch or maybe a two position toggle switch (Momentary/On). To avoid running down the battery might add an LED "ON" warning as well? It will also be necessary to adjust the DC gain to 'calibrate' the meter deflection. There is sufficient space inside IMO to easily accommodate all this. Where, as an old 'tube' man, could I find a suitable circuit or buy a suitable little module to fit module? Help/advice most appreciated. terry You need a transimpedance amplifier, which is a trivial op-amp circuit. Google will find one. --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/ Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/ +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html Important: Anything sent to the email address in the message header above is ignored unless my full name AND either lasers or electronics is included in the subject line. Or, you can contact me via the Feedback Form in the FAQs. |
#3
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Replacement meter has fsd current about three times old damaged meter.
On Sun, 27 Jul 2008 08:24:12 -0700, terry wrote:
some deleted: Have replacement meter (said to be from a Heathkit) approx same dimensions which mechanically fits perfectly; but has an fsd of about 100 microamps (I think) whereas the old damaged one appears to be about 30 microamps fsd. Idea is to fit a small DC amplifier mounted on a piece of vero-board etc. inside to amplify the DC signal that comes through the range and FWD/REV switch from the probes. more deleted: Terry you should check to see if the meter circuit contains a shunt and if so try adjusting it for the coser to the proper range. If you can obtain a movement that is identical except for the scale the original meter face an probablt be placed on the working mechanism to render a more original look. Using an amplifier in a sensitive circuit like a swr bridge is problematic since the amplifier itself will respond with frequency sensitive gains. You might want to go to some of the surplus sites that still do exist like: Fair Radio Sales http://www.fairradio.com/ American Science and Surplus http://www.sciplus.com/ |
#4
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Replacement meter has fsd current about three times old damagedmeter.
On Jul 27, 10:17*pm, Gnack Nol wrote:
On Sun, 27 Jul 2008 08:24:12 -0700, terry wrote: some deleted: Have replacement meter (said to be from a Heathkit) approx same dimensions which mechanically fits perfectly; but has an fsd of about 100 microamps (I think) whereas the old damaged one appears to be about 30 microamps fsd. Idea is to fit a small DC amplifier mounted on a piece of vero-board etc. inside to amplify the DC signal that comes through the range and FWD/REV switch from the probes. more deleted: Terry you should check to see if the meter circuit contains a shunt and if so try adjusting it for the coser to the proper range. If you can obtain a movement that is identical except for the scale the original meter face an probablt be placed on the working mechanism to render a more original look. Using an amplifier in a sensitive circuit like a swr bridge is problematic since the amplifier itself will respond with frequency sensitive gains. You might want to go to some of the surplus sites that still do exist like: Fair Radio Saleshttp://www.fairradio.com/ American Science and Surplushttp://www.sciplus.com/ Thank you for the comments. Re. frequency senistivity. No think that enters int it? The unit has two sensor, that look like diodes mounted in cavities in the brass silver plated transmission line of the unit. The ouputs of the diodes are connected to a 6 position switch. Three FWD. and three REV. power positions. The output of the switch is just two unshielde short leads to the meter iteself. There is also a tip/ring headphone style jack on the side of the unit which disconnects the internal meter and extends the circuit so it can be read by a remote or external meter. So at that stage believe everything 'after' the sensor diodes is low current DC. So it would appear necessary to amplify the DC micro-voltage/current by about 3 to operate the replacement meter. BTW tempoarily arranged the existing (damaged) meter (disconnected from the unit) and the replacement in series with a low voltage source and a varaible resitor. With the old meter at (as far as coule be judged due to the damage) at full scale the replacement was about 0ne third. Haven't measured the DC resistance of each yet because that might have an effect on whatever circuit is used to 'step uop' the DC current to operate the meter. |
#5
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Replacement meter has fsd current about three times old damagedmeter.
terry wrote:
Repairing a used VSWR meter; which has a badly damaged meter. Meter front broken off, most of the meter needle missing etc. If had another similar meter would have a go at fixing the meter and making a scale for it but nothing of similar manufacture! Unit is Bendix Microwave Devices: Micro Match Model 70N? (Nameplate slightly scarred so maybe it is model 7 ON?) Has 3 ranges Fwd and Rev. 30, 75, 300 Watts. 'N' type Load and Transmitter connectors. Have replacement meter (said to be from a Heathkit) approx same dimensions which mechanically fits perfectly; but has an fsd of about 100 microamps (I think) whereas the old damaged one appears to be about 30 microamps fsd. Idea is to fit a small DC amplifier mounted on a piece of vero-board etc. inside to amplify the DC signal that comes through the range and FWD/REV switch from the probes. snip Hi, I have one of those RF Wattmeters. It is a Bendix model 711-N (presumably the "N" indicates type N connectors). It is the same as the M.C. Jones Micromatch model 711N, Bendix bought out M.C. Jones. You can get a manual for the M.C. Jones 711N from A.G. Tannenbaum, but the manual lacks the schematic and circuit description. Data is as follows: Freq Range 25 to 1000 MHz Impedance 51.5 Ohms Accuracy +/- 5% of full scale on each range. RF Power Range 30W full scale, 75W full scale, and 300W full scale, fwd or reflected. There is a jack on the side for a remote indicator, but I have no data for that. The meter movement is 30uA d.c. full scale. My panel meter is marked DEL MS24, there is no panel meter maker's name. Sorry, I have no practical experience with op-amps, but the d.c. current amplifier should be straightforward. You'll probably need a load across the op amp input with a resistance about the same as the original panel meter. I figure the original 30uA meter had a 2K resistance or so (if GE, Simpson or Triplett), or about 1.5K if Weston. Caution: Don't just put an Ohmmeter or DMM across your new panel meter, to measure its resistance, you'll probably destroy it. 73, Ed Knobloch |
#6
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Replacement meter has fsd current about three times old damaged meter.
On Sun, 27 Jul 2008 08:24:12 -0700 (PDT), terry
wrote: Repairing a used VSWR meter; which has a badly damaged meter. Meter front broken off, most of the meter needle missing etc. If had another similar meter would have a go at fixing the meter and making a scale for it but nothing of similar manufacture! Unit is Bendix Microwave Devices: Micro Match Model 70N? (Nameplate slightly scarred so maybe it is model 7 ON?) Has 3 ranges Fwd and Rev. 30, 75, 300 Watts. 'N' type Load and Transmitter connectors. Have replacement meter (said to be from a Heathkit) approx same dimensions which mechanically fits perfectly; but has an fsd of about 100 microamps (I think) whereas the old damaged one appears to be about 30 microamps fsd. Idea is to fit a small DC amplifier mounted on a piece of vero-board etc. inside to amplify the DC signal that comes through the range and FWD/REV switch from the probes. The DC amp. Will require small battery (probably a 9 volt attached with one of those snap on pigtail connectors) that can perhaps be activated by either a push switch or maybe a two position toggle switch (Momentary/On). To avoid running down the battery might add an LED "ON" warning as well? It will also be necessary to adjust the DC gain to 'calibrate' the meter deflection. There is sufficient space inside IMO to easily accommodate all this. Where, as an old 'tube' man, could I find a suitable circuit or buy a suitable little module to fit module? Help/advice most appreciated. terry ============== A transimpedance amplifier is what you need, as Sam suggested. Here's a circuit to start with. You might have to fiddle with exact resistor values, depending on the Op-amp you use, which isn't critical, except for input offset voltage. Select a dual Opamp with low offset voltage, such as the Texas Instruments OPA2227. Since it only sees DC currents and voltages, there are no issues with frequency response. The circuit runs off a 9V battery. Current drain is reasonably low, but if you want to leave it on for extended periods, you'll want to fashion a line-operated DC power supply (a wall-wart is good for this). The bottom half of the opamp is used to split the battery voltage in half to provide a virtual ground. This keeps the opamp well within the linear region, away from the battery's rails. View in a fixed-width font such as Courier. |+9V | | | | +---------+ +---------|--+ Rf +-+ | | +---------+ | | |\ | 33.2K 1% | | | \| | +------+------+- \ | Input Current | \U1A | 0 - 30uA | /------------+ Vout=0 - 1V | / | (0 - 100 uA) +----++ / | | | /| +-+-+ | |/ | | | | | | |Rscale | | | |10K 1% +++ | | | + | +-+-+ | | -9V | | 100uA *** Meter * * * * * * +9V *** | | +-+-+ | | | +++ | | +-------------+ + 10K | | | | 1% | | | +9V | +-+-+ | | | | | |\ | | | | | \| | | +--+- \ | | | \U1B | | | ------+ | | / | +-----------++ / | | | /| | | |/ | | +-+-+ | +++ | | -9V + | | 10K | | 1% | | +-+-+ | -9V Cheers!!! Dave M Never take a laxative and a sleeping pill at the same time!! |
#7
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Replacement meter has fsd current about three times old damaged meter.
On Mon, 28 Jul 2008 20:52:07 -0400, Dave M
put finger to keyboard and composed: A transimpedance amplifier is what you need, as Sam suggested. Here's a circuit to start with. You might have to fiddle with exact resistor values, depending on the Op-amp you use, which isn't critical, except for input offset voltage. Select a dual Opamp with low offset voltage, such as the Texas Instruments OPA2227. Since it only sees DC currents and voltages, there are no issues with frequency response. The circuit runs off a 9V battery ... I don't mean to nitpick your clever circuit, but you've drawn it as running off *two* 9V batteries. Maybe you should have designated the battery terminals at V+ and V- rather than +/-9V. +9V | +-+-+ | | | | +-------------+ 10K | | | | 1% | | | +9V | +-+-+ | | | | | |\ | | | | | \| | | +--+- \ | | | \U1B | | | ------+ | | / | +-----------++ / | | | /| | | |/ | | +-+-+ | +++ | | -9V + | | 10K | | 1% | | +-+-+ | -9V Cheers!!! Dave M Never take a laxative and a sleeping pill at the same time!! Wise words. - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#8
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Replacement meter has fsd current about three times old damaged meter.
On Mon, 28 Jul 2008 12:53:15 -0700 (PDT), terry
put finger to keyboard and composed: The unit has two sensor, that look like diodes mounted in cavities in the brass silver plated transmission line of the unit. The ouputs of the diodes are connected to a 6 position switch. Three FWD. and three REV. power positions. The output of the switch is just two unshielde short leads to the meter iteself. There is also a tip/ring headphone style jack on the side of the unit which disconnects the internal meter and extends the circuit so it can be read by a remote or external meter. So at that stage believe everything 'after' the sensor diodes is low current DC. So it would appear necessary to amplify the DC micro-voltage/current by about 3 to operate the replacement meter. BTW tempoarily arranged the existing (damaged) meter (disconnected from the unit) and the replacement in series with a low voltage source and a varaible resitor. With the old meter at (as far as coule be judged due to the damage) at full scale the replacement was about 0ne third. Haven't measured the DC resistance of each yet because that might have an effect on whatever circuit is used to 'step uop' the DC current to operate the meter. Excuse my ignorance, but wouldn't the 6-position switch be inserting three different series resistances to vary the meter's sensitivity for three different power ranges? If so, then why not just replace these resistors with lower values, assuming the diodes are not affected by the additional current? FWIW, I found this home brew design that uses a 100uA meter: http://www.qsl.net/vu2msy/homebrew/S...newsletter.pdf - Franc Zabkar -- Please remove one 'i' from my address when replying by email. |
#9
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Replacement meter has fsd current about three times old damagedmeter.
On Jul 29, 4:10*am, Franc Zabkar wrote:
On Mon, 28 Jul 2008 20:52:07 -0400, Dave M put finger to keyboard and composed: A transimpedance amplifier is what you need, as Sam suggested. *Here's a circuit to start with. * You might have to fiddle with exact resistor values, depending on the Op-amp you use, which isn't critical, except for input offset voltage. Select a dual Opamp with low offset voltage, such as the Texas Instruments OPA2227. Since it only sees DC currents and voltages, there are no issues with frequency response. *The circuit runs off a 9V battery ... I don't mean to nitpick your clever circuit, but you've drawn it as running off *two* 9V batteries. Maybe you should have designated the battery terminals at V+ and V- rather than +/-9V. * * * * * +9V * * * * * *| * * * * *+-+-+ * * * * *| * | * * * * *| * | * * *+-------------+ * * *10K | * | * * *| * * * * * * | * * *1% *| * | * * *| * *+9V * * *| * * * * *+-+-+ * * *| * * | * * * | * * * * * *| * * * *| *|\ | * * * | * * * * * *| * * * *| *| \| * * * | * * * * * *| * * * *+--+- \ * * * | * * * * * *| * * * * * | * \U1B * | * * * * * *| * * * * * | * *------+ * * * * * *| * * * * * | * / * * *| * * * * * *+-----------++ / * * * | * * * * * *| * * * * * | /| * * * | * * * * * *| * * * * * |/ | * * * | * * * * *+-+-+ * * * * * *| * * *+++ * * * * *| * | * * * * * -9V * * *+ * * * * *| * | * * *10K | * | * * *1% *| * | * * * * *+-+-+ * * * * * *| * * * * * -9V Cheers!!! Dave M Never take a laxative and a sleeping pill at the same time!! Wise words. - Franc Zabkar -- Please remove one 'i' from my address when replying by email.- Hide quoted text - - Show quoted text - Great replies: thank for the thoughts and information. Also for the unit specification; had no indication is was 25 mhz. up. But the small size/length of the transmission line unit inside it might have been some indication. Seems to boild down to a) Look at and modify/jigger the values of the resistors in the DC part of the switching circuit to increase meter deflection 'as is'. |
#10
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Replacement meter has fsd current about three times old damagedmeter.
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#11
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Replacement meter has fsd current about three times old damagedmeter.
terry wrote: Great replies: thank for the thoughts and information. Also for the unit specification; had no indication is was 25 mhz. up. But the small size/length of the transmission line unit inside it might have been some indication. Seems to boild down to a) Look at and modify/jigger the values of the resistors in the DC part of the switching circuit to increase meter deflection 'as is'. Hi, Another hint: I see that the shields of the coax carrying the d.c. voltage from the fwd and reflected detectors are returned to the + side of the panel meter. So the + side of the meter is grounded and the detectors put out more a more negative voltage as the rf power increases. 73, Ed Knobloch |
#12
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Replacement meter has fsd current about three times old damagedmeter.
On Jul 29, 4:54*pm, Edward Knobloch wrote:
terry wrote: Great replies: thank for the thoughts and information. Also for the unit specification; had no indication is was 25 mhz. up. But the small size/length of the transmission line unit inside it might have been some indication. Seems to boild down to a) Look at and modify/jigger the values of the resistors in the DC part of the switching circuit to increase meter deflection 'as is'. Hi, Another hint: I see that the shields of the coax carrying the d.c. voltage from the fwd and reflected detectors are returned to the + side of the panel meter. *So the + side of the meter is grounded and the detectors put out more a more negative voltage as the rf power increases. 73, Ed Knobloch Excellent info and advice. Thanks Ed. |
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