Email received Wed 26 Mar 2008 05:58:32 PM BST
> Subject: ADCOM GFA-585 SUCCESS!!!
>
> Jim: The amp is repaired and completly stable. The key was obtaining 2 NOS
> adcom ic's , not the new items they sell as replacements or the ones I tried.
> These may be the last two on earth, but they are in your set. There will be no
> charge for parts or labor, just shipping.
The amp was shipped Parcel Post (6-10 weeks) rather than Priority Mail (5-10 days), so it only just arrived. While the right channel now stabilizes at -.013 VDC, the left channel still behaves exactly as reflected in the logs and chart attached to my earlier post (stabilizes at -1.35 VDC and sees double-digit voltages when switched off). After a full year and five transatlantic trips at my expense, I'm giving up on the GFA-585 and sticking with the GFA-2535.
> Subject: ADCOM GFA-585 SUCCESS!!!
>
> Jim: The amp is repaired and completly stable. The key was obtaining 2 NOS
> adcom ic's , not the new items they sell as replacements or the ones I tried.
> These may be the last two on earth, but they are in your set. There will be no
> charge for parts or labor, just shipping.
The amp was shipped Parcel Post (6-10 weeks) rather than Priority Mail (5-10 days), so it only just arrived. While the right channel now stabilizes at -.013 VDC, the left channel still behaves exactly as reflected in the logs and chart attached to my earlier post (stabilizes at -1.35 VDC and sees double-digit voltages when switched off). After a full year and five transatlantic trips at my expense, I'm giving up on the GFA-585 and sticking with the GFA-2535.
Code:
Time Function Value Unit
05:52:21 PM DC 0 V
05:52:22 PM DC 0 V
... (switch on)
05:52:23 PM DC 0.08 V
05:52:24 PM DC 0.2 V
05:52:25 PM DC 0.13 V
05:52:26 PM DC 0.12 V
05:52:27 PM DC 0.09 V
05:52:28 PM DC 0.07 V
05:52:29 PM DC 0.05 V
05:52:30 PM DC 0.03 V
05:52:31 PM DC 0 V
05:52:32 PM DC -0.02 V
05:52:33 PM DC -0.04 V
05:52:34 PM DC -0.06 V
05:52:35 PM DC -0.09 V
05:52:36 PM DC -0.1 V
05:52:37 PM DC -0.12 V
05:52:38 PM DC -0.15 V
05:52:39 PM DC -0.16 V
05:52:40 PM DC -0.19 V
05:52:41 PM DC -0.2 V
05:52:42 PM DC -0.23 V
05:52:43 PM DC -0.25 V
05:52:44 PM DC -0.27 V
05:52:45 PM DC -0.28 V
05:52:46 PM DC -0.31 V
05:52:47 PM DC -0.32 V
05:52:48 PM DC -0.35 V
05:52:49 PM DC -0.36 V
05:52:50 PM DC -0.37 V
05:52:51 PM DC -0.4 V
05:52:52 PM DC -0.41 V
05:52:53 PM DC -0.43 V
05:52:54 PM DC -0.45 V
05:52:55 PM DC -0.47 V
05:52:56 PM DC -0.48 V
05:52:57 PM DC -0.5 V
05:52:58 PM DC -0.52 V
05:52:59 PM DC -0.54 V
05:53:00 PM DC -0.55 V
05:53:01 PM DC -0.57 V
05:53:02 PM DC -0.59 V
05:53:03 PM DC -0.61 V
05:53:04 PM DC -0.62 V
05:53:05 PM DC -0.64 V
05:53:06 PM DC -0.65 V
05:53:07 PM DC -0.67 V
05:53:08 PM DC -0.69 V
05:53:09 PM DC -0.7 V
05:53:10 PM DC -0.72 V
05:53:11 PM DC -0.74 V
05:53:12 PM DC -0.76 V
05:53:13 PM DC -0.77 V
05:53:14 PM DC -0.79 V
05:53:15 PM DC -0.8 V
05:53:16 PM DC -0.82 V
05:53:17 PM DC -0.84 V
05:53:18 PM DC -0.85 V
05:53:19 PM DC -0.87 V
05:53:20 PM DC -0.89 V
05:53:21 PM DC -0.9 V
05:53:22 PM DC -0.92 V
05:53:23 PM DC -0.93 V
05:53:24 PM DC -0.95 V
05:53:25 PM DC -0.96 V
05:53:27 PM DC -0.98 V
05:53:27 PM DC -1 V
05:53:28 PM DC -1.01 V
05:53:29 PM DC -1.03 V
05:53:30 PM DC -1.04 V
05:53:31 PM DC -1.06 V
05:53:32 PM DC -1.07 V
05:53:33 PM DC -1.09 V
05:53:34 PM DC -1.11 V
05:53:35 PM DC -1.13 V
05:53:36 PM DC -1.14 V
05:53:37 PM DC -1.16 V
05:53:38 PM DC -1.17 V
05:53:40 PM DC -1.19 V
05:53:40 PM DC -1.2 V
05:53:42 PM DC -1.22 V
05:53:42 PM DC -1.23 V
05:53:43 PM DC -1.24 V
05:53:44 PM DC -1.26 V
05:53:45 PM DC -1.28 V
05:53:46 PM DC -1.3 V
05:53:48 PM DC -1.3 V
05:53:48 PM DC -1.3 V
... (warm up)
05:57:01 PM DC -1.35 V
... (switch off)
06:10:23 PM DC -1.41 V
06:10:24 PM DC - O.L V
06:10:25 PM DC -10.24 V
06:10:26 PM DC -10.33 V
06:10:27 PM DC -10.47 V
06:10:28 PM DC -10.57 V
06:10:29 PM DC -10.72 V
06:10:30 PM DC -10.82 V
06:10:31 PM DC -10.91 V
06:10:32 PM DC -11.07 V
06:10:33 PM DC -11.17 V
06:10:34 PM DC -11.32 V
06:10:35 PM DC -11.42 V
06:10:36 PM DC -11.57 V
06:10:37 PM DC -11.67 V
06:10:38 PM DC -11.82 V
06:10:39 PM DC -11.92 V
06:10:40 PM DC -12.06 V
06:10:41 PM DC -12.16 V
06:10:42 PM DC -12.31 V
06:10:43 PM DC -12.41 V
06:10:44 PM DC -12.55 V
06:10:45 PM DC -12.64 V
06:10:46 PM DC -12.79 V
06:10:47 PM DC -12.88 V
06:10:48 PM DC -13.02 V
06:10:49 PM DC -13.11 V
06:10:50 PM DC -13.25 V
06:10:51 PM DC -13.34 V
06:10:52 PM DC -13.43 V
06:10:53 PM DC -13.58 V
06:10:54 PM DC -13.67 V
06:10:55 PM DC -13.8 V
06:10:56 PM DC -13.89 V
06:10:57 PM DC -14.02 V
06:10:58 PM DC -14.1 V
06:10:59 PM DC -14.2 V
06:11:00 PM DC -14.27 V
06:11:01 PM DC -14.37 V
06:11:02 PM DC -14.44 V
06:11:03 PM DC -14.54 V
06:11:04 PM DC -14.61 V
06:11:05 PM DC -14.71 V
06:11:06 PM DC -14.77 V
06:11:07 PM DC -14.86 V
06:11:08 PM DC -14.92 V
06:11:09 PM DC -14.91 V
06:11:10 PM DC -14.9 V
06:11:11 PM DC -14.81 V
06:11:12 PM DC -14.74 V
06:11:13 PM DC -14.62 V
06:11:14 PM DC -14.54 V
06:11:15 PM DC -14.39 V
06:11:16 PM DC -14.28 V
06:11:17 PM DC -14.1 V
06:11:18 PM DC -13.98 V
06:11:19 PM DC -13.79 V
06:11:20 PM DC -13.66 V
06:11:21 PM DC -13.47 V
06:11:22 PM DC -13.35 V
06:11:23 PM DC -13.17 V
06:11:24 PM DC -13.04 V
06:11:25 PM DC -12.92 V
06:11:26 PM DC -12.74 V
06:11:27 PM DC -12.62 V
06:11:28 PM DC -12.45 V
06:11:29 PM DC -12.33 V
06:11:30 PM DC -12.16 V
06:11:31 PM DC -12.05 V
06:11:32 PM DC -11.88 V
06:11:33 PM DC -11.77 V
06:11:34 PM DC -11.58 V
06:11:35 PM DC -11.42 V
06:11:36 PM DC -11.01 V
06:11:37 PM DC -10.64 V
06:11:38 PM DC -9.96 V
06:11:39 PM DC -9.46 V
06:11:40 PM DC -8.69 V
06:11:41 PM DC -8.17 V
06:11:42 PM DC -7.38 V
06:11:43 PM DC -6.85 V
06:11:44 PM DC -6.06 V
06:11:45 PM DC -5.55 V
06:11:46 PM DC -4.83 V
06:11:47 PM DC -4.38 V
06:11:48 PM DC -3.94 V
06:11:49 PM DC -3.28 V
06:11:50 PM DC -2.89 V
06:11:51 PM DC -2.37 V
06:11:52 PM DC -2.05 V
06:11:53 PM DC -1.76 V
06:11:54 PM DC -1.36 V
06:11:55 PM DC -1.13 V
06:11:56 PM DC -0.81 V
06:11:57 PM DC -0.63 V
06:11:58 PM DC -0.4 V
06:11:59 PM DC -0.27 V
06:12:00 PM DC -0.12 V
06:12:01 PM DC -0.06 V
06:12:02 PM DC -0.01 V
06:12:03 PM DC 0 VADCOM GFA 565 DC OFFset
I have 2 GFA 565 amps, one blew out a speaker and after checking I found a leaky cap on the input board. I have now replaced all electrolytics (except the two large filter caps) on both amps. I cleaned off some leagage on the bad board, then found a bad zener so I replaced 4 zeners and the op amp (LM329DZ and OP97FPZ from Digi Key) on the bad amp only. The good amp caps did not appear leaky but I replaced them anyway.
The bad amp (with leaky cap) now starts with an offset of 120mv and settles at 45mv after a half hour. The good amp (no obviously leaky caps) starts with a 340mv offset and settles at 180mv. (unfortunatly I do not have readings from before changing the caps).
Do you think an ultrasonic cleaning will help? I do not have access to a cleaner so the next step will be expensive to purchase one.
Any ideas or thoughts will be helpful
Herb S
I have 2 GFA 565 amps, one blew out a speaker and after checking I found a leaky cap on the input board. I have now replaced all electrolytics (except the two large filter caps) on both amps. I cleaned off some leagage on the bad board, then found a bad zener so I replaced 4 zeners and the op amp (LM329DZ and OP97FPZ from Digi Key) on the bad amp only. The good amp caps did not appear leaky but I replaced them anyway.
The bad amp (with leaky cap) now starts with an offset of 120mv and settles at 45mv after a half hour. The good amp (no obviously leaky caps) starts with a 340mv offset and settles at 180mv. (unfortunatly I do not have readings from before changing the caps).
Do you think an ultrasonic cleaning will help? I do not have access to a cleaner so the next step will be expensive to purchase one.
Any ideas or thoughts will be helpful
Herb S
Herb,
It's possibe that there is still some residue on the board. The (my) GFA 585 will settle to single digit mV offset. However it took rigorous cleaning to get to this point.
Rigorous: remove input board and scrub with undiluted simple green on both sides. Use a toothbrush (not one belonging to a significant other ) and get right in between the component legs and pcb tracks. I have even heard of boards being run in the dishwasher, but there's no real substitute for scrubbing. Dry well.
The final cure for drifting offset on my amp was to remove some of the pcb lacquer. This had absorbed the electrolyte from leaking caps and caused leakage paths between PCB tracks.
Hope that is helpful,
Ed
It's possibe that there is still some residue on the board. The (my) GFA 585 will settle to single digit mV offset. However it took rigorous cleaning to get to this point.
Rigorous: remove input board and scrub with undiluted simple green on both sides. Use a toothbrush (not one belonging to a significant other
) and get right in between the component legs and pcb tracks. I have even heard of boards being run in the dishwasher, but there's no real substitute for scrubbing. Dry well.The final cure for drifting offset on my amp was to remove some of the pcb lacquer. This had absorbed the electrolyte from leaking caps and caused leakage paths between PCB tracks.
Hope that is helpful,
Ed
Ed Holland said:Herb,
It's possibe that there is still some residue on the board. The (my) GFA 585 will settle to single digit mV offset. However it took rigorous cleaning to get to this point.
Rigorous: remove input board and scrub with undiluted simple green on both sides. Use a toothbrush (not one belonging to a significant other
The final cure for drifting offset on my amp was to remove some of the pcb lacquer. This had absorbed the electrolyte from leaking caps and caused leakage paths between PCB tracks.
Hope that is helpful,
Ed
If I may add to Ed's comment:
After removing the board, electrolytics, servo and bias pot, I spray Simple Green on the board (dry) and use a soft clean paintbrush to scrub the board well - try to get bristles into the cap and servo holes - mash the paintbrush a bit so the bristles get under the transistors, diodes, resistors, etc. Rinse with warm water, remove excess water, and spray some more simple green. Scrub the top and bottom of the board as well as you can with the toothbrush. Rinse well with warm water, remove excess water and place on a nearby towel. I dry the board by shaking as much water off as possible and then using a heat gun (not a blow dryer) to dry the board off. Heat up the whole assembly until it's too hot to touch and keep it heated for a few minutes. The idea is to get the whole board so hot that any moisture under the parts evaporates, but not so hot as to damage the parts. Let the boards sit until room temp and then inspect and install.
Again, I have Ed Holland to thank for this (Thanks Ed!) - so far I have repaired one 585 and two pairs of 565s using this technique and all units settled to no more than 7mvDC.
Cheers,
Well I'm finally back. Removed the board and components as suggested and cleaned with Simple Clean a number of times, rinsing and blow drying. After letting the board rest a few weeks (too busy working) I reinstalled it yesterday. Upon startup I read 180mv, in a minute or so it drops to 160 and then climbs settling at about 240mv within an hour. Somewhat different but no real progress. Changed readings could indicate a change in the board after cleaning, but also some change in ambient conditions (temp, humidity) so I'm not sure I learned anything.
Has anyone considered building the servo circuit on a new separate board? It's not so many components. Where would you expect the leakage paths to be? Seems like you could take the feedback from pin 7 on the board, feed it to the outboard op-amp and reapply it at the junction of R105 and R106. I guess you could leave the diode reference circuit on the original board and feed it to pins 4 and 7 of the op-amp. Any thoughts?
Has anyone considered building the servo circuit on a new separate board? It's not so many components. Where would you expect the leakage paths to be? Seems like you could take the feedback from pin 7 on the board, feed it to the outboard op-amp and reapply it at the junction of R105 and R106. I guess you could leave the diode reference circuit on the original board and feed it to pins 4 and 7 of the op-amp. Any thoughts?
585/565 servo voltage checks
This is distressing, and I hope I can offer some help. From the above, it seems that the status of the servo IC(s) need to be verified. When it is working (OK), it will null out errors from many sources, such that the net output DC will be equal or less than the offset of the IC used (IC101/151). So if that offset were 1mV or less, then the output offset would be 23mV or less. That's assuming all extraneous DC errors sources other than the IC offset are negligible. The question is, can we verify that this is true? If there are still stray leakages in/around the op amp used at location IC101/151, then the output DC will still be anomalous. That sounds like the case here.
Checks to make:
1) Verify that supply voltages to IC101/151 are +13.8V at pin 7, and -13.8V at pin 4. These can be +/-5% without any problems. It is the simple addition of two LM329 reference diode drops. If these voltages aren't present, check R137, D113, D114 and/or R138, D111,, D112.
2) Assuming step 1) is OK, check ICs 101/151 for DC level at pin 6. This should be 10V or less (much less), and will vary +/- with time, temp, warmup status, etc. If pin 6 is stuck at either rail (i.e., +12V, -12V or more) check IC101/151. The inputs of IC101/151 see 1meg resistances to ground, and pin 3 is very susceptible to leakage from pin 4, which is at -13.8V. Likewise, pin 1 is very susceptible to leakage from pin 1, which is near +13.8V. Consequently, all traces which are common to these nodes must be clean of residue. This includes the underside of the IC itself, or the body between the pins.
When all of this is working correctly, pin 6 of the servo IC (101 or 151) will stay well within a range of +/-10V, most likely a few Vdc. The servo cannot however correct for excessive DC leakages elsewhere. Hopefully, these troubleshooting tips will help you converge upon a solution.
FWIW, yes, all of this could be built up on a small PCB, but the same leakage limiations would still aply there. So, be forewarned.
Good luck with it...
Walt Jung
This is distressing, and I hope I can offer some help. From the above, it seems that the status of the servo IC(s) need to be verified. When it is working (OK), it will null out errors from many sources, such that the net output DC will be equal or less than the offset of the IC used (IC101/151). So if that offset were 1mV or less, then the output offset would be 23mV or less. That's assuming all extraneous DC errors sources other than the IC offset are negligible. The question is, can we verify that this is true? If there are still stray leakages in/around the op amp used at location IC101/151, then the output DC will still be anomalous. That sounds like the case here.
Checks to make:
1) Verify that supply voltages to IC101/151 are +13.8V at pin 7, and -13.8V at pin 4. These can be +/-5% without any problems. It is the simple addition of two LM329 reference diode drops. If these voltages aren't present, check R137, D113, D114 and/or R138, D111,, D112.
2) Assuming step 1) is OK, check ICs 101/151 for DC level at pin 6. This should be 10V or less (much less), and will vary +/- with time, temp, warmup status, etc. If pin 6 is stuck at either rail (i.e., +12V, -12V or more) check IC101/151. The inputs of IC101/151 see 1meg resistances to ground, and pin 3 is very susceptible to leakage from pin 4, which is at -13.8V. Likewise, pin 1 is very susceptible to leakage from pin 1, which is near +13.8V. Consequently, all traces which are common to these nodes must be clean of residue. This includes the underside of the IC itself, or the body between the pins.
When all of this is working correctly, pin 6 of the servo IC (101 or 151) will stay well within a range of +/-10V, most likely a few Vdc. The servo cannot however correct for excessive DC leakages elsewhere. Hopefully, these troubleshooting tips will help you converge upon a solution.
FWIW, yes, all of this could be built up on a small PCB, but the same leakage limiations would still aply there. So, be forewarned.
Good luck with it...
Walt Jung
Don't know if you noticed my comment in an earlier post, but the electrolyte can contaminate the laquer/solder mask on the board. This is a definite cause of leakage paths and resulted in unstable 100 mV or so DC offsets in my amp.
Scraping the laquer away in the servo region (and re-washing) fixed the problem completely.
Ed
Scraping the laquer away in the servo region (and re-washing) fixed the problem completely.
Ed
Yes Ed, I did see that, thanks. My purpose of the voltage checklist was to allow one to observe the voltages around the servo IC, then conclude whether or not it can do the job. Obviously, if it is missing a rail voltage, or blown out (stuck at one rail), it can't do so. So, from Herb S's comments, it seems like he may have a servo that is trying to correct, but can't do so because of remaining leakages. Your suggestions could be just what he needs to finally correct the wandering Dc output.
These amps have been plagued by two types of problems. One is the servo IC and the other the electrolyte leakage contamination. These are really two separate problems, with both producing DC errors of different magnitudes. A failed servo will produce DC of a few mV to a couple of V, while the electrolyte leakage contamination can produce totally uncontrolled output DC, as much as 50V or more, which is easily capable of blowing out speakers. That's what it did for me.
Walt Jung
Assuming you addressed that Q to me Bjsam, and you are referring to that long list of voltages which are above 10V, no I don't think it is the servo. The servo can't correct for gross errors. The leakage must be eradicated before the servo can control the output properly.
This is what I was referring to as two problems. The contamination problem is gross, and it must be corrected before the servo can do its thing.
Look at pin 6 of the servo IC for the channel that has the >10Vdc output, and see what it is doing. Most likely it will be stuck to one rail. Compare this voltage at pin 6 to the corresonding point on the side that has the lower output offset. It should be very low.
Walt Jung
This is what I was referring to as two problems. The contamination problem is gross, and it must be corrected before the servo can do its thing.
Look at pin 6 of the servo IC for the channel that has the >10Vdc output, and see what it is doing. Most likely it will be stuck to one rail. Compare this voltage at pin 6 to the corresonding point on the side that has the lower output offset. It should be very low.
Walt Jung
Well, here goes. I removed the IC, cleaned the top and bottom of the board in the area of the IC and C118 once again with simple green scrubing with a tooth brush and scraping between runs with a pick (remember this amp did not have any obviously leaky caps). I then installed a new OP97 and powered it up.
DC Offset started around 60mv and within 1 min was less than 10mv. After 5min it wanders between -5mv and +5mv (sensitive to air movement, blowing on the board will make it go to 15mv max.
Whether it was the last cleaning or the new IC is anyone's guess. If it was the cleaning, this was the third or fourth time on what looked like a spotless board.
I have a green overcoat pen from MG Chemicals (Acrylic Conformal Coating), do you think it advisable to use over the runs that were cleaned and scraped in the IC area?
Thanks Walt, Ed, Woodman, without your help I would have given up long ago. Hope this confirms to others what you have already written.
Thanks,
Herb S
DC Offset started around 60mv and within 1 min was less than 10mv. After 5min it wanders between -5mv and +5mv (sensitive to air movement, blowing on the board will make it go to 15mv max.
Whether it was the last cleaning or the new IC is anyone's guess. If it was the cleaning, this was the third or fourth time on what looked like a spotless board.
I have a green overcoat pen from MG Chemicals (Acrylic Conformal Coating), do you think it advisable to use over the runs that were cleaned and scraped in the IC area?
Thanks Walt, Ed, Woodman, without your help I would have given up long ago. Hope this confirms to others what you have already written.
Thanks,
Herb S
Herb, that's great news - good on you for persevering!
Your account sounds very similar to what I saw after the cleaning, except I was able to use the original op-amp.
One thing I did notice, was that before I did the final cleaning and laquer chasing was that the offset was extremely sensitive to moisture level. Breathing on the board (not enough to form condensation) made the offset swing around, application of a hairdrier steadied it up. That was what made me consider the laquer, which had discoloured on the board in question.
I truly consider it worth some considerable effort to save these amplifiers, they are very good.
Happy listening,
Ed
Your account sounds very similar to what I saw after the cleaning, except I was able to use the original op-amp.
One thing I did notice, was that before I did the final cleaning and laquer chasing was that the offset was extremely sensitive to moisture level. Breathing on the board (not enough to form condensation) made the offset swing around, application of a hairdrier steadied it up. That was what made me consider the laquer, which had discoloured on the board in question.
I truly consider it worth some considerable effort to save these amplifiers, they are very good.
Happy listening,
Ed
Herb S, it sounds to me as if you are where you want to be. The levels of offset are normal, and due to a host of small things, air currents, etc. I would keep an eye on it for a few days, and if all is stable, wrap it up.
I have had no experience with the conformal coat product you mentioned. The idea is good, but unless the amp is subject to a high humidity place, I'd let it alone.
In any event, the DC voltages I mentioned above will tell you how the servo is working, and it sounds lik eit is.
I agree with Ed Holland... once one of these amps is working (properly), you have a valuable item.
Good luck,
Walt Jung
I have had no experience with the conformal coat product you mentioned. The idea is good, but unless the amp is subject to a high humidity place, I'd let it alone.
In any event, the DC voltages I mentioned above will tell you how the servo is working, and it sounds lik eit is.
I agree with Ed Holland... once one of these amps is working (properly), you have a valuable item.
Good luck,
Walt Jung
End of story.
I have gone back to the "bad amp", the one with the obviously leaky caps that had a DC Offset of about 50mv after replacing the IC and the reference diodes.
I removed the two caps and the ic, recleaned the boards with Simple Green a few times, also used a flux cleaner until the board was as clean as I could get it. Cleaned out the ic and cap holes with a small drill bit.
Powered up the unit and now the dc offset varies between +- 10mv.
Once again confirming that cleaning the boards, especially in the area of the IC circuit, is the answer to dc offset problems when all components check out. Incidently, I found the two 10 ohm resistors reading about 20 ohms and did replace them.
Hope this again confirms what has been written in this thread.
Herb S
I have gone back to the "bad amp", the one with the obviously leaky caps that had a DC Offset of about 50mv after replacing the IC and the reference diodes.
I removed the two caps and the ic, recleaned the boards with Simple Green a few times, also used a flux cleaner until the board was as clean as I could get it. Cleaned out the ic and cap holes with a small drill bit.
Powered up the unit and now the dc offset varies between +- 10mv.
Once again confirming that cleaning the boards, especially in the area of the IC circuit, is the answer to dc offset problems when all components check out. Incidently, I found the two 10 ohm resistors reading about 20 ohms and did replace them.
Hope this again confirms what has been written in this thread.
Herb S
Thanks for the update Herb.
This may bear mentioning again, but the 565 floats above ground. You should measure 110 ohms between the power ground and the input ground. Sometimes (actually, most of the time) the ground float resistors will open to protect the amp if there is a fault. This will certainly give you offset problems as well. Use MOX resistors of the same power rating - 1/4W 100 and 2W 10ohms if I remember correctly.
Cheers,
This may bear mentioning again, but the 565 floats above ground. You should measure 110 ohms between the power ground and the input ground. Sometimes (actually, most of the time) the ground float resistors will open to protect the amp if there is a fault. This will certainly give you offset problems as well. Use MOX resistors of the same power rating - 1/4W 100 and 2W 10ohms if I remember correctly.
Cheers,
Hi,
Advancing Walts comments, which are bang on ....
Adding to the cleaning of these boards information. The issue is that the electrolyte is conductive (it's supposed to be). You need to remove every vestige of this nasty stuff from not only the board, but also any components. The servo op amp is in a location where is almost certainly has been contaminated. No, you don't have to discard the op amp, or the references. Just clean them well. these are much easier to clean than the PCB is.
Any traces of this stuff will show up in a bad way. So, how clean is clean enough? How do I check?
Easy. Heat up your iron and warm up areas where the electrolyte has been. Check in component holes! I clean the holes out by drilling them slightly larger after all the other cleaning. The copper has been eaten away near the holes anyway. Go ahead and touch the tip of your soldering iron near holes and discoloured solder mask. Check both sides of the PCB. Just see if you get any of that odor from the fluid. If you aren't sure what it smells like, you were wearing a pressurized air system while desoldering the parts at the start.
Once done, these are great amplifiers. Well worth keeping in good shape.
One question for Walt.
Under warranty we found a few bad servo amp ICs. Not many and usually along with blown outputs. I haven't run into any that I've had to replace so far years later. Did the warranty program catch most of these? Otherwise, there isn't any point in "upgrading " them. Also the current draw from this op amp is important as you well know.
Otherwise, I hope everything is going well for you!
-Chris
Advancing Walts comments, which are bang on ....
Adding to the cleaning of these boards information. The issue is that the electrolyte is conductive (it's supposed to be). You need to remove every vestige of this nasty stuff from not only the board, but also any components. The servo op amp is in a location where is almost certainly has been contaminated. No, you don't have to discard the op amp, or the references. Just clean them well. these are much easier to clean than the PCB is.
Any traces of this stuff will show up in a bad way. So, how clean is clean enough? How do I check?
Easy. Heat up your iron and warm up areas where the electrolyte has been. Check in component holes! I clean the holes out by drilling them slightly larger after all the other cleaning. The copper has been eaten away near the holes anyway. Go ahead and touch the tip of your soldering iron near holes and discoloured solder mask. Check both sides of the PCB. Just see if you get any of that odor from the fluid. If you aren't sure what it smells like, you were wearing a pressurized air system while desoldering the parts at the start.
Once done, these are great amplifiers. Well worth keeping in good shape.
One question for Walt.
Under warranty we found a few bad servo amp ICs. Not many and usually along with blown outputs. I haven't run into any that I've had to replace so far years later. Did the warranty program catch most of these? Otherwise, there isn't any point in "upgrading " them. Also the current draw from this op amp is important as you well know.
Otherwise, I hope everything is going well for you!
-Chris
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