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Old 11th February 2016, 04:34 PM   #1
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Default Yet Another Adcom GFA-565 Thread

Hello Everyone. I have been posting on a different Adcom thread as I have been working my way through a restoration of an old Adcom GFA-565. Rather than add to the bottom of that long and very old thread, I decided to start posting my project here on a nice new thread..

The initial posts for this project can be found at:
Damaged Adcom 565 Monoamps
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Old 11th February 2016, 04:37 PM   #2
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As provided at the other thread, I had found it somewhat difficult to use the schematic in the service manual because it was separated into two pieces, and also had a bunch of fan control stuff that wasn't of any use to me (my unit doesn't have the fan assembly).

So here, for anyone's use, is the schematic from the manual joined up on a single page.

Click the image to open in full size.

You can also view or download this at:
http://mv-makoto.com/AdcomGFA-565SchematicJoined.jpg
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Old 11th February 2016, 04:45 PM   #3
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In addition, I found it useful to trace the various parts of the circuit so I could better understand what signals and voltages I should find where, and also separate the various sub sections and functions of the circuit.

Here is the schematic with some of the signal and voltage paths highlighted in different colors. Green is ground, light Red is positive DC, light Orange is negative DC, Yellow is full cycle audio signal (which is found at the input to the amp, and at the output), Purple is negative half cycle audio, Brown is Positive half cycle audio, and Dark Red and Grey are the positive and negative tail current lines for the differential input circuits.

Click the image to open in full size.

Last edited by cogeniac; 11th February 2016 at 04:50 PM.
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Old 11th February 2016, 05:13 PM   #4
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Here is a closeup image of the control board that I have been working on.

Click the image to open in full size.

I have replaced the electrolytic caps, and cleaned up some amazingly bad soldering on the back side of the board...

In this photo you can see a number of issues. First, the PO seems to have taken matters into his own hands relative to the bias controls. The two trim pots with resistor assemblies under them apparently are meant to replace R127 and R128, which were 1K 1% resistors in the origin. This contraption allows variation between 980 and 36K ohms...Apparently someone didn't do their resistor value homework...

The next issue is that the precision low noise voltage references D116/D117 and D118/D119 have been replaced by single diodes of some unknown type.

The last issue is that the varistor D105 has been replaced by the little trio of flying diodes.. Yikes!!!

I have new diodes and a varistor on order (as well as a full set of other transistors and a fe other parts), and will see how this works once I have put it back to its original form.

Not shown is the fact that at some point the PO decided to add a 100 uf blocking cap on one of the outputs. I'll bet that sounded great..... :-(

Lastly, for some reason the leakage resistors and the mica bypass caps on the big filter caps were also missing. Those probably didn't do much harm (other than the HF noise reduction from the bypass caps), but one wonders who was mucking around in this amp.. It seems they had SOMe understanding of electronics, but not enough to fix it properly!!
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Old Yesterday, 01:57 AM   #5
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OK, so I got the board rewired and replaced the parts that were obviously bad. I fired it up, and the output is pegged at 60 volts (I am using a 300 watt 8 ohm resistor as a load).

I am not sure if doing this will upset the bias circuits, but I disconnected the signal lines between the control board and the output stages. With this done, the DC offset at the output drops to a nice 24 mv. So that seems promising!

The input board clearly has some pretty significant issues.

A couple of observations:

Q111, the big transistor attached on one of the heat sinks, is running pretty hot. The other one is cool to the touch.

With power applied, the collector of Q101 is at 79 volts (!!), The Emitter is at about 7.5.
The other side of R104 is at 2.2 volts

In contrast the Collector of Q102 is at -11 volts, and the emitter is at 1.1 and the other end of R109 is at 1.2 volts.

I see signal on the bases of Q101 and Q102, but no signal on the collectors of Q101 or Q102.

I think I may remove the front end transistors and test/replace them, and then see how it works.

Anyone have any other ideas???

Scott
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Old Yesterday, 06:23 PM   #6
anatech is offline anatech  Canada
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Hi Scott,
Could you please post a high resolution picture of the control PCB as it is right now?

When you are troubleshooting a DC offset problem, don't connect a load. There isn't any point as we already know it will draw a lot of current. Once the load is disconnected, repeat your observations as to what is running hot and what isn't. With the DC offset pegged to one supply, places that normally would show signal will not. The stage is no longer in a linear mode. Its stuck to the rail.

With DC Offset issues, you should remove the op amp, then try it again. You will have an offset if everything is working properly, but it won't be very high. A few volts worst case.

The problem with the leaky electrolyte would cause exactly what you are seeing. The previous repair person was obviously convinced they cleaned it all up. It is entirely possible that the conductive solution is still there. You would have to clean the PCB, and the parts (or replace them with new). This stuff is incredibly tenacious and difficult to actually get rid of. To clean the board means to strip all the parts off the affected area. That includes the diff pairs, op amp and the regulators. Include any components in that area and be certain there are no original capacitors on that PCB. Then, take a tooth brush and scrub that board using a vinegar solution, then a baking soda solution (acid, then a base). I had a difficult one that took ages to clean. % cleaning sessions with parts removal and replacement each )$*@Q*& time. It was ... frustrating to say the least. I finally got it clean. I can't be sure what cleaning solution did the job to be honest with you. You should also try dish washing soap and water.

That electrolyte that came out of the capacitors was nasty stuff. It will corrode anything metallic that it comes in contact with over time. You have to get to it early, don't leave it sit there corroding away.

-Chris
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Old Yesterday, 07:00 PM   #7
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Hi Chris;

Thanks for responding.

Here are a couple of closeups of the component side of the board around the caps, and one of the solder side There is some foil damage, but nothing too severe. The scratches are from the PO.

I have not yet replaced R127 and 128 but I set the pot networks that replaced them at 1K ohms within about +/-10 ohms. You can also see that the op amp was replaced using a socket. The op amp in there now is one I put in.

It sounds like my next step should be to remove the major components, test them, clean the crap out of the board, and then put the components back in replacing anything that is suspect.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Last edited by cogeniac; Yesterday at 07:02 PM.
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Old Yesterday, 07:18 PM   #8
anatech is offline anatech  Canada
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Hi Scott,
Yes, I'm afraid so.

It sure looks like there was a problem with the capacitor electrolyte leaking.

When you rebuild the PCB after cleaning, lose the IC socket for one, also the network of stuff with the trimmer control. It appears that the previous hack artist had some problems with their soldering skills for sure. I would suggest that you use some liquid solder flux before stuffing the board to tin any iffy areas that need solder. One trick that works well is to use some solder wick that has some solder in it already. Use the flux on the area after it has been scraped clean, then rub with the soldering iron with the solder wick between the iron and the board. The mesh will fine rub the copper and deposit the solder in one go. Then you can solder component leads without overheating anything. I hope this makes your life a little easier.
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Old Today, 02:42 AM   #9
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OK, so I disassembled the control board (took the connector jacks, caps and all transistors off). Wicked and reflowed all the remaining solder joints, and then cleaned the board several times.

I used a couple of different very nasty hydrocarbon cleaners with a plastic scrub brush, and gently blow dried the board between cleanings. I used the little nozzle sprayer to thoroughly wash down the board under and around all the components. We used to use nasty stuff like this in heated ultrasonic baths for electronic components, so I was pretty sure it would work.

Here is one of the cleaners I used (the first was an automotive intake cleaner that had acetone, heptane and toluene. It instantly removed all the solder flux, and a good quantity of cloudy residue. I didn't save it because there was a lot of flux in it (I smeared rosin flux all over the board when I was wicking it). The second was this electronic cleaner, which is mostly various hexane compounds. It is super volatile and turns everything icy cold, so cold you get condensation on the parts unless you blow dry them.

Click the image to open in full size.

Anyway, I was surprised to find that after the intake cleaner freed a bunch of crap (including the flux), the electronics cleaner still ended up with a lot of cloudy crap. Here is the catch bowl after two cycles. I eventually went through this about five times, until the off flow was clean...spraying and scrubbing with the brush, in all different directions, on both sides of the board. I got about three bowl-fulls like this before it came out clean.

Click the image to open in full size.

Here are a few pics of the board post soldering and cleaning. You can see that at some point in its life R125 got smoked pretty spectacularly...

Click the image to open in full size.

Click the image to open in full size.

The board is clearly a LOT cleaner than it was, so good call Chris! And way to go nasty hydrocarbons!

I then checked all the transistors I had removed. Apparently Q112 was replaced at some point with some "equivalent" device. I ordered a new one of that and the mate Q111.

One of the 3478 transistors was shorted (not sure which of Q103, 107, 116 or 118 it was - since they were out of the circuit). I ordered new ones of these

One of the 1376 transistors was open (again, not sure which of Q104, 108, 115 or 117 it was - since they were out of the circuit). I ordered new ones of these

Q110 had been replaced by some "equivalent". I ordered a new 2SC1815.

I was able to buy some of these via Mouser, but most of them I found on eBay (some int he US, and some in the UK - I don't trust buying things from the far east).

So, sometime next week, when the parts arrive, I'l get back to this, re-install the parts and give it another go..

Note too that at some point the PO cut the traces at the 100 UF caps. Very strange.. So I'll have to jumper across those gaps...

Cheers all.

Scott

Last edited by cogeniac; Today at 02:53 AM.
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Old Today, 03:14 AM   #10
anatech is offline anatech  Canada
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Hi Scott,
I think you did a really good job of cleaning that board. I should try some of that cleaner the next time I get a "capacitor job". It cost me more than a ton of time scrubbing to clean those boards I had in. You cleaned the parts too - right? They also get contaminated. Once that is done, rebuild it while your memory is still fresh. Installing the new parts will be much easier when that time comes.

You found a bunch of good reasons why it didn't work, so good work. Check the resistors with a meter. They can open up without showing any problems externally. Take nothing for granted.

Now for the news you don't want to hear. The differential pairs must be matched. You're going to need some way of matching them that is better than the transistor test function in a meter. Transistors are very, very sensitive to temperature changes, so that means your parts should be handled with pliers to connect them to the tester. You must not touch them with your hands as that is enough to throw off the reading. Its a real pain in the neck.

-Chris
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