Help needed with a broken Adcom GFA-5800

[mmadden28]

Hi all, hopefully somebody can help me out.

I just bought a used Adcom GFA-5800. The middleman seller knew it was broken but did not know what was wrong. Being a tinkerer I decided to take a chance and see if I could fix it myself--I know enough of the basics that it's worth a try

Not in the pertiest condition--Missing some lid screws; has some old brittle Dynamat all over the place, inside and out, including some sizeable peices on the outside sides-and not even on straight; The interier needs cleaning, but not too bad overall.



opened her up to have a quick look around...


I immediately noticed some light charring on half of the right channel resistors.


I checked all the fuses (all good) and hooked up a cheap pair of speakers and a source and pressed the power button. I expected nothing, or at least a shower of sparks , but no--it powered up and the internal green LEDs were also lit on both R & L channels (two LEDs for each channel).

Left channel played fine.
But while the right channel was outputting sound, it was very very choppy and crunchy; just not right not sure how to describe it.

Not sure if this matters but I also noticed that when I powered-off, the left channel internal green LEDs stayed lit for a moment then faded out after a few seconds, where the right channel green LEDs remained lit for 20-30 seconds.

Did some resistor measurements and all 8 - 1 ohm resistors in the bank that's charred are OPEN. Also all 8 - 220 ohm resistors in the same bank are also OPEN. All the resistors in the other bank are fine.

I had a heck of a time removing the Right channel module (if you want to call it a module), but I finally got it out. However I have no idea how to get the board off the heatsink to actually do any repairs. It appears that the MOSFETS are screwed down to the heatsink, while its leads pass through a hole in the heatsink and are soldered into the board below. There is almost zero clearance betwenn the solder side of the board and then next heatsink fin. Just doesn't look like its possible to remove it--I'm hoping I'm being completely blind and missing something obvious...I'm working on some pics of the module shortly.

So other than the obvious, what should I look at next?

Does anybody have a Service Manual for this model? (just to be clear this unit has fans).

How to I get the board off of the heatsink?

Any suggestions for brand, ratings and source of the replacement resisistors?

Thanks
-Michael

[mmadden28]

Originally Posted by mmadden28

Here is the right channel module removed and some pics of the detail of how the board and transistors are attached to it. Note that the Heat sink is actually 2 pieces. The only thing holding them together are the transistors and the board.

Also gives a better view of how dirty this thing is. Taking it apart will certainly make it easier to more thoroughly clean it.

[CBRworm]

That is quite a dilemna.

The TO3's aren't socketed are they? Maybe remove the screws on one and try to pull it out. If not we must be missing something obvious.

My GFA 555ii the TO3's are soldered into the board on through the heatsink, but you can access the back side to unsolder them.

I suspect that to fix it properly you will need to replace all the burned 1ohm and 220ohm resistors, the drivers and all the outputs on that channel. As you find damage, keep going one more stage back looking for damage. Chances are good that the IRF244's might be ok, the driver for the IRF244's might be ok, but why take chances with cooking another whole set.

Actually you shouldn't pay any attention to me, not having ever worked on one of those I am at a great disadvantage to some of the others here who will chime in.

As for what replacement parts to get, I would start with the service manual and work your way from there. - my guess would be that the IRF244's are original and the IRF9240 may have been replacements. Does the other channel have all IRF244's? Maybe the previous owner tried to replace that bank and then the resistors smoked at power up due to a short or driver problem.

[Nelson Pass]

You can start by replacing the Source resistor, Gate resistor,
and transistor for each case where there is a cooked resistor.

Also, you will be replacing output stage zeners in the damaged
channel.

You have an issue matching the replacement devices (assuming
that they were properly matched by the manufacturer).
Assuming that, you could remove one and measure the Vgs
and then match that to within .1 volts.

[mmadden28]

Well the good news is that they are indeed socketed.
So I didn't really need to take the heatsink module out afterall, but its OK as this thing needs a good thorough cleaning anyway--I'll probably pull the Left channel out to when all is said and done.



I'm in the process of pulling all the transistors out now.
On the subject of the thermal pads--Are they a disposable item? Reusable? Should I just use CPU heat sink compound when I put them back?
They appear to be fabric reinforced, but they are quite brittle now.

What I meant about replacement parts (on the resistors) was, is there a preferred brand or source (Digi-key, Mills, etc) and rating (1% over the factory 5% or higher wattage over the factory 5w resistors) for this application?

[mmadden28]

Originally Posted by Nelson Pass

You can start by replacing the Source resistor, Gate resistor,
and transistor for each case where there is a cooked resistor.

Also, you will be replacing output stage zeners in the damaged
channel.

You have an issue matching the replacement devices (assuming
that they were properly matched by the manufacturer).
Assuming that, you could remove one and measure the Vgs
and then match that to within .1 volts.

Thanks!
Now I just have to look over the schematic to be sure I know where all those components are and do a little research on how to properly test the MOSFETS.

[mmadden28]

OK I got my 'read on', studied the schematic and also read Nelson's article on MOSFET testing here. I have a better understanding of this thing now--Not completely but better. Ah the old electronic theory brain cells are waking up after a long long sleep (pushing cobwebs aside).
EDIT ** I'm building the test circuit from the article so that I can figure out what the Vgs is of all the good Q's.

If I understand correctly, the GFA-5800 is a Common Drain (source follower) design in Parallel operation. I'm not sure how the N/P channels fit in with each other sharing a common connection but...

Here's the schematic. (sorry it looks odd--it was in two pages an I don't have the right editing tools on this PC)


Everything circled in Red was a bad part (Resistors OPEN, Transistors SHORTED).

Circuit oddities are circled in Orange:
Q9, 10, and 12 (TO92) were all missing--looks to be a deliberate design change. nothing changed with the traces or signal path, just left open. However there are small leads soldered in the holes which you can see in a pic I'll post shortly. Not sure if the transistors were cut out post manufacturing or what. All the oddities are present in both channels.
C7 and C6 were also missing but replaced with a jumper.
Also the schematic calls for IRF240s, when IRF244s were actually used (in both L&R channels).
It should also be noted that R14 (top middle) is a resistor for the ERROR LED. While it appeared fried (couldn't read any color bands), it tested fine-its also outside the main path of damage (and outside the signal path) but will be getting replaced anyway.

I haven't tested any of the other transistors out of circuit, but none of them were shorted. All other resistors and zeners are good. I had to remove a few zeners and resistors from the circuit to test/verify them properly due to parallel resistance factors.
Somebody please let me know if I should pull any more upstream transistors for testing.

So based on the failed part path--any ideas as to what could have caused the failure?

[mmadden28]

Now I just have to get some replacment parts...but I'm a bit confused on the matching of the transistors though. Does the Vgs need to match as close as possible just between the N channel transistors, and likewise between the P Channels? Or do the N's also have to match the P's?
Trying to figure out if I'll be needing to replace all 16 Q's or just the 8 bad ones.

As for the resistors--is there a better choice or brand for replacing these? Do either the 220ohm or the 1 ohm resistors affect the signal quality? I'm guessing yes for the 1ohm resistors. Are Digikey brand sufficient or should I go for Mills or other? I don't want to break the bank for an extramely small subtlety, but if the SQ difference is significant I'll spend a few extra $$.
Should I replace all 16 resistors? Or will replacing just the bad 8 be fine?

And does anybody know a good supplier of these transistors that will do matching? Not the easiest part to find in Google searches. Newark so far has them for $11.81 each. Is that a good price?
With all the resistors and assuming all 16 Q's we're looking at about $225-$250, assuming I can get matched parts.

How about an acceptable substitute? (keeping in mind the working Left channel is remaining as is.)

Thanks all!!

[mmadden28]

Here's some pics of the prepared board--all cleaned up and ready for new parts




All contact points (including the fuses) have been cleaned with DeOxit and treated with DeOxit Gold as well.

Now while I'm waiting to figure out what to do about sourcing the replacements and then waiting for them to arrive-I'll give the rest of this beast a bath.

[mmadden28]

Originally Posted by mmadden28

Now I just have to get some replacment parts...but I'm a bit confused on the matching of the transistors though. Does the Vgs need to match as close as possible just between the N channel transistors, and likewise between the P Channels? Or do the N's also have to match the P's?
Trying to figure out if I'll be needing to replace all 16 Q's or just the 8 bad ones.

As for the resistors--is there a better choice or brand for replacing these? Do either the 220ohm or the 1 ohm resistors affect the signal quality? I'm guessing yes for the 1ohm resistors. Are Digikey brand sufficient or should I go for Mills or other? I don't want to break the bank for an extramely small subtlety, but if the SQ difference is significant I'll spend a few extra $$.
Should I replace all 16 resistors? Or will replacing just the bad 8 be fine?

And does anybody know a good supplier of these transistors that will do matching? Not the easiest part to find in Google searches. Newark so far has them for $11.81 each. Is that a good price?
With all the resistors and assuming all 16 Q's we're looking at about $225-$250, assuming I can get matched parts.

How about an acceptable substitute? (keeping in mind the working Left channel is remaining as is.)

Thanks all!!

Bump--Anybody?

Thanks