Just picked up an Adcom GFA-5800 (broken)

mmadden28 · October 2009

From the same place that I got my Monior 10A's and Sansui SP3500 speakers, last night I picked up a broken Adcom GFA-5800. The seller did not know what was wrong with it.
Not in the pertiest condition--Missing some lid screws; has some old brittle Dynamat all over the place, including some sizeable peices on the outside sides-and not even on straight; The interier needs some cleaning , but not too bad overall.

I decided to take a chance on it, so I brought her home... :rolleyes:

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

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

Some of the resistors measured 'open' (even in circuit).

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

Left channel played fine. :cool:
But while the right channel was outputting sound, it was very very very choppy and crunchy and just not right.

I also noticed that when powered-off, the left channel internal LEDs stayed lit for a moment then faded out after a few seconds, where the right channel LEDs remained lit for 20-30 seconds.

So aside from the obvious resistors--what should I look at next?
Anybody have a GFA-5800 service manual? Schematic?

How does the GFA-5800 rank in the Adcom lineup?

Thanks
-Michael

ben62670 · October 2009

I had a 5800. I think it is a very nice amp. The fans run at different speeds in most of them(no concern). Replace all the the output resistors with some metal films. They need to be specked real close. You can not find 5w resistors in 1%, but if you get some MOX resistors, or even some Mill's you can get them real close. I think there was most likely a short when someone touched speaker wires together. Call Adcom, and they will email you a service manual for free. Keep us posted with what you find.
Ben

ben62670 · October 2009

BTW Michael I am going to replace all my output transistors on my amps. After doing some messing around replacing transistors in my amps I have found the cheap upgrades to be worth while. Also check the resistance on those smaller resistors right next to the big ones. Most likely they are fusible resistors. I replaced mine with non fusible HQ parts and the sound noticeable improved.
Ben

Edit: While you are there also check the very small diodes next to the resistors. A simple way to check them is with a continuity meter. Put the red lead on one end, and the black on the other. Then do it the other way. The meter should only buzz in one direction.

Systems · October 2009

Those source resistors look pooched so I suspect the bank of 9240's will be also.

Systems · October 2009

ben62670 wrote: »

BTW Michael I am going to replace all my output transistors on my amps.

Why?

if they aren't blown there would be no advantage and replacing that amount of those MOSFETS's can't be a cheap undertaking.Or are you meaning your older units that use the BJT's you have a stack of?

disneyjoe7 · October 2009

ben62670 wrote: »

You can not find 5w resistors in 1%, but if you get some MOX resistors, or even some Mill's you can get them real close.
Ben

Not really sure what you need here, but here's .33ohm 1% wirewound resistor.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ALSR5F-.33-ND

Ricardo · October 2009

Nice amp; that's a lot of MOFSET's!!!!!!

ben62670 · October 2009

GV#27 wrote: »

Why? if they aren't blown there would be no advantage and replacing that amount of those MOSFETS's can't be a cheap undertaking.Or are you meaning your older units that use the BJT's you have a stack of?

My bad. I meant resistors.

heiney9 · October 2009

Head over to www.diyaudio.com

Those guys will help you and I'm sure I've seen 1/2 doz posts about the 5800. You can most likely get a schematic there.

H9

mmadden28 · October 2009

Thanks all...I'm going to see if I can pull that Right channel out to do some component testing on it--it looks like a module of sorts.

Man I wish I could just dip this thing in one of those electronic chemical bath thingeys.

They appear to be 1ohm 5W 5% resistors so should be good finding a replacement there. Would it be beneficial going with the 1% here or wouldn't it matter in this application?
Should I go with the Mills 1% at $2.49 a pop, or will the Digikey 1% at $1.38 a pop be more than good enough in this application? If it matters I'll spend it.

mmadden28 · October 2009

heiney9 wrote: »

Head over to www.diyaudio.com

Those guys will help you and I'm sure I've seen 1/2 doz posts about the 5800. You can most likely get a schematic there.

H9

I just registered for an account-Thanks

heiney9 · October 2009

Seriously check out diy audio. These are the kinds of questions they answer all day long. Lots of hand on info/advice. Anatech (Chris) is one of the guru's over there with a lot of knowledge.

H9

EDIT: just saw your post about diy......great!

disneyjoe7 · October 2009

Looking at the picture again the resistor value being Red 2, Black 0 and Gold should be .2ohm 5 watt. Gold 5% but here's a 1% wirewound. Also looks to me as these are wirewound in the amp as they look that way being overloaded.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ALSR5F-.20-ND

mmadden28 · October 2009

disneyjoe7 wrote: »

Looking at the picture again the resistor value being Red 2, Black 0 and Gold should be .2ohm 5 watt. Gold 5% but here's a 1% wirewound. Also looks to me as these are wirewound in the amp as they look that way being overloaded.

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ALSR5F-.20-ND

Its Brown-Black-Gold-Gold--Also just found the schematic over at DIYAudio, and confirmed they are 1ohm 5W.
Red-Black-Gold would be 2.0 ohms, the third band would have to be Silver for 0.2 ohms.

Yep I never saw an overheated wirewound resistor expose its windings like that before--usually its all or nothing, but cool.

disneyjoe7 · October 2009

Ok my bad

ben62670 · October 2009

I would go 1%, or get a few extra 5%. The resistors are there to even out the load on the transistors. If one is under by a fair amount it will be would make that particular output transistor over work.

Systems · October 2009

ben62670 wrote: »

I would go 1%, or get a few extra 5%.

I would stick with wire wounds though,changing to a different type "may" cause problems with stability.

The resistors are there to even out the load on the transistors. If one is under by a fair amount it will be would make that particular output transistor over work.

Yep they are tied to the source of the Fet's and serve the same purpose as emitter resistors in a BJT design.

ben62670 · October 2009

GV#27 wrote: »

Those source resistors look pooched so I suspect the bank of 9240's will be also.

Missed this one. I was thinking that, but hoping they would not be blown. It might be hard to find non counterfeit replacements. Mosfits handle mishaps better than BJT's from what I have read.

mmadden28 · October 2009

I'm posting on DIYAudio now, in the meantime I took the right channel (module?) out--Holy mackerel they didn't make that an easy task. Now I can't figure out how to get the board off the heatsink--Just doesn't look possible.

Anyway, in circuit, the left channel 1ohms measure at just about 1ohm, and the 220ohms measure at just about 220 as well. Did the same measurements on the bad right channel. All 8 of the charred 1ohm and 8 of 220 ohm resistors on the one bank are shot--open. The other bank is fine.

Visually I can't see any other obvious defects.

So I at least know I need to replace them at a minimum. Now the challenge is how to remove the board to actually do the repairs. I suppose at a minimum I could clip the old resistors out leaving just a little lead sticking up and solder the new resistors in that way. Of course that's assuming that's all that's wrong. I guess it depends on how difficult the process is to get the board off the heatsink.

Ben, did you ever do any replacements in your 5800?

mmadden28 · October 2009

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 is 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.

disneyjoe7 · October 2009

It would seem to me (but understand never played with one 5800 myself) the mosfets screw from heatsink removing them (socketed) so you're able to slide pcb out of heatsink.

Also I check all those mosfets, for any shorted ones, mark them so you put them back together as it is now. And maybe just me I would replace all resistors bad with the same value and rating, as if I didn't would feel I would need to replace all so they are the same, then I felt I would need to redo to left channel so they are the same.

mmadden28 · October 2009

You'd think just removing the screws and pulling the MOSFETS out of a socket would be the key, but they are soldered to the board-at least they appear that way-and the MOSFET leads pass though a hole in the heatsink-and there is no way to desolder them with the heatsink in place--so kind of a catch 22.
They have to be socketed-that's the only way this can work. I haven't gotten that far to try that out yet.

Yeah I'll replace all the like components in the other half of the channel as well. Might as well overhaul it while I can.

disneyjoe7 · October 2009

At least that's my thinking, I can't see any other way from the pictures. Don't be shocked for the MOSFETS come out like that

mmadden28 · October 2009

I have to believe they are in sockets--I'll find out when I get home tonight.
If true, I probably could have avoided taking the whole heat sink module out, but hey, it needs a thorough cleaning anyway.

mmadden28 · October 2009

Well that was it, they are in sockets. Pheew! :cool::cool:

Now what about those thermal pads--readily available? resusable? Just use heat sink compound? These look to be fibre reinforced, however they are quite brittle now.

ben62670 · October 2009

The metal cases are energized with around 85vdc. They need to be insulated from the heatsink. Compound does not insulate. I have mica insulators if yours are torn. Glad to see those cans are socketed. I chose to reuse my insulators over the mica's.
Ben

Systems · October 2009

I tried a few places but only found these guy's stocking those devices in the TO3 metal can package.http://www.newark.com/international-rectifier/irf9240/mosfet/dp/06F8462?Ntt=IRF9240. I recommend getting some new mica insulators and coating them with thermal compound.

ben62670 · October 2009

Do you think the mosfits need to be beta matched? I have a ton of mica insulators here.

Systems · October 2009

ben62670 wrote: »

Do you think the misfits need to be beta matched? I have a ton of mica insulators here.

Matching would be benificial and the markings indicate they were at the factory but you'll likely need a few extra's at a mere > $11 a pop.

btw don't forget the fuses in the fuse holders on the end of the board, one is likely open.

ben62670 · October 2009

I was very lucky with my Toshiba BJT's, but there were a few fliers in there. I had 500 to play with. Still have the heath kit beta tester.

heiney9 · October 2009

ben62670 wrote: »

Do you think the mosfits need to be beta matched? I have a ton of mica insulators here.

Yes, matching is important for the best possible performance. Nelson even goes so far as to match them in the actual circuit, as once they are in the circuit there can be minute changes between them.

H9

Just picked up an Adcom GFA-5800 (broken)

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