Adcom GFA-535 troubleshooting...

bought this used about 5 or 6 years ago. had this dude running a DVC sub. noticed that the clipping light on the right channel flickered quite a bit, but according to the manual, it wasn't out of the ordinary, and the sub was hitting pretty hard, so i ignored it.

just got a powered sub and pulled the 535 from the system and figured with my newfound knowledge, i'd check the dc offset before moving it to run my newly repaired nht superones. surprise! it's sending 11.5V to the right channel. 20mV to the left. lord knows how long it had been like that...the hum was probably above the passive crossover point on the sub and that voice coil probably never got any signal. :scratch2:

opened her up (beautiful inside with twin power supplies and adcom custom power supply caps) and immediately see one of the two 4A fuses on the right channel is toast. all i have handy are 2A or 3A fuses. plug a 3A fuse in (just to see what happens to DC offset, no plans to run speaks yet). DC offset drops to 31mV on that side.

Next step is to get a 4A fuse and run some cheap speakers for a while to see what happens.

Anyone got a line on a service manual for an adcom gfa-535? assuming he passes the next test, a quick tuneup would be nice.
 
I have the adcom 5300 service manual and it has very little information that is useful other than a schematic. I imagine the 535 manual would have even less.
 
Blowing rail fuses is the #1 failure mode of many Adcom power amps. I normally replace the positive and negative rail fuses in pairs, even though only one rail fuse may be blown.

Fred Longworth
 
do you increase the fuse rating when they blow? I found a note on the diyaudio forums where someone recommended throwing 15A fuses in there...and Nelson Pass himself said to hard wire the connections with NO fuses ???
 
upping fuse values? bypassing fuses?
From where I stand that sounds like complete idiocy. The fuse is supposed to protect the amp for damage, unless you like watching your valuable equipment go up in flames I would use original values

I regard anyone who puts a 15amp fuse in place of a 4 amp as foolish
I regard anyone bypassing a fuse with hardwire as an insane un-educated moron.

I see far too much damage from people who say "Huh, the fuse blew. Guess it needs a bigger fuse!"
 
jpdylon said:
upping fuse values? bypassing fuses?
From where I stand that sounds like complete idiocy. The fuse is supposed to protect the amp for damage, unless you like watching your valuable equipment go up in flames I would use original values

I regard anyone who puts a 15amp fuse in place of a 4 amp as foolish
I regard anyone bypassing a fuse with hardwire as an insane un-educated moron.

I see far too much damage from people who say "Huh, the fuse blew. Guess it needs a bigger fuse!"

my initial reaction also...I'll just replace all four with new 4A fuses, clean the contacts and have spares if its a common failure. fuses are cheap.
 
Digging up an old thread, but a question regarding these fuses.

Are the 4A 250V rail fuses on the GFA-535 spec'ed for slow-blo or fast-blo? I need to replace a fuse in one of these amps and seem to be seeing conflicting info out there. Right now all four of them are slow-blo, but not being the original owner of the amp I don't know if the fuses are original to begin with.
 
I would go by what the owners manual/service manual reccomends.Sometimes it is posted on or near the fuse holder..If it just says 4A I would go with fast blow...
 
I would go by what the owners manual/service manual reccomends.Sometimes it is posted on or near the fuse holder..If it just says 4A I would go with fast blow...

That was my guess.

The owner's manual doesn't seem to say, and I don't currently have the service manual. I've contacted Adcom to see if I can get one for future reference.
 
Without a doubt, fast blow!, standard SFE 4 Amp, available at any auto parts store. :thmbsp: Been there, done that.
 
Without a doubt, fast blow!, standard SFE 4 Amp, available at any auto parts store. :thmbsp: Been there, done that.

Already got some... just wanted to check on the type to be sure.

The amp is warming up right now with no sign of smoke or fuses blowing. A few more minutes and I'll check the DC offset to make sure things look okay. If all checks out okay, it'll get to play with some Insignias for initial testing.
 
I had found that too, but thanks. The service manual, on the other hand, I am hoping I can buy a copy of from Adcom.

My DC offset readings seem pretty good:

Right channel: -0.2 mV
Left channel: -0.4 mV

Plays pretty cleanly, too!
 
True. Mine say AGC4. 250v. Which I am assuming are fast blow but never understood all the various fuse nomenclature which I think may have changed over the years. These have a rough or zig zag pattern to the filaments but probably the same as the smooth or straight filaments??
 
Just got another one in. Seems to hum on the right but over a pretty balanced output from both channels. Everything looks ok. May be the caps? Blue Adcoms. Ill check offset and maybe blow out some hairs. Maybe ill get lucky.
 
This is for the 535 II, I don't think I have the regular 535 manual anymore.
 

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True. Mine say AGC4. 250v. Which I am assuming are fast blow but never understood all the various fuse nomenclature which I think may have changed over the years. These have a rough or zig zag pattern to the filaments but probably the same as the smooth or straight filaments??

Those are regular fuses, not slo-blo.

Most electronics use regular (fast) fuses, most slow fuses are for motors and such with high initial current needs (and sometimes power supplies). Output fusing would not be slow, you will want them to blow immediately if there is an overcurrent.
 
Thats great. Thank you. This one is wierd. Dc offset seems ok but unstable. The front overcurrent lights are lit. One stronger than the other.
How bad were these blue adcom caps?
 
As stated, 4 amp 250v fuse.....NOT an auto fuse rated at 12v..... The auto fuse will conduct much more ampage than the normal 250v fuse. Don't confuse the two......
 
Ceramic cased 4 amp microwave oven fuses were considered an upgrade option by the designer, iirc.
 
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