Adcom GFA-535... Pics... is THIS Stock??

[777funk]

I just received a great Adcom 535 Power Amp as a gift. A friend bought it on ebay and had it sent to my door. Very cool gift and great friend.

Anyways, I'm wondering if this is stock. It looks like it's had a couple of caps added. I have no idea if they were original or a Mod of some sort. Curious on thoughts. This is one of my first Hi Fi amplifiers.
thanks!!
Nick

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NOTE: the red Poly cap soldered in the same hole as the resistor at the edge of the board.

 

[777funk]

does anyone by chance have a schematic?

 

[Magura]

I don't have a schematic, but I've seen the innards of a few Adcom pre and power amps.

Some literally looked like an accident, or at least a prototype. So those two caps that seems like they've been put as a modification, could very well be from the factory.


Magura 🙂

 

[Nelson Pass]

Don't look at me, I have no idea. I suggest that you contact Adcom for
a copy of the service manual.

😎

 

[777funk]

Don't look at me, I have no idea. I suggest that you contact Adcom for
a copy of the service manual.

😎

LOL! Did you design this one?


btw... if this is not supposed to be in this forum, I apologize. I'm totally cool with it being moved if it needs to be. I noticed other Adcom information posted here so figured this is where the fans of the 80's Adcom stuff would be.

 

[Nelson Pass]

Designing for Adcom consisted of creating an original piece with
documentation. From that they made production pieces with their
own artwork and such. They did have a good technical staff, so the
products tended to mutate over time, and some new products were
created by simply scaling down a larger piece.

😎

 

[MashBill]

My 535 does not have those caps.

Send me your email and I will send you a PDF of the service manual.

Bill

 

[llwhtt]

If you look at schematic for the 555 you'll see two electrolytics, a large one in the F/B circuit and a smaller one in the bias circuit. The large one is bypassed by a .1uf film capacitor. Now look at the picture of the 535 board, if you check out the components around the large electrolytic all of the values are the same as the 555 and appears the film capacitor was added as the bypass after the board was made. Also there are many empty part locations, less outputs and associated components, probably the same board. Anyway that's WHAT it is, how,who, when, and why are still the questions. I wonder if the original 555 had the bypass?

Craig

 

[llwhtt]

That should be a 1uf film capacitor NOT .1uf in my last post.

Craig

 

[adason]

wow, I just noticed the fuses on that thing!!!!!
if that thing blows, it takes everything with it
I would replace those fuses with correct ones if I were you

 

[777funk]

wow, I just noticed the fuses on that thing!!!!!
if that thing blows, it takes everything with it
I would replace those fuses with correct ones if I were you

I had read here on this forum that the B+- fuses to the power transistors can limit the current to them resulting in poor sound quality. Especially with corrosion on the terminals. I read a few people saying that it sounded better with them jumpered. So, that part of the amp is definitely non stock. Good eye! I saw the two red caps and that was what I was wondering about mainly. I'll have to give it a good look over.

My thought on the fuses is what are the chances the power transistors are going to go bad. In the case they do the current will pop the main fuse (hopefully). Most audio amps I've seen don't fuse the power supply to the transistors. So I figured that since the point of this thing is to sound it's best, I'll change parts if something does go.

 

[sonidos]

Those caps are on my unit as well (sorry, I'll have to check the schematic later). Those are stock as I bought mine brand new and never modified.

If you go to diyaudioprojects.com, I'm doing a rebuild of the Adcom 535 psu listed in the solid state section of the forums and upgrading my resistors to Dale Vishay, plus replacing all the electrolytics.

 

[sonidos]

Here's a photo I took before upgrading the resistors, caps, and trimmer.

 

[ostripper]

I had read here on this forum that the B+- fuses to the power transistors can limit the current to them resulting in poor sound quality.

Hmmmm , The fuses might be there to prevent a fire hazard , or to keep the phenolic board from actually burning up. My screwdiver recently slipped and instead of having 8 burned OP devices and exploding screwdrivers 😱 , just 1 $.70 - 7A fuse was all I needed. If you are worried about a current "bottleneck" at the fuse , a couple of 2200-3300uf/63-80V caps soldered right at the PCB OPS rails will still offer protection while giving a little more headroom.I have a {C-C-fuse-C} setup and can see a .1mv drop right after the fuse , but at the last cap (6800u - on PCB),nothing at all.

Or at least a slightly over-rated fuse would offer "dead- short"
fire /PCB protection.

OS

 

[sonidos]

I can tell you from experience that those fuses are there if the speakers are shorted!

One of my stepkids, that to this very day has not confessed, but they removed the cables from the speaker binding posts and put the + and - of one channel in contact with each other. I found the cable sitting on the floor. I immediately hooked everything up and the left channel was dead.

I opened up the amp and one of the fuses had blown. Got a replacement and I was back in biz. As you can imagine, I raised holy heck and nobody has touched my stereo since that time.

 

[sonidos]

Adcom 535 schematic

Here's a link to the schematic



diyaudioprojects forum w/schematic

 

[tortello]

Hello,
could you please post the schematic on this forum?
Many thanks

Marcello

 

[sonidos]

Hello Marcello,

I'll check with policy on posting schematics for commercial products. The moderator for diyaudioprojects.com was nice enough to permit me to post it on his site and I don't want to violate policy on this board.

If I get the okay, I will post it.