DIY Audio Projects Forum

Adcom GFA-535 rework
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Author:  sonidos [ 16 Mar 2009, 21:21 ]
Post subject:  Adcom GFA-535 rework

As some of you know, I've had my little Adcom GFA-535 with me since my starving student days. I've had the amp going on more than 15 years and it started to sound a bit lifeless. Many, many units were sold by Adcom and they can still be found for cheap on e-bay. Mr. Nelson Pass himself said that he designed the first of the 5xx series and it received really good reviews from Stereophile, along with its big brother, the GFA-555.

After cracking the hood open on the amp and getting a copy of the service manual, plus reading info on other forums, this is what I've decided to:

1) Replace the 6800 uF 63V stock electrolytics with 10,000 uF versions
2) Upgrade the 2 rectifiers to beefier versions. The datasheet from the originals indicated that amp output dropped as the temp increased.
3) Upgrade of mylar caps in the PSU to Panasonic propylenes.
4) Upgrade of power resistors (1 W versions) to Vishay Dale power metal film resistors
5) Add .01 uF snubber caps to the rectifiers (Panasonic propylene as well).
6) Remove the stock RCA inputs (old and literally falling apart) with Tiffany type RCA inputs.
7) Remove the stock speaker connectors. They are horrible and required that you run bare twisted wire to secure them. The are being replaced with 5-way binding posts.
8) Replace the 1k Ohm metal film resistor at the input and the 22.1k feedback resistor with Dale RN military type resistors.
9) Replace two sets of electrolytic caps (4.7 uF and 47 uF) with Nichicon caps (step down from the Muze version - they don't seem to be in the signal path so I wasn't going to bother with the extra expense).
10) Remove the A/B speaker switch from the signal path and connect each channel directly to the output. (No worries here - the switch doesn't have safety features - its there for convenience).
11) Replace the internal signal wire (tinned copper) with pure copper wiring for both input and output.

All components will be soldered using Wonder Solder. Not that I have an affinity for the stuff, but a friend sold it to me for cheap many years ago and I have too much of it on hand. Gotta use it up somehow!

Photos to come.

Author:  sonidos [ 16 Mar 2009, 21:28 ]
Post subject:  Re: Adcom GFA-535 rework

Here is the original PSU board with 3 of 4 electrolytic caps removed. I don't know if the brown stuff is adhesive or if some of the electrolyte leaked from them. However, the top of each cap was bulging slightly when I first opened the top on the amp.

Author:  sonidos [ 16 Mar 2009, 22:38 ]
Post subject:  Re: Adcom GFA-535 rework

Angle view. These PSU photos are after a bit of clean up. I noticed a bit of black on the mylar caps, plus some of the wiring between the psu and circuitry seemed like it had gotten soft where the nylon wraps were located. At some point, there must have been a current surge that produced a great amount of heat.

Author:  Gio [ 16 Mar 2009, 23:05 ]
Post subject:  Re: Adcom GFA-535 rework

That sounds like a good plan and it looks like fun! I would guess that is adhesive.

Author:  sonidos [ 16 Mar 2009, 23:23 ]
Post subject:  Re: Adcom GFA-535 rework

Thanks Gio! I'm already pretty psyched.

Ok, so here is a snapshot of the original and the replacement. I could have gone for a Panasonic TS-HA but I'm being a tightwad and after all, this little amp has lots of miles on it. (And most of all, I'm getting pretty close to building the Pass F5).

So I settled on the United Chemicon KMH series. Cheaper and some folks say its a darn pretty good cap.

Note: the replacement cap will not fit on the stock board. Therefore, I've had to design a replacement using ExpressPCB software. I should have the new board in a few days.

Author:  Gio [ 16 Mar 2009, 23:32 ]
Post subject:  Re: Adcom GFA-535 rework

I have used those caps and they are good. Can you post the board layout too?

Author:  sonidos [ 16 Mar 2009, 23:41 ]
Post subject:  Re: Adcom GFA-535 rework

Ahh, Gio, that's good news to see re. the KMH caps.

I'll post the board layout shortly. This is my first attempt at putting something together that is meant as a drop in replacement. After I'm certain that it fits well or needs tweaking, I'll post the board layout and notes (or why I goofed up on the layout).

Author:  RiGuy [ 17 Mar 2009, 07:18 ]
Post subject:  Re: Adcom GFA-535 rework

Nice Project Sonidos. Hope it brings back that sound you were hearing in its younger years. 15years since college? You must be my age. Glad I dont still have my Kenwood receiver! Probably would be happy to have that Adcom though and even more happy that it looks like a simple and uncluttered board. Easy to repopulate with quality new components.

Author:  CrazzyAbtTubes [ 17 Mar 2009, 11:00 ]
Post subject:  Re: Adcom GFA-535 rework

I did the same thing to my old Viking receiver that you are doing to your ADcom amp. The Viking amp has a beastly case and power tranny and TO-3 case transistors, but there were allot of junk components in it. The power supply caps were 4700uF 35V and were running at a voltage of 33V!! :hot:

The Chemi-con capacitors make for very nice replacements, in my case I've used the SMH series which are also very good. :up:

Good luck with your rebuild! :)

Author:  sonidos [ 17 Mar 2009, 13:17 ]
Post subject:  Re: Adcom GFA-535 rework

Crazzy -

Hope my voltages don't run so close to the cap!

Ri- Some of what I'm accomplishing was addressed in the GFA-535 II but I am hoping that by bypassing the output on/off switches and upgrading the two resistors in the signal path that I'll hear some audible gains.

The cool thing about this little amp is that it was built as a dual-mono. One goal is to cut down on the clutter in the box. I forgot to take a photo of the internal wiring before starting, but I'll have a photo once everything is back in the box.

One change to the board was the addition of two mounting holes. The original has 3 and uses the output connectors screwed to the chassis as a means of securing the board from movement. Since I'll be running wire from the pcb to the 5 way binding posts, I made room for adding two more mounting positions for a total of 5.

Several years ago, Nelson Pass commented that if users felt that the sound was compromised by the 4 fuses in the V+/V- rails, that they could be removed. I won't do that because one of my stepkids got very curious about my stereo and removed the speaker cable from the speaker end with the amp still on. Yep, you guessed it - they must have touched the two open wires together. Thankfully, the fuse blew and no one was hurt. Getting a confession on what happened was the most difficult part. I swapped out the fuse for a new one and I was back in business. So, the fuses stay in.

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