Adcom GFA-555II

[the apostate]

Well i just bought this amp on fleabay, got it for 290! Question, I know this amp was released in 1990 and probably will need new caps soon. I understand it has 4 15000uF caps in it I assume for power supply. Can someone tell me exactly what is best and where to buy them?

Also I dont have a great deal of electronic experience, did take DC and AC analysis and lab in college, and have soldering experience. I know these large caps can kill you! They need to be discharged before handeling, I seem to remember shorting the terminals with a 1K ohm resistor to discharge them???

Also need to know while soldering them there isnt a problem with heat from the soldering iron like with IC's right???

Any other info would be great to...thanks.

[SY]

Typically, you solder the connections onto solder lugs, which are then screwed into the cap terminals. Makes like easier!

Use a couple of clip leads and a 1k 2W (or greater) resistor to bleed the caps. 5 minutes or so should be more than enough time (t = RC = 1000 ohm x 0.015F = 15 seconds).

[PrestonTom]

You can get the caps at Apex jr. Although some of the series 2's had bypass caps on the PS caps, not all of them did and these caps are not long-lived. Search the threads for bypass caps on the PS caps. It makes a bit of improvement and is dirt cheap.
Good luck with your project.

[infinia]

Hi
For the big caps no soldering is needed. The caps are already HF bypassed and have bleeders. You could inspect them and take some dimensions for the future tho. IMO I wouldn't worry too much about replacements of those CG caps yet, unless too much heat or. The other smaller electro caps should be replaced first. search the forum for other info on extra bypasing and/or additional mods.

[the apostate]

Ok thanks. I've done some more reading on this amp, and wonder if it would be a good idea to check the DC offset voltage on the outputs and make sure its ok before hooking up a nice pair of speakers to it??? If so exactly how is this done? Sounded like you just turn the amp on and use a volt meter and test the voltage across the speaker terminals of each channel whilst nothing is hooked up to the input??? Then what should I be reading in mV's???

[sjdemark]

To check the DC offet, you need a multi-meter that reads millivolts. While the amp is on with no speakers or line inputs attached you simply connect the multi-meter leads to the positive and negative speaker terminals and read the multimeter. The amp should be running for a little while to settle down and stabilize before you will get a reliable reading. For this Adcom, the DC offset should hover around +/- < 5 millivolts.

[faiyaza]

Hi,

I know this thread started off about the Adcom, but so far the advice can probably apply to all older solid state amps. I have a Sonographe SA-250 (made about the same time as the Adcom) and can probably use some TLC as well.

I was thinking of getting a CapWiz to test each cap in circuit, but they're prohibitively expensive. From what I've gathered from other threads on the subject, common knowledge is to check each cap for warped bodies and replace as necessary. Are there any other tell tell failure signs I should be aware of?

Is there a rule of thumb when replacing components in an old amp? Should I mess with the BJTs (no fets in this amp IIRC) and assume they're all past their sweet spot?

Any help would be greatly appreciated.

Thanks!


F

[EchoWars]

Tel-tale sign of a bad electrolytic cap...anything 20 years old or older.

Seriously, it's a lot easier to replace the things than to test them. They are all cheap, except for the large PS caps. And modern caps are many times better than the originals were even when they were new.

Stick with good caps. Panasonic FC or FM, Nichicon HE or PW. If you need a bipolar, the Panasonic SU seem to work well. If you need a axial cap, I use the BC Components AML-138. All are at Digikey.

[faiyaza]

Originally posted by EchoWars
Tel-tale sign of a bad electrolytic cap...anything 20 years old or older.

Seriously, it's a lot easier to replace the things than to test them. They are all cheap, except for the large PS caps. And modern caps are many times better than the originals were even when they were new.

Stick with good caps. Panasonic FC or FM, Nichicon HE or PW. If you need a bipolar, the Panasonic SU seem to work well. If you need a axial cap, I use the BC Components AML-138. All are at Digikey.

Thanks a ton! After I'm done with my current project (Pass B1, Audio Sector DAC, and Arduino remote control in one unit), I'm going to take on refurbing this amp. The PCB looks very straight forward and not too complex (double sided PCB, thick discrete traces, and spread out component layout).


Thanks again.

[the apostate]

Well anybody know how to figure out how old my 555II is??? I think they were made from around '90 to I dont know when? Also if I completely went thru the amp and replaced all electro caps can that throw any of the circuits out of phase??? I mean even replacing with the same values i dont know if anything is a RLC circuit in an amp??? Maybe i might run into some other component that has changed its value over time which could do the same.