Adcom GFA-555II

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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.
 
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).
 
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.
 
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.
 
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???
 
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.
 
Sonographe?

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
 
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.
 
EchoWars said:
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.
 
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.
 
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Hi Sakas,
since my adcom days i still have 2 complete sets of outputs factory matched from adcom
IGFets or BJTs?

While doing warranty on these, I found it necessary to match the new ones myself. The time was well spent.

I'll also agree that these are nice amplifiers. Something new or a clone might work out very well.

Hi the apostate,
There are some caps that should be replaced, but the filter caps are not in that group normally. There is too much iffy information around about replacing capacitors.

Hi Glenn,
And modern caps are many times better than the originals were even when they were new.
I hate to burst your bubble, but this is not always true. I test them out of circuit and there are times when an older capacitor may test solidly better than a new one.

-Chris
 
thanks,
The amp should be here friday. Im just going to hook it up to some old speaks and make sure it works good and no turn on thump or hum, then ill hook it up to my polk rti 150's. Love these speakers, they really rock! And with this amp i should be able to damage my hearing nicely!!!:eek:
 
ok got the amp today, dc offset is 2mV each channel. So im good there, I hooked it up and its real powerful and great sounding! I am having one problem though, but havent had a chance to see if its the source or the amp. While listening to music im hearing just a little high frequency popping going on in each channel. Once i was done i watched a movie using the same source directv. Was hearing the popping sounding like it was coming from the center channel, tomorrow i will tune into pandora radio thru my blu ray player and see if it still does it!
 
sorry for the late reply anatech

anatech said:
Hi Sakas,

IGFets or BJTs?

While doing warranty on these, I found it necessary to match the new ones myself. The time was well spent.

I'll also agree that these are nice amplifiers. Something new or a clone might work out very well.

Hi the apostate,
There are some caps that should be replaced, but the filter caps are not in that group normally. There is too much iffy information around about replacing capacitors.

Hi Glenn,

I hate to burst your bubble, but this is not always true. I test them out of circuit and there are times when an older capacitor may test solidly better than a new one.

-Chris

i have 3 ( ithink ) completesets of the 555 series which is bjt and two sets fets for the 5800 seriers but unfortunatelly the fets are not matched by adcom .... ( this was the problems after all with the 5800 that were imported in greece at the time )

for some reason adcom didnt use matched transistors resulting frying omelets on the heatsinks ....

so they had to ship replecement sets and we got to keep the old ones ha ha ha ....hapend twice
 
Well after watching 2 dvd's and listening to pandora internet radio i can see its the directv hd causing the popping in the audio. Directv hd hasnt been right since i had it installed 6m ago, only on hd channels it locks up about 4 times an hour for around 2 seconds then starts playinf again. Had them out at least 12 times and they replaced the whole system and it didnt fix a thing! The tech's that came out said it isn't perfect! This is rediculas! So at this point i guess ill have to live with it, but at least my amp is ok!!!
I had this place jumping for a few hours, neighbors love me, already been getting some FEEDBACK today...gotta love 'em!!!
 
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Hi sakis,
Well, those BJTs will always come in handy. I would use them for something a bit more refined than the Adcom. For the Adcom, I would go with some of the newer On-Semi devices. I used up my Adcom output transistors some time ago, no loss there. I'm thinking of amplifiers like the Nakamichi 620 or similar, those would benefit from the original Japanese parts.

When we got mosfets from anyone, we always checked them. Even if they promised they were matched. Now, if you think that mosfet matching is important with an Adcom, you should see how critical it is with a Counterpoint amplifier. No source resistors at all, so they really did have to be quiet closely matched together. You will not enjoy the experience, trust me on that.

I'll say one thing, at least Adcom was honest about telling us the mosfets were not matched and that they expected us to do that work. I can't say I was happy about it, but at least we knew. Some manufacturers maintained that their parts were matched. Having spent time in the industry, I didn't trust them. That saved me a lot of aggravation on several occasions.

-Chris
 
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