I have two GFA-555 amplifiers in working condition. One is driving lows in my DIY active speakers, the other one is a floater used on as needed basis.
I recently used the floater and while it worked fine, I noticed that upon turning it off, it cut the sound within a second or less. I remember it in the past coasting for quite a few seconds after cutting power, which to me means the power supply caps are shot and possibly other electrolytics on the circuit boards are bad too. Considering its age (25 years or more) that would be expected. So my question is, what's the most cost effective way of refurbishing the amps? I'm OK with basic soldering, but I don't do it regularly. Should I seek out professional service providers or is it a straight forward repair and I should give it a shot myself?
I recently used the floater and while it worked fine, I noticed that upon turning it off, it cut the sound within a second or less. I remember it in the past coasting for quite a few seconds after cutting power, which to me means the power supply caps are shot and possibly other electrolytics on the circuit boards are bad too. Considering its age (25 years or more) that would be expected. So my question is, what's the most cost effective way of refurbishing the amps? I'm OK with basic soldering, but I don't do it regularly. Should I seek out professional service providers or is it a straight forward repair and I should give it a shot myself?
I just opened the amp and these are screw terminals capacitors, so no soldering will be required. I can do it myself. Now it's a matter of finding the caps that fit and are the correct capacity. Can anyone help me locate eight 15000uf 100V screw terminal capacitors that measure 2.5" diameter by 3.5" height? I found some 10000uF at a fairly reasonable price, but no 15000uF.
My amp inside looks almost like this pic, but it has bypass caps on the big capacitors.
My amp inside looks almost like this pic, but it has bypass caps on the big capacitors.
Not necessarily a cap problem. Can you be sure the bias current of the amps is the same as it was way back then... has it ever been tweaked in the past.
No, I don't know it the bias is off spec. I will have to measure it out, right after I find the procedure. I see two variable resistors on the circuit board, so it hopefully will go easy.
Those oldsters while certainly powerful were more than a bit Coarse sounding, especially noticable if one tried to use some of their prodigious power
Musical concepts once sold a pricey upgrade (Bypass caps on the electros basically) that helped.. some.
But the complete lack of of Regulation was/is the root problem.
Retrofitting a decent PS circuit was a V good improvement, years ago .. likely still is ?
Dunno if that helps your problem.
But while in there..it could turn this amp into a Keeper.
Musical concepts once sold a pricey upgrade (Bypass caps on the electros basically) that helped.. some.
But the complete lack of of Regulation was/is the root problem.
Retrofitting a decent PS circuit was a V good improvement, years ago .. likely still is ?
Dunno if that helps your problem.
But while in there..it could turn this amp into a Keeper.
Checked the bias and it was just about right on 16mV. A little under on one side, a little over on the other. But while probing the right channel I shorted one transistor and got a nice spark. So it looks like I will have to either seek a pro service anyway or cut my losses and get rid of the amp as is. Too bad because I really like this amplifier.
That's bad luck... don't know what to say to that really.
I mentioned bias current originally because a slight increase in bias current can make a huge difference to how long an amp can "run" off the charge in the reservoir caps.
If the short was on the output devices then you are probably looking at a new set of outputs and perhaps drivers too but its worth seeing just what has failed.
I mentioned bias current originally because a slight increase in bias current can make a huge difference to how long an amp can "run" off the charge in the reservoir caps.
If the short was on the output devices then you are probably looking at a new set of outputs and perhaps drivers too but its worth seeing just what has failed.
the short was on the 2SB554 side, but seeing how they are all connected, there is a good chance the 2SC424 are damaged as well.
Possibly.You would have to do a quick check on them either out of circuit or with the emitter and base isolated.
I once had a tech that often blew amps up setting bias due to probe shorts. He wouldn't insulate his meter probes and I had to fire him. Too bad, he did excellent work, but I couldn't afford him - heck! He couldn't afford him either. I had techs on piecework, otherwise many just coasted along, the good ones made more and were happy.
The Adcom can sound good and is worth fixing. Use the current On Semi outputs. Insulate your probe tip shafts!!!!
-Chris
The Adcom can sound good and is worth fixing. Use the current On Semi outputs. Insulate your probe tip shafts!!!!
-Chris
I have a 5503 what do you recommend I do to improve it?
I'm willing to pay for your time if needed.
Hi cnitty,
When I do that, the first amp I see I have to look at it, feel it out and measure the performance to see where any problems might be. Adcom isn't a brand that you can get anywhere just swapping parts. They used really good parts to begin with and you might stand a chance of it being worse after a parts swap. You have to approach improving Adcom (and Nakamichi) with great care and thought.
On another thread, I had designed a transistor matcher and gave the design to everyone. Two folks designed PCBs. The greatest gains in performance will be when you close match the beta in the input pair. The jig allows you to get within 1% pretty easily. You would see a decrease in distortion and also DC offset. The rest? Leave it alone for now.
-Chris
When I do that, the first amp I see I have to look at it, feel it out and measure the performance to see where any problems might be. Adcom isn't a brand that you can get anywhere just swapping parts. They used really good parts to begin with and you might stand a chance of it being worse after a parts swap. You have to approach improving Adcom (and Nakamichi) with great care and thought.
On another thread, I had designed a transistor matcher and gave the design to everyone. Two folks designed PCBs. The greatest gains in performance will be when you close match the beta in the input pair. The jig allows you to get within 1% pretty easily. You would see a decrease in distortion and also DC offset. The rest? Leave it alone for now.
-Chris
@ anatech
Can we get the transistor matcher design ? It will be helpful for me
Thank in Advance.
Can we get the transistor matcher design ? It will be helpful for me
Thank in Advance.
Sure, it's documented in another Adcom thread, I think the 565 one near the end.
If I can find my files again I'll start a thread on it.
If I can find my files again I'll start a thread on it.
Well, by 2022, swapping out old dead caps might help. I picked up a GFA-555 Pro. The thing has gain controls ( pots yuck! ) on the input board. The balanced inputs just have opamps to sum the signal.
It sounds awful. Tonally correct, but distorts - it was basically unlistenable until I re-biased and cleaned all the controls. Reflowed some solder points. Sounds better now, but still can't image worth a damn and the bass sounds like mud. The filter caps are more than 25% gone based on timing the drain time. Sounds gross man... I haven't tried the TS 1/4" inputs yet. I have adapters, but havent had the wherewithal to try listening to this thing again. LOL
The Pro input/driver board is somewhat different from the standard version. Anyone ever seen a service manual for the Pro? The MK1 service manual is pretty good. There are some 4.7mF film caps on the Pro that are not on the std version, as well as circuit breakers instead of fuses on the rails. Weird.
It sounds awful. Tonally correct, but distorts - it was basically unlistenable until I re-biased and cleaned all the controls. Reflowed some solder points. Sounds better now, but still can't image worth a damn and the bass sounds like mud. The filter caps are more than 25% gone based on timing the drain time. Sounds gross man... I haven't tried the TS 1/4" inputs yet. I have adapters, but havent had the wherewithal to try listening to this thing again. LOL
The Pro input/driver board is somewhat different from the standard version. Anyone ever seen a service manual for the Pro? The MK1 service manual is pretty good. There are some 4.7mF film caps on the Pro that are not on the std version, as well as circuit breakers instead of fuses on the rails. Weird.