Adcoms with lots of DC offset. Lots!

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#43
I am trying not to snip the four input wires to get the input board completely out to replace those 100 ohm resistors. Whoever last worked on the amp glued to rca inputs to the chassis. Impossible to remove unless I snip the input wires now.

Can someone identify the 100 ohm resistors (input ground resistors) on the GFA-555 II input board. I see four 100R blue resistors toward the front - not near the input wires in back), and they are all showing fine at 100 ohms, so these are most likely not them. Again its very hard to work on the board when you cannot remove the board from the chassis.

Someone here sent me the scanned GFA-555 II schematic (Thanks!) but was not able to read it very well.

I tentatively soldered a 100 ohm resistor from input ground to the star ground and all works perfectly now. Within single digits on DC offset.

To those with high DC offset - check resistance from input ground to chassis ground on each channel to confirm around 100R....at least this is what my problem was...
 
#44
My GFA-565 has appropriate resistance through the grounds:
10 from RCA Input ground to speaker ground,
100 from Line Cord ground to RCA Input ground,
110 from Line Cord ground to Speaker ground,
so no easy fixes there.

Any ideas? I'm no expert, and will freely admit that although I'm an "I can fix anything" kind of a guy, most of my electronic repair experience has been looking for the crispy bits and replacing them, so I'd be grateful for any suggestions you've got.

- Eric
 
#45
A must have tool for anyone with leaky caps.
http://www.amazon.com/gp/product/B000K7PT72/ref=olp_product_details?ie=UTF8&me=&seller=

Also check .all resistors. The Schematic from Adcom is poor quality. I got mine directly from them, and it is unreadable on many pages Also check the diodes. Set your MM to continuity. Negative on one side first, then swap the leads. You should read open in one direction, and a little resistance in the other. Order some 970', and 2240's, and replace them. As Chris has stated there is a matched Darlington pair that should not be replaced unless necessary. Nobody should do these repairs without having an ultra sonic cleaner. This doesn't omit having to scrub them, but help ensure that you get all the nasty stuff off the boards. On the 565's, and 585's I recommend replacing the Op Amp with an OPA97. Its a pain for sure to test all the little components on a 565 never mind doing it on a 555, but if you are not thorough you will have to remove the boards, and start the whole process over again. As for the input jacks just snip them, and replace them with some better ones.
check out this site. I bought some of the "A" RCA jacks that are very high quality.
http://www.apexjr.com/
The reason I was basically talking to myself in the beginning of the thread is so others with the same kind of problems could compare my experience with theirs.
Here is another good read for anyone doing Adcom's
http://www.diyaudio.com/forums/showthread.php?s=&threadid=104182&perpage=25&pagenumber=1
Chris is stellar when it comes to repairing Adcom's. I am most grateful for what he has taught me, and I hope to be able to share that info with others.

Thanks ZC😀
 
#46
Okay, having read the "GFA-585 Blown 20W 4.7ohm..." thread, I can sense a pattern.

I need to resign myself to re-removing the input board, finding an US cleaner and vibrating the bejeezus out of it in a soothing bath of noxious detergent, sparkling clean though it may now appear to be.

Two questions before I s-l-o-w-l-y embark on this project:

1. Does anyone have a line on any quick-disconnecting PC board pins (say, from DigiKey or Mouser?) that I could install at the wire connections as I put this thing back together to avoid all that soldering and desoldering? Too much heat is just asking for trouble, in my experience, and I see this sort of thing kind of like an umbrella on a cloudy morning - if I make it easy to take apart, then I won't need to.

2. There was a great deal of discussion of specific output transistor replacements in the above-mentioned post. Even though I have not (yet) blown any (see umbrella reference above...), if there is anyone out there who might wish to supply a set of same, I'd be interested.

Thanks,

- Eric
 
#48
Actually, I meant all of the 15 or so wires that are soldered directly to the input board (about a dozen 20ga, 3 14 or 16ga).
One of them has a tubular friction terminal that slides onto a roughly 18ga pin that is soldered onto the board, all of the others are direct connections (these are aside from the white plastic jack connections for power in and signal out to the output arrays). It seems to me that if I can get a part number or descriptive term for the pins and sleeves, then I can install these on all of these wires, thus making disassembly MUCH easier, as I have a feeling I'll need to do it a few more times.

I'm always afraid I'll burn something irreparably (and not necessarily the thing I'm aiming for...) whenever I start poking at forests of wire with a 700 degree piece of metal.

- Eric
 
#50
Well the amp is done now. New 14/3 power cord. Stock was a 16/2. I added a ground, and upped the Ga. I replaced the little transistors 970's, and 2240's. DC offset gone! Bias set properly 50mv when hot. Also replaced the output drivers. Not the main ones, but the ones that drive the mains. All I had in stock were MJE15032, and MJE15033. These drivers handle slightly less currant, but I am taking it easy till I get the MJE15030's, and MJE15031's. The 32, and 33 have slightly more gain, but are not up to snuff for max power output. I am driving the snot out of my Ipod thingy speakers, and they take it well. Adcom's kinda amaze me. They seem like smart amps that just control speakers well. I have driven my big custom SDA's with one, and then I hook it up to small speakers, and it doesn't blow them yet it cranks. The sound is so sweet. It sounds a bit better than my other 555. I don't know why but it does. The shop that worked on it last cut the power cable, and threw out the fuse holder. I guess they though it was beyond repair I am torn whether or not to sell it or keep it. I want to get rid of the silly ground bar, and upgrade it to point to point wiring. I have another GFA-555 transformer, and was thinking of going nuts with this one. I was thinking of paralleling the 2 transformers to double the va, and doubling the output trannies. Also upping the caps to 68,000 mfd x 4. Stock is 15,000 x 4. I would like to improve the dampening factor, and headroom. This might require making the GFA-565 input boards. It would be very similar to the GFA-585, but stronger. I would need a larger case, and have to do a little milling to make heat sinks, or water cool it. We'll see.
 
#51
MDchanic said:
Actually, I meant all of the 15 or so wires that are soldered directly to the input board (about a dozen 20ga, 3 14 or 16ga).
One of them has a tubular friction terminal that slides onto a roughly 18ga pin that is soldered onto the board, all of the others are direct connections (these are aside from the white plastic jack connections for power in and signal out to the output arrays). It seems to me that if I can get a part number or descriptive term for the pins and sleeves, then I can install these on all of these wires, thus making disassembly MUCH easier, as I have a feeling I'll need to do it a few more times.

I'm always afraid I'll burn something irreparably (and not necessarily the thing I'm aiming for...) whenever I start poking at forests of wire with a 700 degree piece of metal.

- Eric
Click to expand...
A few of those mentioned don't need to be removed from the board. One set is thermal protection led, and another is distortion. These can easily be removed from the front panel. Just straighten out the leads. Also 2 thermal heat sensors can be slid out of the heat sinks. I am pretty sure there are only 3 sets of post that I remove by using desoldering braid from underneath. Watch your heat you don't want to melt the plastic that holds the post. Also when reinserting these post don't be too forcefull. You can push the copper off the board. The Op Amp only needs to be removed once. Hopefully someone will double check my post, but Walt Gung (the original designer) recommends using an OPA97 for the servo replacement.
 
#52
Ben -

Okay, I can see that. I grouped the LED wires (2 pairs) and the thermal sensors (3 pairs) together when I reassembled it. That leaves one ~18ga wire (term. 7), three ~16ga wires (terms. 18, 19, and 20), and the three wire-wrapped pairs.

The 3 larger-gauge wires and the #7 wire I could attach with a nylon pin plug (heavy automotive type), if I wanted to, but there's still those pesky wire-wrapped terminals. I will confess I unwrapped them when I disassembled the board, then cut them short, wrapped them into single loops, and soldered them to the pins when I put it back together. It didn't seem as though the strands would withstand many attempts a unwrapping and rewrapping. I hope this doesn't make me a bad person in Adcom's eyes.

As for the op amp, yes, I replaced it with an OP97, but I used a DIP plug, which Chris has since told me is not a good idea, though I'm not sure why.

Since it's coming apart again as soon as I scrounge an US cleaner, I'll have a chance to try it your way. I like MJR's neat push-on pin and screw terminals, though. They appeal to my sense of order [insert exaggerated German accent].

On the subject of US cleaning, about how long seems to be a good idea? Chris felt that MJR's 1 hour in the US machine was too long, and my 20 minutes of scrubbing with a toothbrush in the sink was too short, but how long is just right?

Thanks,

- Eric
 
#53
I clean the boards vigorously till they look clean with a tooth brush. Then I do 3 5 minute runs in the cleaner, and 2 5 minute runs with fresh water. I dry mine with a heat gun being careful not to get it too hot. Maintaining it a little too hot to hold for 5 minutes is good in my book.
 
#60
I stripped mine also - the mains fuse, not the rails. I had one handy.

Since the one i had was a bit old, the cap part basically pulverized into little pieces. I had to take it out and break the housing into little pieces to get the fuse out...
 
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