Adcom GFA-555ii Repair/Rebuild

[Tejaus]

I've have an GFA-555ii that has the dreaded DC output on one of the channels. Took out one of my JBL 4412a's with it. I'm not sure if the output transistors are fried yet. I did replace all of the with the On Semi MJ21193G / MJ21194G in 2005 since it was too hard to find real original Toshiba's. And I replaced all the small electro caps to back then.

Worked fine for a few years until this DC issue. Part of it I think was the bias adjust trimmers being flaky which I want to replace as well.

I've read some of the older posts regarding the Sanyo 2SC2912/2CA1210 parts crapping out and causing the DC. I did replace one back in 2005 on the input board although it didn't cause the DC. I've also read about the problems with fake ones being sold now.

Echowars posted about substitute parts here in 2007: adcom 555II part sources

On-Semi MJE15032 and MJE15033 will make fine TO-220 drivers, and Sanyo makes the TO-126 2SA1380/2SC3503 predrivers that will do a good job replacing the original 2SA1210/2SC2912 combo (which I think were also Sanyo devices).

So the On Semi MJE15032 and MJE15033 are driver substitutes for the 2SA1380/2SC3503? But if the On Semi's are a TO-220 and not a TO-3PB will that work?

Since that post was close to five years ago the 2SA1380/2SC3503 have been superceded. What's the best pre-driver available to sub for the 2SA1210/2SC2912 combo?

I have a pair of 1/4" Vishay 2k ohm trimmers that are rated at 250v / 0.5 watts. Is this high enough? http://www.vishay.com/docs/51016/t73.pdf
My service manual doesn't say what the originals are rated at.

I'm trying to get all the parts from Mouser if possible as I'm repairing some other stuff too.

[djk]

The MJE parts are for the driver/output amplifier, it sounds like you have the predriver/driver/output version (newer).

I think you could use the NJW series 0302/0281.

http://www.onsemi.com/pub_link/Collateral/NJW0281-D.PDF

[Tejaus]

The NJW's are in TO-3 packages. The 2SC/2SA are TO-126. This is maddening trying to find subs for the Sanyo's

I did see that On- Semi Has acquired Sanyo Semiconductor. ON Semiconductor: Customer Integration

[Tejaus]

I saw in another post that Adcom sent these for replacement.
http://www.datasheetcatalog.org/data..._e/2SC3502.pdf

It looks like the Ic is lower than the originals by 40ma

[djk]

"The NJW's are in TO-3 packages"

TO-3P, for the drivers using TO-3P parts.

Can't help you with the predrivers, sorry.

[embrown057]

Take a look at the Fairchild 2sc3503/KSC3503 for your Vas sub. The part number for the 2k trimmer from Mouser is 652-3329P-1-202LF.

[embrown057]

Take real care when trouble shooting that BIAS stage. Check ever single part in it including the thermal switch. It is a UP-62 norm closed 85C part, you should have <.1 ohm cold. You can use the 90C part if you can not locate the orginal. Make sure you reinstall the copper spring holder for the thermal switch so it sit's firm to the heat sink. Also make sure you also check Q307/257 located on the heat sinks including the wiring too and from the boards. You still may find the orginal Sanyo 2912/1210 parts if you look hard but as we all know there are fakes. The fake's for these parts are easy to spot as they are all plastic with no metal backing like the orginal Sayno's. The 1210 can be had from B&D enterprises but they are out of the 2910's as far as I know. Let us know how it works out.

[embrown057]

Sorry, that last line should be 2912's, I think I posted that at 2 am. I have a limited supply of the 2912's in stock you can contact me through PM if you would like a few. I can also provide matched sets of the diff pair. In the event the outputs are gone make sure you replace ALL of them , not just the one's that are bad. The .22 emitter resistors you can use the Mills MR05-.22. It takes some art to form the leads just right to fit but they are very good parts. The MJ21192/93 are still around they just moved the fad oversea's from Mexico. The power base ON-Semi's are also an opt. I just finished a MKII that had been to another shop, the tech gave up on it. The amp hit the ground from 3ft, yes it was on and blasting. As a few know here it was the amp from Hell, but she now lives another day. There are a few member here that have vast experance with the Adcom line, help is only a click away. Good luck.

[Tejaus]

OK, I'm really starting to think it's the 2K bias trimmers that must be bad. Keep in mind the amp has been sitting on a shelf for over a year after it blew out the JBL $$$$$$$$. I have an old Yamaha M-80 that I virtually stole from a pawnshop schmuck and put that in it's place and forgot about the Adcom.

First I tested the six predrivers doing the ECB dance on the Input Board with the diode check on my meter (BK Precision 5390) and they all checked out. Also checked the diodes, resistors etc.

Measured the outputs on each side for shorts etc., all readings were the same both sides.

Time to breakout the Variac and slowly crank up the amp to 120VAC. Measure for DC on the channel the wasted my JBL's. Around 1.5mv, no huge DC found???? Same on the other channel.

Let the amp warm up and breakout my trusty Laser Thermometer. One side is at 95 degrees, the other is at 78. OK, obviously one is biased hotter so now I decide to check the bias. On the side that "blew" and was running warmer I read about 30mv. On the other side 0.00mv.

I adjust the trimmer to the center of it's range and turn the amp back on and it's a 2mv. If I just bump the trimmer just slightly it jumps way up. And when I say just slightly I mean that when I remove the screwdriver from the trimmer it will jump.

I had adjusted the bias to 10mv a few days before it blew.

embrown057, Mouser it OOS on the 2K trimmer you recommended.
I'm going to pop one of these Vishay T73 1/4" 2K trimmers in and see if the bias is still so touchy. I just have a feeling they got cooked like the small electro caps C104/C154 did from the predrivers. I might take you up on the Sanyos. I had replaced one of them before with a substitute non Sanyo part.

I'd like to put some clip on heatsinks on the TO-126's. Couldn't hurt.


I'll report my results after the trimmer swap.

[Tejaus]

OK. swapped the trimmer. It's still a little jumpy when adjusting, meaning it will jump up about 0.4mv when I remove the screwdriver. But now it seems stable with -1.2mv DC offset on the right channel and 0.5mv DC on the left. I'm not sure what's going on or why it did what it did.