Stereo Repair

[mzacharias]

These models are notorious for bad 2SA1145 and 2SC2705 transistors on all amp channels. I change out all 20 transistors, 10 of each. This helps to ensure no re-do for the other channels failing in the same way. Done many like this. I would suggest there is a chance in excess of 90% that this fixes your problem.

I try to keep a minimum of 20 of each type on hand.

A service manual is available from Elektrotanya.

[staze]

These models are notorious for bad 2SA1145 and 2SC2705 transistors on all amp channels. I change out all 20 transistors, 10 of each. This helps to ensure no re-do for the other channels failing in the same way. Done many like this. I would suggest there is a chance in excess of 90% that this fixes your problem.

I try to keep a minimum of 20 of each type on hand.

A service manual is available from Elektrotanya.

Cool. I got the service manual, it is helpful. So by that reasoning, you would think Q507 and/or Q505 is/are the culprit? Where do you normally order from? Any issues with counterfeit ones from china or the like?

To remove the board from the heatsink, I'm guessing you have to break the little tabs on the PCB, and unscrew the power transistors?

[mzacharias]

Yes, Q505 and Q507 for the left channel would be highly suspect. The typical symptom is erratic noise and often popping sounds. Freeze mist might help confirm the issue.

[staze]

Hmm, I'm not seeing those symptoms. Take a look at the first post... shows the waveform for what I'm getting... but anyway, ordered 10 pair of the transistors from some chinese dealer via ebay. Hopefully I'll swap them out and be fixed... but somehow I'm second guessing that. =/

[David_AVD]

Have you tested the output transistors as suggested in earlier posts?

[SgtRock]

Greetings EEVBees:

--KISS,. IE, Keep it simple stupid, usually, I am the stupid one. Please see the below link, and watch any of the any Amplifier and Receiver repair videos from Maxxarcade. He gets right to the point, and he makes professional videos, with no mumbling, pictures of the back of the hand, or witless computer music, and like our hero, quick and to the point. Save yourself some time, by watching someone who knows. See below link. No point in using the O'scope as a DMM.
https://www.youtube.com/user/Maxxarcade

"Measure twice. Cut once"
Norm Abram 1950 -

Best Regards
Clear Ether

[staze]

Have you tested the output transistors as suggested in earlier posts?

Not yet... I just need to pull the board off the heatsink, which I'm not looking forward to. Will in the next couple days... new baby at home is taking up most of my free time. =)

[David_AVD]

You can test them for basic functionality in-circuit.

[staze]

How? I did diode tests which I posted. Only other way I can think would be hooking an FG directly to them... but not sure if that's kosher.

[David_AVD]

I must have missed the results.  Did they test ok?

Have you checked the emitter resistors too?

[staze]

I wasn't sure specifically what resistors were the emitter ones, but I think for Q1 and Q3, they're either the 100R, or the 4.7K ones, both of which checked out. (this is from memory).

That said, I replaced Q505 and Q507, and it fixed that channel! When testing it, I didn't notice my scope had one channel at 1V/div, and the other at 200mV/div, so I thought maybe the new transistors were just working that much better than the old (I know, I know). So I did the right channel, and the center channel as well. The rear channels are both powered by an IC.

When I saw that changing those did nothing, I then noticed my scope was set that way. DOH! Oh well... they all needed to be replaced at some point.

Biggest issue is the PCB was shit. Had to jump things twice because a trace lifted when unsoldering/wiggling the old BJT out. The rest of them I just cut off, and pulled the pins. Still... shitty PCBs suck.

Thanks all! Especially mzacharias for being on the money!

[David_AVD]

The "emitter resistors" are on the output transistors and are usually in the 0.1R to 0.47R range in value.

[staze]

Ah, those! So in the schematic, attached again, it's ACN1087 (0.33R). On the amp, that's a large white box shared between each of the output BJTs. Didn't check that, but guessing they should be fine (since I didn't have any way to replace them either).

[David_AVD]

Yes, that's a pair of emitter resistors with a common leg.  They are used frequently in hi-fi amplifiers.

They can be replaced with two 5W 0.33R resistors if you don't have access to the original parts.

[staze]

Yes, that's a pair of emitter resistors with a common leg.  They are used frequently in hi-fi amplifiers.

They can be replaced with two 5W 0.33R resistors if you don't have access to the original parts.

Good point. =)

Thanks!