Earthquake Thrust 4.5 Repaired but problems

[KyferEz]

I have the amp in the title that had one channel fail. I replaced the output transistors and it seemed to work fine when I tested it. Original parts weren't available, so I had to substitute, and the suggested substitutions were very close in all specs to the originals.

I then stored the amp for later use. Today I hooked it up and it works, but it appears I'm getting DC voltage on that one output when I turn up the gain past a certain level... I say appears because I haven't tested yet, but once I pass a certain level on the gain knob, the woofer on that channel pulls in hard and stays there.

Below that gain setting level, that channel works no problem, just with less than the desired output.

Could that be a sign that the bias needs to be adjusted?

Any other thoughts?

Thanks!

[Perry Babin]

If no one else replies, post a photo of the inside of the amp.

[KyferEz]

On the left is the RCA inputs. The 3rd channel from the left (with the first channel being under the x-over daughter board) is the one I repaired, you will notice it's transistors are in a different package...

Thanks as always Perry

[Perry Babin]

That's not likely a bias problem (although you can't completely rule anything out at this point).

If you leave the gain at the minimum level and increase the input from the signal source, does the amp still produce DC at the speaker terminals?

Does it produce DC if there is no load but the gain is increased as you did before?

What were the original outputs and the replacements you used?

[KyferEz]

Originally Posted by Perry Babin

If you leave the gain at the minimum level and increase the input from the signal source, does the amp still produce DC at the speaker terminals?

No. And It generates the dc even without any input attached.

I'll check on the other questions.

[Perry Babin]

Will turning the gain down reduce the DC after it appears at the speaker terminals?

[KyferEz]

Yes

[Perry Babin]

On the gain pot for that channel, do you have DC on two of the pins and essentially no DC on the other 4 pins?

[KyferEz]

Will check 2morrow

[KyferEz]

Old part: K D998
Replaced with TIP35C

Old part: K B778
Replaced with TIP36C

Also, I found I was wrong. It's not the channel I repaired that's having that problem. It's the adjacent channel. Ch3 was the one I repaired, Ch4 has the problem.

Also, you seem to be on the right track Perry, the DC appears to go away when I set the gain all the way up. Also, when it's got the DC, the output starts to oscillate when I touch the gain pot's metal post. If the gain is at a level where the DC doesn't occur, the oscillation doesn't occur either. I also noticed that it's not DC. It's a slow oscillation, which speeds up when my hand approaches it or if I adjust the pot.

Question: If I ever plan to bridge this amp, should I also replace the output transistors on ch4 with those I used on ch3?

Thx!