Hartke HA3500, reviving old thread. Hiss noise.

[Indyryder02]

Hopefully I’m going about this the right way.
Trying to revive an old thread of an amp I’m getting a look at again with some new info.

Have this Hartke HA3500 with a hiss noise.
I’ve traced the hiss to the power amp board, and as early as the first unnamed transistors. (Q301& Q067 listed on board)

I have another one of these amps in good condition I compared to. Seems the emitters of these two transistors have 1.5v on them opposed to the .7v on the good amp. I’ve checked all the surrounding components without finding any failures, and the transistors themselves all seem to diode test fine.
what should I be looking for?

thanks in advance!

[The Dude]

I'd first check bias. If an amp is substantially overbiased, any existing preamp noise will be louder than normal. If you don't have bias info, compare it to your other amp.

[Indyryder02]

I'd first check bias. If an amp is substantially overbiased, any existing preamp noise will be louder than normal. If you don't have bias info, compare it to your other amp.

I haven’t done that as I figured it was set… and the bias pots wouldn’t have moved on their own..(carburetor theory) but maybe drift? (Now that I think of it, I’ve seen that on the Ampeg SVT-3 pros a lot, but that’s usually a fizzy distorted sound.. too much crossover distortion, adjusting it till it’s gone resolves those)
I’ve replaced the pair of transistors in other amps with better matched ones before to resolve this similar noise due to them drifting apart. (Usually freeze spray identifies these)
Also I’ve plugged into the effects return without any change. So seems to be isolated to the power amp section.