Acoustic 150b unwanted disortion

[randomair]

I have an Acoustic Control Corp 150b here with unwanted distortion at all volumes. I checked the coupling caps and used substitution method with a known good one, but distortion persists. Any ideas on this? The speakers are not the issue being it has distortion through multiple cabs. Should i be looking at the driver transistors? This amp is a pretty easy setup. Volume, treble , bass and only three small circuit boards to boot! Should be a simple one but for some reason this one has really gotten me. Thanks!

[Enzo]

Is the power supply OK? Is the main voltage up close to what it should be and free of ripple? Is the lower voltage for the preamp up to what it should be and free of ripple? I believe each preamp card makes its own voltage stepping down from the main power amp voltage.

Scope the signal, what does the distortion look like?

Do both input channels have the same distortion? If so the power amp is likely.

In the power amp card, there is a 1000uf 50v cap in series with the speaker load. The main power supply is about 75v. If that is OK, does the amp end of the cap - as opposed to the speaker end - sit at about half that?

Does the cap look OK? No leaks? Probably should replace it anyway.

How does teh signal look through the power amp - there are only two transistors before the four power output transistors.

There are three windings on the small transformer. Power off - is any of them open?

[randomair]

enzo, i checked for continuity on that small transformer and all windings are fine. as well, i checked the power rails and they are reading 70 volts. the 1000mf 50v cap is reading 30 volts. not sure where to turn next. i do not have a working scope here so unfortunately its a crapshoot at this point. thanks for your input.

[randomair]

i replaced the 1000mf cap for good measure...distortion still there. but in reading the manual troubleshoot section for the 17-12 power board i see they have listed: symptom---amplifier distortion at low volume---
cause--excessive crossover distortion--- it then says to check the two thermistors at room temp and they should read 100 ohms. well one is reading 95ohms and the other is reading 4 ohms! do you think this may be my problem? thanks.

being i have no access to replacements of these thermistors what are my options for substituting the thermistors with a resistor?

[acousticcontrolcorporation]

Output driver transformer!

www.acousticcontrolcorporation.com

[Bruce / Mission Amps]

www.acousticcontrolcorporation.com

If you are going to take the time to type... why don't you participate instead of leaving your website URL?
How about just sharing some info like the rest of us.
Otherwise your just another annoying free commercial here and that is not in the true spirit of AMPAGE.

[Enzo]

Especially since the link takes us to an "under construction" web site that was last updated a couple years ago and offers no more than email addresses.