Audiobahn A4004T noise

[cooter1974]

Audiobahn 4004t amp, when turned on, has a "reving up" sound at the speakers. Once that goes away I get alternator whine. Eliminated all possibilities of a ground loop, and scoped rca inputs and they're clean. Power has a little noise. Replaced amp with an identical make/model from friend and problem goes away and it sounds great. Not having a schematic for amp and no experience with them, I'm not sure where to start. Sure, I can identify the power supply section and the amplification stages, but I'm hoping someone will have working knowledge of problem and give me a little guidance. I have searched the forum, and most problems I could find were ground loops, proper installation etc..

[Perry Babin]

Do you have ~0 ohms continuity between the RCA shield ground of the amplifier's input jacks to the secondary center tap of the power transformer? The secondary side of the power transformer is near the TLx94 IC). The center tap is where there are two leads going into the board together.

Also check the resistance from the RCA shield to the chassis ground terminal. What is it?

Have no RCA cables connected to the amplifier when checking the resistance.

[cooter1974]

RCA shields to CT reads 0 ohms (0.12)
RCA shields to chassis ground terminal read 9.7k, and slowly rises as though reading through a cap.

I do notice some horrible solder joints (common with alot of consumer electronics). whether it will help solve the problem or not, I'm going to re-flow them. I've had run-ins with poor solder and residue flux generating noise in RF stages.

[Perry Babin]

I checked several Audiobahn amps and they all went to 10k within less than 1 second when measuring resistance between chassis ground and the RCA shields or the non-bridging speaker terminals. On an A2002T, there is a 10k in parallel with a small ceramic capacitor. They are the connection between the chassis ground and the secondary center tap (same as RCA shield or non-bridging speaker wires).

Did the amp work properly at one time?

If not, look for bridged solder connections.

[cooter1974]

I bought it used off ebay. They said it worked, but you know how that goes. I'll reflow some of the joints and look for bridges. Thanks for your help and I'll let you know how it turns out.