Crest FA901 Power Amp - One Channel Offset and Hum

[TomCarlos]

Greetings MEF members...

A friend asked me to look at his Crest FA901 Stereo Power Amp. I'm guessing this amp is "old" !! Channel B sounds fine (no hum, no dc offset at the output). Channel A has a slight "hum" (it is noticeable) and has 10mv dc offset (that is what I am measuring). There is an ever so small ripple that is barely noticeable on a scope.

As I am poking around the amp, I think it's an odd design. First, having a fan in the middle of the amp doesn't make sense to me- but oh well. Next, I see what appears to be a "Right" and "Left" set of components. When you look closer, you will see the right side is made up of preamp board connected to the large heat sink assembly that contains 6 (qty) 2SA1302 PNP transistors (the green ones). The left side is similar but there you will find the 6 (qty) 2SC3281 NPN transistors (the black ones). The schematic is a bit different for me too - in that the Channel A and Channel B are mixed onto a single drawing. I guess you have to be good to separate everything out.

Anyway... before I get way in over my head, I am wondering if the minimal offset has anything to do with the hum on Channel A.

Thanks !

Tom

[g1]

That is not left and right but the + and - sides of the push pull.
One channel will be the upper layer, underneath will be another layer for channel B. Just a different way of laying it out.

[TomCarlos]

Geeze Louise !! The other set of Power Transistors are mounted on the bottom side of the heat sinks. It's like they are inverted for some odd reason.

So, back to the offset and hum. I see VR1 and the LA2500 IC. I believe that is to set the bias current? Does that impact the voltage at the output?

[Jazz P Bass]

I would limit any tests on that amp to power supply.
Check for any ripple that may indicate a bad cap.

Try to discern if it is a pre or power amp issue.

Push comes to shove you can always send it to Peavey.

[TomCarlos]

Thanks JPB. But if the amp has a bad cap in the power supply, I would think you'd hear the hum in Channel B as well. So there could be a filtering issue downstream (as you said... in the preamp or power amp section).

[Jazz P Bass]

I am just cautioning you.

That is not an easy amp.

The bias is set with that LA2500 (which is basically unobtanium) (
It makes a pseudo Class A output without crossover distortion.

[The Dude]

I wouldn't worry about 10mV offset. That's pretty minimal. Does the hum go away when the volume is turned down? Is the amp configured correctly? It's not in bridge mode is it?

[TomCarlos]

The power supply provides a Red and Green (power) lead to each output board. The Red supply line measures at 73.4vdc with 42mv rms ripple. It appears as a sawtooth wave. The Green supply line measures -73.4vdc with 36mv rms. The power supply also has a low voltage output of 20.8 volts sitting across C11 (1000uf) with 500mv rms ripple! I'll try to take out the P/S board and check that capacitor (probably just replace it).

[The Dude]

FWIW: I agree with your original assessment, "if the amp has a bad cap in the power supply, I would think you'd hear the hum in Channel B as well". Since the same power supply is shared by both amps, I don't think you have a power supply problem.

[TomCarlos]

I tested in the stereo mode and bridge. Now for some strange reason (as it happens on occasion), the amp seems quiet. I will retest later tonight and tomorrow. But I am still concerned regarding the ripple on C11. Thanks!