Sony xm-7547

[mnhiphop101]

this amp has an offset fault. I have all the bias pots turned counterclockwise. When I disable the offset protection and check the test points 3 out of the 4 are ove r the .04-.05mv without adjusting the pots at all. The rages are from .06 to .09mv. I have a working 2 channel version of this amp I can reference from i just dont know where to start.

 

[Perry Babin]

Do both channels produce clean audio?

 

[mnhiphop101]

No

 

[Perry Babin]

Can you be a little more specific?

 

[mnhiphop101]

They produce audio but there is a high pitched hum also

 

[Perry Babin]

What is the frequency of the hum?

Are you using a signal source with a grounded shield?

 

[mnhiphop101]

Ok something must have change since last time I powered it up. Now I get no signal or any hum. There is 13v of dc offset on the terminals. If i connect a load it draws about 5.5 amps of current

 

[Perry Babin]

Intermittently shorted power transformer?

 

[mnhiphop101]

Ok the hum is back after flipping some switches. Its about 25 khz

 

[Perry Babin]

Did you see #8?

25kHz is inaudible.

 

[mnhiphop101]

now there's 3.5v offset on 2 of the channels and 1v on the other 2

 

[mnhiphop101]

I dont think thats the issue. I didnt touch the transformer

 

[mnhiphop101]

when i try to wiggle it nothing happens. Its seated pretty well

 

[Perry Babin]

Are you measuring directly across the speaker terminals for each channel?

 

[mnhiphop101]

Yes

 

[Perry Babin]

Are the 4 voltages from the low-voltage supply steady (don't vary when the output offset varies)?

Do you see any supply voltage (or anything else) that varies when you push on either board?

 

[mnhiphop101]

Where is the low voltage supply?

 

[Perry Babin]

In the area designated 'power supply (pre-stage)'.

 

[mnhiphop101]

Do I measure at these jumpers?

 

[Perry Babin]

Yes. Confirm that there are no bad solder connections in that area. Any solder joint that has a depression or a ring in the center of the joint should be suspicious.