Avalon VT-737sp with 120 Hz Buzz

[echo zulu]

I'm working on an Avalon VT-737sp combo unit that has an audible buzz on the output in any mode.

All the voltages look correct but there's a lot of ac ripple on the 34v rails; 486 mv on the positive rail and 286 mv on the negative one with the PSU connected (it's about 20% of that with the supply disconnected).

The electrolytics have all been checked with both ESR and capacitance meters and appear to be good. The PSU semiconductors seem to be functioning and all junctions check out OK with a DVM and a Heathkit component tracer (similar to a Huntron Tracker).

The 34v rails to ground and to each other at the main board Molex connector show resistance in the megohms range and nothing on the boards gets hot, so I don't think the audio circuitry is drawing excessive current.

Any suggestions? I'm not sure where to go from here...

 

[radardoug]

Missing ground?

 

[pucho812]

Assuming you have the  mains transformer set  correctly. IS the ground link connected?

In the rear of the unit about the IEC plug, there is a audio and chassis ground link that needs to be connected.  I suspect it might be missing.

 

[echo zulu]

Thanks for the replies.

The PSU grounds have good continuity to the chassis ground. Mains are set to 120 vac (I'm in the USA). The ground link is present & securely connected.

 

[pucho812]

o.k.

Might be time for a recap...
 
or I would confirm the wiring to and from the xlr's the do red, black and green for ground. while it is rare I have seen those flipped before.

 

[Audio1Man]

The basic PS is +/- 50vdc cap Input and then to two voltage regulators that are +/- 34vdc.
What are the PP voltages of the 50vdc ( I think a 1-2 volts)? The output of the 34vdc should be mv, not ½ volts. Fix the the PS
Duke

 

[echo zulu]

Thanks. The XLRs are wired properly. I determined that the supply was indeed the problem soon after my last post.

There are a pair of 2200 uf filter caps on the supply bd. The cap for negative rail had 54 vdc on it, the positive rail cap had 5 vdc present. Resistance checks showed an open trace from the positive cap to it's rectifier bridge.

After jumpering the open trace, both caps now have full voltage and the ripple on the positive output is at zero. Unfortunately, this also causes the supply's negative dc output to drop to zero and the ripple on that output to rise even higher than before. And no, I didn't short anything .

If I remove the jumper, the supply reverts to it's original state.

 

[pucho812]

unless there is a damaged trace, there should be no need to jumper anything.

 

[Audio1Man]

Repair the + regulator. basic circuit is attached.
Duke