Audio Research D90 Noisy Power Transformer

[BobCrawford]

Hi everyone!

I've got a problem with an Audio Research D90. The power transformer was buzzing, mechanically. I assumed there was a problem with degraded varnish and took it to a motor rewind shop and had the transformer re-dipped in Glyptol then baked. The buzzing has been slightly reduced (not sure it was worth doing, actually) but now I'm getting quite a decent 60Hz hum in both speakers when the tubes warm up. It didn't do this before. There isn't any appreciable ripple on the power supplies and audio is clean in both channels. I'm seeing 55 volts AC (which I believe is being induced) on the insulated bolts that go through the core and mount the transformer to the frame of the amp, and I believe the audio amp is picking this up and sending it downstream. I didn't notice any grounds to the transformer when I took it out of the amp and the schematic doesn't show an electrical ground to the core. Can I shield this tranny and tie it to ground without blowing anything up?

Anybody have any experience with this unit or one of it's close AR relatives? Thanks in advance.
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[rayma]

This transformer may have leakage current, and could be dangerous.

 

[duncan2]

Test it using an analogue meter as digital meters pick up induced current and register it as voltage .


I take it the lamination's are okay?


If it is what Rayma says then disconnect the winding's from the amplifier and do a HV insulation test .

 

[rayma]

If it is what Rayma says then disconnect the winding's from the amplifier and do a HV insulation test .

Yes, see if you can borrow a 2kV hipot tester. The motor rewind shop should have one.

 

[IceBullit]

Noisy Power Transformer

Hello,

I had a similar problem with my power transformers in the past, it turned out to be a DC offset issue with my home power.

I purchased the Emotiva CMX-2 and it resolved my issue.

Here is the link to their product page:

>>> CMX-2 <<<

At the moment it looks like it is temporarily not available but you may be able to get a used one (which is what I did).

Hopefully this helps out, at least it could be one of the reasons for this happening, it took me a good while to figure it out but through research I concluded this was the problem in my case.

-Arthur

 

[audiowize]

The D-90 is OLD!!! Has this amp ever been serviced? That noise the power transformer is making may be its way of telling you that your filter caps are toast. If you ignore this for long enough, you may end up needing to replace the power transformer if a cap fails catastrophically.

 

[TonyTecson]

The D-90 is OLD!!! Has this amp ever been serviced? That noise the power transformer is making may be its way of telling you that your filter caps are toast. If you ignore this for long enough, you may end up needing to replace the power transformer if a cap fails catastrophically.

i agree, the mechanical buzzing is a symptom of a deeper problem, and the filter caps are the first to get inspected and replaced.....hey, caps are getting better and better these days...speaking from experience..

 

[HollowState]

...but now I'm getting quite a decent 60Hz hum in both speakers when the tubes warm up. I'm seeing 55 volts AC (which I believe is being induced) on the insulated bolts that go through the core and mount the transformer to the frame of the amp,

Years ago I had a similar symptom on a color TV. Holding a steel screwdriver against or very close to the PT's laminations I could feel the pulses. Turned out to be a bad diode in the bridge rectifier. It wasn't totally shorted or open, just leaky. An ohmmeter (or scope) may not find this, but a curve tracer or Huntron Tracker will with the diode partially disconnected. Probably a long shot but this triggered my memory.

 

[Eli Duttman]

Cap. replacement is (IMO) a good thing. If DC crud happens to be riding on the AC mains, "Da Blok" could be a solution.

 

[TonyTecson]

i think, the surest way to settle this is to run the power traffo without any secondary loads, meaning all secondary wires floated, just tape the ends for safety...

if your traffo turns very quiet, then it is not your traffo...

if it still buzz then you need to megger for low insulation resistance...even a 1kv megger will do..

 

[bigfishe]

I have a D-90 and had the same issues. Replaced all the Elco Caps and Diodes
Adjusted the amps completely and now I know why the ARC tech said it was his favorite amp they ever made.
You can also bias it full Class A