PT go BZZZZ

[waspclothes]

Hey guys,
I have a Sovtek Mig 60 and the power transformer is buzzing really badly. I disconnected the different output windings one at a time and the buzz is still there. It has two primary windings, and I used a step-up transformer to test the 220V primary and the buzz is still happening. So I'm pretty certain the buzz is from the transformer itself, regardless of load.

This is the first time I've ever encountered something like this. What causes it and is there anything that can be done aside from replacing the offending transformer?

[R.G.]

1. Does it get hot?
2. What is loose to clatter/buzz from the magnetic field?

[waspclothes]

1. Does it get hot?
2. What is loose to clatter/buzz from the magnetic field?

Hi R.G., Do you want me to test if it gets hot with or without the load connected? I'll have a second look for anything loose.

FWIW, the transformer seems to be functioning properly otherwise.

[Chuck H]

R.G. is wondering if there may be an internal short. If the transformer gets hot unhooked that might mean an internal winding to winding or winding to core short. If voltages are off and the transformer gets hot under load that could mean a short within a single winding.

I've seen (heard) this before with otherwise functional PT's. I have one in fact that I haven't used for a project for that very reason. In the case of things being loose and the PT just seems to resonate during operation I've read that it's sometimes possible to shim tighten the windings on the core. IIRC the thread indicated that even new transformers were sometimes done this way once. I haven't tried it.

[Zipslack]

Could be that some of the varnish has broken loose and you have a lamination vibrating. Does the buzz sound like it's about 50 Hz?

[waspclothes]

1. Does it get hot?

Without load, the transformer draws 5.7W and after 10 minutes the temperature rose 1.5C.

2. What is loose to clatter/buzz from the magnetic field?

Everything in the amp has been tightened. A few of the screws holding the transformer to the chassis were a bit loose, but those have been tightened with no effect on the buzz. I called New Sensor about a replacement, but they said none are available and they "don't know the specs of the original transformer".

[mozwell]

So the TX is ok when powered under no load, but we still have the buzz.
Its a rattling lamination, possibly an air dry varnish was used & they didn't spray it well enough.
Normally a transformer will be dipped in varnish (until the varnish has gone into every nook & cranny in the transformer) & then baked in an oven to cure the varnish. This secures the windings solidly together & also secures the laminations. No buzz with this method.
Air dry varnish is a quicker method and is just sprayed on the outside of the finished transformer. The varnish doesn't impregnate the whole transformer & this method wont be as good for eliminating hum or for securing the winding.
option 3 is the transformer has not been varnished at all.

Get an insulating shim of some sort, poke it down between the bobbin & the steel core. The buzzing should stop. Leave the shim in there, if possible some super glue (or similar) around the end of the shim to hold it in will be good.
If you put the shim in from the top only, see if you can also get one in on the bottom of the bobbin.
If the transformer has bolts holding the laminations together, make sure ALL of these are tight.

[J M Fahey]

Agree and add: I see this often, generally it's the loose center leg of an "E" lamination vibrating, since the outer legs are pressed by the bolts on all 4 corners.

I look at it under good light and drop a little varnish between the laminations and the bobbin/coil former; in fact I do it with the primary connected and clearly hear when the vibration gets damped and dies.

Of course, if you want to play it safe you can just drop some varnish in there with transformer unpowered, wait some time for it to get inside and test later.

There is the slight possibility that this "late" applied varnish does not go deep enough, but this works 80/90% of the time.

[TimmyP1955]

I had a choke doing that in an AC30CC. Replaced the choke and all was well. Later the amp came back - same thing. Whacking on it I could make it come and go - Doh! Finally got it to come & go reliably when flicking the recto tube with my finger. Replaced the recto and I could not make it buzz.

[drewl]

I've seen this on many Sovtek amps.

I usually just dampen the sound as the transformers usually work fine, they just buzz.

[waspclothes]

I've seen this on many Sovtek amps.

I usually just dampen the sound as the transformers usually work fine, they just buzz.

Hi Drewl, thanks for the info. Even when I called Mercury Magnetics about a replacement they said that this was very common. The transformer does in fact work as you said. Has anyone found a fix for what exactly starts buzzing on these things? I tightened the bolts on the transformer and I'm tempted to remove the paper surround to inspect the coils a bit.

[Chuck H]

Hi Drewl, thanks for the info. Even when I called Mercury Magnetics about a replacement they said that this was very common. The transformer does in fact work as you said. Has anyone found a fix for what exactly starts buzzing on these things? I tightened the bolts on the transformer and I'm tempted to remove the paper surround to inspect the coils a bit.

Did you try a shim between the bobbin and the laminates? You can power the tranny up just sitting on the bench between adjustments to shimming and know pretty fast if you're on the right track.

[waspclothes]

Did you try a shim between the bobbin and the laminates? You can power the tranny up just sitting on the bench between adjustments to shimming and know pretty fast if you're on the right track.

Hi Chuck, I did try shimming as you suggested and nothing stopped the buzzing. Talking to someone at Mercury Magnetics, they said the Sovtek PT had a "flaw with them" that caused the buzzing.. It was worth a shot fixing, but seems like it needs to be replaced - my buddy took the news OK though.

I actually owned one of these amps a long time ago before I had the confidence to open them up and do repairs. I gotta say, I'm glad I sold it.. the Sovtek Mig 60 isn't built too well, and the layout does not lend itself towards repairing. All of the electrolytic capacitors are oddball russian caps thread-mounted to the chassis as well, and I've never seen direct replacements for that sort of thing. It's that sort of thing that makes you appreciate the old Fender amps even more.

[Chuck H]

Ok then. There's every possibility that the PT makes noise due to poor winding technique that can't be shimmed out because ALL the laminates are loose or something. Re-potting it with some modern product could still be an option though. And might be easier than the retrofit and redesign necessitated by changing to a non specific unit. I wouldn't give up just yet. I'd look into potting options and see if it worked. Not too much to be lost and a lot of redesign and labor might be saved.

JM2C