What would cause a power transformer to act like this?

[thorpej]

Hey folks --

I'm repairing a non-working Fender Blues Junior as part of a barter. The owner says - "It worked before it went into storage, but as you can see, it was stored badly" -- the chrome has obvious signs of ambient moisture (hooray San Francisco fog).

The initial check indicates that the heater supply is fine, but the high voltage DC at the reservoir cap is kaput (measures very low, and fluctuates). So I disconnected the B+ winding of the power transformer and measured it unloaded. I expected to see 260Vac across the winding (according to the Hammond data sheet for their replacement transformer, which jives with the Fender schematic that says "130.5Vac between test point 31 and ground") but instead I'm seeing 150Vac across the winding (and 75Vac TP31 to ground).

The PT is definitely the domestic one that only supports 120V primary, so there's no chance that it's hooked up incorrectly on that side, and besides, the heater voltages are correct.

It occurs to me that I forgot to check the voltages on the C +/- winding (split rail supply for the output tube bias, the "fat switch" FET, and the op-amp IC that deals with the reverb tank). But I would expect a short there would blow the fuse (or fully-cook the PT -- that's a very low-current winding).

I suspect I just need to replace the PT (not a big deal -- $50 + tax and shipping -- and the owner is paying for the parts) ... but I'm puzzled what would cause the PT to act this way. Can anyone shed any light on that?

 

[knockbill]

Maybe it got wet inside,,, I've found circuits that act very strange, like you're describing that had gotten wet and never really dried out, or the water broke down the insulation to a point that there is a slight short on the HV coil,,, may not have enough voltage to short thru LV insulation...

 

[Lavane]

Time to cook it in the sun or oven.

 

[knockbill]

Years ago when I worked for a heating company, we would take motors and transformers off heaters/boilers that were in flooded basements and dry em in the oven... they were 120v primaries,,, the motors usually dried out well enough to use, some of HV ignition transformers didn't fare too well, and needed to be replaced...

 

[BinaryMike]

I'd say that HV winding is essentially open-circuit --- did you check it with an ohmmeter?

 

[thorpej]

Time to cook it in the sun or oven.

Definitely not the sun -- it's summer in San Francisco, after all

 

[thorpej]

I'd say that HV winding is essentially open-circuit --- did you check it with an ohmmeter?

Oh, huh, good point ... no, I didn't check to see if the HV winding was open. If it was, I guess the readings I'm seeing could be explained by coupling from the other secondaries?

 

[BinaryMike]

Oh, huh, good point ... no, I didn't check to see if the HV winding was open. If it was, I guess the readings I'm seeing could be explained by coupling from the other secondaries?

Well, if it corroded open, then you might still see a bit of leakage current from the corrosion products.

 

[BinaryMike]

... And, if it corroded open, it would have done so at one of the terminations, which means that there's a possibility of repair.

 

[thorpej]

Well, @BinaryMike nailed it -- HV secondary winding is open. Checking the termination is going to be a little tricky because the flux band is kind of in the way. I may simply order a new PT, and then keep the old one around for a repair attempt at some later time / for some future project.

 

[thorpej]

Well, I decided I didn't have anything to lose by giving it a try, so... pried back the flux band and snipped it at the solder joint, and opened up the paper-and-tape wrap around the terminations. Fished out the terminations for the B+ winding, and they actually looked OK ... solder was clean and bright (it was well-sealed in strapping tape). Put my meter across the winding at that point, and it still reads open. So, I guess it failed somewhere in the bowels of the winding.

So, I guess the $64K question is, why did the B+ winding open up the first place? The guts of the transformer, at least to depth I dug down, don't seem to have been fouled by moisture.

 

[peterh]

Hey folks --

I'm repairing a non-working Fender Blues Junior as part of a barter. The owner says - "It worked before it went into storage, but as you can see, it was stored badly" -- the chrome has obvious signs of ambient moisture (hooray San Francisco fog).

The initial check indicates that the heater supply is fine, but the high voltage DC at the reservoir cap is kaput (measures very low, and fluctuates). So I disconnected the B+ winding of the power transformer and measured it unloaded. I expected to see 260Vac across the winding (according to the Hammond data sheet for their replacement transformer, which jives with the Fender schematic that says "130.5Vac between test point 31 and ground") but instead I'm seeing 150Vac across the winding (and 75Vac TP31 to ground).

The PT is definitely the domestic one that only supports 120V primary, so there's no chance that it's hooked up incorrectly on that side, and besides, the heater voltages are correct.

It occurs to me that I forgot to check the voltages on the C +/- winding (split rail supply for the output tube bias, the "fat switch" FET, and the op-amp IC that deals with the reverb tank). But I would expect a short there would blow the fuse (or fully-cook the PT -- that's a very low-current winding).

I suspect I just need to replace the PT (not a big deal -- $50 + tax and shipping -- and the owner is paying for the parts) ... but I'm puzzled what would cause the PT to act this way. Can anyone shed any light on that?

One reason that i find probable is that the electrolytic cap has degraded / shorted during storage.
When power is applied the secondary winding of the power transformer broke.

Do check/ replace the electrolytic caps before applying power on the new transformer !

 

[BinaryMike]

Well, that's an unusual failure. If there's no evidence of overheating, then I would guess that the wire was nicked at some point during construction. Don't ignore Peter's advice, in any case.

 

[thorpej]

Yah, I don't see any visual evidence of over-temp anywhere. So, yah, perhaps a manufacturing defect that showed up later. I went ahead and ordered new electrolytes when I ordered the transformer (had correct values on-hand, but not in the right physical size, grumble mumble printed circuit boards...)

 

[crispycircuit]

Also check the fuse to see if it's been replaced by a larger one. The fuse is the safety limiter on the power transformer

 

[thorpej]

Parts were delivered while I was off on a road trip to see the total eclipse, and I managed to get them installed last night -- a replacement transformer, and replacement electrolytics in the B+ supply. Fired it up without tubes to check voltages, and everything was as expected. Installed the tubes, and everything was fine ... played through the amp for a while, decided to call it fixed.

One thing I noticed -- before I installed the tubes to check voltages -- powered up it seems that quite a bit of noise is coupled to the output transformer -- I suppose I should put my scope on it to determine the exactly which frequency it is. Of course, with tubes installed and my Silvertone plugged in, the stupidly-hot Teisco single-coils pick up noise from my neighbor's coffee maker, but I was surprised about the noise without tubes installed.

 

[BinaryMike]

I was surprised about the noise without tubes installed.

You might try adding a copper hum belt on the power transformer --- outside the core, of course.

 

[thorpej]

You might try adding a copper hum belt on the power transformer --- outside the core, of course.

The transformer already has a flux band (they're just meant to act as a shorted turn for the stray flux, right?). The two transformers are even mounted orthogonally, so I'm puzzled. I'll experiment a little more tonight.

 

[BinaryMike]

You can also try adding a band on the OPT, and magnetically isolating one or both xfmrs from the chassis if it's steel. I have read that OPT bands improve performance as well, but never confirmed it.

 

[thorpej]

You can also try adding a band on the OPT, and magnetically isolating one or both xfmrs from the chassis if it's steel. I have read that OPT bands improve performance as well, but never confirmed it.

Yes, it's a steel chassis, and the transformers are pretty close to each other.