Repair advice needed on potential bad PT.

[tdlunsfo]

I have a Fender Bandmaster drip edge on the bench. Ticket says general service so I did my thing.. cleaned pots/jacks, replaced all the original caps (PS caps physically wet and leaking), new cord with the gnd switch rewire, got rid of all the power tube silliness CBS did to this era, tested tubes, etc. Well I fire it up and the B+ is about 120v low (320v). Checked my work and it looks fine. Checked the OT and Choke with my transformer tester and they fire the neon bulb. Buzzed out the HT line to ground and I get OL. Started pulling tubes and no real change there. So, how do you all handle an amp where the PT is unknowingly dead and you just spend time and money into recapping? I've talked to the kid and he could barely afford the first repair so you can guess how this will go.

Update- PT Secondary tests fine but the Primary fails to fire the neon bulb. Any other tests that you all do confirm a dead PT? I have a PA100 from the same kid that I can jumper in a spare B+ line but rather not break back into that one. When I lift the secondary wires and put 10v through the primary with my variac, I get an imbalance of 22v and 35V between the two ends...hmmm. Thoughts? The tester mentioned above is built off of this circuit.

[Helmholtz]

Was the amp on a variac or LBL (bulb limiter) when you measured the 320VDC?

Does the amp blow its mains fuse when on full line voltage?

[tdlunsfo]

It was on a variac on first power up. It pulls around 500ma, so I tried it in the wall but same B plus. This unit did have an oversized fuse that I caught before the recap. Fuse doesn't blow. PT has had some wax come out around the wires but I see this a lot as well as on perfectly fine transformers.

[Jon Snell]

With the GZ34 removed, have you got about 370 - 0 - 370 on the HT windings?

[Helmholtz]

What current does the PT pull at full line voltage with ALL secondary wires disconnected and isolated. ALL includes CTs, heaters and bias winding.

[tdlunsfo]

This version is solid state rectified. I'll have to report back on the readings.

[tdlunsfo]

Ok. I disconnected all the secondary's but left the HT center tap to chassis. At full wall voltage:

HT Total = 662v at 340mA on the variac. (Shouldn't this be zero mA's?)
HT end 1 = 331v
HT end 2 = 331v
Bias = 49V
Heater total = 6.4v

Also, while I had everything disconnected everything tested fine with the neon bulb except the heater winding. Interesting. I do remember having a broken solder joint at the pilot lamp that had the heater supplies intermittent to all the tubes.

[xtian]

340mA at 120v = 40.8 watts! Yeah, that's bad. I expect PTs at idle to draw ~10 watts from the wall. Can you feel it getting warm when energized but all secondaries disconnected?

[tdlunsfo]

I think I'm going break out the dim bulb and see how the bulb responds. I'll let you know on the heat. I'll also disconnect that center tap and see if anything changes. More to come.

[Helmholtz]

340mA at 120v = 40.8 watts! Yeah, that's bad. I expect PTs at idle to draw ~10 watts from the wall. Can you feel it getting warm when energized but all secondaries disconnected?

That power calculation doesn't work with a reactive load. It gives you reactive power (VA).

I just measured a replacement PT for a BM. Its reactive input power with open secondaries is 30VA, while real power consumed is only 10W.
The current you're seing is the magnetizing current of the PT. This current is mainly inductive, being around 90° out of phase with the voltage.

I think the PT is good, no signs of a shorted winding, though voltages seem a bit (maybe 10%) low.
What was the input voltage while measuring?

[Enzo]

You said all secondaries disconnected, but did that include the tube heaters? If tube haters were still going, that might explain the extra mains current.

So, how do you all handle an amp where the PT is unknowingly dead and you just spend time and money into recapping?

When I first started reading your post, I saw you changed all the caps, rewired something or other, and did a bunch of work, and in my head I was thinking, "He didn't power it up first???" So my answer to you question is this: you don't. Though rare, a bad power transformer is about the most expensive thing that can happen to an amp, and a likely deal breaker. One should always ascertain if the amp is operable up front. THEN go about wholesale replacing parts, etc. And especially so when the customer has expressed money issues. COnsider it a lesson learned.

Having said that, I tend to agree with Helms, sounds like the PT is working. 331vAC rectifies and filters to 460vDC or so. SO in my mind it sounds like either it is loading down or there is a resistance in there somewhere.

The bulb limiter is for when amps blow fuses or draw large excess current. In your case, the voltages are already low. The bulb will only make that worse, so don't bother.

[tdlunsfo]

While measuring the input, I was dead on 120v AC on my variac. I caught a shorted PT once with a dim bulb so was just curious if it would light the bulb this time. It did, but just a little and not enough to conclude anything. Tube heaters, bias tap and HT (plus center tap the second time around) where disconnected for the readings. I have considered something is pulling it down but thought that would show when I checked the resistance on the B plus line to ground which I couldn't find anything. As for how to handle the repair, the amp was said to be working and just needed a new cord. Upon inspection I found the caps actively leaking with wet stuff in the cap cover. I wasn't about to power this thing on to test it and I wasn't surprised to see leaking 1968 caps... So, regardless I would had to put caps in it just to test it. I guess where I messed up was stopping after recap and checking the B+ before moving forward. Just a crappy situation for everyone. What would had been your standard order of business in this case? Willing to learn how to do this better. I'll keep looking for a culprit but this heater tap not passing my neon bulb test has me concerned.

[nosaj]

i would suggest the next time you see leaking caps , to first test the PT then carry on with a diagnosis on the rest of the amp. That way if it is a PT problem you not just wasted doing unnecessary work for someone who might not have the funds.

nosaj

[Helmholtz]

..but this heater tap not passing my neon bulb test has me concerned.

I wouldn't worry about the heater winding not passing the neon test. Flashing the neon requires some inductance to store energy. The heater winding only has few turns, so little inductance, maybe not enough for your neon.
Also, did you use a fresh battery as the testing the heater winding requires a lot of current?

When a transformer has a shorted winding, all windings will look shorted and all voltages will be very low. Also none of the windings will pass the neon test.

Apart from that, the heater voltage looks ok.

[The Dude]

Are you sure you got all of the new e-caps installed with the correct polarity? Especially the bias filter, which is + to ground.