Replacing a transformer

[Tubejunke]

I'm glad that this thread was made, and even more glad concerning the outcome of Schanz's radio. Reason being on the former is that I have an old Delco R-1125 with a #80 rectifier that has been giving me fits. Basically the B+ is really low and I can't find a reason why so far. The transformer gets really hot after about a half hour.

I replaced the filters and a number of the non polarized caps. I haven't done the things mentioned here as far as having all leads open and checking the resistance of windings and shorts between windings or to ground. The main reason is that the wiring is so hard that I feel that it would be destroyed by moving it much. The insulation under chassis is good as it stands. I popped the covers on the trans. and it wasn't very pretty, but didn't seem to be smoked.

Like Shantz, I feel that opting for a replacement trans. would save a lot of time and aggravation. I don't know where I would find one, but if anyone does please let me know. Thanks!

[Schanz]

Could be that you need to replace all the caps first. I don't know that the smaller down stream caps can heat your transformer like that but I'm no expert by any means.

I put mine all together last night finally but when I turned it on I heard a small electrical arc. Uh oh. I found one my 22uf electrolytic caps had come loose from the ground. Repaired the solder but now the sound is gone. Replaced that cap but that didn't fix it. Voltages at the rectifier are fine. All tubes light up but my b+ voltage is missing farther down the circuit if that makes sense. Haven't had time to investigate much yet. Frustrating.

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[dieseljeep]

Originally Posted by Schanz

Could be that you need to replace all the caps first. I don't know that the smaller down stream caps can heat your transformer like that but I'm no expert by any means.

I put mine all together last night finally but when I turned it on I heard a small electrical arc. Uh oh. I found one my 22uf electrolytic caps had come loose from the ground. Repaired the solder but now the sound is gone. Replaced that cap but that didn't fix it. Voltages at the rectifier are fine. All tubes light up but my b+ voltage is missing farther down the circuit if that makes sense. Haven't had time to investigate much yet. Frustrating.

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Make sure that both electrolytics don't go to ground. The C-25 negative goes to the center tap of the transformer and speaker field as the choke and forms the bias supply for the 42 tube. It could be that the R-13, 235 ohm resistor opened. As a result of this resistor opening, there won't be any B+ to the rest of the set, not even a hum. They usually used those flexible type resistors. Replace with at least a two watt wire wound resistor.

[Schanz]

Thanks. I'll check r13. It's one of the few original ones left. C26 is the only one that goes to ground and is the one that popped loose. Quick question though. I can measure 360v across C25 (pos to neg lead) but nothing across C26. It's that proper?

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[maxhifi]

Originally Posted by Schanz

Thanks. I'll check r13. It's one of the few original ones left. C26 is the only one that goes to ground and is the one that popped loose. Quick question though. I can measure 360v across C25 (pos to neg lead) but nothing across C26. It's that proper?

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Impossible to tell without seeing the schematic

[Schanz]

Right.

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[dieseljeep]

Originally Posted by maxhifi

Impossible to tell without seeing the schematic

The field coil and the R-13, is the only return to chassis ground. Did you check the 235 0hm and the field coil. It should read around 1050 ohms.
BTW, the speaker should be plugged in, to complete the B+,B- circuit.

[Schanz]

I get about 1100 ohms on two pins of the speaker plug while disconnected so maybe we're ok there. R13 seems to check out ok also. It's an armored wire wound thing attached to the chassis. Voltage before the field coil was about 230 vdc. It was about 30 vdc after. Still getting approx 30 vdc after r14 but nothing after r15. This was done really quickly on my way out the door this morning. I'll replace both r14 and 15 tonight and measure again.

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[dieseljeep]

Originally Posted by Schanz

I get about 1100 ohms on two pins of the speaker plug while disconnected so maybe we're ok there. R13 seems to check out ok also. It's an armored wire wound thing attached to the chassis. Voltage before the field coil was about 230 vdc. It was about 30 vdc after. Still getting approx 30 vdc after r14 but nothing after r15. This was done really quickly on my way out the door this morning. I'll replace both r14 and 15 tonight and measure again.

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So far, everything in the bias supply, seems to be in order. I wouldn't worry about the R14, R15, at this time!
I can't see why, there is no B+ at the C26, measured to ground.
The voltage chart is on page three of the Rider's.
The B+ measured from the + side of C-26, should be around 240 volts, DC, to chassis ground.

[Schanz]

Hmmm...
Could C25 be bad? It is a replacement that was done some time in the past by someone else.
I believe I will go through this thoroughly when I have more time and write voltages on the schematic.

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[dieseljeep]

Originally Posted by Schanz

Hmmm...
Could C25 be bad? It is a replacement that was done some time in the past by someone else.
I believe I will go through this thoroughly when I have more time and write voltages on the schematic.

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It would have to shorted big-time. You voltages on the bias source seem to be right.
Keep us informed!

[Schanz]

Took some measurements and wrote down some voltages.
The speaker makes a little buzzing and the back of it gets hot. Never really felt the speaker coil before.


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[Giulio Maiocco]

Careful here, Schanz, if the voltages you reported are right, you are pulling way too much current from the 80 rectifier and power transformer. The field coil shoud get warm over a hour or so, but you say the field coil on the back of the speaker becomes hot, this confirms overload.

Just to start, with the radio unplugged, what's the resistance reading between the 80 filament (doesn't really matter which one of the two) and the chassis? With the symptoms you describe, I would expect a very low resistance, if this is the case, this is not right. Check C26, C28, the primary of the OPT and the 42 tube for shorts to chassis, or, failing these checks, the wiring carrying the B+ voltages around in the radio. Avoid as much as possible to plug in and turn on the radio, as this may cost you another power transformer / 80 tube / field coil! Oh, BTW, I hope that 6VAC on the 80's filament is a mistypo, as this would ruin the tube in a short time

Just let me know if I can help you further

Giulio

[Schanz]

Yes Giulio, something was definitely wrong. Your comments got me thinking so before leaving for work this morning I checked everything carefully with a flashlight. I found my transformer had shifted and my make-shift mounting bracket had shorted a leg of the 80 to the chassis. That explains the missing voltage. Amazingly, the radio still works. The transformer will get a permanent mounting solution and all bare leads will get insulated.

And yes, that 6v on the 80 was a mistake. It is actually 5.25.

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[Giulio Maiocco]

I think you found your problem , once you fix your PT in a secure and permanent way, I bet you'll find your 240V or so volts between the 80's filament and chassis and, important thing, you'll find the negative voltage generated across the 235 ohm will be quite less than your -31.6V, it should be roughly -17V.

With that dead short, no wonder you heard a pop/arc!

Ok also about the 5.25V on the rectifier's filament, much much better!

Best regards

Giulio