Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Choan on July 27, 2023, 12:04:08 pm
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Howdy y’all. I’m an amateur tech, hobbyist. Trying to diagnose why I’m not getting voltage from the secondary side of the OT on a 68 Princeton reissue. All tubes are out and secondaries disconnected, and I’m still getting a short on the bulb test. I had figured my short could be a component, but it’s showing up totally disconnected from the pcb. 120v is confirmed going into the primary. Only 181v is coming out of the reds that go to the rectifier, only 4v on the heaters and nada on yellow or the red wire going to the filter caps. Resistance on the yellow wires is super low.
There was a burned 1k 1w resistor on the filter caps upon initial inspection. I replaced it and measured the other resistors out of circuit, they check out.
Any ideas on taking more measurements on the xfrmr? I’m at a loss. Any sage is appreciated!
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the PCB re-issues are not for the faint of heart, a schematic would be great, I have the twin re-issue n the blues deluxe re-issue
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Sounds like it's time to replace the PT.
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Thanks for the expedient reply! Doubled checked and disconnect xfmr with no tubes is
171 on rectifier wires
3.5v heater wires
Nothing on yellow or red
Resistance is reading 5ohm on primary, .4 on secondary
No voltage on filter caps
With rectifier tube in, 171 drops off to 0, and no tube glow. Safe to say replace the rectifier while I’m at it? It’s reading good on an ancient tube tester, but I’m uncertain of the tube tester accuracy. Sometimes I show my lack of expertise by using my meters wrong so I just want to make sure!
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It definitely sounds like the PT bit the dust. What would concern me is why. I would be looking for whatever caused the problem to begin with. Since it sounds like none of the secondaries are working, there may be a short in the primary winding. If that is the case, it could be that the transformer just died, but that sounds suspect to me.
Pull that transformer and check it again outside the amp. Do you have a variac?
Dave
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If the PT is toast, you should also check the rectifier tube and filter caps especially the reservoir cap, and the bias supply cap, in case those were part of the failure mode (e.g., a shorted rectifier tube will pass high VAC onto the filter caps, which electrolytic caps are not designed to handle, and so in turn the caps will short, creating a total short from the PTs secondary High Tension winding to ground. Not saying this caused your problem, but if it did, you’d want to replace the rectifier and filter cap to prevent it from continuing to destroy PTs).
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3.5v heater wires
171 on rectifier wires
that's wire to wire, NOT wire to ground?
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Yessir, wire to wire.
Dave, I do not have a variac yet. Just a bulb limiter and multimeter.
Tubeswell, I checked the caps out of circuit, they’re within 20% albeit pushing the high end of that.
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The reason I said pull the transformer and check it on a variac is because, although it sounds like the PT is dead, it "COULD" be that there is a bad connection to one of the primaries. Maybe check the connections to the primaries and check the on-off switch to make sure that it is actually passing current.
Dave
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Welp I missed a pre transformer .1 cap and 100k ceramic resistor. Both are way out of spec. The cap was a .1 330v 20% metallized poly, is it safe to use .1 630v %5 Dijon cap in the meantime for continuing to troubleshoot the power section?
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That’s almost certainly a red herring.
From your reported measurements, there’s a fundamental problem with the mains transformer.
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Welp I missed a pre transformer .1 cap and 100k ceramic resistor. Both are way out of spec. The cap was a .1 330v 20% metallized poly, is it safe to use .1 630v %5 Dijon cap in the meantime for continuing to troubleshoot the power section?
That resistor should be a thermistor and the cap is a Class X cap. I do not recommend putting a regular coupling cap in it's place. If you want to continue troubleshooting, just remove the cap and replace the thermistor with a piece of wire.