Think I just blew my power transformer

[Beyer160]

I was playing my '67 Ampeg Gemini II, and it suddenly cut out. No death noises, just stopped dead. The fuse was blown, but there doesn't seem to be any damage to any components on the circuit board. I pulled all the tubes, replaced the fuse, and powered up again. The lamp flickered for a millisecond, then the fuse popped. I ran out of time for further troubleshooting, but should my next step tomorrow be to read the impedance of the transformer legs? If it comes up a dead short I'd assume it's time for a new PT.

Anything else should I test? I just recapped the amp a couple months ago, but if I'd done anything really stupid I'd imagine it would have shown up before now.

[The Dude]

The transformer would be last on the list of suspects. Check for shorted diodes first. If you can't find any obvious shorts, unhook the transformer secondary wires and power up. If it still blows a fuse, the transformer is bad. If not, you have problems elsewhere.

[Enzo]

Yes^^^

WAY more likely a shorted rectifier or a shorted or super leaky filter cap than the transformer.

Does it blow the fuse immediately at power up, or does it wait until you flip the standby?

[Beyer160]

Yes^^^

WAY more likely a shorted rectifier or a shorted or super leaky filter cap than the transformer.

Does it blow the fuse immediately at power up, or does it wait until you flip the standby?

Immediately at power up.

[Chuck H]

There are a couple of versions of that model. One uses a full wave rectifier and another uses a bridge rectifier. I'll guess that yours is the bridge rectifier version and that you have a shorted diode as Dude suggests. Probably brought about by bad caps as Enzo suggests. Can you tell us if your amp is using a full wave, or a bridge type rectifier.?.

[Tom Phillips]

If you have the skills to work inside the amp safely the next troubleshooting step I would do is lift the high voltage secondary leads, insulate them so they can't short together or touch anything else and then power up again. (Use a light bulb limiter or bring the voltage up slowly while monitoring the line current draw if you have the equipment)

If the fuse does not blow then you have isolated the problem to the power supply section downstream from the PT. If it does blow, then lift the heater leads and try again. If the fuse blows with all the secondary leads disconnected then you probably have a bad PT. I say probably because there is always a slight change that there is a problem between the fuse and the PT primary. Always good to consider everything.

Attached is the full wave rectifier version of the schematic. If anyone has the FW Bridge version then please post it.

[Chuck H]

Not a very good image, but I didn't find a better one.

[Beyer160]

It appears to be the 2-diode full wave rectifier, the schematic in my amp is identical to the one Tom Phillips posted.

Thanks for all the help so far, I'll do some testing and let you know what I find out.

[Beyer160]

Unfortunately I only had a little time to work on this today but did perform a diode test, and both diodes in the rectifier are reading shorted (.016v and .167v respectively, in both directions). I performed the test with the diodes in circuit, I believe this is OK to do with the diode test function on my DMM.

I'm assuming something else killed the diodes (although I didn't replace them in the recap- they're original to the amp), should I replace them before doing the testing described above, or do it with the shorted diodes in place?

[The Dude]

I would remove the diodes and then, since you have them out, check the power supply output to ground for shorts or a low load. If you detect a short or near short, track it down. Replace the diodes and whatever else you find loading the supply and then bring up the amp with no tubes. If all is good, insert the tubes one at a time so that you know if one of the tubes is shorted. Also make sure the fuse value is correct for the amp to prevent further damage.

[Tom Phillips]

Unfortunately I only had a little time to work on this today but did perform a diode test, and both diodes in the rectifier are reading shorted (.016v and .167v respectively, in both directions). I performed the test with the diodes in circuit, I believe this is OK to do with the diode test function on my DMM.

I'm assuming something else killed the diodes (although I didn't replace them in the recap- they're original to the amp), should I replace them before doing the testing described above, or do it with the shorted diodes in place?

If you suspect that the diodes are bad then just pull them out and replace them. In the test described above the diodes (Good or bad) would have no effect on the test results because the transformer would be disconnected from the rest of the circuit. The idea of the test is to isolate the problem. Then you know where to concentrate for your next troubleshooting test / part replacement.

When one diode fails shorted in the circuit you have (Full wave with single diode on each side) the other is likely to fail due to extra stress so I wouldn't be surprised to find them both shorted.

[Chuck H]

Unfortunately I only had a little time to work on this today but did perform a diode test, and both diodes in the rectifier are reading shorted (.016v and .167v respectively, in both directions). I performed the test with the diodes in circuit, I believe this is OK to do with the diode test function on my DMM.

I'm assuming something else killed the diodes (although I didn't replace them in the recap- they're original to the amp), should I replace them before doing the testing described above, or do it with the shorted diodes in place?

I'll predict that the amp is OLD Bad filters or bias caps caused an old power tube to fail and the resulting short took out a diode that took out the other diode. And all this CAN take out a PT, but usually doesn't.

Hopefully further testing will reveal that the PT is fine. But be prepared to replace ALL THE ELECTROLYTIC CAPS IN THE AMP, the power tubes and the diodes.

Old amp.

[Beyer160]

If you have the skills to work inside the amp safely the next troubleshooting step I would do is lift the high voltage secondary leads, insulate them so they can't short together or touch anything else and then power up again. (Use a light bulb limiter or bring the voltage up slowly while monitoring the line current draw if you have the equipment)

To make sure I understand, the HV secondary leads are the ones leading to the rectifier diodes?

[Tom Phillips]

To make sure I understand, the HV secondary leads are the ones leading to the rectifier diodes?

Yes. They are usually red and the center tap of the HV secondary winding is usually red with a yellow stripe. The schematic in post #5 shows that the center tap is connected to the Standby/Polarity switch. When the switch is in the standby position the center tap is not connected to anything. Therefore, you can just do the test with the switch in the standby mode. No need to disconnect that wire.

More thoughts: You have already determined that the rectifier diodes are both bad. You need to remove them eventually. If you remove the two diodes now then the HV secondary leads will be open circuit and you can proceed with the first transformer test without messing with the transformer wires.

The way this is playing out I think there is a good chance that the problem will just be two shorted diodes.

Good Luck,
Tom

[Beyer160]

So I clipped the diodes and lifted the 220K resistor that led off the HV secondaries (instead of lifting the HV wires themselves). It doesn't blow the fuse now, although the pilot lamp is very dim. I did notice something odd, there was what seemed to be a scorch mark under the 220K resistor and an odd silver blotch on the resistor itself, as if the casing was punctured. Whenever I've seen failed resistors in the past, they've been obviously burned and this one isn't. It's original to the amp so I'll replace it regardless.

So, my next course of action is to check the power supply output for shorts? I'm not exactly sure what the "power supply output" means, is that the filter caps (marked A, B, C, E, F in the diagram above)?