Blowing fuses

[balance]

Hello fellow DIYers,

This is my first post on diyaudio, but I must admit I am a lurker. I’m not new to diy audio, though somewhat new to tubes. I am looking to build a low watt SET some time soon, but am holding off until I can scrounge up the necessary parts. I have an old monoblock amp which I would love to salvage from, but I cannot bring my self to disassemble it since I think it is easily fixed. This brings me to my question.

This amp continually blows fuses on power-up, more precisely, when it is taken out of stand-by. This happens without any tubes installed. I don't think it is the power transformer, because it does not blow fuses without load (disconnected and isolated the secondaries.) I have checked the rectifier diodes with my DMM and they seem okay. However, I do not have any good way to test the filter caps under load. With a regular DMM, the caps seem to charge fine, however I suspect they are leaky under load.

Is there any easy way to test the caps short of swapping in known good ones? Are there other possible causes for blowing fuses like this that I've overlooked?

Thank a bunch. I appreciate any insight.

ed

[anatech]

Hi Ed,
Possibly, should it have slow blow fuses installed? You can check the caps by putting a resistor in series with the diodes and measure the voltage drop. 100K will smoke under load, use 5W or better. The diodes go short and stay short when they break down, so I doubt it's them.

Is there a selenium rectifier stack in there? Normally used for bias voltage connected to the neg. end of a cap, pos. grounded.

Normally I check caps with a variable DC power supply and measure the leakage current across something like 10K ~ 100K depending on value and leakage current. That's if they pass the ohm meter test. I also tend to measure the capacitance as that changes a lot when they go defective in normal situations.

-Chris

[richwalters]

Originally posted by balance

I have checked the rectifier diodes with my DMM and they seem okay.

Welcome to the forum......you don't mention what type rectifier diodes you are using. HV Rectifier diodes have been known to <snap> avalanch when a peak secondary voltage occurs. make sure the PIV ratings are well up. The results may look good in dvm tests...the usual 0.6 V forward drop...but not at high AC. If a short was in an electroytic cap, the results would be more dramatic.

hope this helps

richj