Crate amp hums

[Dave Johnson]

Please help me solve this...Crate Turbovalve 60 hums loudly, and dims pilot lamp as soon as standby switch is flipped. It does this with or without tubes in it. Two caps on the main board were swelled (450v 47uf) I replaced these, no difference. What next?

[selloutrr]

try tube swapping with known good tubes
check grounds and power source (clean power)
recapping all electrolytic caps, jsut cause they didn't swell doesn't mean they are good. if you can test them that is the best way. the next is visual inspection, then the 10year rule of thumb not always true but if it's over ten it's time for new.

[Dave Johnson]

This may sound stupid, but when I unplug the red wire from the board to the output transformer, the hum is gone. OK, no output, no hum. But so is the power drain that was causing the pilot light to dim. My question now is, how can I check an output transformer for shorts with a multimeter? Am I going in the right direction here?

[benoit]

This may sound stupid, but when I unplug the red wire from the board to the output transformer, the hum is gone. OK, no output, no hum. But so is the power drain that was causing the pilot light to dim. My question now is, how can I check an output transformer for shorts with a multimeter? Am I going in the right direction here?

NEVER disconnect a wire while the amp is on unless you know EXACTLY what that wire does and EXACTLY what will happen if you disconnect it. In this case you very likely could do damage to the amp as well as to yourself, though I'm not sure specifically what would happen in this case. Hopefully someone else can explain in more detail.

[rfgordon]

If the amp makes ugly noise WITHOUT power tubes in the sockets, then the noise is 120 Hz power supply ripple, and there's generally only one way that noise gets from the OT primary to the secondary: failed OT.

Some shorts are not detectable with a meter, particularly if there is arcing across space. Some trannies only show the failure under load, and I've seen this in a faulty reverb transformer.

The OT is toast.

[Dave Johnson]

I was thinking OT problems too, but it checked out good. It turned out to be the flyback diodes, they were shorted closed. There's my hum and dimming pilot lamp. A short to ground. I read of a similar problem on another forum, with the same model amp. So off to RadioShack this morning and a couple of fresh 1w 1000v diodes and we're back in buisness. It seems to work fine. Sounds decent, for what it is. Repaired for under $15. Actually not too bad to work on. Chassis comes apart, board slides in and out without a lot of fuss. Thanks to all for the thoughts and suggestions. I appreciate it.

[Structo]

Glad you fixed it.
I wouldn't have thought of that but I don't own that amp.
Good job researching it.

[selloutrr]

TESTING IRON....

drain the filter caps and all other stored voltage from the amp after you unplug it!!! meter the legs to ground and the push pulls to each other as well as to the center tap. here is the thing though if you are getting sound out and it powers up it's probably not the iron.

being that it is a crate amp. and no offense in saying this, they make them to reach a price point and in doing so they cut corners on quality parts. cheaper components and weaker PCB. you problem is caused by

a dying tube, an electrolitic cap that is failing, a shitty trace on the PCB, a poor solder joint, or a bad ground.

if the amp plays clean at a friends house your power is dirty.

I recommend you take it to a tech for service before you hurt yourself! seriously amps are not a place to play. $50 could save your life.