'65 Deluxe Reverb Reissue Super Loud Hum

[dorrisant]

Got this one on the bench again...

Had fried screen grid resistors. changed them to 500 ohm 5watt. Fired it up, sounded great. Turned it back in. Back on the bench for the same issue. Let it warm up and take it off standby... sounds good to me. Music shop says "let it warm up, play for a few, switch into standby, switch back out of standby... super loud hum."
They are right. As you switch into standby you can hear the unbelievably loud hum kick in till it fades (due to standby). Switch out of standby and you get this ungodly, speaker is gonna jump out of the cab, hum. Volume seems to be maxed... not in the pre because it does it with both volume controls at zero. I have tried a different standyby switch and a known good rectifier.
i did initially go over all the solder joints and touched up quite a few.
Don't know why it is fine till you cycle it in and out of standby when warm. It will continue to hum like its gonna explode everytime you take it out of standby until I turn it off and let it go completely cold... then it will be just fine until cylcled in and out of standby again.

Any thoughts?

Tony

[JazzGuitarGimp]

How old are the filter caps?

[dorrisant]

It is a relativley new amp... used just a month or so.

Tony

[JazzGuitarGimp]

Don't know how safe it may be to run the amp in its present condition. But I might be inclined to put it on a dummy load and probe around with the oscilloscope.

[dorrisant]

I will probably try that... Weird though, sounds like no filter caps at all.

Tony

[JazzGuitarGimp]

Yeah, that's what I was thinking. Which might be a bad cap, or an intermittant connection to a good cap...

[Milkmansound]

or the bias cap went goodbye

[JazzGuitarGimp]

or the bias cap went goodbye

Ah, there you go. This happened to my bf vibrolux reverb about five years ago. One lead came off the board from a bad repair.... And no, I didn't do the repair.... Lol

[dorrisant]

Odd thing to me... the grounding switch has never had a spade lug attached to it. There are no marks at all on the terminals. I swapped this with the standby switch to see if the problem followed.

I'm guessing the reservoir cap may be bad since i can hear it even in standby as it fades out, maybe not. I tested it with my Eico 950B and it looked relatively the same as a brand new 40uf Sprauge... leakage looked good as well.

I will try to sub in the brand new Sprague 40uf for the and see if that solves it.

Tony

[dorrisant]

or the bias cap went goodbye

Wait... this will be fist!!

Tony

[xtian]

Standby switch could be partially shorting to ground. Easy to bypass the switch, right?

[dorrisant]

Thanks guys... much appreciated!!

Milkmansound pegged it. Bias cap swapped to new, can't get it to fail now. I will give it a hard workout tomorrow.

I guess I should have looked at that cap or at least been suspect of it, with fried screen grid resistors. Anyone care to explain what might have happened? I need to learn more in this area!

Tony

[JazzGuitarGimp]

Good news, great job!

[martin manning]

I can't figure out why it would have to go through charging everything up and then cycling the standby to get the failure mode. Perhaps it was a cracked solder joint or something rather than the bias cap itself?

[Firestorm]

This amp has the oddball main filter with a 220u/100v stacked on a 47u/500v cap. If the divider resistors go funny, it could subject the first cap to overvoltage, especially on standby. Not that that explains the specific symptoms.