Crate VC50 hot tranny/slight hum...

[rockon1]

Noticed my VC50's PT gets pretty hot. The amp also has a slight audible hum coming thru the speaker even with the volume all the way down. Putting my hand on the PT it seems to feel exactly like the hum coming thru the speaker if you know what I mean. Coupled things-

1)Recently I left the amp on standby for 2 weeks by accident. Cant say for sure if I remember the hum being there before. Might have been since Im keyed in on the amp after

2) doing some pre amp mods to it which I dont think have anything to do with the hum-never know though. Double checked the mods,grounds and looked things over-everything looked good.BTW- The amp really sounds excellent with just a few mods.

PT?,Caps ? Tried swapping out the power tubes with no affect on the hum. Also it will still make the hum without the PI installed. No hum with the power tubes pulled. thoughts? Thanks in advance Bob

[rockon1]

OK one of the caps seems to check bad. Im going to bring it to work and check it on a better meter. Probably going to replace all the caps since they are 20 years old and cheap to start with. Bob

[rockon1]

Ok im shooting in the dark here. Replaced the one cap I "thought" was bad-no change still hums. The retifier diodes look like they must get hot as the board is dark underneath them. Thinking Of replacing them-another shot in the dark I suppose but only a couple of dollars. Anybody want to chime in here? Bob

[Enzo]

Isolate the problem.

Since the hum remains with the PI tube gone, that means the power tube stage is humming. COuld be some hum being injected or it could be the whole thing is 0ut of bal;ance. Is the bias supply free of ripple?

Are all the power tubes working? Are any screen resistors open? Is current draw at idle about the same on the push and pull sides?

[rockon1]

Is the bias supply free of ripple?

Dont know. Not sure how I would check for it.

Are all the power tubes working? Are any screen resistors open? Is current draw at idle about the same on the push and pull sides?

Ive checked screen resistors and they are OK. I need some 1 watt,1 ohm resistors to check current draw and see. Wish Radio Shack carried them.They only carry up to 1/2 watt now. Thing is if I cant get this thing working its history. : > ( Its a cheap amp and Im not spending money to have it fixed. Hopefully I'll learn enough or stumble apon the problem and fix it. Heres a schematic. Your help as always is much appreciated. : > ) Bob

http://music-electronics-forum.com/a...schematics.pdf

One other thing diodes 5,6,7 and 8 all look like they got rather hot a the board is darkened under them. Would these cause the problem if bad? I have these I got at Radio Shack I think will work and will only take a few minutes to install.

Temp range -85 to 320 F
peak inverse voltage-1000v
foward current -1.5A
peak surge current(8.3ms) --50 A
foward voltage drop(VF)at (lf)- 1.1V
peak reverse leakage current at(77* F)-5.0uA
typical total capacitance 20pF
typical thermal resistance junction to lead-77*F/W
typical thermal resistance junction to ambient- 131* F/W

[rockon1]

After replacing the diodes with no luck it occured to me what you said about measuring the tube bias. Well I didnt have any means of doing so so I grabbed a bunch of old EL84's. After trying a few different "sets" the hum was gone. I can only figure I had a very unbalanced set in it or even one wasnt working- Well I need a probe or at least resistors around to check the simple stuff first!
Lessons learned the hard way arent easily forgotten so theres a positive! Plus I got some more practice at desoldering and soldering on a PCB. Thanks again for putting up with my ignorance! Bob

[Jazz P Bass]

EL 34 bias check

As to checking the EL34 bias, if you have a DVM that measures DC current, disconnect one plate wire & place your meter in series with the plate & the output transformer.
Be Careful. Insulate your connection points. B+ bights hard.