Vox UL730 noise and hum

[65basser]

I've got a rare Vox UL 730(as used by George Harrison on Revolver) to work on. Lots of hum and hiss! Someone had just tacked in extra filter caps in parallel with the originals so I pulled everything out and installed new caps and cleaned up the sloppy wiring. Not much improvement was noticed although moving the ground wire away from the P.T. helped a little although this is counter-intuitive. I also replaced a couple transistors in the pre-amp and resoldered several connections. The hiss is reduced about 30% and the hum is still unacceptable. I know that these were noisey and troublesome amps but I was just hoping that someone might have some experience playing and or repairing one of these.
Thanks in advance!

[R.G.]

Solid state + hum + noise = (usually) bad grounding wiring

[drewl]

Tell the owner it's a lost cause and you'll give him $50 for it!!!!

Wow, sweet amp, anyway try and isolate the preamp from the tube output section to see where the noise is coming from.
Always start with the power supply, and if the noise is coming from the preamp, try and isolate the channels and stages to find the problem.
Could be bad coupling caps in the preamp, if they're anything similiar to the US made Thomas Vox amps.

[65basser]

Thanks Drewl,

Unfortunately, the customer can't be disuaded from persuing this repair. Nostalgia, I guess. This amp is so hard to trace out due to the complicated and cramped layout. I've cleaned up someone elses sloppy repair attempt but that is just the tip of the iceberg. There was definately good reason that most of these amps were returned by customers the same year they were bought.

Both channels are noisey(hissy) with one being worse than the other and with the pre-amp disconnected, the hum is reduced by about 80%.

[Chuck H]

The only solid state amp I've ever repaired was a Kustom K-250. I had lots of help here. All the hiss issues were noisy SS amps (discrete, op and IC) and the more I replaced the quieter it got. I did replace the noisiest amps but ALL of the SS devices in that amp are obsolete and some are impossible to find. I didn't use this method but I read that freeze spray can locate a noisy SS amp. Noisy ones get less noisy when hit.

As mentioned, I think WRT hum it needs to be isolated first.