I searched (googled) the issue and found https://www.ssguitar.com/index.php?topic=4118.0

So I bought a PE60 for a very nice price (red flag) that has a hum on the 1/4" input only. I've been using it with an BOSS ME-80 with success taking the mono out into a direct box and then into the lo-Z input on the amp.
  So silly me, I want to fix the hum. The amp works great in all other respects.  The amp is dead quiet when nothing is plugged in. Once a 1/4 " mono plug  is inserted, theres a hum, similar to tapping the hot/tip an open jack.  Using the xlr input is quiet as well.

  As an avid electronic enthusiast, I have built 3 fairly complex amps , recapped and modded my SF Bassman, with rewarding success. I have no scope at this point.



I benched the chassis to look for a blessed "easy fix". It didn't happen. What I did find on the 1/4" input jack was that is a "marshall" insulated type but appears to be a stereo jack. It's PC mounted and has 6 solder lugs into the board. The 2 closest to the chassis face have traces, the middle 2 have traces, and the last 2 go through the board but do not appear to have traces. I know it could be a multi-layer board but honestly looks single layer.  (I stopped myself from starting a big trouble shooting session because it was 11:45PM. I'll put fresh eyes on it in the AM.

  Could it be that the input is wired for a balanced  1/4" input? And that using a standard mono guitar jack is fouling the ground on the amp? I'm looking for a schematic, and will find the manual in my paper pile.
The solder points look great but I'll tap them out a bit and check them with a meter to verify what I can.


From the manual
<<1/4" INPUT (*FOR INSTRUMENTS ONLY*)
Accepts 1/4" output of your pre-amp/processor or amplifier. This specialized input
features an isolation transformer to eliminate ground loops when using the Power
Engine 60 in conjunction with an amplifier. Be aware that this transformer loads down
passive guitar inputs. If you would like to plug your guitar directly into the Power Engine
60, either use an active guitar or any buffered type of effect pedal (Boss®, Ibanez®,
Tech 21, etc.) in bypass mode in order to drive the low input impedance of the amplifier.
*NOTE: DO NOT PLUG THE SPEAKER OUTPUT OF AN AMP INTO THIS
INPUT. Doing so may result in severe damage to the unit or your amp or both>>

Tomorrow is another day

Dez
NYC

This is for  a 1990 era 400RB. There are schematics, layouts and manuals.
Should be here: https://drive.google.com/folderview?id=0B69gKOiNPWleb3g2U1hQVVBJZ1U&usp=sharing

First, Hello to everyone. Its my first post. I have a couple of SS amps, My Tech 21 TM-60, an older Yamaha G-112, An even older Peavey Renown 2-12, a previous and deceased and missed, GK-SEL-2100 (RIP, sniff)
  Somehow I've wound up with my sons RB400, which I took out to a BBQ/jam over the weekend. I set up and powered on but there was no sound. Pilot light lit, speaker plugged in, (swapped cable JIC), but no sound.
  As I inspected the head for any obvious problems, the top of the amp was very warm (not hot) to the touch, even after only a short time. The heat sink was also fairly warm but not hot, considering the under 2 minute run time.
  I traced back the amp usage over the last 3 months and I took it out one other time to power a floor monitor.  The monitor didn't work and I blamed it on the sound guy or the send cable. I'll bet the amp was failing then also.
  Yesterday I got to ask my son if he'd had any problem and he didn't recall, BUT, (big BUTT), there might have been one time last year he was running some music through the amp for a party and there were "Sprinkles" and he covered the amp with a plastic tarp. Possible red flag there I think.
  What might be the initial trouble shooting points on this amp? Output transistors, power supply, (amp lights up). Is there a fuse on the output?
I'm going to rummage up an schematic and get prepared in the mean time.
I've got above beginner skills, not expert. I've built a 2-6L6 tube amp from scratch, (no kit, sourced parts), but the well of knowledge is quite deep and I'm just dipping a ladle in.

Thanks for any and all info,
Dez
NYC