RG-80 112SC Issue

[CD]

Hey everyone.
I am going through a nostalgic phase presently and just picked up this RG80 combo amp with the original foot switch for a sweet deal. I really like the amp and it sounds great with one exception. OK, maybe 2, it does have the channel bleed problem, which I can live with.
This amp has more background noise/hum than any amp I've ever owned- including several tube amps. This noise is always present as long as the amp is on and at idle. Makes no difference if a guitar is plugged in or not. No knob twisting has any affect on the volume of the noise coming through the speaker while at idle- it stays exactly the same regardless of volume, gain, or eq settings. It sounds great at loud volumes but is far too noisy to use as a bedroom amp. No changes when moving to different electrical outlets at my house or my friends house.

I figured that it couldn't hurt to replace the 22 year old P/S filter caps. This seems to have improved the noise slightly but not nearly as much as I had hoped for. I checked the bridge rectifier- no problem there. No signs of problems with cold solder joints, leaking or swollen caps, burnt resistors, transistors, or diodes- in other words, nothing obvious. I even checked DC voltage at the speaker and only had .2 VDC, which shouldn't cause this much noise. I haven't gone through and tested every resistor and diode yet as I am hoping to avoid this, if possible. If I must check each and every component, I will, but hoping someone may have seen and resolved a similar issue before doing all of this.

Anybody have any ideas what may be causing this problem?
This is a 1989 model if it makes any difference.
Would anyone have an accurate schematic for this beast? I looked on the website but didn't see anything for an RG80.
I would very much appreciate any suggestions.

 

[Led Hendriani]

Interesting. I was going to replace all the original electrolytic caps in my 1981 RG80 to better quality new ones to deal with a slight hum that I hear at low volume, but now am wondering if it will be worth it. From what I read somewhere online, it is not usually the filter caps but all the other caps. There was a guy selling a full set of 16 caps to replace all of the original RGA caps for something like $16.99 on eBay, but he does not list those on his eBay site anymore, so he may be out of stock...

From his ad:

Randall RG Series Electrolytic Cap Kit
Models:
RG 80-112 SC
RG 80 PH
RG 80 ES
RG 100ES
RG 100HT
I have repaired many Randall amps over the years and have found that some of the caps used in production were not the best quality and had issues over time with noise and hum. (Grey RGA) These are new stock replacement caps.
Kit includes the follow values and quantities:

Qt. (1) 2200uF 50V, C45
Qt. (4) 47uF 50V, C32, C35, C42, C43
Qt. (6) 10uF 50V, C7, C13, C21, C33, C34, C36
Qt. (1) 3.3uF 50V, C25
Qt. (1) 0.47uF 50V, C17
Qt. (2) 1uF 50V, C8, C22
Qt. (1) 4.7uF 50V, C31

I was really happy to see that someone had already done all the work of figuring out all the values and where they go!

I am definitely waiting to see what you find as a solution!
PS. enjoy the manual too!

 

[Led Hendriani]

If you look in the manual it says that if you don't want the other channel to be heard, turn that channel all the way down to 0. So to me it seems that it will always have the issue and that it was purposely built that way.

If I am wrong and someone has remedied this without adding a switch on the unit, let me know!

I did read of someone using a modified pedal to switch off fully one or the other channels, but that may have been concerning a different amp altogether too..., it's all one big blur...learning how to mod SS amps, tube amps, AND guitar pedals all at the same time...one day at a time...

 

[Led Hendriani]

I know this might be obvious, but have you checked all of the wiring internally to see if it may be different than what was original (I mean as far as pathways and wires running together) maybe one is too near something it should not be...

The power strip I use has a built in fault detector that will turn red if the wiring where you plug into is not right...very handy!!!

 

[CD]

Led,

Yes, I have checked everything as far as the wiring and circuit board goes and everything looks OK.

I will probably try and replace all the caps in the amp and see what happens.
I have momentarily abandoned the Randall for now since I got a smoking deal on a tube head that is 10x quieter than the RG80. Even with the master dimed on the 120 watt tube amp, it's still quieter. Something definitely not happy on the RG80.
I am sure that I will get to cap replacement before too long and report my findings since the wife doesn't like my projects sitting around in pieces very long.
Thanks for the schematic BTW.

 

[krija67]

I know this thread is old but i've owned several RG80ES and 100 amps over
the years. In my old house built circa 1940's all my RG's hummed due to the old Knob and Tube wiring which produced either 60 cycle hum, ground loop hum, or a combination of both. It was much worse in my buddies apartment which has old aluminium wiring. I'm now in a newly built home and "low and behold" the hum is gone, the amps are dead quiet. Now i'm not saying that we had the same problem or to buy a new house for your amp; but it's been my experience that people tend to panic and start replacing parts unecessarily. Start by diagnosing the problem first, then move forward and repair and/or replace parts. That same friend fixed a hum problem with his 80's Marshall JCM 800 by purchasing a EBTECH X HUM ELIMINATOR, boom, done! I can't stress this enough, "Diagnose The Problem First", then move forward with repairs. Hope that was some usefull info, just trying to save you and others some time, money, and aggrevasion.