Randall RG80 Hum

[vidmachine]

Thanks for the advice. I will try to answer in order.



With all controls on zero, I hear the hum, it is low frequency, power line hum. It is there with the guitar plugged in and guitar out.

If I remove guitar and turn up master volume and gain on either channel I get additional hiss and buzz and the original hum is underneath.

If I insert guitar, but do not play a chord, turn up gain full and start to turn up master, I get horrible feedback howl, which I can't take past about 5 on the dial. Actually, standing in front of this it is loud at 5, so I have not tried this thing all the way up yet.



The reverb control adds it's own noise (buzz) on top of the original hum. As you dial up the reverb, it increases in noise. You can play over it, but it is there.



I do have the foot switch. I tried turning down all controls to zero, removing the footswtich, but still get the hum. The footswitch makes some noise like a loose connection when inserted. You have to jiggle it a bit to get it to light the channel diodes, but once you do that, it has the proper functions: it has one switch for jumping between channels, and the other switch will allow you to blend the two channels. The green and red diodes light up on the panel and the footswitch for the appropriate channel.

With the footswitch in place, I still have the original hum. I tried moving the switch around the room, it has a long cable. I also tried turning off the lights (flourescent), and tried another outlet in the garage, but neither of those things made a discernable difference.

My feeling playing the controls is that this hum is getting in after the master volumes, because they don't really affect the hum. Fully cranked, they add their own noise, but on top of the base hum.



The reverb circuit adds more of a hiss, than a hum. The output side of the tank is on the other side from the transformer, also this is a combo and the tank is down at the bottom of the cabinet and the chassis is at the top, so it's probably more than one foot away. I took the tank out and put it on the bench and tried different locations relative to the chassis, but it didn't affect hum. It does seem that one of the male rca connectors is loose, however. I tweaked it with pliers.



I checked the two input jacks, there are two ground from the board that terminate on the #1 jack ground, I re-soldered at both jack and board. There is one board ground that also grounds the reverb in and output line shields, it is held with screw, I tightened that one. The main filter ground is tight, as is the centertap for the transformer and the ground for the .005 cap from the neutral line. I tested speaker outs for continuity. There are of course a lot of other places that touch the chassis, so this task will probably take a bit more effort.



The only shielded wires are the runs to the reverb tank and two wires that run from the panel diodes to the footswitch socket. Moving these later does tended to increase hum.



Not sure what you meant here. I tried to ground out the effects return, but it hummed worse than without. Measuring some oft he voltage listed on the schematic, I got it to quiet down some of the hum when I touched the probe to Q8/9 emitters, and also to the r45 base. Makes me think something is amiss in the power section.

The Reverb send and return connector wires are counterintuitve and criss-cross on the circuit board. The one that comes up near the transformer goes to the connectors that are furthest away. if you swap them, the reverb hisses like crazy unless you turn it off with the reverb footswitch.