18-watt - 60 and 120 hums

[skip62]

Hello all. My buddy has an 18-watt Marshall clone. It was in a combo and he decided to move it to a head chassis. After the move with nothing plugged in, volumes down it hums. Turn the volume up on the normal channel and there is a 60hz hum, well I think it's 60hz, that increases with volume increase. The tremolo channel has a 120hz hum, again I think it's 120hz, that also increases with volume increase. He suspected his heater wiring wasn't done properly. I looked around and he had several things all grounding in different places on the chassis, so moved them all to the PT. This cut the normal channel hum in half. It doesn't get bothersome until after 5 now. I redid the heaters, and that didn't change anything. I pulled the PI and the hum stops, if I'm correct that means it's preamp hum??? I then started pulling preamp tubes 1 at a time and the hum disappears when V2 is pulled. I'm chasing the normal channel hum first, then I'll get to the tremolo channel. Any suggestions where to look next?

It also has an oscillation on the top of high notes. The 5watt resistor that sits on top of the filter cap has a burning smell. I used an infared temp gauge on it and it's going over 350degrees pretty quickly. I don't know if any of this has anything to do with the hums, but I thought I'd better give as much info as I can.

Thanks for helping

[skip62]

Ok, I was reading some more about hums. I think the normal channel is lower than 60hz, and the tremolo is 60hz. I read in Gerald Weber's book that if you use your finger on a guitar input cord it will create the 60hz hum, and that's the same hum as in the tremolo channel of this amp and the normal channel is lower pitched than that. Don't know if this helps, but it's more info.

Thanks

[Valvehead]

One thing you can do is get the amp running and lighly move around all the wires in the amp with a wood pencil or chopstick and see if this affects the hum.

[skip62]

One thing you can do is get the amp running and lighly move around all the wires in the amp with a wood pencil or chopstick and see if this affects the hum.

Ok, just tried that, no change in hum. Thanks.

[stokes]

That resistor that is burning up sounds like the problem,if it smells like it is burning,it is.The way you describe it as "the 5 watt resistor that sits....." I assume it is a power supply resistor between 2 filter caps.The cap is likely drawing too much current,this could very well be the cause of the oscillation.

[skip62]

That resistor that is burning up sounds like the problem,if it smells like it is burning,it is.The way you describe it as "the 5 watt resistor that sits....." I assume it is a power supply resistor between 2 filter caps.The cap is likely drawing too much current,this could very well be the cause of the oscillation.

Yeah it tops out my IR thermometer at over 420degrees. Would it be the filter cap or another part of the circuit causing that? It's an electrolytic can type capacitor with the 2 positive terminals and 1 ground. The resistor goes across the 2 positive terminals.

Thanks for the help.

[Enzo]

The resistor burns up because too much current flows through it. SOmething on the lower voltage end of that resistor is the problem. SO far we have taken several temperature readings, but no voltage readings. We already know it is hot, the temp doesn;t matter. What voltage is on the ends of it?

If it has 400v on one end and 2 volts on the other, THAT is a problem. With such a load on the thing, I don't doubt the thing hums, the poor caps cannot keep up with the ripple from the excess current draw.

From the low voltage end of the resistor - the end farthest from the rectifier - with power off, measure resistance to chassis. Is it shorted? At this point it would either be the filter cap itself, or somthing connected to that B+ node. You can disconnect the resistor from the filter cap section. Now with the one end of the resistor up in the air, tack solder to it instead of the cap terminal any wires that went from that node out to the circuits. In other words we are eliminating that stage of filter cap but leaving the rest of the circuit. Does that cool the resistor?

Remember, the resistor is the symptom, not the problem.

What voltage is at each filter cap in the system? Looking for one hanging really low. SInce this was a chassis swap, something could be touching ground that shouldn't.

[skip62]

Gentlemen you are great. I couldn't find out what the voltages were supposed to be, and my buddy had a new spare cap, so I decided to try it. I undid the old one and realized that the he had crossed 2 of the wires going to the cap. 1 goes to the OT and the other to the underside of the board, since it's kind of blind, I wanted to make sure it was correct. Lifted the board and double check and sure enough the wires were reversed. She's quite as a church mouse now. Thanks for the help.