Marshall TSL60 - Excessive hum

[Gank]

Hey all - Great forum you've got here. I've been reading quite a few threads and there's a hell of a lot of collective knowledge here! I'm hoping someone can point me in the right direction with my TSL.

I purchased this amp new in 2001 and it's been rock solid, but there's always been what I consider excessive hum on the crunch and lead channels. I've swapped cables, swapped guitars and done a retube/bias, with no luck.

I pulled the chassis and gave the board a quick visual inspection and there's some solder joints that look suspect. Near the input jack, L1 and R82 both have shady looking joints. Would bad joints on these two contribute to the hum? Would touching up the joints on anything that looks suspect be pretty a straightforward thing to do? Any ideas on any other things to look for?

I have some soldering experience and some basic tube amp knowledge, but to be honest, the TSL looks a little more intimidating when compared to my Valve Jr.

Thanks in advance for any help!

[Gank]

Does anyone know if these amps require the caps to be bled? Looking at the schematic, there appears to be two bleed resistors adjacent to the filter caps.

[Twist]

I know you said you swapped guitars but are you playing single coils? It never hurts to reflow any questionable joints. Does the hum go away as you turn down the gain? After a few minutes the caps should bleed off and also make sure the standby switch is in the operating position and the AC cord is unplugged while your working on the amp.

[Enzo]

Your voltmeter knows. If you are ever in doubt about a charge in a cap, MEASURE it.

[paulh]

As Enzo said, you can check with a meter. These amps do have resistors across the filter caps that will bleed down the voltage but with any amp I bleed them off myself anyway. It avoids any unwanted suprises. Try different seeing what pots affect the noise. This will help in isolating what part of the circuit is noisy. Preamp tubes sometimes can cause hum. Did you try other tubes or swapping them around to see if the symptom changes?

[Gank]

You're right Twist, the caps do bleed off after a few minutes. I let the amp sit for about 30 mins, just to be safe, and checked the caps out with a voltmeter.

I couldn't find any info on how to drain the caps on these amps, and being both green and very cautious, I figured I'd ask before I go poking around with a multimeter.

There's significant hum when the amp is either of the OD channels, using guitars with humbuckers.

At first glance, I didn't see any joints that looked bad. There is, however, a resistor (R50) right next to the filter caps that looks like it missed the hole during assembly. There's solder in the hole, but no sign of the lead underneath.

[paulh]

Thats one of the resistors across the filter caps that balance the voltage acroos 2 filter caps in series. That is also what is bleeding your voltage to ground. Also it would have nothing to do with the problem you are describing.

[Gank]

Ack, I thought that'd be the case. Guess i'll keep digging.

The hum occurs with input and increases with volume. The tubes are about a month old and have been biased correctly, but the problem existed with the previous set of tubes. Increase with volume = problem on the preamp side?

[paulh]

If you turn up the channel master volume and the hum increases yes, it's in the pre amp section. Does it increase if the gain pot is increased?
Even though you have month old tubes it doesn't mean that a tube isn't causing this. I see brand new tubes fail out of the box all the time.
This amp also has a DC heater supply for the first 3 pre amp tubes. If there is and bad cap etc. in that supply, that could cause this symtom also.

[Gank]

Thanks for the help so far, everyone. I'm learning a lot and I'm loving it.

I'll have to reassemble and test to see if the gain knob increases hum. I'll search the forum for a more thorough troubleshooting procedure - no input, check volume & gain, with input, check volume & gain, etc.

I'd ruled out the tubes as a problem initially, due to the fact that the hum has been an issue for quite a while now, through two sets of tubes. Even so, I'll swap them out and retest, to see if that makes a difference.

[paulh]

You can also try pulling preamp tubes one at a time to see if you can isolate the hum to a section. Start with the first one (usually the one closest to the input jack ) and work your way down the chain till the hum goes away. This needs to be done with the guitar plugged in and the volume on the guitar turned down. If you don't plug in anything into the input, the pre amp will be muted on this type of amp.

[Twist]

If you can get to the pre amp tube sockets with your meter, turn the amp on and first check the DC voltage between pins 4 and 9. Then switch your meter to AC and see how much ripple is between these pins. If you can tell us which controls vary the hum, all I have to go by is a schematic and it only list controls as "VR5" "VR6" "VR7" etc..

[Enzo]

Just an idle question: It hums too much in the lead channel. Does this hum remain when NOTHING is plugged into the input jack? Does the hum remain when a guitar is plugged in but the volume control ON THE GUITAR is turned to zero?