I've mentioned in passing my son's JCM2000 TSL100 which is an early model - all the PCBs are -00 like my JTM60. When he bought it for a fair price some years ago he wasn't aware of the issues so he gigged it. It was noisy so I recapped it with 105C caps with no significant improvement; noisy EL34s appeared to be the issue as replacement made a slight improvement. His first gig running it in anger ended up with V8, V8's 1K 5W grid resistor, some PC track and C46 blown. Following repairs it ran OK a couple of gigs and V8/C46 went again. The temporary fix was to run it with one pair of EL34s as a 50 watter using an 8 ohm cab in the 4 ohm socket*. I did some research and found the problem with grid bias resistors and the whole drifting bias thing. I did some mods (e.g. drill out round the pin and install 5k6 resistors) which seemed to improve his bias stability issues but the thing was still somewhat noisy, so when his band was offered artiste discount by Blackstar he jumped at the opportunity and the TSL arrived in my garage. He didn't try to sell it because he wasn't confident that it was reliable enough and he didn't want to end up with bad feedback. Forward some years and the garage needs clearing out a bit so the TSL came up on the bench. As I came across mention of the speaker socket earthing problem on this board (while researching my JTM60 issues) I installed a jumper over the 16 ohm speaker socket earth and fired it up. The following issues are noticeable: 1. Bias control is not good enough on one side. It could be a dodgy pot but I need to double check that I replaced the NTC bias resistors. 2. After a little while the clean channel develops a hum that is not obvious on the Crunch and Lead channels. Step 1; make sure you have a list of all the connections. I thought this might be useful to some other people too. Please tell me if you spot any errors or if the hum is a common issue. Please don't bother telling me to replace the main board. *Edited to correct stupidity. Edited 8-Mar-21 to correct Clean CON6 and Lead (OD) CON1. Edited 21-Mar-21 to remove duplicate CON1 on Rear board.
The hum is to do with the valve heaters and is a problem that can be cured. If I remember rightly the preamp valves use DC heaters and the phase inverter uses AC. It's this one that causes the hum. You can upgrade the recifier diodes and run a DC heater to the PI. You can also permanently cure the bias drift for about £50 and an hour's work with a board from these guys: Valve Tube Guitar Amps | Amp valves | Guitar Amp Valves If you talk to Stewart there he knows about the heater hum problem too and can advise.
When you pull two output tubes, you run a 16 ohm speaker into the 8 ohm tap. Not the other way around. Or you run an 8 ohm speaker into the 4 ohm tap.
Hi Greengriff. Thank you for the suggestions. If the hum was the PI wouldn't it still be there for the other two channels?
Oops. You're so right. Must be the dementia starting. I'll edit the original post so as not to cause confusion.
C46 was underspec needs to go from 500 to 1K check R6 & R9 on the power board. Those seem to blow all the time as well. If bias won't be close to level, you'll get hum. Figure that out 1st & see if you still have hum.
Next time I'm in my amp I will see if I can validate your connector index, so thank you for providing that. I know V1/2 are DC filaments and the rest is AC. In fact, my next project is replacing the DC filament recto/cap array with a beefier spec. I wonder if I should provide a list of everything I have done to mine? Perhaps that might help everyone with theirs in some measure? I was able to rid the hum from both FX loops. I also shielded the reverb tank which got rid of all kinds of noise. I replaced the silicone diodes in the HT recto with FREDs to rid any additional hash, and installed a choke at R71. Because everything passes through the Lead board, there are a pair of caps which should be 630V, not 63V ratings (a typo overlooked on the schematic). Maybe there is a bottle neck on current which is causing some shenanigans whenever the clean is engaged? Reflowing all major components might help, as I had found a few to be loose on the Rear board. Gimme about a week or so, and I should be able to post an itemized list of what I've done. My amp has no issues so far...
Does the "Deep" switch have an effect on the hum? If so, read this earlier post of mine: https://www.marshallforum.com/threads/dsl-deep-switch-and-general-hum-buzz.56478/page-2#post-1816790
Thank you. I'll look for that when I reassemble it. The Deep switch was not depressed when I noticed the hum.
Looks like on the REAR PCB index you have CON1 listed twice, other than that she looks good. Also, REAR W2 can be used in conjunction with R19 for a NFB control: remove R19 and stick a 100k pot between W2 and W4 and you now have JCM800's 22-27k, the JCM2000's 47k, and the old JTM 100k at your disposal. Cheers!
I was thinking it might have been coupling caps that are downstream from a coupling cap that is already blocking the high voltage DC. But I will have to check that. But going higher voltage rating never hurts for sure.
I checked it out and it affects it only slightly. 15V rails look OK. Ah, I'll fix that. Good to know. When first switched on the hum is negligible to the point where I was thinking "What did I fail to connect?". It comes on gradually and after 15 minutes or so there's about 300 mV pk-pk mains frequency hum on the 4 ohm speaker out. Clean channel only, remember; select another channel and it disappears, and it's unaffected by the tone controls. I can't think what would affect just the clean channel though. The PI crapped out on me while I was investigating; fortunately I had a NOS white box (no name) ECC83 handy (35p, it says on the box).
If you are still getting hum, try a few things: - Main C24, lift a leg or clip altogether (annoying treble freq) - OD C36/37, change with 630V types - Rear 16 ohm jack, close the open ground (should match the other jacks) - PI resistors (right of socket component-side) replace with 2W metal films - Reflow connector/wire sockets under Clean PCB - Filament Bridge Rectifier replace with BR104, and replace (4) 3300u with 4700u low-esr 16V types - Reflow Power Entry PCB connector/wire sockets - Reflow entire Clean PCB - Check values on every component mounted on Clean PCB to see what has failed It's work, but it might work...
