It's a while since I acquired my second-hand JTM60 and replaced the melted valve sockets.. It's been sitting there largely unused for the last year and when I switched it on again I got a rather nasty hum out of it. It may well have developed over the years and I didn't realise how bad it was, but the oscilloscope doesn't lie...
Yep, that's around 16 volts of ripple on the HT smoothing cap. So, time to replace. The poor ventilation is probably the culprit; it's got hot enough inside the chassis to melt the hot-melt glue they use to stop the caps from rattling around.
In 20 years technology has moved on and capacitors have shrunk, so if you want a replica with the correct spacing it has to be tubeampdoctor.com. Of course, they're in Germany. Will I get charged £12 for someone to walk the caps through customs?
'Kinell, trying to get the bits together is a right pain. In the end I've ordered around £40 worth of caps, jack sockets etc. from RS Online.
While I was buying stuff I thought I'd get a replacement mains fuse holder for my son's old TSL100. Looks as though they don't make them like that any more. >Sigh<
Edit: Google to the rescue. Now ordered from eBay, mains fuseholder for TSL100.
Littelfuse
81000000005 Fuseholder for 5x20mm or 6x32mm fuses without cap/holder
81600000005 Cap/holder for 6x32mmm fuse
Main board filter caps now all replaced, hum has disappeared and I'm just left with a pop every time I change channel. With volume up on the dirt channel it's quite a loud pop. So out with the preamp circuit diagram (schematic) as found everywhere online and let's see, why does the dirt channel volume control still work when I've pulled this cable out?
JTM60_IMG_3961.jpg (277.05 KiB) Viewed 454 times
Oh, great, this early (1995) version has a different PCB (JTM6-60-00) where the dirt channel volume control is always in circuit, apparently. Can't find the diagram, so I've emailed Marshall. I'll probably end up having to draw out the circuit by hand. Then work out whether it's worth totally modding to the later spec.
Anyway, I also took the opportunity to replace the Cliff jacks with Neutrik ones. They look as if the switch contacts will work more reliably.
Cliff:
Cliff.jpg (119.78 KiB) Viewed 454 times
Neutrik:
Neutrik_jack.jpg (209.37 KiB) Viewed 454 times
L - R: Cliff, Neutrik:
Cliff_&_Neutrik.jpg (222.28 KiB) Viewed 454 times
Back on the TSL100, the fuseholder and cap have turned up so I can get back to that.
Edit: Rehosted pics.
Last edited by NickS on Wed Apr 20, 2022 10:26 pm, edited 1 time in total.
OK, so I've sorted out the differences and listed them at marshallforum.com with some diagrams. It also appears that Marshall's current diagrams are wrong because:
1. The present diagram shows on the same sheet the circuit diagram of the current version but the block diagram of the original version where the Boost channel volume is after the tone stack.
2. The Boost channel tone stack is shown with the bass pot connected the opposite way up to the Clean channel tone stack.
Next; should I be looking to change my tone stack? Tweak my grid or plate (anode) resistor values? Take out the diode clipping?
Fakir Mustache wrote:Check the resistors in the power circuit.
Do you mean PSU or output stage?
PSU. I've had them go bad. Check them all up to the plates. The ones I've had trouble with are the voltage dropping ones between the electrolytic caps, but it probably wouldn't hurt to check the plate resistors as well. Remove the valves to avoid false readings.
Yesterday I did some surgery and moved the Boost channel volume control to be in front of the last preamp ECC83 V103 as per the final JTM60 schematic, swapped the input jacks for Neutriks and put it back together.
JTM60-R46_reduced_a.jpg (256.54 KiB) Viewed 453 times
JTM60-VR6a.jpg (286.87 KiB) Viewed 453 times
JTM60-VR9.jpg (320.37 KiB) Viewed 453 times
Now that I have a usable circuit diagram (schematic) I've been able to check one of the other issues. 160V on the treble pot tells me that I have a leaky tone/DC blocking cap which explains the thump on switching channels. Unfortunately I have neither 220pF/500V nor 470 pF/500V caps in my box of bits so I'll have to wait for mail order to deliver some.
I might pull the power PCB and check the preamp plate resistor values, and maybe order some other spares while I'm placing an order.
Edit: Rehosted pics.
Last edited by NickS on Wed Apr 20, 2022 10:34 pm, edited 2 times in total.
NickS wrote:Yesterday I did some surgery and moved the Boost channel volume control to be in front of the last preamp ECC83 V103 as per the final JTM60 schematic, swapped the input jacks for Neutriks and put it back together. 160V on the treble pot tells me that I have a leaky tone/DC blocking cap which explains the thump on switching channels. Unfortunately I have neither 220pF/500V nor 470 pF/500V caps in my box of bits so I'll have to wait for mail order to deliver.
I might pull the power PCB and check the preamp plate resistor values, and maybe order some other spares while I'm placing an order.
Don't bother if it's a pain. The ones between the nodes are the ones I've had go bad, not the plate ones
Replacement caps delivered.
Clean channel tone stack changed to current spec (as per some old Marshall tone stack).
With boost channel volume control moved to current position, the gain and volume seem to be far too sensitive so I need to look at other changes.
To recap (geddit?) what I said over at the Marshall forum;
The power board turns out to have some differences to the current design too. I decided to reduce the gain in the post-clipping stage of the boost channel by changing the value of the cathode resistor from 1.5K to 22k as per the current design and that tamed the volume control to a usable level. I decided NOT to increase the gain in the previous triode as per the current design as the diode clipping comes in early enough for my taste as it is. I'm still undecided whether to take it out altogether as many people recommend.
Next; when changing channel I noticed that if the boost channel was quieter than the clean channel (why would you do that?) there was a momentary volume increase switching from boost to clean that disappeared after a fraction of a second. Looking at the circuit, the relay that switches between the two channel tone stacks has an electrolytic cap that delays for a few milliseconds compared to the relay that switches between the two halves of the first valve (US: tube). Why? I don't know. Shorting out the cap caused the same jump in volume I noticed during the switch. I implemented another design change in the later issue; replace the 1M resistor between the clean channel tone stack and the relay with a link. That helped the volume level.
You could still tell there was something strange going on, though, so I swapped the electrolytic cap out for a smaller one to speed up the switchover. That sounds much better. I don't know whether the relay's actually dodgy because it should switch in milliseconds and this volume blip seemed to last for a perceptible fraction of a second.
