Need help with a Marshall TSL60

[mac dillard]

Schematic is here: https://el34world.com/charts/Schemat..._tsl60_60w.pdf

This amp will play for a while and then no sound. It looses the DC filament voltage to the three preamp tubes. The power tubes and the PI have 6.4 vac to them. Measuring the DC voltage to the preamp tubes I get 3.38 vdc pin 4 to ground, pin 5 to ground and pin 9 to ground on V1. 0 pin 4 to pin 9 and pin 5 to pin 9. Same type of reading for V2 and V3. These readings are when the amp has no sound. When it is playing right I get 5.54 vdc pin 4 and pin 5 to pin 9 on V1. 5.42 vdc pin 4 and 5 to pin 9 on V2, 5.64 vdc pin 4 and pin 5 to pin 9 on V3. I unplug the amp, let it sit for a while and turn it back on and it works. Never know when it is going to cut out. May play for 30 minutes or it may play for 4 hours or more. Amp has all new JJ tubes.

Would appear to be a Bridge rectifier problem or a solder joint. Could a Bridge rectifier fail like this? ie: work sometimes and then fail and then start back up. I have not pulled the board yet. Putting that off as long as I can. Amp belongs to a friend and I have worked on it before. (I am not an amp tech, do this for fun), other time I cleaned the filament connections and cleaned the tube sockets. Ran it for 4 hours and it did not fail. Gave it back to him and it failed the first time he used it. He then sent it to an authorized service shop. They could not get it to fail. They put new tubes in it and sent it back to him...

Any suggestions are appreciated. Where in the filament circuit would you begin looking for a cold solder joint if that is what it might be.

[nevetslab]

I would suggest un-soldering and then re-soldering the heater connections for tubes V1, V2 & V3 for starters. Then, if the solder terminals are accessible on the bridge BR100, do the same on those. It sounds like fractured solder joints, and not a tube problem. You mention you're NOT a tech. So, be aware that there are dangerously high DC voltages present still, just after turning off/disconnecting the AC mains. Those of us who work on these have power supply discharge probes (low value power resistors with probes attached to bleed off the stored charge on supply caps). Are you at all skilled with soldering, have proper tools for this?

[mac dillard]

Thanks for the reply and the caution. Yes I have the tools and equipment. The readings I got makes me think that I have a cold solder joint at the BR. There was one joint on the BR that looked like it could be a problem. I reflowed the BR. I'll also do the three tubes..Going to put it back together and see how long it will run. Really wanted to change the BR. I know that if you do not get good contact with the legs it can be a problem, but hate to pay $8.00 shipping for a $2.00 item. Don't have a place close that sells the rectifiers...Makes me miss the Radio Shack..Hate to say that as much as I have complained about them in the past.

[The Dude]

If you do replace that BR, use a higher current version and leave the legs long to keep the heat away from the circuit board. It could be solder joints, but I've also seen these rectifiers give up with heat. To be safe and for reliability, I'd just replace it.

[mac dillard]

The BR is a Rectron BR102 which is 10 amp 200 volt and it does have a small air gap between it and the board. You thinking go to a 104?

[The Dude]

There are some Marshalls with a smaller rectifier (maybe the DSL40?). I must have been thinking of the wrong one. A 10A rectifier should be more than fine for filament supply on 3 12AX's. If there's a gap between the board and rectifier, you should be good. Sorry for the misinformation. They are usually soldered tight to the board. Maybe someone has replaced it already in the past.

[mac dillard]

probably thinking of the DL 50. It uses a BR62 which is 6A... Think they had a lot of trouble with that one. I have the amp up and running. Using a CD player to input into it. Going to let it play for a couple of hours.. then shut it down a try it again tomorrow...See if I can make it fail.

[mac dillard]

Well the dang thing played for 4 hrs. and never missed a beat. Shut it down let it sit fired it back up and it failed in about 15 minutes. Back to the drawing board. I am going to replace the BR. I don't believe that the BR it's self is the problem but just maybe replacing it will clean up some other problem. Not sure what else to do at this time.

[Enzo]

I repaired a lot of that, and I almost always found the original solder to the bridge legs never wetted the legs, they were somehow tarnished and didn't take solder well. I removed the bridge, polished the legs to remove oxidation, and then soldered it back in place.

[mac dillard]

Thanks Enzo, It sure seems that is the case. That part of the circuit is so simple that can't much else go wrong. I thought about reusing the BR but then decided that if I am going to the trouble to remove the board and it I'll just go ahead and replace it. After the new one gets a really good cleaning.

[mac dillard]

I wanted to follow up on this problem and not leave it hanging...Enzo, as usual, was correct. The problem was with the contact of the bridge rectifier...I desoldered the rectifier to get a look at the leg. It would appear than when this amp was assembled the rectifier legs were cut, put in the pads and then soldered. I would normally mount the rectifier with the legs through the pads, solder it and then cut off the excess legs...What happened was that the legs barely extended through the pads. One of the legs extended only to the edge of the pads. That leg only had solder on the very end of the cut off leg...That was the contact problem. Install new rectifier and we are off and running...Thanks again Enzo.