DSL40CR R49 burning up

[JzRepair]

Hi all, new to the forum.

I've got this unit on my bench. When I received it, I had opened it up to find bad solder joints and a burnt R49 resistor. According to the schematics, r49 is a current limiter for the preamp plates. I replaced it, but that 2W resistor is on the verge of burning up again only in standby. It registers over 140F on the temp scanner and would suggest there's an excessive current draw within the preamp stage. I've checked all the caps for the plate supplies and the filter caps (c14/c13). I am getting about 25-30Vac coming through the resistor and this doesn't seem right to me. Also,in standby, the mute isn't working as you can still get signal on the output tubes.

I've got 3 resistors that are registering out of the 1% tolerance, but even at 10%, I can't imagine the current being that high that it would burn through a 2W resistor when it should have been over rated to begin with.

I've swapped V1-V3 individually to see anything changed and I'm not seeing anything significant. Has anyone else seen this issue? Can anyone point me in the right direction? I'm running out of ideas here.

 

[Mitchell Pearrow]

To The Forum
I am no amp tech.. but other more knowledgeable members here should be along to help you.
Cheers
Mitch

 

[Leonard Neemoil]

I don't know how to help, but if you want to move this to the workbench sub forum you will probably have better luck.

Just ask @Dogs of Doom if he'll be so kind.

Best of luck with the repair.


Cheers!

 

[Wildeman]

Alls i got to offer is this sweet music to listen to while you work.

 

[Max Gahne]

R49 also limits current to HT1 which feeds TR2 and TR3 - two transistors after the DFX IC. And TR1 and TR6 which are the resonance gyrator transistors. How does R15 look? And it seems like R11 would burn up before R49. Have you looked at C24?

 

[JzRepair]

Sorry for not replying earlier. It would seem the forum is not emailing me as I've set it up to do so I didn't get any notification of this.

So, here's what's been done. I swapped out each preamp tube individually to see if one tube was faulty. No change. I pulled the caps, C24, C25, C26, C28, C29, C30, C13, and C14. I tested them with an ESR and Capacitance tester I have. ESR is quite high, but I would expect that with Marshall downgrading to Fujicon capacitors which are notoriously bad. However, all those caps tested well within tolerance and presented no issues. I then pulled the legs of R11, R12, R14, R15, and R16. No change. Since C24 was left in place and the only thing that would cause R49 to burn up, I replaced it with a Rubycon cap of equivalent capitance and voltage. It still burnt up. The issue only takes place in Low power output. In High power, it works just fine and has no issues.

As can be seen from the schematic, the power supply for the preamp also powers the output tubes and phase inverter circuits, but nothing seems to connect them other than PGND from what I can see.

I spoke with Matthew at Marshall and he said they've never seen this issue before. I explained everything I'd tested with him and he was just as stumped as I am. The whole thing doesn't make sense. Thankfully, I was able to order a new board through Marshall this morning. Once that arrives, I'll swap it out. That will rule out the main board as the issue, but still doesn't eliminate the DFX1 board or the tubes. Honestly, it shouldn't be the DFX1 or the tubes, but I've also already spent more than 40 hours on this amp and I won't get reimbursed from Marshall for all that time, but I also don't like being this stumped. It cannot be that difficult.

I did pull TR6 and TR1 and tested them. Both were fine. Since it smoked R49 even with TR1 disconnected, I didn't bother checking TR2 or TR3. I'm also still not sold on the filter caps. Even though they tested fine under low power testing, that doesn't rule out an issue under the higher power of normal circuit operation, but still doesn't explain why R49 burns up with only C24 connected and nothing else. With R11 pulled, C24 and PGND are the only things in line, but is incredibly odd it would smoke it.

 

[barun]

Sorry for not replying earlier. It would seem the forum is not emailing me as I've set it up to do so I didn't get any notification of this.

So, here's what's been done. I swapped out each preamp tube individually to see if one tube was faulty. No change. I pulled the caps, C24, C25, C26, C28, C29, C30, C13, and C14. I tested them with an ESR and Capacitance tester I have. ESR is quite high, but I would expect that with Marshall downgrading to Fujicon capacitors which are notoriously bad. However, all those caps tested well within tolerance and presented no issues. I then pulled the legs of R11, R12, R14, R15, and R16. No change. Since C24 was left in place and the only thing that would cause R49 to burn up, I replaced it with a Rubycon cap of equivalent capitance and voltage. It still burnt up. The issue only takes place in Low power output. In High power, it works just fine and has no issues.

As can be seen from the schematic, the power supply for the preamp also powers the output tubes and phase inverter circuits, but nothing seems to connect them other than PGND from what I can see.

I spoke with Matthew at Marshall and he said they've never seen this issue before. I explained everything I'd tested with him and he was just as stumped as I am. The whole thing doesn't make sense. Thankfully, I was able to order a new board through Marshall this morning. Once that arrives, I'll swap it out. That will rule out the main board as the issue, but still doesn't eliminate the DFX1 board or the tubes. Honestly, it shouldn't be the DFX1 or the tubes, but I've also already spent more than 40 hours on this amp and I won't get reimbursed from Marshall for all that time, but I also don't like being this stumped. It cannot be that difficult.

I did pull TR6 and TR1 and tested them. Both were fine. Since it smoked R49 even with TR1 disconnected, I didn't bother checking TR2 or TR3. I'm also still not sold on the filter caps. Even though they tested fine under low power testing, that doesn't rule out an issue under the higher power of normal circuit operation, but still doesn't explain why R49 burns up with only C24 connected and nothing else. With R11 pulled, C24 and PGND are the only things in line, but is incredibly odd it would smoke it.

can u pliz tell me the value and colour code of r49

 

[JzRepair]

can u pliz tell me the value and colour code of r49

I don't have this in front of me any longer. I ended up having to file for a warranty replacement on this amp. I tested what I could, even Matt at Marshall recommended I just replace it after swapping out the main board did nothing. My guess was it was a tube issue, but it's now in Marshall's hands.

For the resistor, it was a 4k7 2W, so the color code should have been Yellow, Purple, Red, Gold. The Gold may have been brown, but I don't remember off hand. The original was so charred from heat damage, the bands were all black. This happened to the replacements as well.

 

[BCD123]

Hi JzRepair and hello Forum,

Just wondering how you got on with this repair - have a guess why.........yes, I'm working on a DSL40CR with the EXACT same symptoms.

One thing I did notice which was baffling was that the B+ changed when I alternately flipped out the fuses for the Low power and High power modes (this is still with the standby off - I know - WTF???) which kind of indicated to me they were interactive. I was using a fairly crappy old meter but it seemed to me there was a high impedance connection between the 2 windings which may be the result of a power transformer failure. What do you think? In "Standby Off mode" I find the amp runs at about 75% (~30w before clipping) and then you switch to High power and you get 40w! Switch to Low power and you get a fried R49.

Also, can someone please clarify the exact value of R49 - Schematic indicates 2k7 and you've mentioned 4k7 - I'm guessing it's 2k7 on the 100w?

Cheers,
B

 

[JzRepair]

BCD123, I ended up having to replace the whole amp. According to the schematic from Marshall for the DSL40CR, R49 is a 4k7 2W FP resistor. I'm not sure about the 100W version.

There was definitely something much bigger going on with this amp, but I was never able to find a solution to it. Since Marshall only reimburses me for a small portion of the actual time spent on the amp, I lost out. I put in over 50 hours on trying to figure this one out, because I'd rather repair than replace. Matt at Marshall was just as clueless as I am with this. It was quite unusual. I do have a feeling there was a short somewhere that was causing this, but I couldn't find it. That is interesting that swapping those fuses saw a change in the B+ voltages. Did it still fry R49?

I painstakingly went through disconnecting a leg of R11, 12, 14, 15, and 16 with no change. It still fried R49. I even removed C24 with R11 lifted and still it fried R49.

I only got paid for 4 hours of work on this amp, so I wouldn't suggest you spend much time on it. Marshall is aware, so I'd reach out to them and get the whole amp replaced if you can.

 

[JzRepair]

Hi JzRepair and hello Forum,

Just wondering how you got on with this repair - have a guess why.........yes, I'm working on a DSL40CR with the EXACT same symptoms.

Looking back over my old posts, you could try removing all the tubes and see if the problem still happens? If it was tube related, then you should be able to narrow down pre-amp or power tubes.

 

[MarshallDog]

Welcome to the forum new MF'r

 

[BCD123]

Hi Jz & Marshall Dog - many thanks for the welcome!

Did you happen to jot down any AC or DC voltages around the HT /B+ end of the amp?

Cheers,
B

 

[JzRepair]

Hi Jz & Marshall Dog - many thanks for the welcome!

Did you happen to jot down any AC or DC voltages around the HT /B+ end of the amp?

Cheers,
B

I actually don't remember, but I was within 20% of what is in the service manual. Do you have access to that?

I may have some from my notes for the invoice, I'd have to check.

 

[JzRepair]

Nope, I didn't write any down. I did jot down everything I tested. PGND is the only thing that links it all together. I pulled all the caps and tested them for ESR and capacitance. They were all low quality caps, but all tested fine. With R11 and CT wire pulled, it was still burning up. I replaced C24, because I have seen many caps pass low power testing, but fail under normal operation. Still burned up. After trying a new board, I had some brief success, but then the customer brought it back, because 1-2 hours of play it burned up again, even though my testing with the new PCB showed everything was perfect. I played it for about 10-15 mins with no issues. After that, I was at such a loss and so sick of trying to work on it, not getting anywhere, I sent word to Marshall to just replace it with a refurbished unit.

Looking back over my notes and the service manual, I'm still at a loss. With R11 pulled, and C24 having been replaced, there's absolutely no way R49 could still burn up, because in order for it to burn up, it would have to exceed 2W, with no path to travel, other than through C24 which goes through PGND. The short has to be something connected to PGND and C24 back through R11 is the path of least resistance. Quick test, what happens if you pull the V1 tube and remove the FS4 2A fuse from the circuit, eliminating LT1 and LT2?

 

[BCD123]

Wow!

So you're saying even with a new PCB the fault is still there.......I'm now thinking the PT has faulty windings. Especially if it appeared after it heated up for a while.

I used a better DMM (Agilent U1272A) to test the connectivity between the 2 x HT feeds with the fuses pulled and it was ~ 8.9M ohms which I would think is incorrect. I have some other transformers/ amps at home that I'll check to see if this is reasonable but I suspect not.

Thanks so much for this info and I'm determined to get to the bottom of this as the retailer and the national parts merchant have responded that they've never heard of this problem before and also that because I've touched it (......I've changed 1 x obviously faulty resistor FFS!) Warranty is void as well as it being 13 months since the date of purchase.

Note that the amp sat broken for about a month or so before it was sent to me so technically it died within the warranty period. Not good enough Marshall!!

Used my 1974 100w Super Bass converted to Super lead at rehearsal last night - original 48 Y.O. transformers and still sounds amazing! If you have one of these or similar do yourself a favour and get a BOSS WAZA TAE and you'll be able to get the goods at less than ear splitting levels.

Cheers,
B

 

[JzRepair]

Wow!

So you're saying even with a new PCB the fault is still there.......I'm now thinking the PT has faulty windings. Especially if it appeared after it heated up for a while.

I used a better DMM (Agilent U1272A) to test the connectivity between the 2 x HT feeds with the fuses pulled and it was ~ 8.9M ohms which I would think is incorrect. I have some other transformers/ amps at home that I'll check to see if this is reasonable but I suspect not.

Thanks so much for this info and I'm determined to get to the bottom of this as the retailer and the national parts merchant have responded that they've never heard of this problem before and also that because I've touched it (......I've changed 1 x obviously faulty resistor FFS!) Warranty is void as well as it being 13 months since the date of purchase.

Note that the amp sat broken for about a month or so before it was sent to me so technically it died within the warranty period. Not good enough Marshall!!

Used my 1974 100w Super Bass converted to Super lead at rehearsal last night - original 48 Y.O. transformers and still sounds amazing! If you have one of these or similar do yourself a favour and get a BOSS WAZA TAE and you'll be able to get the goods at less than ear splitting levels.

Cheers,
B

Yes, if you are not an authorized service center for Marshall and you open up one of their amps, you void the warranty. However, Marshall has one of the best warranties in the business. Marshall offers 5 years on parts, 3 years on labor, 3 years on speakers, and 90 days for tubes.

I'll be honest, there's no way of knowing anyone worked on the amp, unless you did a horrible job soldering it and/or you damaged something else. I'm not condoning this, but if you solder in a new resistor and let it burn up, how would Marshall know you did anything to it? Again, I'm not saying this is what you should do, in fact, I still wouldn't recommend it at all. It's just an honesty thing. That being said, if it's even close to being under warranty, I wouldn't touch it in the future. Have the customer get back with the manufacturer. Also, make sure you warn someone that you working on their equipment will void a warranty if you're not an authorized service center for that manufacturer. What I would suggest, is if you own your own business and have business insurance, reach out to Marshall and see if they will make you an authorized service center. Reach out to all the manufacturers for the same purpose. Personally, I won't even touch equipment unless I can find a schematic for it. Sure, I could reverse engineer it to a point, but without the schematic/service manual, that adds far too much labor I won't get paid for. Plus, getting in with the manufacturers allows me access to all the schematics. However, a lot of them won't work with you unless you've been in business for over a year and have an actual retail store front.

Now, as for the PT, you could definitely be on to something. It is very unique. Changing out the PCB, kept four things the same. The DSP board, the PT, OT, and the tubes. That DSP board shares the PGND connection, so that could have also been the issue. All I truly know is that PGND has to have something to do with it, because it's the common factor within the circuit. It links everything together. So, the PT isn't far off from that.

I am definitely interested in what you find. This is a hole I wouldn't want to go down, but, I get that you're in it now. I'll do what I can to help, just ask.

 

[BCD123]

Hi Jz,
I have some great news of sorts.

It wasn't the transformer, it wasn't the standby switch and it certainly wasn't the tubes.

Unfortunately it WAS the PCB or more than likely the excessively long component legs poking out underneath it. If you still have the faulty one around measure for conductivity between the node of R49, R11 & C24 and the node of C21, C22, R8 & R9.

These nodes were unfortunately connected to each other on this amp - I found it by measuring the voltage at this second node & it was much higher than expected (i.e. it should have been half rail and it was the same as the voltage at the first node.

The unattractive solution so far has been to cut the track just after the R49/C24 junction where the trace heads off to R11 and lift that leg of R11 so that it's flying. then join the junction of R49/C24 to the lifted leg of R11 with a wire and she's good to go!

I'll come up with a more aesthetically pleasing /stable mod before it goes back to the customer but that's it 100% and that's possibly why your second board failed..........probably same batch of badly assembled boards.

You mentioned you pulled R11 - did you find that there was an excessive amount of component leg protruding on the underside of the board? Also lifted one end of R8 where it joins R9 and found the same thing - too much component leg poking through. So at least it's not a conductive FR4 PCB failure like the DSL/TSL main boards of yesteryear but it would seem to be a "stuffing" error by the people assembling the PCBs. I don't intend to pull the board up to inspect the underside to confirm it but if anybody already has one please post a pic.

All the best,
B

 

[JzRepair]

Hi Jz,
I have some great news of sorts.

It wasn't the transformer, it wasn't the standby switch and it certainly wasn't the tubes.

Unfortunately it WAS the PCB or more than likely the excessively long component legs poking out underneath it. If you still have the faulty one around measure for conductivity between the node of R49, R11 & C24 and the node of C21, C22, R8 & R9.

These nodes were unfortunately connected to each other on this amp - I found it by measuring the voltage at this second node & it was much higher than expected (i.e. it should have been half rail and it was the same as the voltage at the first node.

The unattractive solution so far has been to cut the track just after the R49/C24 junction where the trace heads off to R11 and lift that leg of R11 so that it's flying. then join the junction of R49/C24 to the lifted leg of R11 with a wire and she's good to go!

I'll come up with a more aesthetically pleasing /stable mod before it goes back to the customer but that's it 100% and that's possibly why your second board failed..........probably same batch of badly assembled boards.

You mentioned you pulled R11 - did you find that there was an excessive amount of component leg protruding on the underside of the board? Also lifted one end of R8 where it joins R9 and found the same thing - too much component leg poking through. So at least it's not a conductive FR4 PCB failure like the DSL/TSL main boards of yesteryear but it would seem to be a "stuffing" error by the people assembling the PCBs. I don't intend to pull the board up to inspect the underside to confirm it but if anybody already has one please post a pic.

All the best,
B

I've got the old board in front of me. Actually, you nailed it by cutting the trace. R49 should NOT be connected to R8 and R9, but it is. Double check on your board, but R8/R9 should be connected to C21(K) and C22(A), which they do, but somehow, they also make a connection to that trace between R49/R11. I wonder if the phenolic has a bad insulation where the two traces cross over between the layers. On my board, the lead protrusion was perfectly fine. I worked for a PCBa at one time, so I was solder certified. I had a buttload of bad solder joints, but no bad lead protrusion.

For you your fix, I would suggest cutting both ends of that trace from R49 to R11, then route it with a good AWG wire to handle the voltage/current, but I'd mount the jumper wire on the underside.

 

[FretlessTech]

In case anyone gets here with the same problem...I believe I have found the actual cause of this problem. R9 is one of two resistors mounted over the trace that connects R49 to R11. In the amp I am currently repairing the end cap on R9 has degraded. The enamel that covers the the resistor has worn away and revealed the metal of the end cap. The lacquer covering the trace under the resistor had also worn and the two bare spots touched which bridged the node of R49, R11 & C24 and the node of C21, C22, R8 & R9 as referenced in BCD123's post above. This raised the voltage on C21 which caused it to overheat and "dome" the top of the cap (which is what I first noticed when I inspected the chassis...electrolytic caps should be flat on top!). The additional current drain is what caused R49 to overheat and burn out. Should you find either C21 domed or R49 burnt out check to see if this has happened to that amp. The glue that Marshall used to secure the caps to the PCB flowed around R9 which made it impossible to see that bridging impossible without removing R9 and doing some major cleaning! (The photo of the trace is shown after I repaired the trace.) Good luck to one and all.

Steve KS4KJ
Mays Electronics
Atlanta, GA