I just clipped that lead of R9 on the board I still have laying around. I have the same thing on this one. Good eye. I'll definitely keep all of this in mind if I run into this problem again. Seems like an incredibly odd failure point, but by the looks of it, that trace could have been routed above R9/R10 to R11 which would have eliminated the issue all together.
DSL40CR R49 burning up
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I just clipped that lead of R9 on the board I still have laying around. I have the same thing on this one. Good eye. I'll definitely keep all of this in mind if I run into this problem again. Seems like an incredibly odd failure point, but by the looks of it, that trace could have been routed above R9/R10 to R11 which would have eliminated the issue all together.
FretlessTech
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It also appears that R4 and R5 can have the same problem. The unit came back with no HV going from R49 to R4. The end cap on R5 had rubbed of and shorted to the trace that connects C13/R4 to R49 and burned away the trace. The only trustworthy repair for this was a jumper wire on the underside of the main PCB to reconnect the HV to R49. All of the affected resistors had the same glue holding the caps to the board under them so it may be that the glue is degrading the enamel on the resistors which allows them to short to the traces under the end caps. I'm hoping this is the end of the problems as I don't see any other resistors with this glue under them. I wonder of Marshall has seen a lot of these amps with these problems and re-evaluated the way they do things.
And this is being added as I have found one more resistor that could also have this issue. R6 hadn't had a short yet but I found evidence that the glue was degrading the coating over a trace right below where that end cap is located. In time this might have been another short. There don't appear to be any other resistors that were on the glue path. Let's hope not as this is quite enough!
And this is being added as I have found one more resistor that could also have this issue. R6 hadn't had a short yet but I found evidence that the glue was degrading the coating over a trace right below where that end cap is located. In time this might have been another short. There don't appear to be any other resistors that were on the glue path. Let's hope not as this is quite enough!
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FretlessTech
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I was going to DM you about what you had seem but didn't see a way to contact you. I think the trace location would be ok if the raised the resistors up off of the board and/or not let the glue seep under them. I wonder if the changed anything about these areas in later production runs.
Yeah, I have this all set up to let me know when more posts get added to this topic and I keep an eye on it. It is interesting that the solder resist was so weak to allow that to short out there. I've not seen this before on anything else. I haven't seen another one of these units on my bench since I did this one, but I can tell you that the replacement board I put in didn't have raised resistors in those two spots, but that is a really good idea. I'm curious as to why they glued those resistors in place, too. It doesn't make sense to me, because they shouldn't be prone to moving around or worrying about vibrations causing them to break.
I did inform my guy at Marshall about the defect initially, but never heard anything more after this. I later informed them about the thought of the trace shorting to the trace on the underside, but again, never heard anything more. I would hope they'd find this issue, but it's also possible they had a bad batch of resistors and this is something that isn't seen very much. Hopefully, this thread will receive more views when people experience this same issue. It would be interesting to see just how many people experience this. Makes me curious about the replacement board I put in, if this issue will pop up again after a few years. However, I'm truly hoping it was just a bad batch of resistors that were used on a small run and the issue resolves itself, even though I know wishful thinking rarely, if ever, solves anything.
Speaking with the engineer from the states, he was just as stymied by this issue as I was, suggesting he'd never seen anything like it either. The sad thing is, they never asked for the original board back, so I've been hanging on to it, just in case they ever decide they do want it. I'm glad I kept it since you discovered the true cause and I could verify it. This is definitely one for the books and I hope this information helps others who face this. If I ever see another one of these units, I'm definitely going to raise those resistors, even if there doesn't appear to be a problem, just to err on the side of caution.
cablercdd1
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Steve, you nailed it for me. I had one come in with the same problems. When i took the tubes out and shut the lights in my shop, I could see the R9 arcing. Thanks to you and all the others who posted solutions previously
Yes, I had the same problem. Scratching my head for a day before I found this thread. Same deteriorated R9 wearing through and shorting to R49. I'm still not running yet. I blew my only 630ma fuse. Do you know if the fuses are slow blow? The amp worked if I used the Variac to wind the power up but blew F3 if I used the amp power switch. I ordered some slow blow fuses. I hope that is the problem now
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I don't see where it says it's a slo-blow, but general rule, because it's audio is to use a slow blow. You should be good with what you bought to replace it.
Mastershon
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Great job everyone’s efforts to track this down! A very odd and sneaky problem. One of you should contact Marshall with the findings to prevent future occurrences as it’s possible they are still unaware.
Unless someone beats me to it, I will be reaching out to my contact at Marshall soon about another warranty issue. I'll make sure to link him to this thread so he can pass it on to their engineers.
PelliX
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Welcome
@JzRepair !
That's a bold statement, fuses have a letter on the indicating whether they're slow blow or regular, this is stamped into one of the metal ends generally for the common types. "T" indicates slow blow, and IF there's a type one defaults to, it should not really be that
That's a serious red flag if I'm understanding you correctly. The standby switch actually raises, lowers or kills the B+ in this amp. There should be no leakage...
Ugh, try Fender amps for a laugh (not).
It's for easier assembly and doesn't have to do with vibration - that would only perhaps be a consideration for the larger caps, and they're all radial anyway on a DSL.
Quite honestly, from an engineering perspective, the DSL combos (and many others) are designed in a bit of a "ballsy" way; there is a lot of heat coming up from the valves which is absorbed by the PCB and the chassis with zero airflow. The boards are not exactly the very best out there and it all RoHS solderwork. The expansion/contraction combined with vibrations is tough on the components. The boards, the caps and perhaps another odd thing or two will thank you if you provide just a little airflow. Just my
That's a bold statement, fuses have a letter on the indicating whether they're slow blow or regular, this is stamped into one of the metal ends generally for the common types. "T" indicates slow blow, and IF there's a type one defaults to, it should not really be that
That's a serious red flag if I'm understanding you correctly. The standby switch actually raises, lowers or kills the B+ in this amp. There should be no leakage...
Ugh, try Fender amps for a laugh (not).
It's for easier assembly and doesn't have to do with vibration - that would only perhaps be a consideration for the larger caps, and they're all radial anyway on a DSL.Quite honestly, from an engineering perspective, the DSL combos (and many others) are designed in a bit of a "ballsy" way; there is a lot of heat coming up from the valves which is absorbed by the PCB and the chassis with zero airflow. The boards are not exactly the very best out there and it all RoHS solderwork. The expansion/contraction combined with vibrations is tough on the components. The boards, the caps and perhaps another odd thing or two will thank you if you provide just a little airflow. Just my
Hence, "general rule." I wasn't saying it's a definite, but typically, from my experience, audio products usually incorporate slow blow fuses so they don't blow with regard to the peaks of the audio signal riding on the DC. I have seen a few fast blow, but generally speaking, audio products typically have slow blow fuses.That's a bold statement, fuses have a letter on the indicating whether they're slow blow or regular, this is stamped into one of the metal ends generally for the common types. "T" indicates slow blow, and IF there's a type one defaults to, it should not really be that
Don't get me started on cheap amps with their paper PCBs and the cheapest quality components money can buy. It's a business decision and I get that, but I'm old school in believing if you make a product you should make it with quality and pride rather than your first priority of cutting costs at every opportunity. Their boards may not be the best, but there are far worse out there.Ugh, try Fender amps for a laugh (not).
Quite honestly, from an engineering perspective, the DSL combos (and many others) are designed in a bit of a "ballsy" way; there is a lot of heat coming up from the valves which is absorbed by the PCB and the chassis with zero airflow. The boards are not exactly the very best out there and it all RoHS solderwork. The expansion/contraction combined with vibrations is tough on the components. The boards, the caps and perhaps another odd thing or two will thank you if you provide just a little airflow. Just my
Thanks for your two cents. Appreciate that! I consider myself a student and I'm far from mastering all this.
PelliX
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Yup, there are a lot worse, this is very true.
Depends on where in the circuit. The HT fuses are generally regular or fast blow, the mains fuse will be a slow blow often in guitar amps. If in doubt, give it a shot with a fast blow - it it works, great. If not, well, the fuse is cheaper than most other things that could go south - replace with a slow.
This is directly after the PT and before the FW rectifier. According to your earlier statement, I did my due diligence and yes, it is a slow blow. It's a T630mAL250V, which is a timed delay/slow blow. I never knew that with the stamp. Fuses are something that I've not spent a lot of time researching.
I'm trying to think though about the last time I saw an HT fuse blown and I believe it actually called for a slow blow fuse rather than a fast. I could be wrong. Though I do see a lot of blown fuses, I'm often surprised to see a fast blow when I do come across them, which isn't very often. I'm a stickler for sticking to what the original engineer called for in the schematics. Most times, they call it out directly, but most fuses I see blown are on the AC end before rectification. Of course I say that and now Murphy will probably pay me a visit and bring me an amp with a fast blow and on the rectified side of things. But, more opportunity to learn something new.
PelliX
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I didn't mean to start a fuse firestorm. Very glad for all of the extra info though. When my fuses arrive I'll be sure to check them out. I'll also be checking out any other areas on the PCB that are possible weak points. Hard to believe Marshall haven't picked up on this fault yet
I have a question regarding the bias. Doesn't the DSL bias adjustment show voltage rather than milliamps? should I be doing a calculation to get the right bias current? Let me know if this question should go to a different thread.
I have a question regarding the bias. Doesn't the DSL bias adjustment show voltage rather than milliamps? should I be doing a calculation to get the right bias current? Let me know if this question should go to a different thread.
I don't think you started anything. As stated, I'm a student. I'm always learning and that's what got me into my field in the first place. @PelliX taught me something I didn't yet know and provided a great insight into the manufacturing process. I am grateful for his input.
As for biasing, I use bias probes and take the actual readings from the tubes, because then I can see any variances in the tubes and/or the circuit to judge how closely matched the tubes really are. I just personally find it much easier and safer not having to poke around the circuit more than I have to.
PelliX
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mV indeed. It uses resistors to accomplish this and make it easier. While you would *ideally* know your plate voltage, etc, you should be fine simply picking a nice spot somewhere between 30 and 45mV.
As your AC voltage will vary a little from day to day and so on, it will seldom be 'perfect', but a general rule of thumb is to bias it the 'coolest' you can without sacrificing tone.
Thanks again Pellix. I think I see how it works now. So it is the voltage setting and I am reading millivolts. When the DSL started to act strange I tested the bias first but was getting inconsistent readings. It made me wonder if I should have been reading volts. It would have been quicker to turn off the lights and watch the fireworks at R9 instead.
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