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Thread: Marshall DSL401 problem, some questions

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    Marshall DSL401 problem, some questions

    Hi All,
    Hope everyone is well. I have a friend, who has a customer (friend owns a music shop), who has a Marshall DSL401. He loves the amp. The amp stopped working, no sound. I got an email from my friend asking to give some help with replacing the power transformer. His customer has a new one, and wasn't sure about the wiring.
    My first questions were: is he safe to work with high voltage, and 2) is he absolutely sure that its the PT and not something else, or if it is the PT, what would have burned it out?

    So, I did some digging, and found an old thread:
    https://music-electronics-forum.com/...ead.php?t=6650

    and also quite a few other old blog posts with people complaining that "the bridge rectifier burned out". Took some time to get a relevant although not complete schematic. Seems these amps have DC heaters on the first few tubes, and the bridge used to feed them, at least in the earlier versions, had no heat sink and was socked down tight to the circuit board, which helped lead to their demise.

    I found this schematic which seems to be the most complete:
    https://drtube.com/schematics/marsha...02)%20iss9.pdf

    Found bridge BR201, and it leads to valve V3. What I can't find is how the heater supply connects to the other 2 preamp tubes. There are two connectors labelled W30, W26. But V2 has 4 connectors, labelled W 24, 25, 28, 29. And V1 has 2 connectors W27, W23. But I can't find any of these connectors elsewhere on the board.

    So, Im thinking maybe there are jumper wires, e.g. from W26, and W30, to W28 and W24 (to V2), then on down. But Im usually wrong.

    If this is the case, then they are not "elevated" heaters, like a few blog posts say, but just "dc" heaters, is this correct?

    Finally (sorry for one of my long rambling notes) I don't even know that this is the problem, its all being done remotely through email right now, but should be look for a higher current replacement rectifier? The marshall schem above doesn't have any specs, it just says "BR102".

    The schematic is a 201, all I could find even though it was marked 401 in the link
    I spent quite a bit of time looking through links on the schematic request page, but quite a few are old and hard to tell which ones have which amps.
    Thanks!
    Mike

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    Supporting Member The Dude's Avatar
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    Funny,...... The first thing I thought of when I saw the title of the thread pop up was, "Oh, there's another DSL401 with a bad bridge rectifier". Besides bad tubes, it's really about the only thing I've seen go wrong with these amps. It's a relatively common problem. That said, every amp is different and your symptom could certainly be caused by some other problem, especially given that we don't even know what the symptom(s) are. If it is the BR, I'd replace it with a higher current version and/or add a heat sink and leave the leads long enough to stand off the board a bit.

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    Cool thanks Dude! Yeah, I get the feeling, after all the blog posts, that the guy bought a PT not having diagnosed the amp. I just sent a long note to my friend to get more info before he dives in and replaces the PT, and do some testing with his VOM.

    I read that fairly simple, leave the standby ON (no HV), turn the amp on, and look at the small tubes V1-V3. If the power tubes are glowing at the bottom, and the small tubes are not glowing, then shut down, drain the filter caps and test the bridge.

    I read around and blogs said its BR-10 10A 200V
    https://www.mouser.com/ProductDetail...9Lqqj2T2GIjQyV

    He should go up one size in amperege whatever that is? Looks pretty tight in there, should not get a bigger physical unit I think?

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    OK I feel stupid: BR-10 means 10 amp. Reading . . .

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by mikepukmel View Post
    ......I read that fairly simple, leave the standby ON (no HV), turn the amp on, and look at the small tubes V1-V3. If the power tubes are glowing at the bottom, and the small tubes are not glowing, then shut down, drain the filter caps and test the bridge.......
    ^^^^^^Yes that. I'll add a couple things. Testing the bridge with a DVM out of circuit, the bridge will often test ok. It dies under load. The best way is to measure AC voltage into the bridge and DC voltage out. If there is AC in and no DC out, the bridge is bad. Also, there is a temptation to just resolder the bridge because there are often cold solder joints on its legs. This usually doesn't last. The rectifier may run for a few minutes and then die and the unit comes back to the shop. It's best to just replace it. It's a cheap part, you already have the amp apart, and it's good piece of mind to just do it right the first time.

    Edit: I was typing during your last post, so for clarity: 10A should be plenty. Some of these did not have 10A rectifiers. It's supplying 3) 12AX7 tubes, so total current should be less than 1A (edit: more at initial power up). The problem is not so much current rating of the rectifier as it is that the rectifier has no heat sink to dissipate the heat. Adding a heat sink is a good solution. Another is using a larger rectifier with more body mass. And again, leave the legs longer on the replacement part and keep it off the board. This will help keep the solder joints from baking loose and let more air flow around the rectifier.

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    10A is plenty. Like any diode, the bridge has a surge rating, which would be well over 10A. For example the common 1N4007 diode is rated at 1A, but it has a 30A surge rating. Yes, 30A.

    I found that many times the bridge wasn't "bad", but the solder never wetted the legs well, so the connection failed. Scrape the legs clean and resolder often fixed it. But bad or merely failed solder, try to avoid deciding any failure in an amp requires re-engineering the circuit.

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    Quote Originally Posted by Enzo View Post
    10A is plenty. Like any diode, the bridge has a surge rating, which would be well over 10A. For example the common 1N4007 diode is rated at 1A, but it has a 30A surge rating. Yes, 30A.

    I found that many times the bridge wasn't "bad", but the solder never wetted the legs well, so the connection failed. Scrape the legs clean and resolder often fixed it. But bad or merely failed solder, try to avoid deciding any failure in an amp requires re-engineering the circuit.
    Thanks so much for this. I figured the bridge rectifier on a TSL60 that was blowing heater fuse was within specs so didn't want to pull it out. When the board was out I've seen that the pins were pretty much not making contact with the solder. Scraped them from all sides with a scalpel and resoldered properly; it worked.

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    Lifetime Member Enzo's Avatar
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    A lot of folks replace the bridge, and now the amp works, so they assume the bridge was bad. But if they had removed the old one, cleaned the legs, and reinstalled the same bridge, it likely would have worked as well.

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    FWIW: I removed the old bridge, cleaned, and reinstalled it in a couple of the first of these that came into the shop. Both returned with the same problem. Solder still good, I checked voltage at the bridge after it died. AC in and no DC out verifying that the bridge was indeed failing after warmup. Sure, it could have been luck of the draw, but I've learned not to take chances on these. A BR doesn't cost that much and, at the very least, you can leave the legs longer on the replacement.

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    That is true, cheap insurance, but I have never had one come back on me.

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    Quote Originally Posted by Enzo View Post
    That is true, cheap insurance, but I have never had one come back on me.
    I've seen examples of what both you and the Dude are describing. To your point, Enzo, failure of the solder joints have been the most common failure I've come across. But I still think the Dude's suggestion of replacing the part is the best solution. Marshall mounts these flush to the board, and there isn't much length left on the existing legs. By replacing the part, and lifting the bridge off the board, should reduce the risk of the bridge and solder joints failing again due to heat.
    Isn't this the model which Marshall could not seem to pull their heads out of their asses to provide an adequate bias design? If this is the model I'm thinking about, the service literature was next to useless to help distinguish between the early models and the revised one.

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