Marshall JCM 2000 DSL 201 - Filter Caps

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by SackvilleDan, Apr 21, 2015.

  1. SackvilleDan

    SackvilleDan Member

    Messages:
    2,007
    Joined:
    Nov 15, 2009
    I had the bridge rectifier replaced in my DSL 201 as it was overheating quite a bit.

    My amp tech suggested I change the filter caps in the amp, as I am getting a huge amount of hiss and white noise on the clean channel of the amp (but surprisingly, not the OD channel). He's thinking these caps went when the bridge rectifier went.

    I would like to do the work myself to keep the costs down, and I'm pretty handy with a soldering iron, but I don't know which caps need to be replaced!

    I'm hoping someone is feeling rather charitable and can tell me which components are the filter caps for the clean channel on the below schematic? I'd kindly repay you in lots of internet gratitude!

    [​IMG]

    [​IMG]
     
    Last edited: Apr 21, 2015
  2. Cirrus

    Cirrus Member

    Messages:
    2,459
    Joined:
    Oct 23, 2009
    It's the ones hanging off the HT line - C41, C42, C61, C60, C59, C66 and C58 by the looks of it.

    There's probably at least another 1, probably a higher capacitance value maybe 50uf+, immediately after the rectifier which I'm assuming is on page 2 of the schematic.

    White noise only on the clean channel doesn't sound particularly like filtercaps going though - have you tried just changing valves?
     
  3. SackvilleDan

    SackvilleDan Member

    Messages:
    2,007
    Joined:
    Nov 15, 2009
    My tech had all of the tubes tested, and replaced as required. The power tubes were changed and I believe several of the pre-amp tubes, but I can't remember exactly which ones off hand. I will post page 2 of the schematic if I can find it!
     
  4. SackvilleDan

    SackvilleDan Member

    Messages:
    2,007
    Joined:
    Nov 15, 2009
    Edit: Just added second page of the schematic. Thanks in advance @Cirrus for all your help!
     
  5. Cirrus

    Cirrus Member

    Messages:
    2,459
    Joined:
    Oct 23, 2009
    C32 on that second page.

    I would be surprised if it was the filter caps producing noise on one channel and not the other, though I'm not that familliar with the DSL circuit. The filter caps smooth out power to each valve stage so if they start to go they'll affect pretty much everything unless there's one particular valve stage that's only used by the clean channel - I'd say more likely it's a bad opamp/transistor/diode/resistor/coupling cap; something that gets bypassed from the signal path once you switch to the second channel, just from the description you've given.

    Best of luck with it all the same! :)
     
  6. SackvilleDan

    SackvilleDan Member

    Messages:
    2,007
    Joined:
    Nov 15, 2009
    Thanks Cirrus! My problem is I'm not all that great at tracing circuits, so I can't figure out the areas that are in the clean signal path only... I'm just going on my tech's advice about the filter caps but if there's another handful of components that it could be, I may as well replace them too...
     
  7. xtian

    xtian Member

    Messages:
    1,671
    Joined:
    Dec 15, 2010
    Location:
    NorCal
    Yeah, hiss and white noise are not caused by failing filter caps. Bad filter caps cause persistent hum, weak bass, ghosting, etc.

    Has this hiss always been a problem with the clean channel, or is it a new condition? When did it start?

    If you can absolutely confirm it's not the preamp tubes, then you start looking at plate resistors and coupling caps. But you can further localize the problem by pulling V1. Does the hiss stop? If yes, then the issue is part of the circuit involving those first two triodes.

    How about the FX loop? If you plug a signal into the FX return, does that bypass the hiss?
     
  8. donnyjaguar

    donnyjaguar Member

    Messages:
    4,244
    Joined:
    Nov 4, 2005
    Location:
    Canada
    Agreed on the hiss. High voltage rectification does cause hiss and the filter cap's generally get rid of it. However, whenever there's a high-gain amplifier in a circuit (op amp) that's where I'd start looking for the hiss first.
     
  9. SackvilleDan

    SackvilleDan Member

    Messages:
    2,007
    Joined:
    Nov 15, 2009
    The hiss is a relatively new problem. I have owned the amp for a couple years and noticed about six months ago that the volume was dropping out on the clean channel (I never used to use the OD channel, just pushed the clean really hard with a Tim). It would get really farty and drop out, sometimes coming back in but it became very unreliable. Shortly after this, the clean channel gain pot got really noisy, like you could hear white noise as the wiper of the pot moved.

    As I did a bit more research, I found out that these amps overheat quite a bit. So I reached out to a guy to do some work on it for me. Turned out the bridge rectifier was toast, so we replaced it. The tech also reflowed a bunch of cold solder joints and some that were noticeably cracked. The underside of the board looks great now. The gain pot was also replaced, as were most of the preamp tubes and the power tubes, which were also re-biased a bit colder. My tech had all of the tubes tested and replaced the ones which didn't perform to spec.

    After we did all this work, the amp doesn't cut in and out anymore, and I don't have the volume drop problem that I had before. BUT, even with the new pot, the clean channel gain still has a foolish amount of white noise as the wiper travels, and a lot of hiss. Switching over to the OD channel is like hitting mute - all the noise goes away. So we determined that there's an issue in the clean channel preamp somewhere (as it's independent of the OD channel and the master volume section). I haven't jumpered the effects loop to see if that's an issue since the repairs were made, but had done that before and it didn't seem to affect anything.

    So I'll add an op-amp to my shopping list as that should be pretty cheap and simple to replace. Any other thoughts on culprits for this noise?
     
  10. Cirrus

    Cirrus Member

    Messages:
    2,459
    Joined:
    Oct 23, 2009
    Might be totally wrong/ superfluous info, but I seem to remember that pots can crackle when moved if there's voltage across them - which might be the case if a coupling cap or nearby dodgy opamp chip was leaking DC voltage.

    Like I say, half remembered stuff here...
     
  11. SackvilleDan

    SackvilleDan Member

    Messages:
    2,007
    Joined:
    Nov 15, 2009
    Thanks for all your help with this Cirrus! Do you know which caps are the coupling caps? And which op amps I should change?
     
  12. xtian

    xtian Member

    Messages:
    1,671
    Joined:
    Dec 15, 2010
    Location:
    NorCal
    Example coupling caps are C74, C98, C72, C77.

    Start by measuring for DC voltage on the tone pots. You should see no more than, maybe, 25mV between any of the pot's pins and ground. Another sign is that the pots sound scratchy when turned, but not from dirt.
     
  13. SackvilleDan

    SackvilleDan Member

    Messages:
    2,007
    Joined:
    Nov 15, 2009
    Getting ready to order replacement components for this amp to start getting rid of the hiss! Does anyone have any other ideas besides the coupling caps and filter caps? And what op-amp(s) should I be changing?
     
  14. xtian

    xtian Member

    Messages:
    1,671
    Joined:
    Dec 15, 2010
    Location:
    NorCal
    What? No, I don't think so. Solid state diodes as rectifiers can sometimes introduce switching noise, and that's why some folks prefer the UF (fast switching) models, or put snubber caps across each diode. But hiss is broadband noise, and I can't imagine how your amp's HV rectification and filtering would cause hiss. What are you talking about?
     
    pdf64 likes this.
  15. RussB

    RussB low rent hobbyist Silver Supporting Member

    Messages:
    11,071
    Joined:
    Jul 3, 2004
    Location:
    CT
    My advice is to rethink your goals here. You don't know where the "problems" are, and you can't even identify the parts, but you want to start swapping parts out. I've been there, and done that. Here's a couple thoughts from experience,

    1. If you don't know where the problem is, you can't fix it
    2. Cheap PCB's are difficult to work on, and more damage is often done
    3. Trying to save money often costs more money that if you had it done right the first time.

    Your tech replaced the bridge recto, correct? Why can't he trace the circuit and find the source of the noise for you?
     
  16. pdf64

    pdf64 Member

    Messages:
    5,150
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    My understanding was that the rectifier overheating issue with these was primarily regarding the heater supply, BR1 in the DSL201 schematic above?
    I suspect this whole thing is a mess up of mis-understanding / mis-diagnosis / incompetence, and novice diving in there replacing parts is likely to make any eventual repair more expensive or even uneconomic.
    I suggest getting a second opinion from a competent tech.
     
  17. Tone Meister

    Tone Meister Member

    Messages:
    2,663
    Joined:
    May 1, 2010
    Location:
    NC
    Posts #15 and #16 offer solid advice that will save time and money in the long run. The PCBs in these amps are quite fragile. I agree with the suggestion to get a second opinion from a competent tech.
     
  18. SackvilleDan

    SackvilleDan Member

    Messages:
    2,007
    Joined:
    Nov 15, 2009
    Thanks for chiming in guys! I'm good with a soldering iron, but not a great amp tech. I don't understand the circuit, so I can't determine what is a filter cap vs a coupling cap, but I can handle the mechanics of pulling components and replacing. I've built a few pedals before and have done work on PCBs in the past. The reason I want to do it myself is because there's really only one way to learn. I would just rather try and figure this out on my own instead of paying the tech to do more work on the amp. That's why I was looking for advice on what components could likely be the culprit - I can buy a bunch of parts from DigiKey or Mouser and spend the time troubleshooting the amp as a hobby / learning experience. I am not into the amp for a lot of money, and simply don't want to spend more on repairs. Worst case scenario, if I screw the PCB up and somehow take out the transformer, I still have a good cab and speaker to build a nice 18W with, which will be another learning experience!

    He could likely do that, but I want to try and figure it out myself! When he replaced the bridge rectifier, he re-flowed a bunch of paths in the circuit near where stuff had started to overheat, and it doesn't seem to be that bad a PCB to work on, so I'm confident I can replace the components myself, provided I know which ones to change.

    Yes, the original issue with the amp was that the volume would cut and drop while playing. After this was happening for a while, I noticed that the clean gain pot would give off static during path of travel, but not otherwise. When the tech replaced the bridge rectifier, he also changed this pot. Now I don't have the initial volume drop anymore, but the pot still hisses during travel and I have a ton of white noise / hiss in the clean channel only (it's perfectly quiet on the OD side). THIS is what I'm trying to troubleshoot myself - eliminating the noise and hiss from the clean side of things.

    The tech suggested filter caps were a possibility but I did not ask him to trace the rest of the circuit out and source it for me. Thanks to Cirrus, I now know which caps are filter caps, and will order replacements. Then, above, coupling caps were suggested and pointed out to me (thanks XTian!) and I will order and replace those too, as needed.

    What I'm looking for now, are other opinions as to potential causes of the hiss I'm dealing with - op-amps have been mentioned but I'm not sure which ones!

    As an aside, the guy who replaced the bridge rectifier does good work, so I wouldn't say that he's an incompetent tech. For reference, here's a couple shots of one of his amp builds:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    Thanks again folks!
     
  19. pdf64

    pdf64 Member

    Messages:
    5,150
    Joined:
    Aug 9, 2008
    Location:
    Staffordshire, UK.
    It's a difficult amp to learn on, that schematic seems convoluted and unclear.
    Dr Dan's handiwork looks great, but building a nice classic amp and fault finding a channel switching PCB amp are different skills don't necessarily go hand in hand; whatever, a second pair of eyes is often helpful.
    Have you tried a different, known good tube in the pre-amp slots, especially V2?
    Is the level of the noise on the clean channel affected by the gain setting of the overdrive channel?
    Are you competent to take voltage readings on an open, live amp chassis?
     
  20. SackvilleDan

    SackvilleDan Member

    Messages:
    2,007
    Joined:
    Nov 15, 2009
    Thanks for the kind words.

    I was assured that the tubes that are in the amp were all tested, however I will pull a 12AX7 from another amp that's working well and try it in the DSL 201 just to confirm.

    I'm farily sure the level of noise on the clean channel is NOT affected by the gain on the OD channel, but I will double check this too.

    And I have a multimeter. I'm not sure about working on the live amp - I had planned on powering it down and discharging it, replacing groups of components and then firing it back up again. Taking readings of the pots would be the only thing I'd be doing while the amp is live, no?
     

Share This Page