Marshall Bass 12 - Repair Help

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  • barnsleyboybarnsleyboy Frets: 26
    ..... the saga continues :/

    Replaced the caps, sorted out the heatshrink to the transistor, but still exactly the same problem.Last call for anything else that I could easily replace, before I throw in the towel and take it to my friendly amp tech?

    I'll post an audio file of what it sounds like, which might help with a bit more diagnostics (need to work out how to do that!)
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  • barnsleyboybarnsleyboy Frets: 26
    Audio Files are here:


    Volume control swept from 0 to 10 a few times on each clip. As you can hear, the sound diminishes at mid point, then re-appears at 10. With the guitar plugged in the crackly interference sound only appears when the volume is at zero.

    Another thing to note is the big pop sound at the end of clip 1, which is when I switch the amp off - not sure if that is relevant in the overall scheme of things, but it is pretty loud!

    Once again, any assitance would be very much appreciated!
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  • ICBMICBM Frets: 56973
    Hard to say. The ‘null point’ behaviour means the same noise is getting into the circuit both before and after the control, so it cancels when the gain is at a particular point. This is either going to be a filtering issue (you’ve changed all the relevant caps) or a grounding issue, most likely... there’s not a lot else that does that. The interference when the control is at zero does suggest grounding.

    Check the continuity of the mains cable earth wire. It’s not likely, but just possible, that it’s broken inside. (If so it will be at the cable clamp
    on the chassis.)

    The pop when turning off is normal, but it could be worse if the earth is missing as well.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • barnsleyboybarnsleyboy Frets: 26
    ICBM said:
    The interference when the control is at zero does suggest grounding.

    Check the continuity of the mains cable earth wire. It’s not likely, but just possible, that it’s broken inside. (If so it will be at the cable clamp on the chassis.)


    Continuity of the mains cable earth wire is all ok. Could there be a dry solder joint somewhere on the board? is it worth me going over all the connections with my soldering iron? Or is it likely that there is an issue with the transformer?
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  • ICBMICBM Frets: 56973
    It's worth inspecting the whole board carefully, but I wouldn't just reheat everything - you can actually cause more problems than you solve like that. It's not a transformer issue, it's something to do with noise either being generated or not properly suppressed on the board.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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  • barnsleyboybarnsleyboy Frets: 26
    .... Clutching at straws here, could it be the volume pot itself that is faulty? Compared to the other pots it grates a bit as you turn it, whereas the others are fairly smooth in their sweep. Like I said earlier in the post, I was going to change all the pots anyway due to their pinched together splines. I think I'll get some new pots ordered, get my magnifying glass out and see if there is anything obvious, but this is looking more and more like a trip to the professionals!
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  • Danny1969Danny1969 Frets: 6870
    Have you got a meter ? if so check for DC on the pots and check for AC on the rectified side of the power supply. I wouldn't expect a small value cap failure on a low voltage board like this but it's worth checking
    www.2020studios.co.uk 
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  • barnsleyboybarnsleyboy Frets: 26
    Danny1969 said:
    Have you got a meter ? if so check for DC on the pots and check for AC on the rectified side of the power supply. I wouldn't expect a small value cap failure on a low voltage board like this but it's worth checking
    I've got a meter, but I'm not really sure how to do that!
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  • robinbowesrobinbowes Frets: 2420
    Can you get the op amp out easily? ie. is it socketed? If it is, and you pull it out, does the hum go away?
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  • barnsleyboybarnsleyboy Frets: 26
    Can you get the op amp out easily? ie. is it socketed? If it is, and you pull it out, does the hum go away?
    Op amp is soldered on to the board. I might do more harm than good trying to unsolder it.
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  • robinbowesrobinbowes Frets: 2420
    Try grounding pins 2+3 - is it quiet if you do that?
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  • barnsleyboybarnsleyboy Frets: 26
    Grounding 2+3 at the same time makes no difference. (I'm assuming the dot on the op amp indicates pin 1?)
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  • robinbowesrobinbowes Frets: 2420
    Try replacing R17 with a 1K resistor. You can simulate that without soldering by connecting a 1.5K resistor in parallel.
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  • (I'm assuming the dot on the op amp indicates pin 1?)
    Yes, pins are numbered anti-clockwise starting from the dot
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  • barnsleyboybarnsleyboy Frets: 26
     Are you suggesting that R17 might be faulty? R17 is reading 2.7K which is correct.

    Not sure if it is relevant, but there is a space for C11 next to R17, but there isn't a component installed
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  • robinbowesrobinbowes Frets: 2420
     Are you suggesting that R17 might be faulty? R17 is reading 2.7K which is correct.

    Not sure if it is relevant, but there is a space for C11 next to R17, but there isn't a component installed
    No, I'm suggesting you try setting it to 1K :)

    R.
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  • barnsleyboybarnsleyboy Frets: 26
    Right you are! I'll see if I can locate an old resistor to try. I was just curious  =)
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  • barnsleyboybarnsleyboy Frets: 26
    I know Maplin lost their way at the end, but at least you could pop down the shop and buy a couple of resistors or a bit of wire at short notice if you needed them!! 
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  • barnsleyboybarnsleyboy Frets: 26

    Before I order up a 1/1.5k resistor to test out @robinbowes R17 replacement, I thought it best to do a check on all the other resistors to see if I need to add anything else to the order. I checked them "in circuit" and pretty much all were in tolerance. Anything out of tolerance I have highlighted in red. R4 stands out as a potential problem. R7 & R8 has a pink 1st band, but are measuring 87k, so the pink could be white or grey. R21, 25 and 27 are solid grey and are stamped with their values, but are too small in value to read with my meter.

    Could the resistors be my problem? and in particular R4? Any advice as always gratefully accepted.


























































































































































































































































































































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  • ICBMICBM Frets: 56973
    Even without checking the schematic which I don't have with me just now, resistors being 'out of spec' in-circuit is almost always due to being effectively in parallel with other components. It's extremely rare for a typical carbon or metal-film resistor to measure below tolerance - they almost always drift high or fail outright. (I'm guessing that R1/2 and R4/5 are in positions where they're affecting each other, in fact.)

    Pots are usually quite wide-tolerance (typically 20%), and won't make any difference. The only one that doesn't look quite right is R22, but even then it's unlikely such a small discrepancy could be a problem.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

    "Just because I don't care, doesn't mean I don't understand." - Homer Simpson

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