Marshall Bass 12 - Repair Help

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  • Yep, point taken about the series/ parallelI issue. CSE electronics was a long time ago. I have a vague recollection of making a small radio and a circuit that made some lights flash. Guess I'm just clutching at straws and trying to put off the inevitable trip to get it repaired professionally! What do you think of @robinbowes R17 replacement idea?

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  • robinbowesrobinbowes Frets: 3021
    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!! 
    Have you got any spare resistors at all? 

    R.


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  • sadly not! I should really get a few typical values in to have knocking around for these types of circumstances.
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  • .... don't even have an old radiogram to rob some from!!!
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  • robinbowesrobinbowes Frets: 3021
    edited July 2021
    sadly not! I should really get a few typical values in to have knocking around for these types of circumstances.
    Yeah, definitely worth having a few standard values. Not expensive to buy. Something like this:

    https://www.bitsbox.co.uk/index.php?main_page=product_info&cPath=83_289&products_id=2002

    Or search on Amazon: https://www.amazon.co.uk/s?k=metal+film+resistor+kit

    R.
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  • I'll have one of those - it's in the basket right now! Probably a rookie question, carbon film or metal film does it matter?

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  • robinbowesrobinbowes Frets: 3021
    I'll have one of those - it's in the basket right now! Probably a rookie question, carbon film or metal film does it matter?

    It probably won't make much difference in most situations, but metal film are "better".

    R.
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  • They're on order. I feel some heavy duty tinkering coming on :)
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  • Try replacing R17 with a 1K resistor. You can simulate that without soldering by connecting a 1.5K resistor in parallel.
    Box of goodies arrived in the post today. 1.5k resistor in parallel with R17, no change. Same noises..... Grrrr!!
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  • robinbowesrobinbowes Frets: 3021
    Try replacing R17 with a 1K resistor. You can simulate that without soldering by connecting a 1.5K resistor in parallel.
    Box of goodies arrived in the post today. 1.5k resistor in parallel with R17, no change. Same noises..... Grrrr!!
    Bugger...
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  • barnsleyboybarnsleyboy Frets: 39
    edited July 2021
    I guess I'm going to have to face the inevitable and get a proffeshnial to fix it!
    Many thanks to you all for the assistance!! I bet it's something really simple like
    a loose wire or something!!!
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  • robinbowesrobinbowes Frets: 3021
    FYI, you might find this interesting: http://guitarder.fc2web.com/other2/o2-28-bass12.html
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  • A great find @robinbowes I have given it a quick skim through, but I'll have a proper read tonight. Sounds like exactly the same problem, and explains the old R17 trick!
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  • I've had a good read through the Japanese chappies post and where feasible I'm going to check through each step to try to eliminate the possible sources of the hum. I posted on the Marshall forum too and somebody mentioned reflowing solder joints for jacks, pots and anything that carries stress onto its PCB solder connections. I've done that but to no avail.

    The vibration of the transformer is an interesting thought. Comparing it to my recently acquired Lead 12, the Bass 12 one is significantly louder and you can feel the vibration through the chassis. Could a replacement transformer fix this? I'm tempted to try running the Bass 12 off the Lead 12 transformer to see if that is where the problem lies. The model numbers are not the same:

    Bass 12:


    Lead 12:

    I'm assuming the voltages would be correct though. I'm wondering if any components are picking up the vibration or EMF from the transformer?

     I've looked into repair costs and at labour costs of £59 per hour (min. charge 1 hour), I think I'll stick to trying to resolve this myself.
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  • .... Oh, and if anyone is interested, in episode 34: 

    After being beaten by the El of water 
    both Gills and Agito retreat. Shouichi becomes bedridden with injuries and Ryo must protect him even though both of them known they are no match for the deadly Unknown!!!
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  • robinbowesrobinbowes Frets: 3021
    Good luck!
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  • I may have fixed the shortwave interference type noise by seperating circuit and chassis ground paths! Big respect to Dave H at music-electronics-forum for suggesting "rectifier current pulses which could cause a ripple voltage between circuit ground and chassis ground". The hum is still there though, so in another desperate attempt to get to the bottom of my noise issue, I recalled Johan Segeborn doing a Bass 12/ Bass 20 shootout where he showed some pics of the circuitboard for each amp. I took some screenshots off youtube which admittedly are a bit on the fuzzy side, but I suspect that there may be some differences in resistor values between his Bass 12 and mine. Oh happy days, could this be the golden bullet? It then occured to me that there must be a few members out there that have Bass 12's that are working perfectly and don't suffer the hum issues that I have.

    Would any of you kind souls mind dropping the chassis out and taking a few decent hi res photos of the board so that I can make a decent comparison and check the resistor codes? I suspect that when uncle Jim was churning these out of the Milton Keynes noise factory in the 1980's that perhaps the QA procedures weren't quite up to scratch, and they may have used what they had in the component bins at the time!



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  • Should have mentioned mine is a 1988. I've found a 1992 version on Reverb that looks proper different to my one! R&D advancements maybe?
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  • ICBMICBM Frets: 71963
    It’s worth posting some pics. Unlikely anything is visible, but you never know.

    I think there are two different versions - hence the cap numbering anomaly earlier.

    "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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  • JMP220478JMP220478 Frets: 421
    Ive got a working Bass 12 - all be it the one with an 8" speaker - give me a few days - as Im away til Wednesday evening 
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