Need help with buying the right pots for '83 jcm 800 (john phillips?)

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by yoyohomieg5432, Jan 21, 2008.


  1. yoyohomieg5432

    yoyohomieg5432 Member

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    im going to replace all the pots on my '83 jcm 800.

    im looking here: http://store.metroamp.com/index.php?cPath=22_33_66

    im not sure which ones to buy.

    im pretty sure that my knobs are set screw, there is a screw on the knob on the outside. somebody in the previous thread said that the pots in the link are only for push-on knobs.

    i'm soo confused:jo
     
  2. AdmiralB

    AdmiralB Silver Supporting Member

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    If your amp is an '83, it doesn't have PCB-mounted pots. Any decent solder-lug pot with a solid shaft and a 3/8" bushing will do the job.

    '84 and later Marshalls use the pots in the link, although the 800 series amps did use solid-shaft pieces that (apparently) are no longer made.
     
  3. yoyohomieg5432

    yoyohomieg5432 Member

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    so i can use one of the ones in the link above? which ones am isupposed to buy? i dont know what the differnce is between a 22k and a 4.7 and which ones my amp uses
     
  4. John Phillips

    John Phillips Member

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    First, stop. You do not need to do that.

    Unless all of them have been snapped off (it can happen!) the only thing you're likely to need to do is clean them. Even if one or two of them don't respond to cleaning (the gain and volume most often, because they get the most use) you just need to change those, not all of them. Pots do not just 'decay' with age.

    Get some contact cleaner/lubricant - the lubricant part is important, don't use plain cleaner, DeOxit, WD40 or anything else - and spray it into the pots through the little slot in the casing. Rotate the pot fully a few times and that should be it.

    With respect, if you're still at that stage of learning about electronics, changing the pots yourself wouldn't be a good idea anyway. Knowing what you're doing and a high standard of workmanship are essential for this sort of job or you'll just make things worse.

    I've seen this sort of thing so many times, unfortunately - usually perfectly good components that just needed a little maintenance taken out and lower-quality ones fitted badly instead.

    BTW, have the filter caps been replaced (including the bias caps)? If not, that's something you should look into, since they do decay with age. It is also not a job for a beginner though.

    No offense intended.
     
  5. yoyohomieg5432

    yoyohomieg5432 Member

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    i took it apart and went thru all of the pots with a product similar to deoxit and rotated them. this didnt help at all.

    i dont think the caps have been changed.
     
  6. John Phillips

    John Phillips Member

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    Try proper contact cleaner lubricant (Servisol or Electrolube) instead. Not all stuff sold for 'cleaning electronics' is the same... :)

    You probably should think about it. At 25 years old, it's getting to the point where they will need doing sooner or later, and if you're going to take it apart to change the pots, it's easiest to replace the cap can under the board at the same time - you have to remove the pots to do that anyway.

    Start with cleaning the pots again though, for some reason the ones Marshall used around this time seem to get quite scratchy and often need more than one go to clean them up fully.
     
  7. yoyohomieg5432

    yoyohomieg5432 Member

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    im going to open up the amp again tonight and try cleaning it out again. could you give me a link to the caps i should buy if the cleaning doesn't work? i did a little bit of reading, many people seem to like F&T caps, but there is so many different numbers and product names im not sure which ones to be looking at.

    i should also add, filter caps are the bigger blue things that stick out of the amp like tubes right? if so, i think they have already been changed on my amp, stock filter caps are blue, the ones in my amp are black.

    i can take some pictures of the guts of the amp if it would help at all
     
  8. yoyohomieg5432

    yoyohomieg5432 Member

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    ok guys, i got some pics. i tried cleaning it more with my electronics cleaner and it didnt help it all. it still makes a really cloudy sound every time i turn the presence knob. on the high input channel, there is also a significant drop in volume sometimes. like it will be loud, and i i turn one of the knobs the volume just cuts dramatically for no apparent reason. the low sensitiity channel, it sounds great except for the fuzziness of the presence knob.

    here's the guts
    [​IMG]

    this is the knob that is making all the trouble. the presence knob.
    [​IMG]

    here is the black capacitors(i think?) i was talking about
    [​IMG]
     
  9. Groovey Records

    Groovey Records Member

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    good luck but I think every one has shared their best advice with you

    no offense yoyo just having some fun at your expense

    a bump gets you to the top again.

    What are you using to clean the contacts?
     
  10. Groovey Records

    Groovey Records Member

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    no offence yoyo but i think every one has offered their best advice- just not what you wanted to hear.

    I apologise if I offended you G
    I was just having fun at your expense

    So what contact cleaner did you use? using the good stuff is step one

    Abump gets you to the top
     
  11. yoyohomieg5432

    yoyohomieg5432 Member

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    its a radio shack brand of contact cleaner. it says it's meant for cleaning PCB's etc.

    im probably just going to take it to a tech. my uncle said it might haev already had a mod done to it where the high input channel runs thru the low input channel, which might be the reason why the high sounds so bad.

    i forget what he called the mod but it was something like i said above. the low input channel sounds very nice, i may just use that.:messedup
     
  12. phsyconoodler

    phsyconoodler Member

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    You haven't really said what it is about the amp that you aren't liking.I notice that there is no mods around the inputs from your pictures.The bright cap is still there,as well as the treble peaking R/C couple.
    If you can describe what is is you don't like,we can point you in the right direction.
    Your amp is one of the best marshall's there are out there.It likely will just need a good tune-up and you will be ready to rock.
     
  13. Groovey Records

    Groovey Records Member

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    +1 beautiful amp

    Where do you live maybe someone here can reccomend the right man for the job to bring it back to spec.

    Spend a little green to make as good or better then new
    Think Long Term Investment

    Regards
    Groovey Records
     
  14. yoyohomieg5432

    yoyohomieg5432 Member

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    the low input channel is very bassy, powerful and punchy. it was knocking stuff off the walls in my basement. this is how the amp is supposed to be. the high input channel doesnt have any of that powerful punch to it. it sounds very thin/trebly, the little bit of distortion is very weak. i get a very ballsy blues tone with the low input channel up to lighter gain rock music. the high sensitivity just seems like a very sterile clean sound.
     
  15. Lonely Raven

    Lonely Raven Member

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    Just take it to a reputable tech and have him service it. The amp looks great! You don't want to go replacing parts yourself and wind up taking it to a real tech anyways to not only fix the problem, but fix what you did to it as well.
     

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