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  1. #1

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    Yes, I know there are many more modern solid state amps out there. I like my Polytone MB II and I want to fix it. I have built a number of amps from kits, so I feel like I can make the repair if I can identify the culprit. The issue is that I have little to no experience diagnosing amp problems. If anyone here has experience fixing amps and could shed some light on the problem I describe below, I'd be very grateful! I know at least a few of you have some real experience working on Polytone amps. I'm looking at you Greentone .

    First off, the amp is a Mini-brute II of the diamond tolex variety from the 80's. After not using the amp for some months, I plugged into it a few days ago and here's what happened. I turned the amp on, turned amp volume up to about 4, and got no sound initially. The amp was extremely quiet, but clearly on: as, for example, the pilot light was on. I strummed a bit, but got no sound. I tried turning the amp on and off once or twice to no avail (This shows the extent of my diagnostic skills. Hey, it does seem to be a magic trick to fix computers). With the amp volume at 4, I strummed a little bit more and eventually (after a couple minutes) heard a loud, unpleasant sound that is hard to describe precisely, then the amp seemed to work just fine for the remainder of the time that I practiced.

    The next day when I went to try the amp again, I had to go through a similar scenario. No sound at first, tried turning on and off. I tried to be a little more careful with the volume this time (keep it at 2 to 3 on the amp), so as to avoid the unpleasant sound. The one difference this time though was that I thought I may have heard some of the strumming I was doing coming ever so faintly through the amp before it started working properly: what I thought I heard was very quiet and possibly sounded a bit distorted. Then after a few minutes with the amp on, my plucking on the guitar started coming through the amp as it should. I may have detected some slight fuzz/distortion sound while the notes were decaying away, but I'd say the amp sounded very close to completely normal for the rest of the time until I shut it off.

    Does anyone have any thoughts on what could be causing this delay in the amp coming to life and amplifying my signal?

    I'd be happy to run any tests that anyone feels are necessary to help narrow down possibilities. Thanks again!!!

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  3. #2

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    Loose/corroded conection somewhere. It will take some troubleshooting to find the exact connection.

  4. #3

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    This can sometimes happen with op amps, perhaps transistors as well, truthfully I'm not really sure as most of my experience is with tube equipment. However, I've run into this many times. For instance in a mixing console, you'll get no sound until you jack up the fader and it comes back to life. usually replacing the op amp fixed the issue.


    I think there are a couple different power amp designs for that model. It would at least help to narrow it down to if the problem is coming from the preamp or the power section. In the back there should be a preamp out and perhaps a power amp in. Plug in your guitar and see if you get anything coming out of the preamp out, that will give you a good idea of where the problem lies.


    PS, if someone here recognizes the above symptoms, it would be appreciated if you reminded me of what it is. Something to do with the bias if I recall correctly.

  5. #4

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    When I bought my Polytone, it was listed as not working, for parts only. If I remember correctly I paid about $80 for it, thinking I might be able to get it to work.

    When I got it and plugged in, I heard a very faint sound of the guitar. I unplugged and sprayed contact cleaner in the pots.

    >presto<

    It has worked flawlessly since. I had an amp tech check it out, he said it’s in great shape, never been altered/touched, bring it back in another 50 years for a checkup. I’ve gigged with it, it’s awesome.

    Try contact cleaner.

    Good luck!

    -Jon

  6. #5

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    Quote Originally Posted by jonh
    When I bought my Polytone, it was listed as not working, for parts only. If I remember correctly I paid about $80 for it, thinking I might be able to get it to work.

    When I got it and plugged in, I heard a very faint sound of the guitar. I unplugged and sprayed contact cleaner in the pots.

    >presto<

    It has worked flawlessly since. I had an amp tech check it out, he said it’s in great shape, never been altered/touched, bring it back in another 50 years for a checkup. I’ve gigged with it, it’s awesome.

    Try contact cleaner.

    Good luck!

    -Jon
    I've had similar results. Try contact cleaner.

  7. #6

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    With the amp unplugged run all the pots up and down full travel. Turn it on and see if it helped.

  8. #7

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    Try contact cleaner on the pots. Cramolin is a great product. I suspect two other things: the power supply capacitors and the input jack.

    After having been dormant for so long, the power caps need to be slowly reformed. This is done with a Variac to bring the voltage up slowly. Your input jack may have oxidised from lying dormant for so long.

    If you are getting sound your amp works. The power caps may be borked.

  9. #8

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    Whenever encountering this type of a problem, it's a good idea to service the amplifier.
    It's fun and easy to do it yourself, and you sound like you are up to the task...
    MB 11's are (to my ears) the best sounding Polytones, just instant classic jazz tone.

    1st thing to do is get this book.... Electric Guitar Amplifier Handbook by Jack Darr...
    It is the BIBLE for guitar amp troubleshooting, servicing and repair. Do no accept imitations.

    Get it from Antique Electronics Supply and keep the extra $5 Jeff B takes to buy it from
    you know where...

    Become a disciple of Darr.. Then whenever you encounter a problem, ask yourself..
    "What would Jack do?"

    Also check out "the Truth about Vintage Amps" podcast, from Jason at Fretboard Journal
    and legendary amp rat Skip Simmons..insane amounts of great technical knowledge
    but more fun than anything,

    Please help diagnose my Polytone's problem-jackdarr-jpg

  10. #9

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    Thanks a ton, everyone! I cleaned the pots earlier today and it seems to have done the trick. The amp started up just as it should. I'm going to test it out again tomorrow, but I'm optimistic it'll work just fine again. I appreciate all of you offering your time and knowledge. I'm glad it was such an easy fix. I definitely plan to get that book by Jack Darr too, Papawooly.

    I did have one more question that arose when cleaning the pots. I used Deoxit D5 that I bought a while back, but after doing a little research, it looks like maybe I should have used their F5 formula. Anyone have opinions on this? I read that the D5 is more harsh and can remove some grease in the pots that you want there. I thought I looked into what variety of Deoxit to get when I purchased it, but maybe I made a mistake.

  11. #10

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    Congrats Greg for the gorgeous Polytone and solving its (first) mystery!

    The book seems to be online too. At least You can check it before buying.

    https://www.trinityamps.com/ForumGal...r_Handbook.pdf

  12. #11

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    Deoxit Faderlube is your new best friend, not F5. D5 is a corrosion remover, and pretty much relieves the pot of any “grease” the manufacturer stuck in there. Well known audio pot trick is D5 followed by Faderlube which will reintroduce slippery stuff into the pot without you having to disassemble it. Lube (or grease or whatever) would have been put in three places... the shaft bushing for a nice smooth turn, the wiper on the resistance track to reduce wear, and on the back of the shaft to lube it where it’s held in place by the back bushing or back plate.

    Old tech joke... cheapest pot becomes more expensive with thicker lube.

    The Faderlube will work with carbon or plastic based pots, helps both won’t ruin them. And definitely lubes the shaft/bushings.

    F5 is an odd ball. It’s marketed as good for pots but if you dig into the tech sheets it’s kind of a Jack of all trades specifically for plastics. Deoxit recommends it for the touch pad on laptops which I dunno just doesn’t seem like a product designed for the shaft/bushing needs in a pot.

    if you want to read some amazingly big fights about lubing pots, head over to audiokarma.com. They make our political threads look like calm quiet sunny afternoons)))

    good kuck
    d
    Last edited by jazzkritter; 01-24-2021 at 03:39 PM. Reason: Yucky grammer

  13. #12

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    Hi, jazzkritter. Thanks for the advice. Can you share a link for the right stuff to get? I did a quick search and some of the Deoxit product out there actually says Faderlube and F5 on the same label. They also have some labeled as Deoxit Fader, Deoxit Fader F5, etc. So it's somewhat confusing. I just want to make sure I order the right stuff. Thanks again!

  14. #13

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    Hi Gregmath
    Showing my age, there used to one product called faderlube but yea I see CAIG has gotten fancy. This the guy I was referring to: DeoxIT Fader, #F100S-L2.
    At Amazon as:
    https://www.amazon.com/dp/B0002EHP66/ref=cm_sw_em_r_mt_dp_phSdGb4T1VXX
    Or at parts-express as https://www.parts-express.com/CAIG-F....-57-g-341-254

    FaderF5 is a combination of the F5 cleaner and the Fader lube, an all in one thing.
    According to the the CAIG Material Safety Data Sheets (MSDS) its interesting as D5 contains 20% cleaner whereas both F5 and Fader are 1% cleaner. D5 shouldn’t have damaged anything, just needs the lube follow up. The product info is all at CAIG.com.

    By the way, I have a minibrute IV. Given the quality of the components (and the age) the pots are likely pretty robust compared to the lightweight stuff they make now. So after the lube you should be in good shape. Great amp!