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Thread: Polytone Mini Brute II (2) Needs Repair

  1. #1
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    Polytone Mini Brute II (2) Needs Repair

    Picked up a Mini Brute II in sad shape to restore and I could use some guidance.

    First off, I need a schematic, still searching and hope to nail one down.

    After some visual inspection it looked okay to bring up on a variac and bulb limiter. As I dialed up the voltage (less than half way) the bulb lit up bright.
    I am surprised the two fuses are not blown. I better check the values of the fuses, although I do not know what values are supposed be be there.

    Right now I have the amp section out of the cab and I wanted to check the health of the MJ802/MJ4502 transistors. Can I test these in-circuit? I tried some continuity and diode readings and got mixed results. I need some clear directions on how to test these (I have not done this enough and need to ask).

    Some observations. Some bent pot shafts. A loose pot. Reverb ground connections on preamp board not good. 10 meg continuity from preamp board ground points to amp chassis, not sure this is an issue but was expecting good continuity.

    Thank you, MarkO

    Some pics attached.

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  2. #2
    Stray Cap DrGonz78's Avatar
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    A picture of the underside board of the power board would be a good idea too. Here are a couple of schematic that may or may not relate to what you have there. What we need to figure out is if your power amp board has the LM39IN chip or if it is like this one that I am posting. Hopefully the preamp schematic I posted might help too, we'll see.
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    I am seeing the 2N5210 pair. Is that in place of the LM39IN?

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    Don't forget the joker g1's Avatar
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    No. The other side of the board the pots are connected to in your first post.

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    Stray Cap DrGonz78's Avatar
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    I think that is the correct schematic or at least close. The Polytone amps that I have worked on had the LM39 chip on the power amp board. It says 378c for the preamp board, the one G1 is referring to. If you're seeing those 2N5210 pair then I think it is the correct schematic and it won't have the LM39 chip on that version.

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    Does the bulb limiter light up with no speakers connected?
    Also, is there any significant DC on the output?

    Some amps can be misleading when using a bulb limiter (some Sessionettes, Trace Elliot etc..) - they swing over hard to one supply rail. I don't know if the Polytone is one of these.

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    Last edited by Mick Bailey; 08-18-2019 at 09:44 AM.

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    Stray Cap DrGonz78's Avatar
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    Yes do check for dc voltage on the output and personally if there is dc on it then I would not connect a speaker load yet.

    C2 looks to be in that picture you took 10uf 35v but on your amp it is the black cap 10uf 50v. Also C6 is 50uf 35v but in your picture it is a 47uf 50v. Pretty sure someone was in there and replaced those caps. Those two caps trace back to the two 2N5210 transistors which makes me think you have the correct power amp schematic. Which by the sound of things is where most of the repair is going to occur. You're going to have to test Q3 & Q4 as well as any and all diodes resistors that are around the output transistors on the schematic. The U07 and U57 are obsolete but there are other transistors that can be subbed out for those if need be.

    I am not sure but I think G1 wanted a picture of the preamp too. Perhaps to match it up with the preamp schematic.

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  8. #8
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    My preamp board says "578 C". Let me know if you still want me to remove it for a picture.

    I had tested with the speaker connected. Will disconnect speaker and look for DC on the output. Should I have a dummy load on it?

    Thank you

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    Old Timer J M Fahey's Avatar
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    In due order:

    I am seeing the 2N5210 pair. Is that in place of the LM39IN?
    }Yes.
    You are lucky, this one is more repairable.
    The LM391 one requires one of them, of course, and are as rare as hen´s teeth.
    If you're seeing those 2N5210 pair then I think it is the correct schematic and it won't have the LM39 chip on that version.
    Yes.

    Does the bulb limiter light up with no speakers connected?
    Also, is there any significant DC on the output?
    That is a problem and usually means shorted power transistors and/or their drivers and or bridge rectifiers and/or main filter caps, in that order.
    One or more of the above, specially in an amp as abused as this one.
    Some amps can be misleading when using a bulb limiter (some Sessionettes, Trace Elliot etc..) - they swing over hard to one supply rail. I don't know if the Polytone is one of these.
    Many amps do that if turned on with bulb limiter and a speaker or any load attached; I have never seen one do that unloaded.
    Not that it´s impossible, but so far ......

    NOTEne chassis end, the right bottom one, is cracked and practically fully split off.
    The amp is "usable having the chassis held by the other 3 bottom screws, just be careful with handling, but eventually you´ll need to find some guy with an TIG or MIG machine and the proper wire to weld Aluminum.

    Plan B (although Plan 1 is strongly recommended)

    cut and bend a piece of sheet Aluminum, 2mm thick, same size and shape as the compromised chassis face, apply on the "upper side", the one opposite what we see in the picture so chassis can seat "as before", "stitch" both with Pop rivets so you have rebuilt the compromised side.

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    Sadly the crack is nearest the transformer, which is the heaviest component there.

    Plan C: get an L shaped Aluminum extrusion, say 1" by as much as possible, at least 2",3" would be better (or whatever height the chassis has) and generously stitch it to the side of the cracked chassis.
    Ugly looking but easier to home make, just requires a hand saw, a drill and a pop riveter.

    Side note: you´ll have to replace all rusty parts such as jacks, switches, etc. they are beyond mere cleaning, pots and slide switches are highly suspect too (they depend on clean mating surfaces to work), include bias pot.
    How´s the speaker?
    I fear it being a rusted hulk too.

    Typical baked paint used by CTS and then Eminence is good as an initial air-to-metal blocker, but once a pore lets oxygen in, plus ambient humidity, rust starts "walking" under it.

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    Juan Manuel Fahey

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    The 15" speaker is good.

    So. This time I tired the amp with no load (no speaker). The bulb limiter did not light up all the way to full AC. Current draw was negligible. I have 43 mVDC on the output.

    Safe to hookup the speaker and try the amp?

    Thank you. MarkO

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  11. #11
    Old Timer J M Fahey's Avatar
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    Yes.

    1) turn amp ON, with bulb, without speaker

    2) recheck you have acceptable offset (43 mV is fine)

    3) without turning amp OFF, connect speaker

    4) if everything fine, apply some signal,play music but at low power, say 1 - 2 Watt tops.

    If fine, repeat but plugging straight into mains.

    Add: rising volume to rehearsal levels, then up to clipping , just forn testing purposes.

    If nothing weird/wrong, pull a can from the fridge and enjoy.

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    Thank you all and Juan. She is working! Got through all the steps!

    I am surprised all the pots are working. Got a lot of cleanup to do on those pots, fix the crack on the PA chassis, check/fix the reverb cables. Etc. Will report back as I make progress.

    MarkO

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    Old Timer J M Fahey's Avatar
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    Cool

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