Polytone Mini-Brute "Project"

Discussion in 'Shock Brother's DIY Amps' started by Engine Swap, Feb 16, 2020.

  1. Engine Swap

    Engine Swap Tele-Meister

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    So, I picked up this pile of Polytone for $25.

    Looks like the pre-amp is a 1978 Mini-Brute II and the power section is possibly from a Polytone 103.

    Some pots are missing, the wiring harness is hacked up, and at least 1 chip is missing from the preamp section. Should be fun....

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  2. nojazzhere

    nojazzhere Doctor of Teleocity

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    I see you like a challenge.....good luck, and keep us updated.
     
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  3. Engine Swap

    Engine Swap Tele-Meister

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    OK - did an initial assessment of what I have here. The seller didn't know the history - basically it was a box of parts that was included in an auction lot.

    Looks the the preamp is from 1978. It's the simple earlier version w/o the dreaded distortion circuit. The pots are missing or wrecked and the unobtainium Ratheon RC4739 has been poached.

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    pre-amp schematic

    Wiring harnesses have been chopped up as well. Color codes don't match between preamp and power.

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    The power amp, interestingly, is not from a Mini-Brute. It appears to be from a Polytone 103. Lots of missing hardware and the wiring is a mess.

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    Power amp schematic

    Going to start with the power supplies to see if the power TX is any good...
     
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  4. Engine Swap

    Engine Swap Tele-Meister

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    Got the wiring harness all reconnected. The color codes didn't match, so I had to buzz it out. PITA.

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    Had to reconnect all the wiring to the filter caps in the power amp and find screws to secure the PCB. Really had to dig deep in the parts bin to come up with all the necessary parts.

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    Before powering up, I removed the power transistors, so I could test them. They're good. Check out the 1975 date code.

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    Brought everything up slowly on the variac. The +/- 17 VDC for the preamp and +/- 30 for the power amp both look good. Quite a relief, since The power TX was an unknown.

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    Gonna put in a order to replace all the electrolytics, since the TX is good. Stay tuned...
     
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  5. Engine Swap

    Engine Swap Tele-Meister

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    While I'm waiting for the electrolytics to arrive, I've turned my attention to the missing Raytheon RC4739 chip. I was poached by a previous owner.

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    It's a dual opamp with a weird pinout. On top of that, it's long out of production and expensive if you can find it.

    Rather than deal with that, I've decided to remap the socket so I can use standard 8-pin opamps. This will let me try different chip flavors

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    I put the jumpers under the socket so they won't be visible.

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    Tah-dah! I had to cut a trace from an un-used pin to ground and add a jumper, but otherwise, no biggie.

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    Next, I'll put in the missing tone pots and see if I can get the preamp working.
     
    ocduff likes this.
  6. televillian

    televillian Tele-Holic

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    I for one, am impressed
     
  7. tfarny

    tfarny Friend of Leo's

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    Wow! I was recently given a Polytone from the 90s that was likely on its way to the dump. All I had to do was about 5 minutes of contact cleaner spray, and some new knobs in the mail. Yikes!
     
  8. Engine Swap

    Engine Swap Tele-Meister

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    Had a smidge of time over the weekend, so I worked on replacing the tone pots.

    Polytone uses PCB mount pots in the preamp. Both tone pots were off the board, loose in a bag of parts. One was trashed, so I decided to just replace them.

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    Since the tone circuit is a Baxandall (center = flat, boost/cut), I'm using 100K linear pots. Going with flying leads, since it's difficult to find qualtiy PCB pots.

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    Got the pots in - the preamp is now working! Here's a shot of the input vs. output.

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    Next, I'll move on to the Power amp. Not sure what I'll find there...
     
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  9. Andy B

    Andy B Friend of Leo's Silver Supporter

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    Nice old scope. I have a slightly different version a 30MHz mdl. 1474.
     
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  10. LudwigvonBirk

    LudwigvonBirk Tele-Holic

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    [​IMG]

    Nice - on this amp this is what you want to see, clean and round. Good work!
     
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  11. Engine Swap

    Engine Swap Tele-Meister

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    Been busy, but had some time over the weekend to work on this.

    I decided to tackle the power amp, since I have the preamp working.

    Everything is an unknown with this amp, so I removed the power transistors to check them out of circuit. They both tested fine. I'll leave them out so I can check the drive signals going to the bases.

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    I double checked all the wiring and tested all driver transistors in the circuit. They all checked out fine. The +/- 30V rails are in spec.

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    One interesting thing I noticed was each drive transistor has a diode mounted on top of it of covered in thermal compound. This is an interesting self-biasing scheme for the drive transistor.

    Basically, the diode has the same thermal co-efficient as the transistor, which regulates the bias as the temperature changes to keep things stable.

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    The moment of truth: I ran a test signal into the power amp input.

    Here the negative drive signal:

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    Here's the positive drive signal:


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    And here's both drive signals overlaid - crossover distortion is minimal.


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    At this point things are looking good. Next I'll put the power transistors back in and see if I can get some sound out of this thing.
     
  12. Engine Swap

    Engine Swap Tele-Meister

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    Things have been a little crazy lately....


    I had ordered filter caps and bi-polars so I replaced all the electrolytics.

    Here's the preamp board. The big caps are for the power rails, the green ones are bi-polar coupling caps.

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    The power amp has 2 honking big 4000uF electrolyitcs for the 30V rails. Yes, the xformer is a bit crusty...

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    The choices are a bit limited for 4000uF caps, but I found some that I was able to squeeze in. I also replaced 4 smaller electrolytics on the power amp.

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    Here's a group shot of all the old electrolytics - not at bad as some sand amps.

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    The amp is sounding great, and very quiet. I'll try to get some soundclips soon.
     
  13. Engine Swap

    Engine Swap Tele-Meister

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    Hooked up to a 12" Eden Cab for a quick sound clip. Definitely has the classic Polytone sound. Plugged into the "lo" input, controls set flat. Need to find a cab for it.

    [​IMG]


     
    Jlwctn likes this.
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