power supply (Roland Juno 60 synth)

Discussion in 'The Projects Forum' started by jahsquare, Jan 17, 2013.

  1. jahsquare

    Thread Starter New Member

    Apr 21, 2008
    Hi all,
    I'm troubleshooting a power supply issue in my Roland Juno 60 synthesizer. Here's the circuit:

    The +5/+10V rails are fine, but something funny is going on with the +/-15V rails. When everything was assembled in-circuit, I was getting about -15.6V on the negative rail but something like -0.7 on the positive (testing at points 9 and 10). I disconnected the board and tested: The AC inputs are hot - about 19.4Vac at points 5 and 6 from the transformer. I'm getting +/-25Vdc on the +/- outputs of D3.

    I thought TR1 might be the issue so I changed it out (changed out TR2 as well but had the same result after) and still had an issue. I pulled TR1 again and tested it on a breadboard apart from the circuit and now I'm getting +/-25Vdc at points 9 and 10....

    I'm beginning to think IC2 is the problem (and I have a replacement) but I don't want to pull it if I don't have to. Admittedly I'm not an expert and may be misunderstanding, but does anyone have an idea where I might be slipping up?
  2. jahsquare

    Thread Starter New Member

    Apr 21, 2008
    I'm just not sure if I'm testing properly in-circuit.. As far as I can tell all the passive components are fine - no shorts and no visible leaks. The BE/BC resistances across the power transistors is high and the BE voltage when energized looks normal (~1V). The regulator chip is a mystery - all I can find online is a pinout image partially in chinese, no full datasheet...

    Should I be concerned that the voltage in/out of D3 is a bit high?
  3. jahsquare

    Thread Starter New Member

    Apr 21, 2008
    Scratch that - the BE/BC voltages for TR2 are ~0.15 (tried on 3 different transistors) vs. 1.2V for TR1.

    I don't know what could be causing the problem. Could the bypass capacitors be an issue? They measure as open circuits.
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    Most importantly you should remove any load from the +/- 15v supply rails, and THEN test the voltages.

    The entire PSU is linear and does not need a load, so if there is a plug, just disconnect the PSU from the rest of the synth and measure the output voltages.
  5. vrainom

    Active Member

    Sep 8, 2011
    What he ^ said.

    Tr1 and Tr2 are pass transistors to increase the current output.
    Here's a bit more info on the chip.