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Preamp power supply revamp

Discussion in 'd.i.y.' started by RichardH, May 8, 2018.

  1. RichardH

    RichardH Bodging pleb

    I alluded to this in another thread, but I thought I'd start a thread about it.

    I have a full width power supply that was originally built for me many years ago (probably around 1986 or so?) by JemHayward of this very parish.

    This PSU contains - in effect - two hicaps. Over the years I have tinkered with it as my own DIY attempts got more enthusiastic - I've had some ALW Superregs in there, and then a few years back I went the whole hog and split all the rails in my 32.5, and added a heap of Avondale TPR modules into the PSU. I also did the Mr Tibbs earth mod so ran all the earths into the preamp as well into the star point on the PCB.

    I ended up with 7 rails of power going to the preamp, arranged thus:

    PSU1 channel 1 - 2 regulators - gain L & gain R (321 boards)
    PSU1 channel 2 - 2 regulators - Time align L & Time align R (729 boards)

    PSU2 channel 1 - 1 regulator - phono L (323 board)
    PSU2 channel 2 - 1 regulator - phono R (323 board)
    PSU2 channel 2 - 1 regulator - 325 plus tape buffer

    For the 325 board, I ran a separate feed from the rectifier to a separate cap to feed the regulator board to try and isolate it from the phono supply a bit.

    I had one TPR module start playing up, so got in touch with Les to get a replacement ((I suspected the trimpot, but thought it would be just as well to build a fresh module) .... short story, I ended up with a full set of TPR2 PCBs, so I can now fettle the whole lot.

    Before I start rebuilding, I wonder if in fact I would be better off rearranging things a bit. I don't know how critical a mega supply to the time align boards is...

    Perhaps I could drop to "just" 6 regs, and powering the 325, tape buffer and 729 boards from PSU1 channel 2. - that would leave me with the gain boards having a channel to themselves, and the phono boards a channel each.

    Any thoughts from those who know better?
  2. Dowser

    Dowser Learning to bodge again..

    I don't profess to know better, but would assume keeping a separate supply for relay and tape buffers to be beneficial.

    But to add complexity you didn't ask for - are you running a single dual secondary transformer for each PSU? I found a surprisingly large benefit in my own DIY dual rail supply (for a NAC62) when I moved from a single transformer with dual secondaries feeding a bridge rectifier per channel, to 2 transformers in centre tapped mode and full wave rectification. Do you have space for extra transformers? :)
  3. RichardH

    RichardH Bodging pleb

    Yes, one transformer for each PSU, each with dual secondaries. So 3 separate supplies in effect.
  4. RichardH

    RichardH Bodging pleb

    Here's a quick snap of how it looked before this round of mods


    ...and a couple of shots of the terrible things I have done to my 32.5


    ... here's what I am doing with the TPR2s... the original plan was to just strap them down to the base of the case, but the hole spacing meant I would end up with having to drill holes and making a mess. So I was inspired by the heat spreaders that Les supplies with the full module - I bought some 6mm aluminium bar from eBay


    The countersunk holes are so I can bolt the transistors to the spreader and then bolt the whole thing to the case (so the ones in the pic are upside down). The end holes are at the right distance to drop straight over the holes in the tray so I can bolt the modules in pairs down on to the base of the case.
    Suffolk Tony and Mark65 like this.
  5. RichardH

    RichardH Bodging pleb

    Hum..... - well, no hum actually.
    Been fiddling around redoing the connectors on the capacitor rack - they were just stripboard pins so a little annoying to solder to. At the smoothed end I ahve put screw terminals in (as I so happened to have some in the parts bin), and I have put spade connectors on the other end to make things easier to connect/disconnect.

    The Avondale bridge rectifiers are in, and I've wired them up to the transformers and capacitor array.


    However, one of the transformers keep popping the fuse as soon as it is powered up - obviously it never used to do that. I've checked that there are no shorts between the two secondary windings, but apart from that, I'm at a loss really.....
  6. S_J

    S_J pfm Member

    A faulty rectifier board perhaps? Try swapping the boards over (left to right and vice versa) and see which fuse blows. Also power the transformers up when disconnected from all bridges, to check for transformer problems.
  7. RichardH

    RichardH Bodging pleb

    Have already tried powering the transformers without connecting to the bridges, and got the same issue. However, just tried dropping in a different rectifier (bog standard one) and that seems to work OK.... more investigation needed!
  8. RichardH

    RichardH Bodging pleb

    ...wierd - would appear that the diodes are knackered - they read short circuit in either direction - either that or they are the wrong pinout for the PCB. They had been in my bits box for years, so will try fresh diodes....
  9. laverda

    laverda pfm Member

    I'd replace the LED resistors while your in there with a higher wattage variants and install them well clear of the board. But you probable know or are doing that?
  10. RichardH

    RichardH Bodging pleb

    I was going to comment on that actually - the scorch marks were from the first iteration of the build, and when I did my first session of twiddling I did indeed raise the resistors off the surface -so the marks are from early days. I also replaced them with 1W versions this time as well.

    I toyed with the idea of chopping off that end of the board and reworking the layout, but in the end decided that as everything fits I would just leave it as it was, and left it at upping the resistor wattage.
  11. martin clark

    martin clark pinko bodger

    Richard, I can spare some Qspeed diodes for this - they’d drop right in being TO220 format. I’ll put some in that other parcel I need to send you...
  12. JemHayward

    JemHayward pfm Member

    Nice to see there is life in the old girl yet, though I think you've somewhat overtaken me on the PSU design expertise now!
    RichardH likes this.
  13. Arkless Electronics

    Arkless Electronics Trade: Arkless

    Beware when testing high power Schottky rectifiers that they measure as partially conductive in the "wrong" direction, which would lead to a conclusion of dud if they were conventional ones. Obviously if they measure short circuit they are dud!
  14. RichardH

    RichardH Bodging pleb

    Ha! I doubt it - I'm just an assembler :)

    Thanks.... I think I have worked out the problem - the diodes I put on those boards (goodness knows when) were the wrong pinout... they need to be a dual common cathode type (MBR20200) rather than MSR1560.
  15. RichardH

    RichardH Bodging pleb

    Thanks Martin, but I have some MBR20200 arriving from RS, so I should be sorted :)
  16. RichardH

    RichardH Bodging pleb

    And we're back in the room! All sorted with a diode swap.

    Getting a nice 36V from each channel.

    Now got to start fitting the TPR2 modules into the case without releasing any magic smoke....
  17. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    You could easily convert that cap bank into a CRCRC by unsoldering a few caps and adding some resistors.

    I find it really quietens down the supply.

  18. RichardH

    RichardH Bodging pleb

    Hmmm - you intrigue me..... What spec on the resistors (wattage, ohmage)? I assume I would just split the banks into three and add two resistors per bank?

    In the meantime we have the modules mounted to their heat spreaders:


    The resistor between the 0V and +out is to load and stabilise the output (as per Les's instructions!).
  19. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    The value depends on the headroom between your supply, output voltage and the current drawn.
    A few ohms would be a good place to start say 10 - 47 R wirewound 3w and check you have enough voltage for the regulators to work.

  20. RichardH

    RichardH Bodging pleb

    Well I am getting 36V from the supply, so I have plenty of "drop" room - Les recommends no more than 7V difference between input and output voltage, so I could do with losing a bit anyway!
    I have also realised I can add the resistors without losing capacitors, as I can just break in to the rail running along the +ve terminals and drop the resistors in.

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