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Townshend Merlin Help

Discussion in 'd.i.y.' started by colasblue, Jun 8, 2019.

  1. colasblue

    colasblue pfm Member

    A friend of mine has one of these which has conked out.

    I'm having a look at it and conceptually it looks quite similar to a valhalla but clearly isn't the same.

    The obvious fault was that a large wirewound resistor had cooked itself off the board and disconnected from the track to the next component.

    Easy fix I hoped but unfortunately not so much. I remade the connection and the resistor heated up nicely but no joy with the unit as a whole.

    It looks like it's trying to work since about 80V ac appears at the output if it's switched to 33 or 45 and goes to zero if in the "off" position but it doesn't seem to have enough grunt to turn the motor. I get a bit of vibration on either setting but no rotation, and giving it a push doesn't help.

    Also a 100V dc offset appears at the output on power up and then drops away quite quickly. On that basis I'm guessing some of its caps might be shot.

    Anybody got a schematic or know anything helpful about them?

    It has very few active components so if all else fails it's worth having a go at replacing all of them, but on the basis of what it's currently doing I think the digital side might well be working properly so it's likely a fault with the output section. I've already tried swapipng out the output transistors to no avail, mostly because I had some suitable ones in my bit box.
  2. davidsrsb

    davidsrsb pfm Member

    Photos would help
    Does this unit have two outputs or just one to a phase shift network? Measuring both winding voltages and the voltage between two outputs allows calculation of the phase between them, should be about 90 degrees
  3. colasblue

    colasblue pfm Member

    Single phase.

    The motor phasing is done outside the unit by using the tt 's normal 110V supply option.

    I'll try to sort out pics just as soon as I've bought some batteries for the camera!
  4. colasblue

    colasblue pfm Member

    Right, got pics

    The main problem seems be with the two big wirewound resistors.

    I lifted the smaller one off the board about 10 years ago but the bigger one has now cooked the track off the PCB and disconnected itself.


    I've repaired it (I think) but I'd really like confirmation that the two wirewound resistors actually aren't supposed to connect together at their closest point. I'm pretty sure they're not but if anybody has a Merlin they can look at confirmation would be good.


    The other issue (which I'm hoping is a non issue) is that the m706b1 chip seems to be long obsolete so is there a drop in replacement if I need one?


    PS can people tell me if the pics and links work properly since they've just changed photobucket and the new site seems to me to be an even bigger PoS than its predecessor
  5. davidsrsb

    davidsrsb pfm Member

    Is that an electrolytic next to the burnt resistor? There is no way it will be healthy after being cooked.
  6. colasblue

    colasblue pfm Member

    indeed it is and the output couples through it.

    I've ordered a complete set of replacement lytics since the thing is getting on for 35 years old but those two axial 4.7uF caps are definite suspects.

    The issue with that theory is that it was working OK until very recently so I'm not confident that there isn't something else a bit more fundamental amiss. It would be a strange coincidence if a gradual deterioration has suddenly become the secondary failure mode.
  7. colasblue

    colasblue pfm Member

    Woohoo it's working now. Not bad for a bit of guesswork without a schematic! Pretty sure one or both of the 4.7uF caps was the second issue.
  8. JimmyB

    JimmyB pfm Member

    Can you heatsink those big resistors at all or heat-shield the caps? Nice repair, I like the touch of snaking the wire to match the track!
  9. colasblue

    colasblue pfm Member

    The "snaking" is actually practical rather than artistic since it ensures the wire's insulation doesn't come into contact with the track running across the bottom of the board in the same direction which terminates hot running wirewounds. Hence meltdown related shortcircuit issue avoided at outset!

    I think lifting that resistor off the board helps a bit as heat rises and it's now above the new cap. Obviously I've replaced the one that had "shed its skin" and with a slightly higher wattage to avoid a repeat performance.

    The original caps have lasted 35 years so I'm not overly bothered about making additional improvements to extend their life further.
  10. harnfield

    harnfield Member

    Max Townshend is very approachable in my experience and I visited him twice. Although Townshend Audio stopped selling turntables, he might answer your query. Perhaps he didn't design this circuit himself, but maybe he did. Max's phone number at the factory: 020 8979 3787
  11. colasblue

    colasblue pfm Member

    I no longer have a query - the Merlin is fixed and back with its owner!

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