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Leak Stereo 20

Discussion in 'classic' started by Tony L, Jan 31, 2012.

  1. Tony L

    Tony L Administrator

    Looks like a nice tidy one, though I'd not personally trust it yet as a) it is missing its 'safety drop-out' green resistor and b) the two big dual electrolytic caps that go through the chassis look original and they will be way off spec by now. Those two things could I suspect spell a nasty fail. I'd address them as a priority as if I understand things correctly (a big if!) if the caps are off tolerance that stresses the rectifier and in turn can cook the mains transformer, and that is something you really do not want to do. This is the point the green resistor is meant to heat up and drop out - it exists to protect the transformer.

    Has the rectifier got any etched codes? They can be faint and hard to spot so try using a torch. Usually two lines e.g. F32 (F3 is a GZ34, 2 is second revision), then on the next line something like B7D5 (B being the factory, in this case Boackburn, 7 being the last digit of year, in this case with a four dugit code that would be 67 or 77, D5 being week, day).
  2. Dion

    Dion Member

    Hi Simon

    I also just got one of these recently and mine also has a 8162R mains trafo.

    Going through the component values and comparing them with the circuits available out there, I've discovered that the capacitor values for C3 and C5 are different to anything I've found listed out there.

    R5 = 47k
    R12,13 = 470k
    C3, C5 = 82n

    I see your C3 and C5 caps (the bright blue ones) have been replaced, but I can't see the values. Can you confirm what the values are?

    Here's a picture of mine [​IMG]
  3. snowman_al

    snowman_al pfm Member

  4. Dion

    Dion Member

    Hey Alan

    For some reason I can't imbed images in my posts, but I wanted to confirm that I found a picture of an orignal Leak Stereo 20 with an 8162R trafo where C3 and C5 caps were 0.1uF.

    Thanks for your feedback

  5. Tony L

    Tony L Administrator

    To my knowledge there were two different mains transformers, two different output transformers and three actual variants. I'll need to edut this post later with the numbers but in gist:

    * First version: early mains transformer, early output transformers, early component values.

    * Second version: early mains transformer, later output transformers, later component values.

    * Third version: later mains transformer, later output transformers, later component values.

    As I understand it the changes were made as the earliest ones had a real tendency to overheat and were running the Mullard EL84s right on the edge of their capabilities, so tended to fry less good EL84s. As such they changed the output transformers and went for a more moderate set of values so as to give the EL84s an easier ride. Finally they introduced a different mains transformer which outputted less voltage and ran a little cooler (overheating mains transformers were an issue in very hot countries if the amp was too tightly boxed in and powering its pre and tuner, I understand they certainly had issues in Australia where they sold a lot).

    Mine, dated 1961, is the second variant, i.e. was built with first mains, second output and second component values. Some original resistor values in the amp actually differed to its own manual schematic which confused the hell out of us when rebuilding it, hence my knowing this for sure. I also understand that Leak rebuilt early amps sent back for service with the second set of component values.

    PS Instructions as to how to link images are in the FAQ beneath the site logo.
  6. snowman_al

    snowman_al pfm Member

    No problem.

    Here is a picture of a near original one. (it's on ebay right now)


    I believe all versions of the St 20 and T/L12+'s had C3/5 at 0.1uF. (I've never seen anything else) I 'think' there is a typo on one of the St 20 circuit diagrams.
    Correction: Since this date I have seen a very early example with 0.02uf caps in the C5 position so they were built. And obviously not a typo on the early diagram. In any case it should still be replaced with a 0.1uf cap when replacement is needed.

    There is loads in this thread at around page 15 that explains it too. Post #223 for transformer info Tony is talking about ^^.

    Try 'tinypic' for easy / quick and dirty picture posting...

  7. Tony L

    Tony L Administrator

    A very original looking Stereo 20 has just gone up on eBay here. Looks to have lost some magic smoke at some point, but may well be totally stock. I can't see which output transformers it has though. Looks like an early mains transformer.
  8. snowman_al

    snowman_al pfm Member

    I was doing well moving a few of my less used amplifiers etc. on to new and more appreciative owners. (That included my 2 Leak TL12+s that went off to Hong Kong.)
    Then, I'm afraid common sense left me for a moment when this near original Stereo 20 beckoned. And I had all those Leak 'spares' in the drawer with no home?

    [​IMG] [​IMG]

    A bit scabby in places and suffering from decorators paint spot measles, but underneath all tidy and un-messed with.

    The previous owner said it did 'work' to a degree, but was a bit crackly and smelt a little hot. He was correct.
    A quick check showed 38 volts on one EL84 cathode with the others not far behind.
    It did work on both sides, but very distorted at anything above low volume as you might expect.
    Time to do some component checks then.
  9. Tony L

    Tony L Administrator

    Superb, a really nice one and a top quality restoration candidate. It likely needs nothing more than a sympathetic recap (Russian K40Y-9 PIO would be my choice) and a really good clean. It will be the final spec output and mains transformers so make sure you have the right circuit diagram!

    PS Decorator's paint spots will likely just pick off with a fingernail. I've found a fairly flexible nylon guitar pick really useful for this in the past, doesn't scratch at all but can usually pop paint spots off!
  10. John_73

    John_73 pfm Member

    Very nice! Looks like a very late model too, it has the extra components fitted across both R5 resistors: a 4K7 and (from memory) 300pF silver mica cap. I've only seen this on a few ST20s. Supposedly aids stability under certain conditions.
    Anybody know if this is worth retro-fitting to any ST20?
  11. snowman_al

    snowman_al pfm Member

    Thanks, I spent some time lifting the spots off, using an ice lolly stick cut to a chisel shape. Most did not flick off, but wore away with a light sponge with water.

    Sadly nearly every resistor is out of spec by at least 40%, some by 100%. (All the 47k's read between 94 and 97k? The 91k's read 111 and 115k and the 100k's read 127 and 128k. And one cathode 270 ohm resistor reads 489 ohms. And so on...) Then all the caps are leaky or well down on capacity. (The smoothers are between 18 to 25uf and the 0.1uf couplers are good 100k resistors!)

    John, yes the date, on the under side of the tag board is 'WE 16/1/1965'. Paint finish is not nearly as good or as thick as the earlier gold/bronze ones I have experienced.


    More pics here, including stripping and tightening the output transformer sockets: https://goo.gl/photos/G4dc6NnypmZYGLrH8
  12. Tony L

    Tony L Administrator

    Wow, that is some strip-down! I guess doing it that way might make it a bit easier to repopulate. I found it quite hard to get to some of the resistors. Why did you feel the need to open the transformers? I just used an inter-dental brush and a bit of Deoxit on the impedance plug sockets on mine and they seem to work ok.

    Is the paint thick enough to take a gentle T-Cut polish or equivalent? You'd have to be very gentle over the lettering, but it might be possible to get a bit of a shine to it. On my bronze one the chassis paint is just beautiful, the transformers less so, obviously a different quality/supplier.
  13. snowman_al

    snowman_al pfm Member

    There is plenty of shine as you can see, I think my camera makes it look a bit flat too. I gave it a really good wash in the sink in Simoniz car wash then buffed up using a couple of goes with their spray wax. I will live with the result and once everything is re-installed you will hardly notice the 'scabby' bits...
    (Personally I wouldn't dare use abrasives as the lacquer is so thin.)


    I cleaned the tag board as well as possible and removed the all the connections to the 'pre'amp' socket. (One of the reasons for removing the board. It makes re-wiring the new input sockets so much easier.) Also treated the surface rust on the screws with Hammerite rust gel and they have come up really well. The darker colour suits the charcoal chassis too. Then refitted the tag board into the chassis and wired up the new inputs.


    More here: https://goo.gl/photos/QQNzMkXuZKfrzHb6A
  14. Tony L

    Tony L Administrator

    I really like the way you are approaching this. I wish I'd seen this before doing mine as I'd have been very tempted to remove the main board, though would likely not have stripped all the components off as I like taking one old component off and putting the new one in its place one at a time. Less potential for error that way! Is there any sonic gain in removing the connections to the old Leak octal pre/power link? I kept everything stock though it was a right PITA to solder the resistors in that area in situ. I actually had to get Si (Chops54) to do one bit as it was just beyond my skill level/steadiness. They have all been replaced, every last one!
  15. snowman_al

    snowman_al pfm Member

    I'm not sure if there are any immediate sonic benefits, but the input wiring is now so much shorter, so less chance of 'stray' pick-up, and removing the heater loom removes another chance of hum.
    As I am never going to use a Varislope there is no need for them AND it just looks so much tidier.
    I'm putting the Octal socket back in, so it appears as before and if anyone does want to use it in the future, its an easy job to reproduce everything above the board, so no need to pop it out again.

    By the way it is easy to take the transformers apart off the chassis. Mainly I wanted to see inside the mains transformer and be sure it had not overheated at all. (Its all fine, no smell and no scorching.) Then I had to polish up the bolts and nuts so they had to come out. So just a matter of minutes to check the output transformers too and 'fettle' the impedance tap connections.

    It was 1996 when I last touched one of these and had forgotten how well designed they were and what a pleasure to work on... Alan
  16. snowman_al

    snowman_al pfm Member

    A bit of progress here.
    Some re-population of the board, still waiting on a few bits, and a rattle can paint job for the F&T reservoir caps.


    I've treated the transformer mounting screws with the 'green gloop' and I'll be putting them back on this morning.

  17. Tony L

    Tony L Administrator

    Excellent, I'm pleased to see you are sufficiently obsessive to paint the caps! Are you retaining Harold's magic green safety resistor?

    PS You have the capacitor clips on upside down!
  18. snowman_al

    snowman_al pfm Member

    Yes they have to be grey!
    I would normally 'gut' the old ones and secure the replacements inside, but the F&Ts are the same diameter, so not possible. (Anyway the old ones can go in the 'original parts' box.)

    Yes the magic green resistor will go back on then I've mounted and wired the mains transformer. Saves knocking it back off whilst getting to the loom!

    ''PS You have the capacitor clips on upside down!'' - True, but only for them to have their picture taken, or they fall over!
  19. Tony L

    Tony L Administrator

    What spray colour did you use? Worth noting it here for others as it looks like you found a very good match. Volkswagen Sand Metallic is spot on for the older bronze Stereo 20 like mine, the only visible difference is the modern caps are rather shorter.
  20. snowman_al

    snowman_al pfm Member

    For the Charcoal / Grey paint I chose Toyota Decuma Grey.
    It is not exact, but good enough when the rest reflects back.
    Another close colour was Peugeot Steel Grey metallic, but I had to decide from the colour on the can and Decuma had the similar metal fleck finish.

    Managed to get most of it back together today. Still waiting on a few caps (ex USSR).


    More pics here: https://goo.gl/photos/CiDXLAyZb6XETHoG6

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