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Mike's Stereo 20 refurbishment thread.

Discussion in 'classic' started by Mike P, Apr 23, 2019.

  1. Mike P

    Mike P pfm Member

    If you were wondering who bought the Leak Stereo 20 that was recently in the classifieds, it was me :).

    Before I go any further, many thanks to Mark for a smooth and pleasant transaction.

    I hope it won't be considered too indulgent of me to start my own thread rather than tag onto Tony's, I just thought it would be less confusing for all if I kept them separate.

    So far I've only allowed myself the pleasure of some gentle cleaning with the ubiquitous Servisol foam cleanser.

    Mine appears to be completely original and untouched as far as I can tell so far.

    [​IMG]P1110098 by Michael Pickwell, on Flickr

    It came with a full set of Mullard valves, although I've no idea what state they're in as yet.

    Looks to be 1961 although confusingly the mains transformer is marked 81 62.

    [​IMG]P1110095 by Michael Pickwell, on Flickr

    The output transformers are both marked 8778.
     
    Taff63, John_73 and Hoopsontoast like this.
  2. Mike P

    Mike P pfm Member

    So far I haven't found anything drastically wrong but there's certainly lots of work to do before I can think about listening to it.

    The mains plug is smashed to bit so I need to source a replacement. There are a few on ebay at around £30 if I can't get one from a fellow fishy.
    [​IMG]P1110097 by Michael Pickwell, on Flickr

    This red wire has broken loose. It looks like it should be soldered to the same solder tag as the other red wire.
    [​IMG]P1110087 by Michael Pickwell, on Flickr

    The big electrolytics look ropey.
    [​IMG]P1110090 by Michael Pickwell, on Flickr

    As do several of the smaller ones.
    [​IMG]P1110089 by Michael Pickwell, on Flickr

    And the PCB under Harold's magic green resistor looks like it has been lightly toasted.
    [​IMG]P1110096 by Michael Pickwell, on Flickr
     
  3. Tony L

    Tony L Administrator

    Looks superb and exactly the same spec as mine (also ‘61). The codes on the transformers are types, not dates. You have the revised 8778 output transformers and the earlier 8162 mains transformer. As I understand it the output transformers plus some resistor changes were made to stop blowing marginal EL84s (the first variation ran them right at the edge of the Mullard spec), and then with the dark grey Stereo 20 the transformer was altered to give 280V rather than 300V, I suspect to stop it melting in hot countries like Australia and South Africa, where IIRC quite a few Leaks were exported.

    When rebuilding be very aware that there are two different schematics for the Stereo 20 and you definitely want the second one. If you have the manual (as I do for mine) the schematic is likely wrong to what was actually fitted in the amp! Make sure you go from a schematic that shows 280V out for the mains transformer as that has the later ‘kinder’ resistor values and is very likely what is already fitted to your amp.

    PS My serial is 103/2/61. I’m pretty sure the ‘61 is the date, but not sure of the rest.
     
  4. Mike P

    Mike P pfm Member

    My first priority is going to be sourcing a copy of the correct schematic and I also need to start compiling a shopping list.

    The big electrolytics seems as good a place to start as any as they're definitely going to get replaced. The originals are 32uF + 32uF and 1 3/8" diameter.

    I've found three options for 32+32uF

    F&T from Hi Fi Collective
    https://www.hificollective.co.uk/catalog/fandt-lfaz-dual-section-radial-electrolytic-capacitors.html

    ARS from Watford valves:
    https://www.watfordvalves.com/product_detail.asp?id=2889

    MA from Crinklewood:
    https://www.cricklewoodelectronics....ganKefeN33pDKusJkTvmsblGMU3QTVNhoC6ogQAvD_BwE

    The F&T and ARS are also available in 50+50uF which will still fit nicely.

    I'm drawn to F&T as I have faith in the quality of their capacitors but I'd be very interested to hear other's opinions. Is there any significant advantage in going for the higher capacitance 50+50uF over the standard 32+32uF?
     
  5. Mike P

    Mike P pfm Member

    Ah, many thanks Tony, great info.

    This is the only version of the schematic I've found so far:
    https://www.hifiengine.com/manual_library/leak/stereo-20.shtml
     
  6. Tony L

    Tony L Administrator

    I used F&T. They have a great reputation. Don’t be tempted to up-rate them as you stress the GZ34. Stick to 32+32uF. Be aware that the Stereo 20 runs fairly close to the edge as-is, be *very* wary moving from Leak’s spec at all as proper Mullard GZ34s are about £130 now and it doesn’t take much to cook the mains transformer!

    PS After googling and checking pics of my grey S20 it appears that whilst the transformer did change at some point from 300-0-300V to 280-0-280V the number remained at 8162. As such I’ve not a clue which we have, but it doesn’t matter as the circuit is the same. Just pick a schematic that says 280-0-280V as that has the right resistor values.
     
  7. Tony L

    Tony L Administrator

    Both circuit schematics are up on Jim’s site here.

    PS Is the serial maybe unit-number/week/year?
     
  8. Mike P

    Mike P pfm Member

    Great, I've just saved a copy of the correct 280v circuit diagram.
     
  9. Tony L

    Tony L Administrator

    Broken transformer red wire is indeed soldered with the other red wire. Visible on mine here. Bulgin mains plugs are sadly rather expensive due to being used on vintage Marshall, Hiwatt, Vox etc guitar amps, but regularly available on eBay for about £30-35.
     
    kevinrt likes this.
  10. George J

    George J Herefordshire member

    Fantastic!

    After my old 1957 Troughline coming home from repair last week I have no doubt that this amp will bring you so much pleasure!

    Best wishes from George
     
  11. Mike P

    Mike P pfm Member

    Thanks George.
     
  12. Mike P

    Mike P pfm Member

    I've just hit 'Buy it now' on a replacement plug on ebay. That's one more small piece of the puzzle sorted.
     
    John_73 likes this.
  13. John_73

    John_73 pfm Member

    Great stuff, look forward to seeing this project progress as I’m also in the middle of rebuilding my Uncle’s ST20, which sadly needs rather a lot of work. Have you started measuring the resistors yet? Sadly in the end all but four of mine where out of tolerance (plus several broken tags from the previous ‘repair’ guy), so I’m now torn between the vintage-esque looks of TE 1W and 2W carbon films, or the slightly lower noise and higher tolerance of the Kiwame (aka KOA Speer) types, but losing the original underside looks somewhat. Have you any thoughts on components yet? I can highly recommend the Russian K409-Y paper in oil caps - bomb-proof with similar looks and sound to the original TCCs but far more reliable.
     
  14. foxwelljsly

    foxwelljsly Keep Music Vile

    Is the bulgin mains plug the same as those for ESLs? If so, I have a couple. PM me if interested. Was going to pop them up for £20 each.
     
  15. Mike P

    Mike P pfm Member

    Doh! Too late I'm afraid, I just bought one.Thanks anyway.
     
  16. Mike P

    Mike P pfm Member

    I haven't started on the resistors yet but I'll definitely be replacing as many carbon comps as possible with carbon film. At this stage I fancy either the Kiwame (KOA SPR) or Takman.
     
  17. Chops54

    Chops54 pfm Member

    The Takmans look more correct but sound wise I think the Kiwames have the edge. Horses for courses.
     
  18. Mike P

    Mike P pfm Member

    I don't suppose anyone has a BOM for replacing the resistors?
     
  19. Tony L

    Tony L Administrator

    There is one here, though no mention of version so I’d check it against the 2nd schematic.

    PS When I eventually come to do my early TL12+ I’ll be using Kiwame for no other reason than their having the values clearly printed in text rather than colour bands! It makes checking your work afterwards so, so easy! I’ve no idea when this will be as I’m kind of waiting for another one to turn up to make a stereo pair.
     
  20. John_73

    John_73 pfm Member

    Here’s what I used for mine - I’ve yet to double-check it however!

    Kiwame 2W or Takman etc:

    R1 - 1M (x2)

    R2 - 22k (x2)

    R3 - 2k2 (x2)

    R4 - 100R (x2)

    R5 - 47k (x2)

    R7 - 91k (x2)

    R8 - 1k (x2)

    R9 - 22k (2)

    R10 - 100k (x2)

    R12 / R13 - 470k (x4)

    R18 - 12k (x2)

    R19 - 47k (x1)

    R20 - 4k7 (x1)

    R21 - Retain 3W 100ohms original (if still fitted!)

    Carbon comp:

    R16 / R17 - 10k (x4)

    Wirewounds:

    R14 / R15 - 270R (x4) Y
     

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