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Thread: Radford STA 12 Rebuild

  1. #11
    Join Date: Feb 2010

    Location: Berkshire, UK

    Posts: 2,818
    I'm Tom.

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    My recommendation (as someone who did exactly this when I got hold of an even rarer STA100 and one transformer was duff): get Will to wind you a pair of matching output transformers for this, don't mess about with second hand ones (even if it's possible to get them - you could wait a very long time). Will's will be 100% the correct Radford spec and good for another 50 years. I found the price for the STA100 ones very fair indeed.
    Last edited by montesquieu; 29-11-2017 at 19:22.
    Main: Speakers 'RFC' Tannoy Canterburys / custom crossovers; Amp - Radford STA100; Pre EAR 912; Vinyl: Thorens TD124 MkII + Ikeda IT345-CR1 / Ortofon 309 Limited tonearms; Cartridges Stereo - Miyajima Madake, Ortofon SPU Royal N + Shure M3D Jico stylus; Mono - Miyajimas - Zero 0.7, Premium 1.0 retipped by Ana Mighty Sound bamboo cantilever, Spirit '78' 3.0 conical from Ana, also vintage GE VRII, Shure M44 strapped for mono with several Expert Stylus conicals for different eras of 78s; Phono stage and SUT from EAR 912; Esoteric Sound Re-Equalizer for equalisation of early mono and 78, switchable in from tape loop; Digital: Audio Note CDT2/II transport, heavily modded AN Kit DAC + Mutec MC3+USB.

    Study: Speakers - Tannoy DC6; Amp: Duncan 'Tuberhunter' 200w class D; preamp - Pass DCB1; Vinyl: Garrard 401 with AT1503 tonearm and vintage SPU-GE; SUTs - Ortofon 2-15k & Bob's Devices; Phono Stage: Schiit Mani; Digital - Trichord Genesis III CDP + Arcam rBlink; Schiit Multibit DAC.

  2. #12
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 4,059
    I'm Alan.

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    That is such a good suggestion, full marks

    You would then have a spare transformer to sell, could help someone in the same position of needing one transformer.

  3. #13
    Join Date: Apr 2012

    Location: South West of England

    Posts: 220
    I'm Will.

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    You have mail
    Radford Revival
    www.radfordrevival.co.uk

  4. #14
    Join Date: Oct 2017

    Location: West Wales

    Posts: 28
    I'm Alan.

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    A quick pictorial of progress.

    Sripped and cleaned the octal sockets. The GZ34 looked like this...





    I replaced the B9As, but the new ones did not have a central spigot. Used the ones from the original holders.




    Refitted to the rails now.





    Started to tidy the boards too. I've stripped and cleaned them. The resistors all measure well within tolerance except for a couple of damaged ones. Re-done some of the side wiring and started on replacing the caps. And a shot of how one of the boards looked before...


  5. #15
    Join Date: Feb 2008

    Location: Middlesex, UK

    Posts: 3,210
    I'm Alex.

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    Nice work, those metal plate where the valve holders are mounted look in good condition.

    What are the central spigots for on the small valve holders?
    Spendorman

  6. #16
    Join Date: Jan 2009

    Location: Essex

    Posts: 17,194
    I'm openingabottleofwine.

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    Quote Originally Posted by spendorman View Post
    Nice work, those metal plate where the valve holders are mounted look in good condition.

    What are the central spigots for on the small valve holders?
    They're not connected to anything, so act as a useful local 'stand off' tag if components need to be wired close to the valve base. They were used a lot in old valve TVs.

    Love the way you have aligned the slots in the screw heads to be parallel. (Alan, you didn't work for Marconi in a former life did you?)
    Have you listened to this month's choice in the Album Club?

    Barry

  7. #17
    Join Date: Feb 2008

    Location: Middlesex, UK

    Posts: 3,210
    I'm Alex.

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    Thanks Barry, I can now see that those spigots could be useful. Not looked in my STA25 for a few years now.

    As you might recollect, I was a Marconi man, nearly 20 years there.
    Spendorman

  8. #18
    Join Date: Jan 2009

    Location: Essex

    Posts: 17,194
    I'm openingabottleofwine.

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    Ah, I had forgotten that. You will thus understand my comment on the alignment of the screw head slots. Also when lacing up wiring looms, so as to conform to the top level Marconi standard, all the wires had to run parallel and not criss-cross one another, and the spacing between the lacing turns had to be equal, to make the loom look neat and tidy.
    Have you listened to this month's choice in the Album Club?

    Barry

  9. #19
    Join Date: Oct 2017

    Location: West Wales

    Posts: 28
    I'm Alan.

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    A day off from Christmas prep today, so spent a little time populating the Radford boards. And re-doing the valve holder rails.





    I've also made a small bracket to mount the 35mm reservoir cap correctly, so I'll do a picture of that next time.


    ''What are the central spigots for on the small valve holders?'' They are needed as common ground locations for several components (and maybe add a little shielding) round the EF86.

    ''Alan, you didn't work for Marconi in a former life did you?'' No, though I did visit the Chelmsford site a few times. I worked for Burroughs Machines in the late 60's then for Xerox from 73 to 2001.
    I just like to get everything 'tidy'. (OCD?)

    Will says he may be able to provide a couple of later output transformers, so fingers crossed.

  10. #20
    Join Date: Oct 2017

    Location: West Wales

    Posts: 28
    I'm Alan.

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    Well it has been a little while since I made any progress with the old STA12, but a couple of weeks or so ago Will sent me a brand new pair of output transformers for my amplifier. So many thanks to Will, and time to re-start the rebuild...




    The new transformers are slightly larger and heavier than the originals fitted, but fit the original footprint with just a little elongation of their mounting holes.

    Now a design / engineering problem of the original STA12 needed to be corrected. Difficult to see from pictures, but everything on the STA12 is thinner and lighter than the STA15.
    Especially the chassis plate, where the transformers and choke fit, which is tinned mild steel and only has the folded edges to add any strength. This is how mine looked when it arrived... In fact all of the pictures I have found of STA12s exhibit a sagging top plate.




    I added a couple of aluminium angle braces directly under the edges of the plate like this.




    Now with the transformers mounted, time to add a bit of wiring back in. The chassis ground / earth post, centre/bottom, is a bit busy with 13 connections!




    And on top.




    And a few more pics here: https://photos.app.goo.gl/rPJOEbrehSVqL1l12

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