Radford STA 12 Rebuild

[snowman_al]

Hi Everyone,
I'm new to posting on Aos so please treat me kindly. And I hope I have chosen the right place to post... So here goes.

As a long time fan of Radford, I recently acquired a very sad STA12.
This one had suffered more than most. Clearly in the past it had had the reservoir/smoothing cap explode. There was sticky residue and flakes of paper and foil under everything. I guess that was the start of 'someone' fixing it up and 'modifying' it. Triode connected outputs and shared cathode bias resistor, components dangling on bits of wire and so on.
Then it had clearly been dropped and the transformer chassis badly bent together with the outer frame.

Only one thing to do, a total rebuild. This is where I am to so far, the is chassis stripped, cleaned and straightened:-


http://i63.tinypic.com/20781v6.jpghttp://i65.tinypic.com/2u41014.jpg http://i64.tinypic.com/i2ulqe.jpghttp://i63.tinypic.com/30lfamx.jpghttp://i68.tinypic.com/119qah0.jpg

I'm missing a few parts, especially an output transformer, so please look out I will be on the scrounge!
Alan

[brian2957]

Hi Alan , welcome to AOS . I'll be watching this thread with interest :popcorn:

Enjoy the forum :)

[spendorman]

Very interesting. I shall be following too.

I wonder if Radford Revival can help.

I'm a long time owner of an STA25 III, probably more than 40 years.

[walpurgis]

I wonder if Radford Revival can help.

That idea crossed my mind. I'd expect they'd have some spare parts and could possibly match the enamel too for touching in purposes.

[snowman_al]

Guy's thanks for the Radford Revival tip, and Will says, ''rarer than hens teeth!''
My best bet will be a pair of later T2063s, anyway I've put a Wanted Ad up just in case.

[spendorman]

Rare, yes, but I would have thought an STA20 was even rarer. I just happen to know someone that owns one! (not me).

[walpurgis]

It's worth keeping an eye on ebay. There may be a compatible transformer from a non Radford design, although obviously an original would be desirable.

[spendorman]

Circuit diagram here:

http://www.audiomisc.co.uk/HFN/Radford/circuits/circuits.html

[Firebottle]

Welcome to AoS from another Alan :)

Great start to your project. Good luck with the hunt for a transformer.

[spendorman]

There is a lot of interesting info here by Jim Lesurf: http://www.audiomisc.co.uk/HFN/Radford/revisited.html

[montesquieu]

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.

[Firebottle]

That is such a good suggestion, full marks :thumbsup:

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

[Radford Revival]

You have mail :)

[snowman_al]

A quick pictorial of progress.

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


http://i66.tinypic.com/2sacg14.jpg http://i68.tinypic.com/r1y0c8.jpg


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


http://i66.tinypic.com/27y25n7.jpg http://i63.tinypic.com/2yyto1z.jpg

Refitted to the rails now.


http://i67.tinypic.com/2n9e14y.jpg http://i64.tinypic.com/2vsenmc.jpg


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...

http://i67.tinypic.com/2s1ub8g.jpg http://i65.tinypic.com/2zy99ur.jpg

[spendorman]

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?

[Barry]

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?)

[spendorman]

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.

[Barry]

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.

[snowman_al]

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

http://i65.tinypic.com/2mx3dpg.jpg http://i65.tinypic.com/1j41o8.jpg

http://i66.tinypic.com/ngcap.jpg http://i65.tinypic.com/2mnljm1.jpg

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.

[snowman_al]

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...

http://i64.tinypic.com/wtsoie.jpg


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.

http://i68.tinypic.com/ie0xm9.jpg


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

http://i66.tinypic.com/110i00n.jpg


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!

http://i64.tinypic.com/4r81av.jpg


And on top.

http://i67.tinypic.com/2s96qa8.jpg


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

[walpurgis]

Going well there Alan. The bracing angles were a good idea. I'm looking forward to seeing how this turns out.

[snowman_al]

... well it has taken a while.

I started this back in 2017 and have had several more 'false' starts to get it completed. So having cleared everything else it was time to concentrate on finishing the old Radford.
It is an STA12 which is a very close relative to the original STA15 (EF86, ECC83 and EL34 etc.)

Recapping a little (Tinypic is closing soon so the original pictures might vanish soon...), the first problem was someone had modified this amplifier in a very unsympathetic manner, then the chassis was distorted due to the much flimsier metal used on these models. This distortion then leads to the component boards also bending and so on.

https://i.imgur.com/uxDgo8x.jpg

https://i.imgur.com/OYvY8UN.jpg

Excellent work here, not.

https://i.imgur.com/BHBctnn.jpg

https://i.imgur.com/vCwc060.jpg



I previously reinforced the chassis plate and 'reverse engineered' the component boards from an original STA15 diagram and a couple of rare STA12 photos on the net. That and a few obvious, and some less obvious, differences in the circuit / layout.


https://i.imgur.com/KURMRb0.jpg

https://i.imgur.com/SlkluB8.jpg


Then my next problem were the output transformers. The new replacements are for the Series 3 STA15 having different output taps to the originals, so required different impedance switching etc. But they fit very well.

https://i.imgur.com/xAW1bZd.jpg

And that is where I had to halt for some time, until last week in fact. So to start with I had to meld the earlier and later impedance switching using the Series 3 as my template.


https://i.imgur.com/F07pPwa.jpg

https://i.imgur.com/lFVzhpz.jpg

https://i.imgur.com/8X6Xmex.jpg



Then wire up one of the circuit boards switch on and hope... for the best.
I was greeted with a low 'far*ing' on initial power up, but reversing the primaries fixed that. Then music at last if only in mono.

https://i.imgur.com/0tlQaOK.jpg

https://i.imgur.com/IkQR5l6.jpg


I left it running for an hour or so and all was good. Happy everything was well - on to the other channel asap.


https://i.imgur.com/1vR8uzA.jpg

https://i.imgur.com/u3NujRe.jpg

https://i.imgur.com/9AKNpLo.jpg

https://i.imgur.com/t1JskXj.jpg

So there we are, it has been running for a couple of days now and it is an astonishing amplifier. It really does the Radford thing very, very well. So much detail and control all through the volume range and no noise. Not bad for an amplifier made in 1960. Even the mains transformer is much quieter than my STA15 III. It has become my instant favourite.

Still a few of things to do though.
First is thank Will at Radford Revival, it could not have happened without him.
Then it needs a base plate, it came without one and it's on 4 rubber feet at the moment.

Alan

[Barry]

Beautiful work done there Alan. Well done! :)

[struth]

Bravo... Top Job

[Radford Revival]

Great job, glad to be of assistance with the OPTs and happy it all worked out :D

[Firebottle]

Superb job Alan, that must have taken a good few hours (weeks?).

Another thumbs up for Will :thumbsup: Great to have such committed engineers on AoS.

[Cycleallday]

Another well documented rebuild Alan.

Your care and attention to detail is amazing - most would be rushing to get the job done rather than doing it properly.

I'm still enjoying the World Designs KEL 84 you rebuilt and documented in the same manner as this.

Mel

[JohnJo]

Great to see another of these superb amplifiers resurrected and good for years to come. The Radfords really do seem to be an amp for life as they are so serviceable especially with Radford Revival on hand. Really good stuff :)

[IHP]

That idea crossed my mind. I'd expect they'd have some spare parts and could possibly match the enamel too for touching in purposes.

Beautiful work Alan. May I please ask if you got a result on the matching enamel ? I have a 25 in lovely condition but with 2 minor blemishes on the casework I'd like to touch up. Airfix don't seem to do a 'Radford blue' !

[snowman_al]

No sorry I did not. I decided to leave it 'as is' to show some of its age...
I have a metal plate for the bottom now so will give that a coat of something close from Halfords or the motor factors. It does not need to be perfect, it will be underneath.

The other thing is that it is totally different in colour to my STA15 III (which is two tone and I guess will be similar to your 25). The closest picture to the real STA12 colour is the 'rear view' (one from last) the white balance on my camera is not the best.

I have found that the Humbrol (or Airfix) enamel actually can be mixed to match lots of things with a bit of time so that might be the way to go if the areas are not too large or obvious?

Alan

PS thanks for the comments everyone.