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A DIY Radford STA25 restoration

This morning I tested all of the resistors and on the boards and found that they were generally well within spec still. There are only a few that I'm going to bother replacing and even those aren't far out.

Can anyone tell me what the minimum power handling and voltage requirements are for R32 (68K), R9 (33K) and R31 (8K2)?


I intend to replace all of the electrolytic caps but out of curiosity I measured a bunch and found that they were also remarkably within spec too.

On that subject: I've noticed that the new improved boards have significantly higher capacitance for most of the electrolytics than the original circuit diagram e.g. 100uf vs 50uF for C4, 470uF vs 250uF for C12+C11 etc. Any advice as to whether I should stick to the original values for upgrade to the higher values?
 
Mike P said:
This morning I tested all of the resistors and on the boards and found that they were generally well within spec still. There are only a few that I'm going to bother replacing and even those aren't far out.

Can anyone tell me what the minimum power handling and voltage requirements are for R32 (68K), R9 (33K) and R31 (8K2)?
Click to expand...

I tend to use 3W resistors for the 68K and 33K screen divider supply, and 2W for the 8K2. Technically higher than they need to be but they run warm so good to be a bit overkill here. I also use 2W resistors for the phase splitter anode loads (33k and 39k), as well as the 8k2 common cathode resistor. Originally they are under-specced and you often find they are discoloured in STAs with heavy use.

Mike P said:
I intend to replace all of the electrolytic caps but out of curiosity I measured a bunch and found that they were also remarkably within spec too.

On that subject: I've noticed that the new improved boards have significantly higher capacitance for most of the electrolytics than the original circuit diagram e.g. 100uf vs 50uF for C4, 470uF vs 250uF for C12+C11 etc. Any advice as to whether I should stick to the original values for upgrade to the higher values?
Click to expand...


100uF in the position of C4 has a bit of a tradeoff - it does increase loop gain at LF so technically you get a slightly better damping factor at around 20Hz, but at the expensive of a slightly less damped LF pole with the feedback loop closed. Nowadays I have gone back to 47/50uF in that position, I ought really to remove that picture from the website, it is incredibly out of date.

As for the high voltage caps I now just use 10uF good quality 450V caps as I buy them in bulk.

On the STA25 the bypass capacitor value doesn't matter too much as it's in parallel with such a low value resistor in the cathode, I just use 220uF now but going higher won't cause any harm. It is however possible to overdo it on the STA15 as it moves the pole-zero pair a bit close to the others as experience has taught, bigger isn't always better.
 
Radford Revival said:
I tend to use 3W resistors for the 68K and 33K screen divider supply, and 2W for the 8K2. Technically higher than they need to be but they run warm so good to be a bit overkill here. I also use 2W resistors for the phase splitter anode loads (33k and 39k), as well as the 8k2 common cathode resistor. Originally they are under-specced and you often find they are discoloured in STAs with heavy use.




100uF in the position of C4 has a bit of a tradeoff - it does increase loop gain at LF so technically you get a slightly better damping factor at around 20Hz, but at the expensive of a slightly less damped LF pole with the feedback loop closed. Nowadays I have gone back to 47/50uF in that position, I ought really to remove that picture from the website, it is incredibly out of date.

As for the high voltage caps I now just use 10uF good quality 450V caps as I buy them in bulk.

On the STA25 the bypass capacitor value doesn't matter too much as it's in parallel with such a low value resistor in the cathode, I just use 220uF now but going higher won't cause any harm. It is however possible to overdo it on the STA15 as it moves the pole-zero pair a bit close to the others as experience has taught, bigger isn't always better.
Click to expand...

Thank you for the detailed reply, that's extremely helpful!
 
Mike P said:
I intend to replace all of the electrolytic caps but out of curiosity I measured a bunch and found that they were also remarkably within spec too.
Click to expand...
My STA15 is still stuffed full of those silver TCC jobbies, some dated 1965. Only one failed to make the cut when it was serviced two years ago.
 
I've now ordered most of the parts I need but I'm struggling to find anywhere that has stock of F&T 8uf+8uF dual section capacitors that I want to use.

Still, there's loads to be getting on with while I wait for those to come back into stock at HiFi Collective.

@Tony L could you perhaps modify the thread title to something along the lines of 'A DIY Radford STA25 restoration'?
 
Mike P said:
I've now ordered most of the parts I need but I'm struggling to find anywhere that has stock of F&T 8uf+8uF dual section capacitors that I want to use.
Click to expand...

Couple on eBay here, but in Canada. May still be faster than HFC restocking.
 
This site contains affiliate links for which pink fish media may be compensated.
15488432.jpg
Found these at a company called Modulus £4.80.Picture shows 15+15uf but they do 8+8
 
Continuing with the strip-down today.

Impedance selector switches removed. The contacts were filthy but cleaned up well with a small brush and IPA. I'll lubricate them before fitting.

P1110611 by Michael Pickwell, on Flickr

Transformers and choke removed (more on that later).

P1110615 by Michael Pickwell, on Flickr

The main filter caps removed. One shows signs of venting.

P1110620 by Michael Pickwell, on Flickr

And the boards have had a bath.

P1110624 by Michael Pickwell, on Flickr
 
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