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Radford STA25 Mk 3 modifiactions



 
 
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  #1  
Old August 26th 19, 07:43 PM posted to rec.audio.high-end
Howard Stone
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Default Radford STA25 Mk 3 modifiactions

Someone is offering me one of these for a good price. HOWEVER a Radford expert says that it has been modified

1. It appears to be using HT capacitors in series with balancing resistors (probably to make up the voltage rating as the original needs 500V caps).

2. Someone has made brackets to mount the extra two capacitors on top the output transformers.

3. The caps have been changed on the main boards too though judging by their appearance they look like 80s vintage caps (as do the replaced main smoothing caps)

4. The cap for the mains voltage selector is missing, presumably the tap has been selected via a solder link somewhere within the amp.


I'm sure the amp works but what I don't know is how serious these changes are, whether they will change the character of the amp in an important way and whether.

Your thoughts much appreciated.
  #2  
Old August 26th 19, 08:53 PM posted to rec.audio.high-end
Peter Wieck[_2_]
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Default Radford STA25 Mk 3 modifiactions

Three things:

a) I do not own, nor have I ever used any Radford Product.
b) This is a 6550 (KT-88) based amplifier, operating at the bleeding edge of what modern-production 6550 tubes can deliver.
c) From what I can see of Radford amps contemporaneous to that one, they are reasonably well made, if a bit crowded.

So, based on my experience with 6550-based power amplifiers (Dynaco, Scott):

1. The voltage selector is the least of your worries, assuming it is correct for your location. In point-of-fact, if it is a typical rotary switch vs. a fixed plug, it becomes a weak point in the system. Good that it is gone. NOTE: Dynaco and Scott typically changed voltage by reconfiguring power-transformer windings - permanent, IOW.

2. Using dropping resistors to reduce voltage to filter caps is altogether a bad idea from many perspectives. Putting them where they are seems to involve additional leads, exposing HV B+ where it should not be - another bad idea.

3. Nothing intrinsically wrong with 80s-vintage caps - I run a number of them myself. However, and since then, the chemistry has gotten remarkably better than before. If you are going to take on this device, consider a recapping with caps of the proper voltage and screened to be close in value channel-to-channel.

Cutting to the chase: you are offered an amplifier of reasonably good repute that has some hair on it. That, overall, you would like to restore to its original reliable condition and design.

Amp costs X
Restoration will cost Y in labor (presumably yours) and Z in parts.

X - (Y+Z) would be the fair price of this amp. Writing entirely for myself, I love a challenge and would think nothing of going through the necessary restoration steps. That is not everyone's position, however.

Good luck with it!

Peter Wieck
Melrose Park, PA
  #3  
Old August 26th 19, 09:07 PM posted to rec.audio.high-end
Howard Stone
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Default Radford STA25 Mk 3 modifiactions

Cheers Peter. This is similar to the advice I got from the engineer who fixed the Krell from me, he basically said that these things shouldn’t make too much of a difference and anyway they could easily be reversed at a later date. So I’m going to go for it.

  #4  
Old September 2nd 19, 11:38 PM posted to rec.audio.high-end
Howard Stone
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Default Radford STA25 Mk 3 modifiactions

Well it arrived and was hooked up to the Rogers JR149s!

And wonderful, I listened to it twice and both times after it had warmed up for 15 minutes the sound was just glorious! And it helped greatly make the bass on these little speakers.

And then . . .

.. . . and then A CAP EXPLODED. Very dramatically with lots of smoke.

Thank goodness for ebay buyer protection. In this case the seller wants to take it back and fix the problem and I'm going to give him the chance.

Of course it's a very old amp, and of course I expect to spend a lot of money to get it rebuilt. But I'd like it to work for more than four hours first given that it was sold as being "in good working order!"

  #5  
Old September 3rd 19, 09:10 PM posted to rec.audio.high-end
Peter Wieck[_2_]
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Default Radford STA25 Mk 3 modifiactions

We have apples: "Good working order".

And we have Oranges: "Of course it's a very old amp...".

That it was only a capacitor is remarkable.
That the amp was modified is, likely, a contributing factor.
That it is not the seller's fault is also very most likely the case.

I will lay dollars to Krispy Kremes that the seller ran the amp for 20 minutes or so, and had no issues. That, in eBay speak is "good working order".

And, for the record, had that been my amp, it would have spent its first 8 hours or so on my metered variac, being watched very closely. Likely, I would have seen the rise in current prior to the cap going *POW*.

This is why, when introducing a new component into the herd, a certain amount of precaution must be taken.

If you have no skills personally, this will be costly, because the 'proper' fix is to restore it to its original condition. If you have such skills, less than US$100 should more than cover it in parts.

Best of luck!

Peter Wieck
Melrose Park, PA
  #6  
Old September 4th 19, 02:09 PM posted to rec.audio.high-end
[email protected]
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Default Radford STA25 Mk 3 modifiactions

On Tuesday, September 3, 2019 at 4:10:54 PM UTC-4, Peter Wieck wrote:
> We have apples: "Good working order".
>
> And we have Oranges: "Of course it's a very old amp...".
>
> That it was only a capacitor is remarkable.
> That the amp was modified is, likely, a contributing factor.


That it's made by Radford is at least as likely a factor.

In the 1970's I was an authorized service center for Radford
and made, at the time, a fair piece of change repairing their
stuff under warranty. That is until they decided to stop paying
for warranty repairs altogether. And, at that point, I stopped
accepting Radford product for warranty repairs: I'd be damned if
I was going to eat their problems.

The biggest issues were failed power supply capacitors and,
strangely enough, failed power supply transformers. There
was no apparent correlation between the two: Lots of caps
went nova, and two or three transformers went as well: but
in those cases, the caps were okay.

Beyond that, almost every one of the Radford preamps I
encountered has bad pots (all noisy, several with dead
spots).

Not the most confidence-inspiring product line, to be sure.

> Best of luck!


Indeed!
 




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