• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Radford STA25 service UK

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C

coolmaster

Member
Joined 2005
aquapiranha said:
wont let me mail you? the guy says this.. " I had the output valve bias
voltages checked and they were a good way out"...
Not sure what thuis means, I am no expert!
Thanks..
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Likely the bias resistors or capacitors have aged. Happens to old amplifiers which may still have it original resistors and caps.
Old resistors change value, old capacitors dry out and these impair the performance in varying degrees.Don't worry...easily tackled if nothing else is wrong.
 
aquapiranha

aquapiranha

Member
Joined 2007
barretter said:
The STA25 has variable fixed bias. There are test points and potentiometer shafts, accessible through a hole near the test points, which can be varied with a screwdriver. Has your friend tried this? They can go off quite a way over time.
Click to expand...


I will pass on the info thanks. I assume like most amps it is a case of measuring from the "-" speaker terminal and the measure point? do you know what the measurement should be?

Many thanks, Steve
 
B

barretter

Member
Joined 2003
One of the test points is at chassis/ground (Radford uses the chassis for ground) level and the other is positive. The potentiomters should be adjusted until you get a reading of 2V (I think it says this on a plaque near the test points). There are two adjustments for each channel. I found it easier to attach the ground lead of a multimeter to the chassis with a crocodile clip whilst poking the positive lead into the appropriate socket and adjusting the pot with a screwdriver.
 
B

barretter

Member
Joined 2003
If Peter Comeau has the brand now, what happened to Woodford Electronics who used to service Radfords? What I would really like are some replacement unstuffed circuit boards for my STA 15 : the originals are very hard to work on as the tracks tend to come unstuck and peel off.
I see that the STA25 Mark VI was announced over a year ago : is there any newer news? I tried to join the forum but it wouldn't play ball.
 
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barretter

Member
Joined 2003
Having now found some photos of the Radford STA25 I have to correct my advice. All the test points are positive and correspond to the potentiometer shaft closest to them. Attach the negative lead of the voltmeter (multimeter) to the Radford's chassis (loudspeaker negative would be most convenient as pointed out), poke the positive probe into the test point socket and rotate the pot shaft until you get a reading of 2 volts. Repeat until you have readings of 2 volts at each test point. If you can't achieve this, something else is wrong. For instance, one of the valves might be knackered. After that it gets more complicated.
 
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