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Old 8th April 2007, 04:31 PM   #1
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Default Need help with a Radford HD 250 amplifier


I have an integrated amplifier Radford HD 250. I did replace its powers transistors and drivers.

I don't have the diagram of this amplifier, on the circuit board of the drivers and pre-driver, there is three triming potentiometers; so from the left to the right its; P6, P7 and P5. I presume that one among them is for adjusting the symmetry or for the quiescence current, but I don't know what are P6, P7 and P5 and how much millianpères is the quiescent current.

Here's a link to a photo of the circuit board of drivers and pre-driver of my Radford with the tree triming potentiometers;

Anybody have an answer or does have the diagram of this amplifier ?

Thank you

Gaetan, Canada
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Old 8th April 2007, 04:38 PM   #2
Netlist is offline Netlist  Belgium
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It shouldn't be too difficult to reverse engineer the board. From there I might be able to help.
Get that pencil and paper.

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Old 4th July 2007, 05:59 PM   #3
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Here's some news about that Radford that I repair for a friend..

I did take, for some time, a brake from the Radford HD250 repair. And yesterday I did finish the repair.

I did found the service manual on the web.

I have used MJ15003 and MJ15004 for output, and BD139 and BD140 for drivers, there was some wrong value resistors on the board (my friend try to fix it himself), so I replaced them by the righ value.

I've put back everything and do a first test, P5 did adjust the clipping to be symmetrical.

P6 don't do very much for adjusting crossover spikes since I can't see any spike on the scope.

For the bias, I did goes first with 45 ma. for each channel, but it was sound a bit agressive, so I goes up to 75 ma bias and now it sound very good.

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