problem wit a Radford integrated amplifier

Hello 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 board of drivers and pre-driver, there is three triming potentiometer; so from left on the right its; P6, P7 and P5. I presume that one among them is for adjusting the symmetry and one for the quiescence current of but I don't what are P6, P7 and P5 and how much millianpres is the quiescence current. Do you know someone that knows all it and who does the diagram of this ampli have, if yes, are you able to forward him my email ? Thank you Gatan

Reply to
Gaetan Mailloux
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Reply to
Michael A. Terrell


Same answer as on the other group. Without the shop manual, it is difficult to determine the precise quiescent/bias current required without a bank of instruments and enough time to allow the outputs to heat (or not) indicating the proper setting (or not).

On the other hand and especially if you closely match the drivers and outputs, the 'factory' settings should be adequate for most situations. Keep in mind that (part of) the balance is between heating and distortion... so with a good design, you can afford to err towards the cool side without having the amp sound like crap, and that is also typical of factory settings. If you do not match the pairs, it should not be the end of the world, but it does give additional margins for error.

What are the outputs and drivers? I am guessing either 2N3055 outputs, driven either by paired TIP-31/32 or 2N3054 drivers? And what, if any, symptoms is the unit displaying that has you in there doing these repairs?

You have already managed the other typical sources of problems? Old electrolytic cap, power-supply problems, drifted resistors, and such- like?

Related: I have quite a bit of vintage AR electronics about. Early production amplifiers and receivers used carbon-element pots for bias adjustments, two for each channel. Naturally, they drifted over time causing all sorts of ruckus usually ending with blown outputs (and sometimes drivers). As it happens, AR developed a wretched reputation due to these failures. Even though back in the day they had an indefinite warranty on their amps and receivers so-equipped, and would replace the pots with fixed resistors after a bit of checking for the ideal fixed value. Their shop manual reflects this process and how that value is determined. One can 'shotgun' it at a given value, but the precise balance will be lost even if 'safety' is achieved.

This is why I hesitate to give you anything exact or to have you alter the factory settings without a manual.

Peter Wieck Wyncote, PA

Reply to

Hello Gatan,

First, set the regulated supply to 70 volts, this is the pot on the power supply board. I also suggest that the amp be set to 120V, not

110V at the transformer, as you don't want to exceed the rating of the main filter cap.

P5 is supposed to be set for symmetrical clipping. I have found that the pot introduces so much noise that you are better off replacing it with a 47K resistor. The amp will clip symmetrically.

P7 is the bias pot., though the instructions say P6 is for adjusting crossover spikes. The factory procedure is:

With input of 70 mV into the tuner input (All other buttons off and they do not specify where to set the input level controls or volume.) and an 8 ohm load connected Adjust P6 (Crossover spikes) and P7 (Distortion)

"Connect Distortion measuring set (D.M.S.) to amplifier output and oscilloscope to the D.M.S. output. Reject fundamental and measure distortion. Adjust P6 for minimum amplitude of crossover spikes as observed on the oscilloscope and P7 for minimum distortion.

The original Z170 and Z171 output transistors are long obsolete. I'd use MJ15003 and MJ15004 for outputs.

Regards, Tim Schwartz Bristol Electronics

Reply to
Tim Schwartz

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