Hello
I have an integrated amplifierRadfordHD 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
Gaetan:
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
