[Gaetan Mailloux]

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 circuit board of the
drivers and pre-driver, there is three triming potentiometer; 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 millianpres is the quiescent current.

Do you know someone that knows about all that and who does have the
diagram of this amplifier, if yes, are you able to forward him my email ?

Thank you

Gaetan, Canada

[Gaetan Mailloux]

Gaetan Mailloux ) writes:
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 circuit board of the
drivers and pre-driver, there is three triming potentiometer; 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 millianpres is the quiescent current.

Do you know someone that knows about all that and who does have the
diagram of this amplifier, if yes, are you able to forward him my email ?

Thank you

Gaetan, Canada

Hello

Here's a link to a photo of the circuit board of drivers and pre-driver of
my Radford Hd 250 with the tree triming potentiometers;

http://img440.imageshack.us/img440/8...rdboardpj3.jpg

Thank

Gaetan

[Peter Wieck]

On Apr 9, 6:11 pm, (Gaetan Mailloux) wrote:
Gaetan Mailloux ) writes:
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 circuit board of the
drivers and pre-driver, there is three triming potentiometer; 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 millianpres is the quiescent current.

Do you know someone that knows about all that and who does have the
diagram of this amplifier, if yes, are you able to forward him my email ?

Thank you

Gaetan, Canada

Hello

Here's a link to a photo of the circuit board of drivers and pre-driver of
my Radford Hd 250 with the tree triming potentiometers;

http://img440.imageshack.us/img440/8...rdboardpj3.jpg

Thank

Gaetan- Hide quoted text -

- Show quoted text -

Gaetan:

In an absolutely perfect world, you would need the shop manual to set
these correctly. Without that manual, you have several options:

a) closely match the transistor pairs. This requires the proper
transistor-tester capable of doing this. Heath made a very nice one
that I happen to use for the purpose. Closely matched pairs will be
more tolerant of not-quite-perfect bias current and may do just fine
on existing settings. Unmatched pairs will load unevenly with "mixed"
results depending on your luck and the phases of the moon. If you can
match, try it at cold and at 'pretty warm' to see if the levels
change. NOT HOT. You can do pretty warm with a soldering iron on the
flange until you can just hold the transistor *comfortably* between
thumb and little-finger (the little finger is usually more sensitive
to heat).
b) measure the present current levels. Determine quiescent operating
temperature of the unit with the new transistors in place. It should
be pretty-much dead-cold. If not, you will need to decrease the bias
current a bit until things do cool down. This is by-guess-and-by-god
electronics, not good practice. There are all sorts of compromises/
balances between temperature, distortion and output levels that
require a bank of instruments to optimize.
c) note that most designs even from the 70s onward used fixed
transistors for the purposes you describe. Once you do optimize, you
might try that route.

What does this beast use for drivers? My guess from the vintage would
be 2N3055 outputs with 2N3054 drivers? Or TIP 31/32 drivers?

And are there any untoward symptoms that make you want to make these
adjustments?

Good luck in any case.

Peter Wieck
Wyncote, PA