----- Original Message -----
From: "Max Cotton" <airra...@dsl.pipex.com>
To: <drakelist@zerobeat.net>
Sent: Wednesday, October 31, 2012 11:10 AM
Subject: [Drakelist] R4C Hum and audio transistor
I have a very nice R4C it had a hum with bad audio
distortion so I decided to investigate, whilst taking it
apart I noticed that if I touched the noise blanker the hum
would alter, I removed the NB board and the hum was still
there, I found eventually that the audio transistor had
desoldered itself, do they really run that hot? so I
resolded the wires, now the radio does not work anything
like as well and has a high amount of white noise, has that
audio transistor died and if so where do I get a new one, is
there an equivalent for that power transistor?
73, Max M0GHQ
Which transistor desoldered itself? Its possible it was
overdriven because the bias was wrong. The three
transistors in the audio amp are direct coupled so a problem
with any of them will affect all.
Check the power supply voltages and the amount of ripple
on each. While a scope is the best tool for looking at
ripple any AC voltmeter capable of reading perhaps ten
percent of the DC voltage will do. Distortion is likely if
one or more of the transistors is bad. Check the caps
around the audio amp. You will have to check them for
leakage rather than ESR. You will probably have to
substitute them.
Unfortunately, the only schematic I have for the C is
from BAMA and is so low res that I can't even read the
voltages or part designators.
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
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