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SQ9NRF > TECH 30.12.02 03:25l 55 Lines 2570 Bytes #999 (0) @ WW
BID : A80149SQ9NRF
Read: GUEST OE7FMI
Subj: Audio hum in Yaesu FT-101 ZD
Path: DB0MRW<DB0PV<OE2XOM<OE5XBL<OE3XSR<OK0PBX<OK0PHK<OK0NAG<OK0PPL<RZ6HXA<
SP7MGD<SQ5BAQ<SR9ZAA<SR9BKR<SR9BKA
Sent: 021229/2325Z @:SR9BKA.#KAT.POL.EU #:42357 [Katowice] FBB7.00g $:A80149SQ9
From: SQ9NRF@SR9BKA.#KAT.POL.EU
To : TECH@WW
Author: Jose Miguel Fernandez EA4BQN - JOSEMFG@santandersupernet.com.MODIFI
CATION.NET
I have always liked transceivers with tubed final stage, for several
reasons, but mainly because it is nice to hear from your correspondent: -
you have a very good modulation, ... it sounds different, ... the tubes'
job is noticed.
In addition of that I have to say that tuning these transceivers is not a
trouble for me, and also that up to now all the servicing needed by my3
tubed transceivers ( 2 Yaesu FT-101 ZD and 1 Yaesu FT-102) has been
performed by my self.
One day I noticed in one of the Yaesu FT-101 ZD a little hum which could
be heard at minimum volume level and when the radio was switched to
transmission with the PTT. Tt wasn't a great trouble but when you want to
have things working as better as possible you are forced to try finding
the problem.
The first suspicious were the electrolytic capacitors of the power supply,
and they were all replaced by new ones, even the pair of 200 uF, 500 volt,
on the high voltage rectifier, but unfortunately the hum was still there.
Then the following step was to checkthe final audio amplifier located in
the PB 1964 af unit, replacing the output module UPC2002, but no
improvement was obtained.
After checkingthe parts around this module I could verify that the problem
was in the 2 wire shielded cable connecting the input of Q01 (UPC2002) to
the potentiometer of volume in the front panel.
I desoldered this 2 wire shielded cable at potentiometer end and connect
there, two new 1 conductor shielded cables, soldering together the braids
of these wires at the af board end to a good ground point, for instance to
a solder lug inserted in the screw fixing the UPC2002 to the board.
Then I desoldered one end of the resistor R06 4.7 K and the negative end
of C07 1uF and soldered there the corresponding wires from the
potentiometer
After this operation the audio hum was removed.
I suppose this hum could be picked up by the original shielded cable along
its way to the volume control
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