[Collins] Audio squeal problem in KWM2

[James Miller]

From:  http://www.angelfire.com/de/vk3kcm/kwm2.html

24.
A few KWM-2 transceivers have shown signs of an audio squeal at the
instant the transceiver returns to receive from the transmit function.
Reducing the microphone gain momentarily will stop the squeal but it may
start again at the end of the next transmission. There is a feedback
loop being created from the microphone through the balanced modulator
and into the receiver first IF implifier and then to the speaker. A
check of this condition may be made by operating the transceiver in the
receive mode, disconnecting the antenna, set the MIC and AF GAIN
controls at 12 o'clock
and speak into the microphone. If the audio heard from the speaker is
quite loud, the following change will eliminate the effect. Install
R122, a 47k ohm 2 watt resistor (CPN 745-5722-000), from vector turret
E40 terminal D, to tube socket XV3 pin 7. This will put positive voltage
on the cathode of the microphone amplifier cthode follower tube when in
the recieve mode, thus cutting off the audio feedthrough. This change
will have no effect on the audio voltage to the VOX circuitry. (refer
ASAB No. 1006)

To see ASAB 1006 go to  http://www.collinsradio.org/html/asab.html

How did I find this?  Go to www.google.com and search for a few words
that represent the thing you want to know about.  For example, I entered
'kwm audio squeal' (without the '')  and in half a second google
returned three web pages that had the desired information.  It is the
best search engine I know about.  Good luck

73 Jim N4BE


Yong Surk Lee wrote:
> 
> Hello;
> 
>       I am repairing a KWM-2A for a local ham here in Korea. The radio shows
> the following symtom.
>      When the radio returns to receive from transmit, a loud audio squeal
> occurs from the radio speaker for about two seconds. Reducing any of the
> Mic Gain or AF Gain or RF Gain eliminates the squeal. I am aware that the
> R-122, 47K 2W resistor supposed to cut off the feedback path but the
> squeal still occurs. I tried decreasing R-122 to 25K but this only reduced
> the squeal, not eliminated it.
>       Someone mentioned about screen grid emission but what does the screen
> grid emission has anything to do with this problem?
> 
> Yong Surk Lee,Ph.D
> Professor of Electrical & Electronic Engineering
> Department, Yonsei University, Seoul, Korea
> Ham; WW6N & HL1FB, Director of Korean Amateur Radio
> League in charge of international affairs,
> Former head of KARL Laboratory
> Email ; yonglee@yonsei.ac.kr
> Homepage; http://mpu.yonsei.ac.kr
> Phone; 02-392-7194 Cellular; 017-236-2872
> Fax; 02-312-4584
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