[Collins] KWM 2 issue

[Dr. Gerald N. Johnson]

On 1/31/2017 7:37 PM, Larry Shumway wrote:
> The RF gain is fully clockwise
> It worked normally until this issue popped up so that eliminates wiring
> issues.

Doesn't elliminate an RF gain pot with a broken resistance element on 
the low voltage end of the pot or a broken resistor in that part of the 
circuit.

Time to check all the DC voltages and then figure out what is wrong from 
them. See if the odd voltages change when pulling a tube.

The recovery when turning off still makes me think its AGC. The receiver 
is muted while transmitting with that same DC bias voltage so if the 
muting relay contacts aren't opening that would be a cause. VOX relays 
in S-line tend to be dependent on the two 68K cathode bias resistors 
from the low B+ to the relay tube cathode that over heat and over the 
decades go high in value and the VOX relay tube doesn't shut off. They 
should be replaced with a 33K 5 or 10 watt metal oxide film power resistor.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
>
> thanks
>
> -----Original Message-----
> From: Collins [mailto:collins-bounces at mailman.qth.net] On Behalf Of Dr.
> Gerald N. Johnson
> Sent: Tuesday, January 31, 2017 8:24 PM
> To: collins at mailman.qth.net
> Subject: Re: [Collins] KWM 2 issue
>
>
>
> On 1/31/2017 6:47 PM, Larry Shumway wrote:
>> This issue started recently.
>>
>> When I power up my KWM 2 the meter pegs and stays pegged.
>>
>> Nothing happens and the meter stays pegged until I power it down.
>
> Sounds like the RF gain control is full off either by being turned full
> counter clockwise or there is a wiring problem putting lots of negative
> voltage on the AGC line. I'd start with the power supply and make sure the
> bias supply filter capacitors are good and that the selenium rectifier has
> been replaced with a good silicon diode like a 1N4004 or 1N4007. The
> selenium rectifier went bad 30 years ago.
>
> The S-meter uses an IF tube as a vacuum tube volt meter to indicate AGC
> voltage which is generally zero at maximum gain. The voltage can go positive
> from grid emission but that pegs the meter below the zero setting. RF gain
> is reduced by applying a variable negative voltage to the AGC line.
>
> Low screen voltage could also be a problem but I think its a way negative
> AGC voltage. Trace the voltages with a vacuum tube voltmeter.
>>
>> As it powers down the I can hear the receiver for just a second.
>>
>> Any ideas where I should start.
>>
>>
>>
>> Thanks in advance
>>
>> 73
>>
>> Larry, N5DHY
>>
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
>>
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