[Collins] KWM-2 + 312B-5

[Dr. Gerald N. Johnson]

I can't see how it would hurt the PA tube. The two tube heaters are in 
parallel, never changed by external wiring. And the heater circuits in 
the 312B-5 only are switched to show which VFO is in use by the dial lamps.

The big problem is that if the 312B-5 mode switch is in the RX 1, TX 2 
(up position) R275 is connected to -70. Worse than shorting the -70. 
Sure to blow the electrolytics on the -70 volt circuit from reverse 
polarity and high voltage together. They may have actually exploded. 
That's pins one and two. I see no electrolytics in the rig, so those are 
in the power supply. C6 and C7. The selenium bias rectifier is probably 
also gone, CR-1. Its best replaced with a three terminal terminal strip 
and a 1N4004 or 1N4007 or something in between those. There are chances 
that the resistor and the bias adjust pot in the power supply have 
suffered too. Presuming 516F-2 power supply.

Then pin 4 which may be grounded by the rig mode switch is connected to 
pin 2 (-70) in some positions of the VFO select switch.

For certain trying to transmit with the -70 gone will make the PA tubes 
draw current like they are shorted, more so with +275 on the -70 circuit.

And the KWM-2 acts very weird with low voltage on the -70, much of the 
TR switching is upset and it has parts of both the transmit and receive 
circuits on at the same time.

Unlike the 75S receivers, the bias voltage is not used for the audio 
output tube. It has cathode bias, so loss of -70 should not affect it. 
So the 312B-5 in the NB connection shouldn't affect receive audio though 
it may have affected the low voltage section of the power supply and 
power distribution in the KWM-2.

The switch contacts in the 312B-5 may have suffered.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association

On 9/4/2011 7:08 AM, Gary H. Harmon, Jr. wrote:
> He did plug it into the NB socket.  Took out one final, a resistor
> somewhere, and maybe the audio transformer.  Damage to the 312B-5 yet
> undetermined.
>
> Gary H. Harmon, Jr. - K5JWK
> 6003 Archwood
> San Antonio, TX 78239-1504
> 210.657.1549h / 210.884.6926c
>
> "Retirement means every day is a Saturday, except Sunday".
>
>
>
> -----Original Message-----
> From: collins-bounces at mailman.qth.net
> [mailto:collins-bounces at mailman.qth.net] On Behalf Of Dr. Gerald N. Johnson
> Sent: Sunday, September 04, 2011 12:28 AM
> To: collins at mailman.qth.net
> Subject: [!! SPAM] Re: [Collins] KWM-2 + 312B-5
>
> Foot in mouth. Its a 9 pin miniature plug. Same as used for the noise
> blanker next to the VFO socket which has a shorting plug in it. Sure it was
> in the right socket? Some of the pins are the same and some are wildly
> different. Also check for missing pins on the miniature plug.
>
> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association
>
> On 9/4/2011 12:03 AM, Dr. Gerald N. Johnson wrote:
>> Sounds as if he plugged it in with the power on. Not a good idea
>> because there's supposed to be a jumper plug when the external VFO is
>> not connected that changes the B+ and the heater wiring. Those are
>> necessary to allow the VFO selector switch in the 312B-5 to pick which
>> oscillator has power and which VFO dial is illuminated. From the early
>> KWM-2 schematic in the yellow book I can't see how it should blow one
>> final since the final heaters are in parallel. There probably is an
>> electrolytic being charged abruptly when the VFO is plugged in and
>> something to check is whether the guide key on the plug is present and
>> in the correct position. And when running on a 516F-2 all the tubes
>> are in parallel running on a 6.3 volt AC supply.
>>
>> 73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.
>>
>> On 9/3/2011 12:13 PM, Gary H. Harmon, Jr. wrote:
>>> A friend HAD a working KWM-2.  He plugged in an untested but clean
>>> original 312B-5.  There was a bang, an essence of fried something,
>>> loss of receiver audio and one final not lit.  Also, the cable from
>>> the 312B-5 was warm on the plug end.
>>>
>>>
>>>
>>> Anyone been down this road?
>>>
>>>
>>>
>>> 73,
>>>
>>>
>>>
>>> Gary H. Harmon, Jr. - K5JWK
>>>
>>> 6003 Archwood
>>>
>>> San Antonio, TX 78239-1504
>>>
>>> 210.657.1549h / 210.884.6926c
>>>
>>>
>>>
>>> "Retirement means every day is a Saturday, except Sunday".
>>>
>>>
>>>
>> _