{Collins} KWM-2A Very erratic transmit and "more"? What a mystery!



I've been working on a KWM-2A for several days now and it has nearly beaten
me, make that it has beaten me!

When I first got it from the owner, it was dead as a stone. Previously it
demonstrated erratic transmit and a delay before pulling plate current and
transmitting. He even made a video of it occurring to show me the problem.

The "dead as a stone" part was relatively easy to figure with an open 1 watt
resistor on the +275 volt line caused by a 2 watt that went from 47K to less
than 100 then taking out a 1 K 1/2 watt. R157, R122 and R12 in that order
affecting the receive +275 volt line and V3A where the R12 and 122 were
located. This brought the receiver back to life on all bands. The only
exception to this is that as volume was advanced past 2 o'clock, the audio
sounded like it was motor boating, perhaps a clue to what else is going on
or a separate problem. I was told it was doing this before it completely
quit.

As to the transmitter erratic operation, it continues to take a very long
time before it will transmit if at all. Max power is about 90 watts but only
briefly and then it fluctuates from zero to anywhere in between. In the
video it was about 5 seconds before RF output but now it may not show power
for a minute if at all, so it is getting worse.

In the transmitter the only thing I found so far is that C269, a .02 uf cap
had the top broken off but it did not seem bad as measured. This is the 6CL6
plate bypass cap. I changed it.

So what works and what is going on with the erratic operation? I have done
nearly everything I can think of and some "clues" as follows:

Note: All testing has been done in CW but voice does the same erratic thing

1. I have shorted out the ALC and removed the rectifier the 6BN8 V17A. I
measured ALC voltage to be about -1.5 vdc. I do see a very slow recovery to
receive from transmit similar to what is described in SB6 as excessive
screen emission

2. I have looked at every stage with the scope and see erratic operation at
every stage past and on the plate of V-4 the Xmtr IF amp

3. I have broken the signal path and injected signal at J22/23 pulling the
1st transmitter mixer V5 the 12AT7 same erratic operation

4. I have pulled the 6DC6 V7 Rx/Tx RF amp and injected drive from my HP8640B
to the 6CL6 driver and it is still erratic.

5. The side tone on transmit is clean and clear until volume is turned past
2 o'clock

6. I have looked at the PTO output and it is clean and stable when not
transmitting

7. I have looked at the VIF output and it is clean and stable

8. Now here is a clue...I see when transmitting what I think is a 120 Hz
waveform that all signals rides on and it is very erratic. I see it at all
stages when transmitting from V4 onward. 

9. I see that waveform (.5v p-p) at the junction of L36 and L37 and C127 at
the grids of the 6146's. That ripple stays there even with ALL tubes pulled
trying to eliminate any cathode to other element shorts!

10. I see that waveform on the power supply with .4 Vac ripple on the +275
volt supply and 5 Vac ripple on the 800 volt DC supply. Now, another clue,
when I try and transmit, the ripple on the 800 volt DC supply significantly
increases to 25 Vac p-p or more and jumps even higher than that. It also
jumps up and down on the +275 volt supply increasing to about 4 or 5 Vac. 

11. OK, it seemed like the motor boating and the ripple sure ought to be the
power supply but one scope I was using would not trigger on it and I
dismissed the supply (before I saw the ripple) because the filters looked
new. Later with my second scope, I saw the ripple and was convinced it was
the supply. I borrowed another supply today and saw the same garbage.
However, the borrowed supply has original filters and was not used for a few
years but could I be that unlucky? Also the voltage on the 800 volt supply
dropped to as low as 650 Vdc when this erratic attempt to put out power
occurs on the original supply, less so on the borrowed supply.

12. I did note a couple things that seem wrong such as C149, the PA plate
coupling cap to pi-network is a 4700 pf disc and not a 1000 pf as the
schematics show. That does not seem to be a factor does it? 

13. A clue of oddity. This KWM-2A has had the open frame relays replaced
with the enclosed. I checked that the coax's were on the right pins but here
is the odd thing. In transmit, I see a direct short from the pi-network
stators to the RF out connector with power off manually closing K3. BUT, in
receive, I see about 40 ohms from the RF output connector as well as the
Receiver Antenna connector back to the stators of the pi-network! I can't
figure this one out either! I've double and triple checked my pin
assignments to no avail.

14. I have swapped all tubes including the 6146's and 6CL6.

15. I have tightened all tube socket ground screws and shield can nuts.

At this point, I don't know what I am chasing. Do I have one BIG problem or
several little ones? Is this radio possessed? I am truly at the end of my
rope on this one. I've never had a radio beat me yet but this one is giving
it all its got.

I really appreciate hearing from anyone who has seen something like this
before and what was done to fix it, even if it was to bury the radio in the
garden!

73, Roger, NJ2R








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