Re: {Collins} KWM-2 No Grid Drive


I take it that the receiver works fine since you do not say anything about
that not running well.
That means the XTAL OSC is running fine in receive and since the B+ for the
XTAL OSC is derived from the TR +275 line and does not change, let's assume
for now that the XTAL OSC is running well.  That grid level is a little low,
but explore that later.

I also take it that the rig switches back and forth between T & R so the VOX
is not stuck on.  The VOX plate being down at 15.2 and not up at 82 says
that you probably have the VOX  CONTROL UP A BIT - Check that and turn it
off for now.

The grid of the TRANSMIT MIXER V6 being at +5.3 volts as opposed to -0.5
worries me. What is the voltage in transmit mode (TUNE LOCK) on Pin 8 of
that same tube V6 (12AT7). Also give me a reading on Pin 6 of V6.

See if you can measure R31 with the rig turned off.  Before you do that,
tell me what the Pin 2 of V6 reading is in TUNE LOCK.

Do you have a scope?

Standing by for some data.

Bill Carns, N7OTQ (Trustee K0CXX)
Past President, Collins Collectors Association
Founding Board, Collins Radio Heritage Group
Editor, Signal Magazine (Retired)
Wimberley, TX
512 618 2762  (Cell)
512 847 7010  (Home)

-----Original Message-----
From: Collins [] On
Behalf Of Randy Hollingsworth via Collins
Sent: Friday, October 13, 2017 1:45 PM
To: collins@xxxxxxxxxxxxx
Subject: {Collins} KWM-2 No Grid Drive

I have a very clean KWM-2 that I purchased several years ago. About 2 years
ago, I finally got around to re-capping the KWM-2, and following that the
unit was working fine. I was out of town for a couple of weeks, and when I
returned and attempted to tune the rig, there was no grid drive. Given other
priorities, the unit went back on the shelf for a couple of years, and I am
just getting back to it. When tested, the unit still has no grid current at
all in tune on any band. Following previous advice on this reflector, I
pulled all the tubes and checked them on a high quality Hickock tube tester,
all showed good. I also exercised the band switch and tightened all the
chassis screws as instructed with no affect. I then proceeded to take
resistance measurements per the manual. Resistance values in general were
high although most were still inside the 20% tolerance. About a dozen were
outside that range, with 4 readings that were nearly double the spec.
However, none appeared to  be so wildly off that they would cause the unit
to have no grid current at all. 

As such, I proceeded to take voltage measurements. For these measurements,
the KWM-2 was running off a variac that was set to be sure that the unit was
getting the spec 275 volts in from the 516F-2. power supply. As I guess
would be expected, many of the voltages were lower than the spec, although
again, most were in the ballpark with respect to the specified value in the
manual. Of the tubes that appear to be in play when the unit is in the tune
mode, the most suspect readings were from pin 3 on the 6146W finals - only
80 volts vs. the 240 volt spec. I traced the voltage level back in the
schematic, looking for where it dropped off. The voltage was at the correct
level at the jumpered J5 - J6 jacks, but prior to the DR7 diode, the voltage
was down to the 80 volt level. The only thing in between these points is the
S9A mode switch wafer, so I applied deoxit to that wafer switch and
exercised it multiple times hoping that oxidation on the switch wafer was
the cause. Unfortunat  ely, this had no impact. I then checked the value of
R70 and it was well within spec at 21.8K. The resistance value for pin 3 of
V9 and V10 was also in spec at 14.7K vs 15K, which would indicate that R69
is also in spec. So, those two resistors do not seem to be the cause of the
low voltage level at pin 3 of the finals. At this point, I am at a loss to
explain what might be the cause of the low voltage level on the finals. 

The only other transmit voltage levels that appear to be seriously off are
- pin 7 of V6 (2nd mixer), where the voltage is 5.3 volts vs. the spec level
of 0.5 volts
- pin 2 of V13 (xtal osc) where the voltage is-0.5 vs the expected 2.5
- pin 7 of V14 (vox) where the voltage is 15.5 volts vs the expected 82

So, any help on where to go from here would be greatly appreciated. All of
the resistance and voltage measurements are available in an Excel
spreadsheet if someone wished to look them over. 

Randy, K0FA
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