[Collins] Fw: Some notes on troubleshooting my KWM-1

[Dr. Gerald N. Johnson]

I hate to see micas going bad, but they can. They can have been bad from 
new also because mica sheets are a natural product and not always 
perfect and inclusions sometimes are conductive.

Coupling capacitors are always suspect, no matter the material because 
they do drive the grid more positive than designed which in many tubes 
tends to increase gain and plate current. In the audio output stage that 
increased plate current makes the tube run hotter and in some cases has 
fried the audio output transformer as well as the tube.

The toughest test of a capacitor is to hook one wire to a high voltage 
supply, as high a voltage as available up to the voltage rating of the 
part and ground the other terminal with a VTVM on a voltage scale. I've 
always used a Heathkit V7A with 11 megohms DC input resistance. I start 
with the meter on a range that includes the test supply voltage and as 
the capacitor charges, I increase the meter sensitivity. I consider a 
capacitor very good if I can apply 450 volts DC and the VTVM stays on 
scale on the 1.5 volt range. That's a leakage resistance of a few 
gigohms and orange drops will do that. Some of the voltage I see comes 
from line voltage changes changing my test supply's output voltage 
because its not regulated.

Positive going grid voltage can be clamped or limited by the grid 
conducting but most of the time there's enough negative bias developed 
in the cathode (with the exception of the early S-line audio output 
stage with no cathode resistor) so leakage or faulty bias (check the 8 
mf capacitor in the S-line bias circuit, if it leaks it cooks the audio 
output tube). The control grid doesn't clamp until is more positive than 
the cathode voltage.

Another source of a positive going grid bias is the tube itself. If 
cathode material gets sputtered on the control grid wires (that's 
enhanced by running at high current) the grid will emit and drive the 
bias positive. I've seen this happen in my 75S-3B several times due to 
grid emission of 6BA6 IF tubes that counteracts AVC voltage. There has 
been a tube tester specialized for checking grid emission (most common 
testers do not check for grid emission) and I've know TV service shops 
to depend almost entirely on that grid emission tester finding more set 
faults with it than any other piece of test equipment for tube sets. The 
handiest indication of grid emission in a receiver with AVC is the 
S-meter zero drifting negative. That can be zero circuit drifting but is 
more likely grid emission in an AVC controlled tube.

As for meter resistance, Collins recommended the HP410B VTVM which has 
about 100 megohms input resistance on DC. The Simpson 260 input 
resistance varies with range, being 20K per volt of full scale. So its 
100K on 5 volts, 1 megohm on 50 volts, and 10 megohms on 500 volts while 
VTVM inputs tend to have the same resistance on all ranges.

The temperature stability of the miniature ceramic may not be up to the 
normal stability demands of the KWM-1. It may prove necessary to go to a 
silver mica or a high stability disk ceramic. General purpose ceramics 
can show wild variations in capacitance when heated. Like changing 75%.

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

On 5/19/2011 12:44 PM, AI2Q wrote:
>
> ----- Original Message -----
> From: AI2Q
> To: kwm-1 at yahoogroups.com
> Sent: Thursday, May 19, 2011 1:43 PM
> Subject: Some notes on troubleshooting my KWM-1
>
>
>
> I have made progress with troubleshooting and repairing my late model KWM-1.  I had put it aside for a few days in order to get a fresh approach. It was becoming a headache. So far, I think I have spent about 100 hours or more troubleshooting this rig.  Many evenings. No progress---until now.
>
> I had been grappling with the problem that the receiver's sensitivity drops off over a period of ten minutes, accompanied by a very slight occasional crackling noise. The first IF following the rig's mechanical filter at V13 includes a gain pot R136 in its cathode circuit, and it always had to be cranked full open to get enough sensitivity. Nonetheless, all the voltages around this tube proved to be okay.
>
> The plate output of this 1st IF amplifier has a parallel-tuned LC circuit L16 that resonates at 455 kc, and it's de-Q'd by a 100k parallel resistor. But I could not ever really peak it up. I had taken the coil out of the can and out of the set to eyeball it, and I was able to adjust it nicely in a series test circuit using my signal generator and scope, so I knew the LC circuit was good. I re-installed it.
>
> I opened the AGC loop by pulling the AGC rectifier tube, and made lots of IF strip tests. No joy.
>
> Then I noticed that with a high impedance voltmeter there was a few millivolts of positive DC on the grid of the second IF amplifier!
>
> NG.
>
> So, I cut the 510-pF mica coupling cap from the 1st IF plate circuit, and---lo and behold---with my high input-Z DMM or old Heathkit VTVM, I could see positive voltage on the other side of the cap that fed the grid of the 2nd IF.
>
> So, I installed a new cap---and now the receiver works like a champ.  Yipppeee!   Indeed, I had to crank the gain pot back, otherwise the IF stage had so much gain that it would take off.
>
> As for the troubleshooting analysis, it gets interesting. When the mica cap was in the circuit, the grid showed a few millivolts of positive voltage. That positive voltage was my tip-off. Prior to that I was measuring plate and screen voltage, and when I poked at the grid I may have used my old Simpson 260, which is a 20,000ohm/V meter. That loading didn't let the meter reveal the millivolt level on the grid.
>
> Later, I made my tests with an old Heathkit VTVM and a Fluke DMM, both of which have much higher input impedances, on the order of 10 Mohms and higher.
>
> When the capacitor was cut open on the grid side, the voltage on the open end jumped up to about 100V or so (the preceding stage has about 280V on the plate).
>
> When the leaky cap was in place, it must have been drawing electrons from the cathode. This current through the cap might be quite a bit. It wouldn't necessarily affect the B+ of the previous stage, which is stiff, but the cap path would be "stealing" current from the following IF amplifier. In effect, the grid of that second amplifier would be acting as a defacto plate or screen. The voltage drop in the 2nd IF's cathode circuit would be low to begin with (as it essentially sets the bias for the tube), and so the voltage on its grid would also be quite low (as the IR drop inside the tube was probably quite small). Thus the millivolt reading---which I overlooked. Simple series circuit Ohm's Law, eh?
>
> The effect that kept me coming back to that 1st IF was that its LC circuit in the plate would tune very, very broadly. Also, the gain pot associated with the stage was at its extreme setting.
>
> After enjoying listening to the KWM-1 receiver, I decided to see how the TX section worked. It worked for a while, but the RF output was erratic.  Finally, the output disappeared entirely. I traced the problem to an inactive audio oscillator.
>
> With some trepidation, this morning I removed the small block mica cap C111 from the audio phase shift oscillator V20B. There are a number of RC elements that ultimately give the requisite 180-degree phase shift, but my supposition was that that cap connected to the plate of the 6U8 would be most susceptible to leakage due to B+ stress.
>
> BTW, phase shift oscillators produce very clean sine waves. As such, they can be applied to a balanced modulator, and the latter will deliver a carrier. If you applied other than a pure sine wave, the balanced modulator would create sidebands, as with voice.
>
> Anyhoo, on my old Eico cap tester the 430-uuF block mica closed the green eye almost immediately when I applied B+ for the leakage test.
>
> I cut it out (had to unscrew the interfering headphone jack) and replaced it with a miniature ceramic disk cap.
>
> BINGO!
>
> The oscillator started immediately and produced a nice sine wave at the balanced modulator. The attached image shows the probe point, the scope trace, and the density of components under the chassis.
>
> The sinewave serves two purposes. First, it generates a carrier for tune-up and for CW operation. Secondly, it provides sidetone in CW mode.
>
> Perhaps these notes will be of help to others.
>
> Vy 73, AI2Q, Alex
> Member: ARRL, FOC, RSGB, CWops, QRP-L, Antique Wireless Association, Wide Area Amateur Radio Network
> http://home.roadrunner.com/~alexmm
>
>