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

[AI2Q]

----- 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