Re: {Collins} KWS-1 Drift

On Nov 24, 2014, at 5:48 PM, Chuck Curran <ccurran@xxxxxxxxx> wrote:
> ... the 75A-4 is rock stable, while the KWS-1 drifts high.  When I open the lid and place a cooling fan on the RF deck of the KWS-1, transmitter starts drifting low.
> I expect I have marginal mica caps in the PTO--any advice or comments on this issue?


I look forward to replies from folks with more experience with this transmitter than I have, but I do have two suggestions and some comments:

The 70E-23 PTO uses the type 5749 tube.  This is an industrial equivalent to the 6AU6 WA.  Folks have reported that in the S-line some modest drift using 6AU6 tubes was reduced considerably when the 5749 or 6AU6WA tube was used.  So: if you find a 6AU6 in the socket, and have or can get one of the other types, try that.  At the very least you can try any other similar tube type you have on hand.  Some folks run a 6AH6 instead of the 6AU6 type in the S-line.  Note that V208, the crystal oscillator is a normal 6AU6.  Check that socket for a different tube number, and swap the two to see if it makes a difference.

The PTO plate voltage is fed from the two OB2 regulator tubes. It could be that these are not working right, or not working at all.  A digital voltmeter on that supply point may show you varying voltage as the thing heats up or cools down.  (A 7-pin tube extender is very handy for this sort of test.)

The variable inductor in the PTO is resonated to frequency by the parallel combination of one 335 pF and two 10 pF fixed capacitors, with a 20 pF cap isolating the grid from the tuned circuit.  The parts list in my manual does not show these parts, but I bet that the two 10 pF caps are temperature compensating caps, possibly selected during the manufacturing of the PTO.  If one of these has gone into retirement, you might well have frequency drift during warm up or cooling down as you report.  All four of these caps are inside the PTO housing and are not easy to access.  However, except for the (suspected) temperature compensating nature of the two 10 pF parts, and possibly being unmarked as to value or tempo, they are not magical, and dis assembly of the PTO housing may lead to solving the problem.  This would involve taking the PTO out of the transmitter.

The two screen bypass caps are 4700 pF caps and may be causing trouble.

Note that this PTO does not have an oven heater element as do some other Collins PTO’s.  The nature of the tuning slug and tuned inductor are likely quite stable with temperature, but it may not be at it’s age now.  My guess is with the PTO tube as it is most susceptible to the heating and cooling you tell about.

The filament regulator tube, type 3TF4A, manages the PTO filament current.  It has possibly gone into retirement also.  (If it’s open, the PTO tube will not light up.)  Being basically a temperature sensitive device, the filament regulator tube might cause frequency drift if it’s not working right.  The PTO will respond to varying filament voltage/current by changing frequency, at least to some degree.  (Note that the PTO filament is raised to a portion of the positive regulated plate voltage by R-131, R-132 and R-133.  We can assume this is to control hum being induced into the cathode circuit by heater cathode leakage or the like.)

The plate and screen dropping/isolating resistors R003 and R002 may be drifting with temperature changes.  These are more accessible in the PTO assembly than are the parts inside the can, I would think, but still would require removing the PTO from the transmitter.


Roy Morgan
K1LKY Since 1958

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