KWS-1 Mechanical Filter Change

[n8fvj]

The KWS-1 only has 3kHz bandwidth on SSB or AM. (Block diagram shows no connection to filter on AM, but schematic does not). I plan to change the 250kHz center frequency 3kHz mechanical filter to 10kHz with Collins Radio Mechanical filter F250 F100 526-9410-00. However, the original filter is like nothing I ever seen and is a large cylinder. The replacement is like any 1960s mechanical filter. The original filter states input and output is the same impedance. Schematic shows only input wiring, but not output wiring. It just shows goes to tube V201. V201 shows input to grid and a coax grounded shield. This means the Collins filter has one side of filter output grounded and other to the V-201 grid?

Also, D-104 unamplified will not work to 100Hz unless it sees about 3.5 meg ohm input. I plan to change the mic amp 12AT7 grid resistor from 100K to 3.6 meg ohm. Any stability issues with the much higher grid resistor?

[ki4nr]

James

The filter can go in either way, the magnetostriction coil operates symmetrically. The newer filters are square. But more importantly I found this Collins service bulletin #7 that pertains to what your doing and is very important on AM/CW at lower power levels. Looks like you absolutely need to do this modification. The mic input grid leak resistor R-102 shows 1 megohm on my schematic. I'm not a tube circuit expert, but I believe you can change it to a 4.7 megohm with no ill effects to properly load the unamplified D- 104.


Collins service bulletin #7 on Eham and was recently posted

https://www.eham.net/article/48576

[n8fvj]

Thanks, will perform the filter coax mod.

[ki4nr]

Here's a decent schematic in PDF. This schematic shows the input mic resistor R-102 at 100K  the other manual I have shows it at 1 meg. But I think 4.7 meg will be fine. Maybe some tube experts could chime in on that.

[Tom W2ILA]

It looks like you have your KWS-1 up for sale/trade.  Did you implement the AM mods?

[n8fvj]

It looks like you have your KWS-1 up for sale/trade.  Did you implement the AM mods?

I have not performed the mods yet with idea to swap for a real AM transmitter. and new owner may want it for SSB, not AM. Negotiating a pair 4-400 modulated by pair of 4-400 HB transmitter. States size of a refrigerator and 1000 lbs. I hope it is on casters. All I want is 250 watt carrier this transmitter appears to be capable of 500 watt carrier- whew!

Update: The Bauer transmitter had the low level RF circuits removed and needs a 20 watt carrier from a transmitter. Total bust for me.

[kb3ouk]

Solid-state transmitter or transceiver makes good stable 20 watt driver. Been done by many before.

[ka1tdq]

I prefer marshmallow filled rice crispy square treats. Drive the grid of a tube rig well when coupled through an impedance matching device, such as an air coil. Slap a few turns around the cold end and 40 windings should do the trick to drive the grid into saturation. Word.

Jon
KA1TDQ