KWS-1 Restoration

I found this KWS-1 at an Estate sale. Unfortunately there was no power supply with it so first thing I had to do was build a power supply....

This is a very close copy to the original Collins KWS-1 but it is built into an enclosure almost half the size. It stores easily under my desk.

Here is the LV chassis. It has the LV supply, filament supplies and screen supply with regulator. Every thing is overrated in true Collins fashion. Those black chokes in the center are the LV (275V) chokes! LV transformer to the right. Screen supply transformer, choke and regulator are on the left.

Here's a look underneath. All the wiring is Teflon! The PA filament transformer has series Thermistors installed to bring up the filament voltage slowly. The umbilical cable to the KWS-1 transmitter feeds in at lower right.

Here's a shot of the complete PSU in its cabinet. The LV deck is on top. The HV PSU and blower are mounted in the bottom. Front panel controls are identical to the Collins PSU (I need to find some white rub on letters for the front panel). The cabinet and panels have been black wrinkle powder coated (this used to be a rusty old cabinet!).

Here's a look in the back. This was taken before the PSU was completely finished. It is now neatly wired and the back door installed. A hole in the back door allows the blower hose to pass through. The HV transformer is clearly visible at lower left and is a genuine KWS-1 HV transformer (I was lucky enough to find this at an estate sale).

Here's a look in the front at the HV section. That's the HV swing choke on the right. HV rectifiers are bolted to the right side wall. Smoothing cap in the center. HV meter multiplier to the left of the cap and HV fuse to the left of that. Bleeder resistors are bolted to the left wall. The blower is mounted to a bracket behind the fuse and the HV transformer is visible behind the swing choke. All the HV wiring is done with 30KV+ silicon jacketed HV wire ($1.50/ft from Ebay).

The back door is hinged but also secured with screws. If you want to get inside this PSU, you have to get your screwdriver out. If you do that, you have to know what you are doing. You may have noticed that I have not included any interlock switches to prevent operation with the panels removed! Only the very foolish would poke around inside something like this with the power applied. I like to think I know better!

Updated 11-4-02 

The Power supply is now finished and I have started on the KWS-1 itself. There may be a more efficient order in which to do this but I'm going to describe things in the order I actually did them (this is my first time inside a KWS-1)!

This particular KWS-1 was acquired at an estate sale.  The matching Power Supply was no where to be seen. I have absolutely no idea of the history of this rig. The umbilical cable that connects to the Power Supply was included and appears to have been simply cut off right where it exited the Power supply! I was very happy to get this cable and connector with the rig. Having now had the opportunity to look it over in some detail, I can say that it looks to have been used very little and generally well taken care of. I don't think it will take much to get this looking and working like new again.

First step was to remove and test all of the tubes. Weak ones were replaced and all were put to one side. The tubes and shields were all cleaned.

All of the front panel knobs have been removed, cleaned and polished. The white lines were also refilled using a white paint stick.

                                        Paper caps replaced in Audio chain.

I usually replace all of the paper caps in my restorations. There are many reports of these failing though I have yet to remove a bad one myself! In the KWS-1, most of the paper caps are in the audio section in the lower right section of the chassis. There is also one cap in the PA driver section, one in the PA compartment and a couple behind the front panel. So far I have replaced just the ones in the Audio section and PA driver.

        If you are restoring a KWS-1 you might want to check the 4.7K screen resistor pictured here

I went through the transmitter checking most of the resistor values with an ohm- meter. All looked good in this transmitter except the driver screen resistor, a 4.7K 2W, it measured 6K and was replaced! A bit hard to get at but a lot easier with the side panel removed. There is sign of the driver cathode choke overheating at some time but the choke is still good.  I also cleaned up the wiring where the driver feeds the PA. It looks like someone worked on this before, The correct values were installed, just a little messy. I have not reconnected the driver to PA grids yet as I intend to remove the PA compartment - one of the tuning shafts is stiff, a common problem that causes people to over tighten the adjustment knobs which usually forces them out of round and chews up the gears! I'm sure it just needs lubricating but I'm not sure where. I can also see that a few screws are missing from the side of the PA compartment. These were probably missed when the HV feedthru cap was worked on. I'll remove it and attend to these problems. This would be an ideal opportunity to swap out the HV feedthru cap so I might do that also - it is a reported high failure item.

A few people have indicated to me that it is quite common for the mechanical filter in these rigs to drift off frequency. That got me worried, especially as I have no knowledge of the working history of this rig. I connected a signal generator into the filter and measured its response. Thankfully it is centered bang on 250 KHz and appears to have a very symmetrical response!

As you can see in this picture, I have removed one side and the top and back. All of the removed panels have been cleaned, touched up and made ready for re-assembly. The chassis on this transmitter was relatively clean to begin with but I still used Q-tips and Windex to clean a thin layer of dirt off the chassis.

I will be removing the front panel and other side panel soon. I also plan to remove the PA cage. Getting to the PTO in this rig looks like quite an ordeal and since I will have much of the rig already dismantled, an ideal time to work on it is fast approaching. With this in mind I needed to test the PTO! I hooked up a 12 V filament transformer to the PTO filament circuit and +275 V from a bench supply to the LV line (all the tubes are removed except the PTO, and PTO regulators mounted on top of the PTO). The PTO output is easily monitored with a frequency counter connected to the chassis BNC monitor connector. 

My PTO worked fine but had a run-out of 5 KHz from one end of the tuning range to other. The run-out appeared to increase linearly across the band, which is good.  There is a small adjustable inductor that sticks out the front of the PTO that allows errors of this magnitude to be trimmed out. Turn it 1/2 turn or so and re-check. This allows you to determine if you are turning in the right direction and also to estimate the number of turns to trim out the error. After a little bit of trial and error I have it down to less than a few hundred Hz across the whole band! Once you have the range fixed, loosen the tuning shaft to zero the KHz dial and retighten. Warm up drift looks to be just a few hundred Hz, quite normal I think.


I got the front panel off and PA cage out today.

Started by removing the balanced modulator assembly. Just a couple of wires to remove then off it comes.

Got to remove this to get to the screw underneath that holds the PA cage in place. There was a small piece of fiberglass insulation here (you can see the glue in the pic that held it in place). I have no idea why they installed this fiber glass?? 

Next unhook the wires feeding into the PA cage

I already unhooked the grid connection into the driver (sorry no picture of that).

Also had to remove the dead man switch on the side of the PA cage and the neutralizing wires that feed through the side of the cage into the anode compartment.

Next loosened screws holding back panel in place and then removed all screws holding PA cage in place.

Out she comes!

At first I was a little nervous about disturbing the PA cage gears but after having done so I can say that there is nothing to scared of! There are no stops or anything to worry about, the only synchronization required is the 2 loading caps which should both be at full mesh at the same time and the roller inductor sync as outlined in SB6.

I removed the front of the PA cage, removed gears, then cleaned and lubricated the bronze bearings. I had to re-position one of the loading caps slightly to get the gears to mate correctly.

Checking the small rotary inductor, I could see that it was not making a reliable contact (you can't use an ohm- meter for this - I used an inductance meter). I cleaned the contact wheel (removed tarnish with silver polish) and also cleaned the shaft it rides on. I cleaned the rotary coil itself with Windex and a clean rag. After I did all of this, the contact was 100% and the loading control turns very smoothly!

I concluded that the stiff PA tune control was due to the PA rotary coil. It was disassembled from the rear of the PA cage as you can see below.

This operation is a little tricky but avoids having to undo the front connections to the inductor. I removed the rear bracket holding the inductor in place first and then removed the rear porcelain section. the coil itself is then simply pulled out after first loosening the front flexible coupler. All the contact and bearing surfaces are cleaned. The coil itself is cleaned with Windex and a clean rag (I take a lot of time to clean between the turns). I also use a stiff bristle brush to clean between turns. The contact wheel was removed and cleaned with silver polish (remove tarnish). The rod that the wheel travels on was also cleaned with silver polish to remove the tarnish. You have to be careful with this polish as you can polish off the silver if you go at it long enough! As soon as the tarnish is gone, buff it out and use no more polish. after cleaning, I could see that one side of this shaft is wearing very thin on silver (the side bearing the brunt of the force of the wheel)  but by simply rotating it 180 deg, I will have a fresh silver face contacting the inner surface of the wheel (even though this rod is not prevented from turning, it appears that the springs hold it firmly in one position).

On the subject of what to lubricate the contact surfaces with, I concluded that silver conductive grease is the thing to use. I found some at a local hardware store, ~ $15 for a tiny syringe full! Its a bit pricey but seems to work quite well. Usually when you spin a roller inductor, you can see the resistance between the roller and coil jump up and down. After you smear a little of this grease on the wheel shaft  the contact is solid! I used used a small amount on the contact surfaces and on each end of the rotating inductor (do not use on the coil itself!).

After re-assembling the PA tune roller inductor it does seem to turn a little smoother than before but it is still quite stiff. The silver conductive grease helps but gives it a different feel. Without grease it is stiff and predictable. With the grease it turns more freely but is a little jumpy. I almost removed the grease but concluded that on the whole, it is better to have it. It certainly improves the contact resistance when adjusting. I can detect no breaks in continuity when adjusting both inductors which should make tune up very predictable.

I decided to go ahead and replace the HV feedthru capacitor.

I used a HV feedthru capacitor from Tusonix. It is rated at 2500V.  I took it to work and applied a 3KV spike a few times just to make sure and it survived without incident. It is much smaller than the original so I mounted it to a small piece of aluminum. I'm not sure of its RF handling capabilities so I also used a 1000pF RF doorknob also to decouple the HV line. The HV connector has been changed to a Millen connector. None of these modifications required any extra holes.

While I had the PA cage apart I also cleaned and aligned the plate connections to the 4CX250's. The plate connection ring snaps into a clip that looks like one half of a fuse holder. One of these connections was very poor, the clip was not gripping the anode ring tightly. I cleaned the anode rings, re-aligned the clips and adjusted (bent) the clips so that they gripped the connection on the anode rings tightly. Underneath the PA tubes I added a 470 Ohm series resistor in the screen supply (SB6).

The PA cage is now completely back together, rotary coils and capacitors are synchronized and it is ready for re-assembly back into the transmitter. 

I decided to go ahead and service the PTO - it squeaks a little when tuning. This might be just the slug rubbing against the coil form but it might also mean that a re-lube is required. The latter is all I really expect to do to it. I'll replace the bearings in the front if needed but I suspect they are fine.

It was an easy job to remove. First I had to remove the metal band the rotates the PTO assembly between USB and LSB. Then de-solder the connections, remove the 4 screws attaching it to the front and out it comes. It looks to have never been opened (the seal is till intact). 

-The PTO was rebuilt:

Here it is in pieces:

The squeaking I reported earlier turned out to be the slug moving on the lead screw. I removed the lead screw to clean and re-lube, also to inspect the PTO bearings. This is a fairly easy job. After removing all the screw on pieces there is a cir-clip on the front of the lead screw that must be removed. Just pry it off:

After removing this and the plate at the back of the PTO, the leadscrew and slug will fall out. The inside bearing should come out with the lead screw. The outside bearing will remain in the PTO housing.

Remove the slug form the lead screw (screw off the end) and clean the lead screw. I run the lead screw back in the slug several times without grease to pick up the old grease. I then apply new grease, run the slug up and down several times and clean it again - just to make sure I get all the old crud out of there. I then re-grease and re-assemble. I use a Teflon based synthetic grease but any high quality grease should be fine.

Inspect the bearings. Mine feel fine and so I will not replace. The bearing that came off with the lead screw will just slide off the lead screw. The bearing left in the housing will push out easily but need not be removed for inspection. Just make sure they both turn smoothly with no noticeable play or bumpy feeling. 

There are no paper caps to worry about in this PTO that I can see so one less thing to change.

As you can see, the desiccant bags (there are 2 in my PTO) are pink indicating that they have absorbed moisture. I have now removed them both and placed them in the oven on low heat. Hopefully they will dry out and turn blue. I'll reinstall them then. After ~ 5hrs, they are a nice dark blue!

When rebuilding the PTO there is a trick to re-installing the circlip on the front of the lead screw. You must install the circlip after you attach the rear plate:

There is a spring washer between the inside bearing and the inside circlip on the lead screw. This washer applies pressure between the 2 bearings and ensures no side play. In order to reinstall the front circlip you must apply pressure to the end of the lead screw to compress this bearing. You can see a small hole in the end of the lead screw that facilitates this procedure. 

I place a small center punch in my bench vise with the point pointing upwards. Carefully position the PTO above the punch with the end of the lead screw sitting on the tip of the punch. Place the circlip in position on the front of the lead screw. Push down on the body of the PTO and at the same time push the circlip into position. 

Apply some grease to the cam and follower mechanism (frequency tracking corrector) and check that everything turns freely.

Clean the rest of the parts ready for re-assembly.


After about 5 or 6 hours in the oven, the desiccant packs turned a nice deep blue color. I also baked the PTO itself for several hours at ~ 130 deg.F prior to re-assembly.

Here you can see the packs re-installed. I used Kapton tape to re-secure them. This is a high temperature tape used in the printed circuit business. I applied a very small coating of Vaseline to the 'O' ring seal and then replaced the cover.

The front bearing on the PTO cradle is a tight fit in the PTO body so warm up the PTO body prior to reassembly and it will slide on easily. The wires to the regulator tubes were re-attached first.

Here is the finished PTO, good for another 50 years!

I checked the front panel controls and all were OK except the carrier level pot, it was noisy (resistance jumps all over when turning). Luckily I had a spare in my junk box and it was replaced today. 

I removed all the parts from the front panel and cleaned it up and touched up the paint. I do this under a microscope with dental picks. You would be amazed how much dirt gets in the paint wrinkles and how much nicer a panel looks after cleaning. This particular panel had a bunch on solder on it (splash) which I originally thought was chipped to paint (to bare metal).


The 3 screws that attach to the PA cage in the audio compartment are shown below. They are just about impossible to get at with a regular screwdriver. I swapped them out for Allen head screws. This allows them to be torqued down easily. You'll notice that I cleaned up the glue and removed the fiber glass insulation in this area.  It serves no useful purpose.


After re-installing the PTO, you must reconnect the band that rotates the PTO with USB/LSB select. Here's a trick to doing this. Route the band and loosely install the 2 screws that hold it in plce (one in PTO, other in switch as shown below). Connect one end of the spring to the end of the band not shown in the picture below. Take a piece of lacing cord and first thread it through the hole shown in picture and attach it to the other end of the spring. Pull the lacing cord to extend the spring and attach to the other end of the metal band. After it is attached, cut and remove the lacing cord.


Well as you can see, it is starting to look like a KWS-1 again. Everything underneath is back together. I still have a few things to do topside and the front panel to reassemble and install.

The KWS-1 is all back together now! My wife borrowed my camera so I'll have to post pictures later.

One of my PA load/tune knobs is bent oval from being over tightened. I ended up putting a strip of shimstock in the hole before I inserted onto the 1/4" shaft. I used this one on the Load adj which is now very smooth. It is a little tricky installing these knobs. You want them to turn freely with not binding or mashing of the gears but at the same time if you make them too loose they may jump out of sync.

Today I powered her up for the first time! Only the LV so far;

Voltage checks came first. Exciter filament is 6.7V. A little high, I plan on adding a series resistor in the power supply to drop that down to 6.3V. LV is 288V, close enough to 275 for me. 

It is important to set the PA filament voltage right at 6V. When set to 6V I found that my filament monitor meter indicted 5.4V! I shunted R707 with a 39K resistor and the meter now reads 6V. This made me want to check the other positions. Here's what I found;

  Actual Indicated
Screen current 25mA 29mA
Grid current 8mA 9mA
Plate current 450mA 420mA

If I take the front panel off again I will pad the screen and grid shunt resistors. There's not much I can do about the plate current meter without opening up the meter. I already tried to open this meter to clean the inside glass but I could not separate it so I left it alone before I broke something! It's close enough.

The PTO alignment was slightly off since I baked it. A slight adjustment of the trimmer brought it right in. Warm up drift is about 300Hz drifting low from power up. Dial accuracy is well within +/- 0.5KHz across the band!

The HF crystals all look good and within +/- 1KHz of desired frequency (there are no trimmers to adjust them bang on).

Exciter alignment is progressing but not as quickly as I had hoped. I am able to obtain grid drive on all bands but I've found a few things that concern me;

The mic input stage oscillated at ~ 2KHz with no mic connected. Appears to have way too much gain. All of the circuit values/voltages check out good. With mic connected it does not oscillate but position 2 on the mic gain is already too high! I moved a few components and re-seated the grounds around the mic input stage and this appears to have stopped the oscillation. I'll probably substitute the 12AT7 mic input tube for a 12AY7 which has just a little less gain.

I have cleaned the band switch with De-oxit using small pieces of chamois cloth glued to toothpicks (this works much better than Q-tips, no lint). I scrub the moving plate of the wafer switches with de-oxit. The de-oxit is supposed to leave a lubricating film behind. This seems to have helped and I believe the bandswitch is OK now.

For some reason the 80m slug alignment does not converge as it should. I can not find any out of tolerance components. I was able to tune it and get acceptable performance (a compromise).

All I need to do now is verify the Neutralization adjustments and I'll be ready to turn on the HV!

Here's a shot of the unit on the bench. You can see how I finished the back of the Power Supply.


It's all back together!

I used all new hardware to secure the panels. Several of the original screws were chewed up. Touch up paint and new hardware really makes a restoration look nice. 

After spending a lot of time trying to track down an intermittent drive problem I rebuilt the crystal oscillator with new components. The oscillator output level would periodically jump around. I suspect it was a bad mica cap but it was just as easy to replace everything as the caps were buried underneath the resistors and chokes.

I just tuned her up into a dummy load and I'm getting about 550W out on 15m and almost 700W on 80m (if you can believe my Drake W4 wattmeter)!

Tomorrow I will add a soft-start circuit to the PS, trim the filament voltage and center the PA filament pot (it is almost at its end stop for 6V). 


All done! 

I made a few minor modifications to the power supply;

I added a .06 ohm resistor in the 6.3 V filament line and the filament voltage at the KWS-1 is now exactly 6.3V.

I added a 12 ohm resistor in series with the PA filament voltage adj control and it is now centered with exactly 6V at the 4CX250's.

I added a simple soft start circuit to the HV supply. It consists of 25 ohm resistors in each leg of the 240V to the HV and screen transformers that are shorted out by a relay after a short delay once the HV is turned on. I used a 120V DC relay to do this. A simple series diode/resistor and 56uF cap powers the relay and provides about a 200mS delay. This limits the inrush current to the HV transformer and should ensure a long and trouble free life.

I changed the antenna relay circuit to supply 115VAC as I am using a 115VAC antenna relay. The original Collins relay is a 115VDC! By removing the diode and capacitor in the relay supply circuit but leaving the 100 ohm series resistor in place, I get exactly 115VAC at the antenna relay. The series resistor should limit the inrush current and help protect the contacts on the internal relay.

I then added a small fan to the back of the KWS-1 - those 6CL6 drivers give off a lot of heat. I mounted a 12V fan to a bracket and secured in place using the left rear hinge mounting holes as anchor points. The fan is powered from a small bridge rectifier and smoothing cap soldered across the indicator light in the multimeter. This is a simple mod and is 100% reversible.

I'm a little concerned that the blower I'm using to cool the 4CX250's is not strong enough. According to info I found in an old QST, a pair of these tubes needs about 1.25" of back pressure and I have only got about 3/8" (measured with a very crude manometer). I'm not sure how much better the Collins blower is but I suspect not too much different. In order to generate that kind of back pressure you need to spin a blower at ~6,000 RPM and that would be very noisy I think. I'm going to look for a better blower that I can mount in the next room. If anyone has one that might do the trick I'd love to hear from you.

I went through the alignment one more time and then moved it all into the shack! I have paired it up with a very early 75A4 (S/N 019). This was my first piece of A-line equipment. It has all the mixer mods and SB's installed and works great.

Here it is all plumbed in. The Power Supply fits nicely under the operating table. I have not used this too much. It works perfectly and puts out a strong signal but the blower is a bit too noisy for my liking. I have acquired a a new blower that can supply more back pressure (required for 4CX250) and I plan to place it in the adjacent room and run a hose through the wall to the KWS-1.