EB5AGV's KWM-2A repair notes

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Collins KWM-2A

Collins KWM-2A

These are articles about my KWM-2A, posted by me in the Collins and BA lists.

Date: Thu, 2 Jul 1998 20:09:03 +0200
Subject: KWM-2A+516F-2+312B-4+MM-1 RECEIVED!!!

Hi gang,

Some time ago, I sent a message about a Round Emblem KWM-2A with accessories I had been offered... Thanks for all your messages of advice. Well, I finally bought the unit and the set arrived today.

The KWM-2A is S/N 39773, and seems a late unit, as it is RE, has plastic trim ring, plug-in relays, teflon wiring and dial brake. In some components there are mid 1974 date codes, so it should be from late 1974. Its front panel is in very nice shape, with all the knobs in good condition. Only drawback is that the S-meter crystal is broken (but the meter seems to move freely :-) ). It has one tube missing (V17, 6BN8), but I have spares here. Chassis is dirty, but it will clean nicely (I've checked it). All the Collins labels are there and in astounding good condition. Tube covers are also placed. Main trouble with the unit is that it is missing ALL the standard crystals (I knew it before buying, so the price reduction). But it has 9 extra crystals on the upper bank. If anybody is interested in swapping them for standard crystals, I will be very happy!. The frequencies marked on the crystals (in kHz) are: 7555, 9755, 11155, 14755, 9377.5, 12955, 13156, 9577.5, 10577.5. The unit is rack mounted in what seems an original Collins rack, as the transceiver fits perfectly on it, has a professional appearance, and it is painted in the Collins grey. The rack mount itself is in so nice condition that I plan to leave it rack mounted.

The 516F-2, also RE, is S/N 62655 and is also rack mounted, but has the cabinet (without feet). It uses a 'trick' to keep the unit attached to the rack and to the cabinet same time. It seems original Collins manufactured also. It has both tubes, no SS converted :-), and it works at 230VAC or 115VAC (nice for me, as in Spain the standard is 220VAC). As in the KWM, the trim ring is made of plastic.

About the 312B-4, it is mounted same way as the PS. The internals are really nice, and the labels are all like new. It is also RE and has a plastic trim ring.

The MM-1 is also RE, and it has cleaned to a shinny silver color. It seems a everlasting mike!. By the way, the support is mounted in the 312B-4 rack panel.

Some interesting things I found attached to the transceiver and power supply are Collins labels. There is one 'Collins Quality Control Record' 'to assist Collins in evaluating customer satisfaction' (I wonder what will happen if I return the label to Collins now ;-)!) , a 'Tested OK' label for the PS, a warranty card and a red label which seems related to the thread of the wood pieces in power supplies; it reads: 'IMPORTANT - Remove all packing supports that are inside this unit before placing into operation. Read your instruction book carefully'. Just curiosity items, but I had never seen any of these!.

Well, now I have some 'new' toys to play with!. I'll keep you informed about its restoration. The seller told me it has been stored for 10+ years, so it is time to fill the space again with its melodious RF notes ;-)!.

Best regards form a happy camper.


Date: Sun, 5 Jul 1998 18:28:34 +0200
Subject: Help: 516F-2 wiring

Hello Collins gang!

I have been working on my recently acquired KWM-2A set. Once the KWM-2A is finished (it is a 9+ looking unit :-)!. More information once I finish with the complements), I have taken over the 516F-2. I have all my Collins manuals at my winter QTH and now I'm located in my summer QTH :-(. So I have now here the 516F-2 schematic (I have one in my S-line). As here the power supply is 220VAC, I would like to know if my unit is able to work at 230VAC and, in that case, if it is properly wired. The reference of the power transformer in my unit is Collins 662-0249-010 (manufactured by Freed, reference 38578). By the way, the 516F-2 power supply of my S-line can't be wired to work at 230VAC, only 115VAC, so I use there an step down transformer. I remember that in the 516F-2 schematic is the info I need. So, I wonder if somebody could send an scanned schematic or tell me the wiring colors and their setup for 115 and 230VAC work.

Thanks a lot for your help!

Best regards.


Date: Mon, 6 Jul 1998 20:27:43 +0200
Subject: Help: 516F-2 trouble

Hi gang!

Today was the GREAT DAY!. After a complete cleaning of the KWM-2A set, I have connected the power to the unit and, after a while (about 20 s), I got a blown fuse :-(!. I was able to see an spark inside V1 (5R4) just before the fuse blew. As nothing appeared to be wrong with the unit, I put another fuse (I know sometimes these PSs are prone to arc, as happened to me with a KWM-2A I had some years ago), I replaced V1 with a brand new tube and connected again. Now, all the tubes in the KWM-2A lighted up and the S-meter went up, stay there a while and then went down. I could hear some background noise. NICE!!!. Well, it seemed to start working but, a moment later, I got another spark at V1 (this time the fuse didn't break). I guessed that perhaps the HV was a bit high, and measured it: 1050VDC instead of 800VDC and 340VDC instead of 275VDC... Filament voltage was right, about 6.35VAC (by the way, my PS has the dual primary winding, and I'm running it at 220VAC). I have an scanned schematic here (thanks, Jay!), but I have no idea of what happens in the PS to get a voltage so high. Any idea will be highly appreciated!.

Best regards.


Date: Wed, 8 Jul 1998 12:34:10 +0200
Subject: KWM-2A restoration (I)

Hi gang!

This is the first of a message series about my works on the KWM-2A set. Hope not to bore you too much :-)!.

By the way, I have a KWM-2A manual in the mail, but I need desperately (hope you understand ;-) ) some basic data to start troubleshooting the rig... Could somebody scan and send by e-mail the alignment instructions?. As there are lots of pages, perhaps the voltage and resistance charts, along with the first pages on alignment could keep me busy until I get the manual. The schematics would be also very interesting. THANKS A LOT for your help!.

Thanks a lot to all the people (15+) who helped me with the 516F-2 troubleshooting. Summing up, reported possible causes to the higher than usual voltages were, in order of nominations:

-Shorted choke(s)
-Shorted 0.05uF/1000V capacitor (the one that is across the HV choke)
-Shorted 6146B
-Non standard electrolytic capacitor values

As I have an S-line, I took its 516F-2 to start working on the KWM-2A. But I did some checks to the 'defective' 516F-2 and the chokes seem fine: 150 Ohm for L1 and L2 and 17 Ohm for L3. Also, the 0.05uF capacitor is not shorted (at least at low voltages). But I wanted to check the KWM-2A, so I left the PS trouble apart for a while...

Well, then I did what, IMHO, should be done first: check tubes!. I didn't do it before as my tube checker was not at my summer QTH shack, but I went to my winter home and got my TV-7C/U along with some spare tubes. The results were amazing for me. The unit had no less than ELEVEN bad tubes (from a total of 18)!!!. There were shorted ones (including a 6146B) and weak ones. For your records, FOUR 6U8As were weak. Some time ago I read about 6U8A high failure rate. It seems true!.

So, once all tubes were carefully checked (including the replacement ones; hint: don't asume that a NIB tube is good. I had a 6AZ8, manufactured in the 80s, which was bad), I connected the KWM-2A with my 'good' 516F-2. No sparks. Nice!. And the KWM-2A receives (sort of ;-) )!. The calibrator signal is audible up to 14 MHz, but _very_ low. I could hear some strong BC SW stations near 7MHz, so the basic functions should work. USB and LSB work on AM signals. The S-meter had a broken glass and the arrow completely bent; I repaired it with a transparent plastic piece (until I get a correct replacement for the glass) and lots of patience to carefully straighten the arrow and paint it black. The result is very good :-). It works (it goes full up on power on, then full down after a while), but I get no indication on received nor calibration signals (probably as they are too weak). Low B+ voltage is about 280VDC, which seems very good. How I wish I had the schematics to continue my work!!!.

Well, that is all for now. Keep tuned!.

Best regards.


Date: Sat, 11 Jul 1998 14:08:23 +0200
Subject: KWM-2A restoration (II)

Hello gang,

First than anything else, thanks for your help, specially to Walt W2OKF and Ian VK3KCM for the information they sent by e-mail and FAX. Now I have Chapters 3 and 4 of the KWM-2A manual and also the schematic.

Good news is that the KWM-2A is starting to work. I get good receiving signals at 80, 40, 20 and 15 meters. I've removed one mod made to T1 (an added 390 Ohm resistor in parallel with the output and a capacitor in parallel with the input). But now I'm having trouble with the alignment procedure. When I put the rig in transmit and tune it, in LOCK mode the GRID current starts falling fastly (from a very low value), until it reachs 0, independent of MIC GAIN position. In TUNE mode, the GRID current also decreases, but doesn't reach 0. As in the aligment procedure I should use the GRID current in LOCK mode to tweak the slugs and trimmers, I can't do it properly. I've tried it in TUNE mode, but of course something is not working fine. It seems as an ALC voltage decreases the power output gradually. As a sample, I can get about 60W in 3.7MHz at start, in LOCK mode (once properly tuned in TUNE mode). But immediately, it starts to going down, to reach about 20W. If I change from LOCK to TUNE mode with the meter in GRID current, it starts to raise its reading gradually, so I need to lower the MIC GAIN to maintain it at low levels. Any idea of where to look at?.

Thanks for your help.

Best regards.


Date: Sun, 12 Jul 1998 20:53:42 +0200
Subject: Help:KWM-2A unstable grid drive

Hello gang,

My last message about the trouble with my KWM-2A was replied by two person. Probably the explanation was not too good. I'll try to clarify it a bit. Sorry if first message was not too clear...

The problem is that the grid drive is not stable, i.e. I can't get a constant reading in the meter. In TUNE mode, as I increase the MIC GAIN (once properly tuned), I get an increase in grid current, but it starts to decrease immediately. Power output decreases same way as grid current. In LOCK mode, I get just a second of grid indication, then it goes zero and remains there. But power output continues decreasing from about 60W (at 3.7Mc) to about 20W. Using the mic, I can get peaks of about same power (60W). Looking at the schematic, I saw that V17 (6BN8) acts as an ALC rectifier, so I changed it with a new one, without any change. Indeed, without V17 plugged, I got exactly same response from the rig. I have tried to change the driver tube (V8, 6CL6), both PA tubes (V9, V10 6146B), and also both TX mixers (V5, V6 12AT7) without any difference. In fact, checking the signal at the driver (V8 6CL6) plate, shows a clean and constant one.

Please, any idea will be welcomed!.

Thanks a lot and best regards.


Date: Tue, 14 Jul 1998 07:54:15 +0200
Subject: KWM-2A story continues...

Hello again!

I've received some help messages about the trouble I have on my KWM-2A (thanks a lot!). I've done more checks and here you have some results:

-Screen voltage varies up to 38V, when tuned for 'maximum' power (60W in my case, which soon decrease). If I put the oscilloscope in the PA screen, I can see the variations as I talk in the mic. Is this voltage supossed to be constant?.

-Following the advice of Scott Prather, I disabled the HV to the PA by pulling the 5R4 in the 516-F2, and cut the jumper in the KWM-2 between J5 and J6 (this disables screen voltage to the 6146's). Once done, I checked the grid indication and, to my surprise, I could get stable readings, both in TUNE and LOCK mode. By the way, once there, I performed the alignment procedure (slugs at 3.7Mc and then all the trimmers in all bands). The rig is now receiving very well. Of course, replacing the PA tubes returned the grid drive trouble.

-The signal at the 6CL6 plate is of about 50Vpp... Is this enough?. This signal is steady, BTW.

So it seems my trouble is at the PA cage... Could a leaking capacitor be the culprit?. Which ones I should look at first?. One thing I found is that the resistor in the PA screen supply had drifted from 820 to 1655 Ohm... I'll replace it today. Which is its power rating (I have not part list)?.

Some of you asked about the test equipment I have available, in order to assist me better. Well, in my current location (I'm not at my 'main' shack), I have a ME26D/U VTVM, HP-5328A frequency meter, OS-2100 30MHz dual channel scope, ADVANCE HF generator and a Fluke 77 DVM. I have also a TV-7C/U tube checker along with spares for all the KWM-2A tubes. So the trouble is not the test equipment, but the user :-(

Thanks again for all your help and, please, forgive me to bore you so much.

Best regards.


Date: Tue, 14 Jul 1998 23:33:00 +0200
Subject: KWM-2A... WORKING :-)!!!

Hi gang,

I can't take out the smile from my face ;-)!

I'm sure some of you will also be happy to stop receiving my boring messages ;-) ;-)

Well, let's go BA!. Starting by the end, the KWM-2A is generating 120W in 80 meters and about 80W in 10 meters. Near 100W in the other bands. The finals could be a bit weak, but for my use (mainly 80 and 15 meters) it is just fine.

Now with the details. It has been an accumulation of 'bugs', to call them some way:

(1)The 516F-2 power supply is NOT defective. So, how did I measure 340VDC in the 275VDC line???. I can only guess that, as one final was shorted when I did the test (of course, I was not aware of that then), my readings were distorted.

(2)The 'good' power supply (the one I have usually attached to the 32S-3) is fine BUT the downconverter 'transfomer' (220 to 115VAC) was not so good (in my other QTH I have a real 750W isolating transformer which works flawlesly). It is not only a transformer but, literally, a 'power stabilyzer'. Well, I had observed too much screen voltage decrease when transmitting, modulated by voice peaks... Trouble was NOT at the secondaries but in the primaries of the 516F-2!!!. The 'stabilyzer' (which I have just trashed ;-) ) was not able to follow the demand variations of the PS, so the output voltages were also 'dancing'... The result was a poor screen voltage regulation, a decreased HV and also some flicker in the dial lights... I know I should have been suspicious on that one!. But I was polarized towars a KWM-2A failure and didn't see the obvious trouble... Live to learn :-)!.

Well, so the KWM-2A _really_ had these troubles:

*10 bad tubes (8 weak, 2 shorted).
*1 missing tube.
*Broken meter glass and bent meter pointer (carefully repaired; now it looks very good except for the glass, which has been temporally subtituted by a transparent plastic piece).
*1 resistor out of specs (R148 in the PA screen circuit; 1655 Ohm instead of 820 Ohm).
*Modification in T1: one resistor changed and a capacitor added. It has been removed.
*Dust in K2 and K4 relays. Visually, they were perfect!. Bright gold... But I got intermitent S-meter readings and the sensitivity was not constant... Tapping the relays it returned, so I opened them and cleaned contact by contact with paper soaped in contact cleaner. Now they work fine.

I'm not sure if the rig needed a complete alignment. But I have done it already and it seems to work fine, so it doesn't matter too much. Anyway, after a 10+ year storage, I think it is a good idea.

I'm very happy now, not only for the working rig ;-): I have learned a lot about the KWM-2A circuit, and this is for me very good. And I have also found lots of people willing to help. Without your support, I would be completely lost!. THANKS!!!.

Best regards from a _very_ happy camper. 23:35 local time... time to go bed... and have nice Collins dreams... a working KWM-2A on the desk... ;-)


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