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KWM-2/2A Information

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Information index:
KWM-2/2A Feedthrough.
RIT for the KWM-2/2A
Rejuvinating problems
Getting the mid dial on the PTO to zero beat with the xtal calibrator.
The trials and tribulations of SB8
KWM-2/2A Audio and AGC problems.
VOX problem
ALC meter readings
VOX problems.
PA load trimmer adjustment
A bit of trivia..1977 price list

KWM-2 IF Feedthrough document Here is the KWM-2 IF Feedthrough document from Dick Benson. It's in a PDF file which means you need the free download from Adobe to read/view a PDF file. It should print out as a polished professional looking document with good looking diagrams, etc. * For anybody who's new to this "Adobe Acrobat Reader" and PDF file business, here's where you get the reader. It's a "must have" so take the time to do the download and instal the software as it produces beautiful near professional quality printed documents and screen viewed documents. I'd suggest downloading it to your hard drive and opening it from there rather than trying to open it directly from the Web site, but it can be opened here without any trouble.

RIT for the KWM-2/2A This is also in a PDF format similar to the above document. If you have Adobe reader (free at you can read this document in a nice clear form. Actually, it even prints better than it looks on the screen. RIT document

Rejuvinating an old KWM-2A I would like to share an experience encountered while rejuvinating this old KWM2A. This deals mostly with the problem of old/over-heated resistors and the problems they can cause. I would recommend spending some time to check the values of any resistor in your KWM2 (or any old radio for that matter) which appears to carriy power to tube circuits, especially 1 or 2 watt resistors. In my unit, I have discovered that several resistors feeding power to tubes in several stages had gone bad internally with little or no evidence of exterior burning! I had to use two ways of discovering bad resistors (other than clues caused by misbehaving circuits): (1) When moving the resistor around slightly with a pair of needle nose pliers, it breaks or crumbles without a lot of unusual force applied. or (2) An ohmeter check shows a change of value outside of tolerance. I say again: these resistors looked perfectly good and clean on the outside, no evidence of burning or swelling you usually see when a resistor has experienced heat failure. Also, in some cases, a circuit was apparently designed to use resistors which were rated too low in power, and these were replaced with higher wattage values. The following are some of the resistors in my KWM2 that had to be replaced eventually after discovering their internal failures or drift in value (all of these looked OK to visual inspection): R-148 820 ohms 2W - The value had drifted up to 1400 ohms. Replaced with 820 ohm 5-7 watt wirewound. )This resistor and a diode was installed in the PA screen circuit as a part of a service bulletin. This resistor heats during transmit.) R4 (68K) and R7 (47K) both 1/2 watt - Both had internal value drift as a result of over heating. Replaced with 1 watt units (caused mushy transmit audio). R175 (4700 ohm) - One day the transmit audio just quit. This resistor had broken down internally, no external evidence, however. R50 (47K 1/2 W) (in the tone osc.) - Signs of aging. Replaced with a 1W unit (also recommended by Collins mod.) R20 and R47 - Two 68K 2W resistors in parallel - Replaced with 34K 7W (or another 2 68K of higher wattages) - Caused overheating and instability in VOX. There may have been others I lost count of. Of course, the old capacitors can also be a culprit, but I have another long list for that. Another note: My KWM2 had been run for an undetermined period of time by the previous owner with a 516F2 that had the solid state mods (bad news in my opinion). These failures could have been caused by excessively high B+ (supposed to be 275V). Of course, I have removed the solid state mod on the 275V side and am back running the tube rectifier, as well as a series dropping resistor (in lieu of a variac or equivalent on the AC side). Bottom line - If it's a large power resistor (1 or 2 watt or greater size), check it with an ohmeter. If it looks like a resistor that is in the power side circuit feeding any tube plate or screen, check it with an ohmeter. If it's a resistor like this, and is 1/2 watt and appears bad or out-of-tolerance value, replace it with next higher power rating. Or when in doubt, just replace them anyway (with next higher power ratings).

Getting the mid dial on the PTO (ie 100 on the dial) to zero beat with the xtal calibrator. It may be that since I have repaired the coil T301 after damaging it I may have inadvertently changed it to my advantage. Now I have perfect alignment and it peaks at 2.630Mhz. To get the resonance down when the dial was aligned. I cheated a little by adding a 5pF cap across T301 to bring the resonance to 2.630Mhz. (The PTO output increased also.) This was the compromise which balanced the drop at both ends of the dial (ie 000 and 200). Seem you will need to make a compromise. Or play around as I did. You have to add the 5pf across the entire T301 or from the center tap Wondering if I can achieve the same by adding it externally at the PTO output to avoid going into the box. Also it looks like you had to remove the guts from the little PTO box to get to T301. I found that with all screws taken out, it all seems to come out in one glob. But it isn't comforting to do that! I had to get the PTO's gut out and it's not comforting!I put the 5pF cap across the whole coil. Do this only if the resonant frequency,after you align the dial, is moved up in frequency. The cap will bring it back down. It's a pain!!! You can run the PTO out of the box. This is how I roughly aligned it.When you put it back in the box it will only shift by 20 to 40 hz! so do all tuning out of the box. I had unsoldered the PTO from the M2 and tack soldered it to the bottom side of the M2 so I could work with it. Then at the end put it back in the box and resolder the wires. Well I tried your procedure and there seemed to be some improvement but I'm still off a little at the mid point. Mine was reading about 1.1 khz low at 100, but I got it to about 700 hz off now. T301 seems to have an effect on the PTO output amplitude and in fact that may be its main purpose (as an output tuned circuit for the PTO). You may notice the S meter level change as you adjust it, indicating the PTO level is changing. I couldn't get the frequency pull enough with T301 to get the linearity any better without loosing an S-unit or two on the meter. But it is a little better for linearity now anyway. I'm well "within specs." but perfect tracking would be nice. My most recent KWM2 manual shows a change to a capacitor C304 from 200 to 50 pf, and a test select for resistor R301 inside the PTO to "improve tracking and stability" it says. You may have mentioned that before too. I don't feel up to going inside mine again right now though. I had to rebuild the PTO T301 coil on my KWM-2A. In the process I had to re-align the PTO. I was able to get the 000 and 100 and 200 on the dial to exactly line up on the xtal calibrator. 1. Follow exactly the VFO end alignment procedure as outlined in the manual. 2. But, in between going from 000 to 200, stop at 100 on the dial and tweak T301 until you have the xtal calibrator zero beat at 100 then goto 200 and start again back to step 1. then 2. then 1. then 2. until the dial readings are exact. Took me 25 minutes of back and forth adjusting! It's painful! But you'll have a very accurate dial. I would recommend tuning in WWV to calibrate the xtal calibrator first, then do the above. Disclaimer: If T301 slug 'does not turn' do not force it (like I did) you will break the coil form (like I did)!!! You will need to disassemble the PTO and remove the coil and free up accurate at 000 and 200. Now it is exact. Hope this is useful. John Bielefeld KA4DGF Jim N4BE

The trials and tribulations of SB8 Does any one know the time constant of the decay time on the AGC of the receiver side of a KWM-2A? Does any one have this info? (With the SB-8 change.) I do not have another M2 to compare to. But my AGC seem to decay rather fast. So with the RF gain control set to full (clock wise) I get some AGC pumping action which is anoying in SSB reception. John KA4DGF John: It sounds like you have other problems. I've done the mod to several KWM-2's, and I'd judge them to be superior to the S-Line after it's done. The decay seems to be somewhere between the slow and fast settings on my 75S-3, which are 600 and 300 milliseconds, respectively. I can't find the spec. on the KWM-2 either, not even in Rev 9 of the manual. George Maier - K1GXT George, If what you wrote, that the time constant is 300 to 600mSec then, I assume that if the AGC meter is at let's say s-9 +40 it would take 500 to 600mSec to go to 0. then my M2 may be Ok. That is what I'm seeing. I'm used to the AGC on my Kenwood R2000 short wave receiver which has a slow AGC time constant (aprox 2sec.). I like it better. John KA4DGF Sounds logical, John. Collins never got into the real slow time constants in the S-Line...600 MSec was the longest on the 75S-3B also. Of course you have the option of putting in a bigger cap if you like. I'm going to do just that to my 75S-3 on the "slow" position one of these days. George - K1GXT I have experimented over the weekend and come up with the results recomended by Jim MIller. Change C256 from 0.01 to 0.068uF and R180 from 680k to 47k. Makes a real difference. John What was the real difference? Less punchy audio, less distortion, faster attack or faster release??? Also, before you made the changes did you verify the the tube/s in the circuit were testing in good condition and the voltages to the pins were within specifications. thanks,Dave Dave Yup, there was a real difference. No pumping of SSB signal. Less distortion on the audio. Again this is a mater of your likes as far as theAGC response. There are two time constants in the AGC circuit: One fast (680k and the 0.01uF), and one slow (1.5M and the 0.47uF). The fast one is the problem. Of the AGC voltage produced 30% of it is as a result of the fast circuit. This circuit causes the pumping action on SSB signals. It rises and falls with the voice peaks. To me this very annoying! If you bypass the fast circuit (clip a jumper lead across the 680k resistor)you will hear the difference. The audio clears right up. but you will still need some fast AGC to cover any over shoot. Jim Miller's change is a good compromise. There are advantages to both designs. The original is better for coping SSB with a lot of QRN. You can hear a Weak signal behind a stronger SSB signal/spatter because of the faster decay of the fast time constant circuit. But, for casual rag chewing it's very fatiguing. I prefer the long time constant circuit. The tubes I use on the rig are all new. I do check them often. I have a tube checker.I also check them for in circuit performance. I just swapped a few of them to get the best Signal to Noise out of the M2. I got 13dB S/N with a 0.5uV input. I had before is 7dB. I found that the 6DC6 was good but it was noisy! I gained 6dB better noise performance just by replacing it. I put the noisy one in the box the replacement came out of and labeled it 'Good but noisy.' I will use it on something else. Again the AGC is a personal preference. Tune it to the way you like it. I found after experimenting that Jim Miller's circuit was a good one. John KA4DGF

KWM-2/2A Audio problems. There have been several postings on KWM2A audio and perhaps someone who has been successful can shed some light here. I have distortion on loud signals, especially when the audio gain is up past 12. I blamed it on my speaker, but then I saw the SB8 posts and decided to add 8B and 8C. 8A was done at the factory.Audio problems remained so I changed to a better speaker. That didn't clear it up. I decided to get aggressive. I changed C102. Still no improvement; I got out the scope and sure enough, I could see the problem: Flat topping on one polarity only once the volume got high. I followed it back from the output transformer to the AF amp V16. Everything looks ok at pin 2, the input grid. No distortion at any volume level. At pin 3, 1st stage plate, I see a slight break in the positive rising part of the waveform. Everything looks ok at low volume and on the negative part of the wave. But, it's at the second stage grid, pin 7 where I see real clipping. Not subtle, the wave form is ok below 5V but at 5 V and above, it's flat topped just like a conducting diode. On the negative side, there's no clipping and at full volume it goes to about -15 V. I replaced C103, thinking it might be breaking down, but that wasn't it. So, what do I try next? What's making this tube become a rectifier instead of an amplifier at high audio levels? Shel WA2UBK Check/replace the cathode bypass on the driver (half) of the output tube. Substitute a known good tube, that's about all you can do, again, I'm assuming that the audio is clean and nice on average signals with the RF gain at 4 oclock and the audio at about 10 oclock,if not, you may have other problems. With all the MODS (8A,B,C, non-sanctioned audio mods, etc) the agc rises to fair and the audio quality good on most signals, indoors with low ambient noise. If in doubt, compare your radio to one known to be in good condition. You could have some bizarre problem that everyone's overlooked. Walter S Delesandri I am very pleased with my KWM2A audio and AGC after the SB8 mods (and after some minor changes I have implemented in the SB component values). While the adjectives used would be more than accurate for this KWM2 when I first got it, they certainly do not apply now. I enjoy listening to it now as much as I do my state-of-the-art ICOM! Really! here is also a mod (from the Dennis Brothers video tape) that places a 100 ohm resistor in series with the top end of C235 (a .01 which is connected to the plate of V16B), and adds a .05 from the screen (pin 8) of that same tube to ground. Some other things to consider. The caps and even resistors in these old radios WILL go bad over time. Caps in strategic areas of the circuit may need replacing as a general rule, using good quality modern caps with sufficiently high voltage rating. Corrosion in switches, tube sockets and chassis hardware, and power connectors can also cause strange things to happen in signal quality. I have even discovered that gunk and grime collecting between the pins of tube sockets can offer enough impedance to affect the behavior of these high impedance tube circuits. I thoroughly cleaned the underside of my unit (yes it takes time and concentration) and improved things a lot. And, in spite of the claims of high Collins Q/A and excellence, I found four (count 'em) - four unsoldered or loose solder connections on mine FROM THE FACTORY. Good solder connections can help wonders. How do you find them? I invested in an illuminated magnifying glass, a set of dental picks for close up fiddling and pulling on leads, one of those bench lamps with the big magnifying glass, an oscilloscope, a function generator, a counter and a high impedance DVM. Then some sleuthing to trace circuits, signals and isolate probable sources of distortion. Then replace suspect components or tubes, and hope that you guessed right. And if you didn't, replace some more, and try again. You will probably need all of these to get an old radio like this working again as it should. But I am proof that a KWM2 can sound just as good as a modern rice rig with a little effort, and it probably sounds like it did new (after all the SBs are installed). If you're interested in what I changed in SB8 to improve things in the AGC, at least an improvement in my opinion, email me and I'll let you know. And yes, the Walter also mentions different versions. I do have at least 3 copies of KWM2 schematics here which don't all exactly agree in some areas, I suppose depending on the date of manufacture. The most recent, and presumably the most accurate, is the official KWM2A manual published by Rockwell, available from SSN, which I think is the circuit as it appeared on the very latest releases. Wonder which one is the best? Another thing. If you have one, then get rid of the solid state replacement for the 516F2 V2 low voltage rectifier as fast as you can (my opinion only, remember), and use the original rectifier tube. Also, put a 130 ohm 25W power resistor in series with the low voltage rectifier output in the 516F2. This brings the low B+ down to 275-290 volts, which is where it should be for heat reduction and longer component life. May even help with distortion? These are all my opinion only, but the radio sure sounds sweet. 73 and have fun! It takes time and work to be a collector. Jim N4BE Could it be that whoever wrote SB8 was trying to duplicate the 75S3 AGC circuit Within the KWM2, but didn't quite make it? (Just my opinion.) I'm continuing to compare the two AGC circuits to see what gives. I would be interested in other's analysis and comparison of the two AGC circuits. It does appear that SB 8 is trying to make the KWM2 AGC duplicate the 75S3 AGC, even to having the 1N458 diode and R213 resistor fast attack/slow decay circuit for the RF Amp. So why does the KWM2 SB8 AGC behave even better without the 680K R180 resistor, or with a much smaller value? Interestingly enough there is even a typo (?) in SB8 on page 2. In step (h) they call R180 a 680 ohm resistor. Elsewhere it is called 680K. I liked 680 better! ) (Jim N4BE) Kenneth J. Lopez wrote: If you're interested in what I changed in SB8 to improve things in the AGC, at least an improvement in my opinion, email me and I'll let you know. OK, inquiring minds want to know! Seriously, I would like to know that and any Other information you can lend. Ken, N6TZV Let me go over the S-line and KWM-2 AGC time constants. The radios use a dual or stacked time constants consisting of a fast section and slower section in series. The slow time constant charges to the average agc value for the incoming signals, the fast time constant then acts on short noise pulses as well as voice peaks so that average agc voltage is not overcharged by a pulse minimizing the amount of information to be missed. I think the first schematic you will se this technique used is the KWS-1 ALC circuit, similar circuits can be found in the KWM-2 and S-line alc. The old timers around the company (yes there are a few left) will tell your that the fast time constant should be 10% of the slow one, if you look at the values on the S-line and KWM-2 you will find that the fast constant in much faster than that. I think 3 or 4 %. I have increased the time constant values in my radios to this ratio and they do work better, I think that several of the list members have published some values that are pretty close. I have found the KWM-2 and S-line agc to be superior when copying weak signals with strong off frequency QRM and during the summer when lots of thunder static is present. I think its no good for round table QSOs with strong signals present. In 1973 I purchased a new Drake C-Line (wanting 160 meter SSB rig), I was also at the time a NCS for the Iowa phone net. I used the new toy to call the net one summer evening, an boy was it rough. The Drake AGC was tight, there was very little audio output difference between weak signals and strong ones. The AGC compressed the thunder storm static that was present and the sideband signals so that there very little difference between the two, never did that again back to the KWM-2 for phone nets. Then there was the time in Arabia, a KWM-2 was installed at an oasis, when the pump ran it put out all kinds of RF noise although difficult signals were always copied when the pump was running. Well some of our oil dollars purchased a new solid state 718U transceiver for the oasis (year 1975). The 718U had a nice agc with a time constant of approximately 1 second decay time. The 718U would not copy signals when the pump ran because the AGC charged up to noise peaks and the desired signal could not be heard. It took us a month to get it fixed to the customers satisfaction and get our passports returned. (RFI line filters, different ground for the radio , relocate the antenna etc) Oh yes the radio was in the pump house. There are design trade offs for everything, the KWM-2 AGC is compromised towards Weak signal performance in the presence of interference. Today we have DSPs. Hope this helps, with out stiring up to much of fuss Gayle K0FLY

VOX problem One of my KWM2's started acting up last night. When I first turn the rig on, instead of the meter going right scale and then slowly coming back to zero as it warms up, the meter slams downscale at once and pegs below zero. Your problem sounds like the classic VOX Relay amplififer problem caused by the change in value of the two 68K 2 watt resistors in parallel the cathode of V4b (6az8). This is caused by undersizing of the resistors and the ambient heat around them. Check them out to be sure you have 34k in the cathode circuit. If not, replace them with one (1) 7w 34k resistor and should solve your problem. You can go back to two (2) 68k 2w resistors in parallel, but your problem will come back later on. If fthis is not your problem, it needs to be done anyway. The resistors involved are R20 and R47 on your schematic.

PA load trimmer adjustment. Hello Gentlemen, have a KWM2A and would like to know how the PA Load trimmers are adjusted. For example when I am on 3.900 Mc, The main PA load cap C151 is at its max disengagement (min capacitance) and I am not at 230 ma plate current. Dip and loading appear ok, just can't load to 230 ma. Could not find load trimmer adjustment in my manual unless there is a page missing. My 32S-3 loads up on the same setup just fine. Have not tried other bands yet. Thanks....... Herm Meyer KC2DFJ Subject: Re: KWM2 PA Load Trimmer Herm: What you can do here is trial an error if not in the manual. I sold mine off a while back. If you are at min capacitance and cannot load then what you want to do is loosen the trimmer cap that is coupled to it on the band in question. I just did it by trial an error in the past so that on a dummy load it set up at the 50 ohm mark. Kurt Keller At 10:37 PM 2/9/99 -0500, you wrote: Thanks for the reply Kurt. I found the adjustment procedure in an Army Tech Manual dated 1976 and it is close to your method which I also thought about doing after looking at the schematic. Only difference is they put it in "Lock Mode" then raised the Mic gain just until its starts to produce grid current. Then they adjust the trimmers for 230ma plate current at dip. Thanks again .....Herm Just a follow up not to everyone on this subject. I now remember that there was a mod performed by the military on the KWM-2A which made it easier to load it out of the ham bands. If you look in the PA cage you will see the two gang variable capacitor with a lead off of each section that goes straight down through holes to under the chassis. To install the mod you disconnected one of the gangs (I think the back gang) and tied the two gangs together leaving only the front gang connection to the bottom of the chassis intact. The above may explain why someone may have a unit that is loading way off the 50 ohm mark when it is feeding into a know 50 ohm balanced line or dummy load. Kurt Keller

A bit of trivia...
Amateur Equipment Price list March 1977 Collins Radio Group Rockwell International Equipment COLLINS PART NO. LIST PRICE KWM2A TRANCVR 522-1792-000 $3533.00 KWM2A TRANS W/BLKR 522-1792-011 4427.00 75S3C RECVR 522-3317-000 3000.00 32S3A TRANS 522-2956-000 3250.00 30S1 LINEAR AMP 522-1286-000 7049.00 30L1 LINEAR AMP 522-2375-000 1689.00 ACCESSORIES 136B-2 NOISE BLANKER 522-1661-000 490.00 180S-1 ANTENNA TUNER 522-0651-000 1570.00 302C-3 WATTMETER 522-1696-000 493.00 312B-3 SPEAKER 522-1166-000 95.00 312B-4 CONSOLE 522-1167-000 648.00 312B-5 CONSOLE/VFO 522-1668-000 1616.00 350D-3 SHOCKMOUNT 522-2782-002 185.00 351D-2 MOBILE MOUNT 522-1726-000 731.00 351E-1 TABLE MOUNT 522-1479-003 88.00 351E-2 TABLE MOUNT 522-1480-003 79.00 351E-3 TABLE MOUNT 522-1481-003 72.00 351E-4 TABLE MOUNT 522-1482-003 71.00 351R-1 RACK MOUNT 522-2665-000 128.00 351R-2 RACK MOUNT 522-2666-000 128.00 399B-4 CRYSTAL CONTROL 522-1780-000 161.00 399B-5 CRYSTAL CONTROL 522-1781-000 230.00 440E-1 POWER CABLE 522-2051-000 123.00 440F-1 POWER CABLE EXT 522-2114-000 55.00 516F2 POWER SUPPLY 522-1170-000 440.00 637T-2 DIPOLE 772-5477-002 793.00 CC-2 CARRYING CASE 597-0393-000 264.00 CC-3 CARRYING CASE 597-0403-000 264.00 CP-1 CRYSTAL PACKET 597-0404-000 731.00 DL-1 DUMMY LOAD 522-2771-000 270.00 MM-1 MOBILE MICROPHONE 097-5945-000 65.00 MP-1 12V POWER SUPPLY 522-2750-000 680.00 PM-2 POWER SUPPLY 522-2639-004 680.00 SM-3 DESK MICROPHONE 099-3288-000 167.00

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