KWM-2 Low Power Out

Discussion in '"Boat Anchor" & Classic Equipment' started by W3TA, Oct 17, 2021.

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  1. W3TA

    W3TA Ham Member QRZ Page

    Somewhat of an interesting turn of events last night. I was poking around checking various resistances and not finding anything unreasonable. Turned it on and started checking some voltages which all seemed ok. Then decided to check the filament voltage in the finals which measured 5.5 VAC. Hmmmm, Checked it on other tubes and it was 5.5 VAC throughout.

    I have another 516F-2 (with solid state mod) and connected it - same filament voltage though, 5.5 VAC. Keyed the radio and wow, I have more power output. Checked all bands and each was right at 100w except 10m which is 90w (plate tuning hitting the clockwise stop before being tuned properly - I need to address this). Pluse I now have close to 230mA of plate current vs 200mA with the other supply.

    Both 516F-2's have the same solid state kit installed (by me). I'll have to dig into the differnces between the two but that'll be another time. It was getting late and I didn't think it was wise to be poking around HV at that time.

    So a twist - in a good direction..........

    KL7KN likes this.
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Check the calibration of your AC voltmeter used for the heater voltage measurement?

    516F-2 was designed when household line voltage was 117Vrms or lower; today it's 120Vrms or higher. Heater voltage would be expected to be a tiny bit high, but not low....

    Of course maybe the line voltage in your home is on the low side. Maybe use the same meter to test that.
  3. W3TA

    W3TA Ham Member QRZ Page

    Have AC controlled with a variac set to 115 VAC (measured using a different meter for setting voltage...) . Using Fluke DMM for AC measurement - not in any calibration cycle. I will check with another meter (VOM) but don't think the fluke is being a fluke.... I can also bring a Fluke home from work that is in cal.

    I did turn the AC up to 120 V but the filaments didn't go up much, maybe to 5.7v

  4. KK6IYM

    KK6IYM Ham Member QRZ Page

    What is the filament voltage with no tubes installed? It sounds like something is dragging down the filament voltage. I would suggest you clean the antenna relay in the front of the cage. You have to remove the cage to do this. While in there, clean the switch wafer contacts. My power supplies put out 900v plus and I always thought that the pin voltages given on the chart were for a power supply that is higher than what the manual says for the 516. The power out numbers you have are low, although the best KWM-2 I have puts out 110 watts on a good day for the lower bands. New finals are probably in order. Make sure you change all the finals cathode and screen resistors. The resistors in any old radio that need attention are the plate and cathode. Screens and grids are of lesser importance in my opinion.


    N8ZL likes this.
  5. W3TA

    W3TA Ham Member QRZ Page

    Hi Norm,

    As part of going through this radio I did go through all of the relay contacts and clean them. This unit has the open frame relays and they took some time to get cleaned and operational. Went through the same thing with the RF relay and did the neutralization cap mod when I had the cage off. I also removed all of the covers nd cleaned the band switch - not to say it is perfect and it may be worth cleaning again.

    I agree, something is hanging on the 6.3v pulling it down. As you suggested, tonight's plan of attack is to remove the tubes and start from there.

    Both of my 516F-2's are putting out around 830vdc and drop to the 780 range when keyed. I need to monitor the screen voltage as well.

    Cathode R's are good to go (I learned this going through my 32S-3 a couple years back).

    Just got done making an Excel chart for resistances and voltages so I can walk though the whole radio and record them. As I said early on, the RX side is is working great so I'm not too concerned at looking at those circuits.

  6. W3RU

    W3RU Ham Member QRZ Page

    Is the idle plate current properly set to 50 ma as per the manual on both of your 516F-2's? This would not impact the filament voltage deficiency, however.

    Jack W3RU
  7. W3TA

    W3TA Ham Member QRZ Page

    Update from last night - I definitely have a power supply issue vs an M2 issue. With the "good" supply and the bias correctly set, I get 100 - 110 watts out 80m thru 15m with plate current right around 230mA. 10m is approx 90 watts and 200mA of plate current.

    I have three 516F-2's, this one was used with the 32S-3 and the one that has issues was for the KWM-2. The third is an original that needs new caps (good price, couldn't pass it up.....).

    Both supplies were updated to solid state at the same time with the same pcb. Only difference being with the questionable one I put the rectifiers in removable plug so I can remove the HV form the radio by pulling the plug (no pun intended...)

    Both HV and screen voltages are low in this supply so I need to do some digging into the cause.

    I had mentioned about the filament voltages being around 5.5v. I pulled the tubes and it goes up to 6.3. With each tube insertion it goes down as I would expect. I had the variac set to 115 v but raised it to 120 and the filaments are now around 6.2v. This is the case regardless of which supply I am using.

    I'm going to run through the alignment one more time with the good supply and call it done for the KWM-2. Then turn to the power supply and see what is going funky in there.

    Thanks for the tips.


    PS: I think my SX-28A will be next up on the bench. Have several radios in the que to keep me busy. Funny, anymore I find that I like working on the radios better then operating them.
    KL7KN and KK6IYM like this.
  8. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

  9. W3TA

    W3TA Ham Member QRZ Page

    Don't understand
  10. W3TA

    W3TA Ham Member QRZ Page

    Couple pics of the finished product (before going back in the cabinet). Also a close up of how I replaced the electrolytic caps under the chassis and left the original cap in place.

    Back to the bench...........


    Attached Files:

    KF4NU likes this.

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