Vintage: KDK FM-2030 again!

[majoco]

Same radio as previous post but with another problem - more of a philosophical problem really! On the front panel is the pseudo LED "S" meter that shows the output power as well as the incoming signal strength. The instruction manual warns against trying to squeak more power than rated, 5 watts low, 25 watts high, but unfortunately I don't have any such luxuries as a Bird 43 inline power meter or anything that will measure power or voltage at 145MHz at all. I have a couple of HF twin meter VSWR meters that do indicate something and one of them says to set the calibrator control to 1.8 at 144MHz to measure the power - but it says the low power is 11 watts and the high is 22 watts so I don't believe that, especially when the ammeter on the power supply says only 2 amps when delivering 22 watts. It does show that my 2m antenna has a very low VSWR at mid-band so I'm pleased with that. About the only thing I have that could be used is a Heathkit RF scope probe and measure the peak half wave rectified signal on a scope or a digital voltmeter - I do have a 12watt 50ohm load and BNC Tee piece..
Anyone got any suggestions on how to achieve setting the power to 5 watts so I can calibrate the VSWR meter? Thanks for your ideas.

 

[techman210]

Dirty pots can cause “low power” to indicate and deliver “high power” and also can be caused by some previous owners. That’s a pretty old radio as you know.

If you have an “out of spec” power meter and can run a known power through it, and note the power, then you can match or compare that with the other radio.

As noted; 2A DC with 22 Watts RF is pretty suspicious and not at all correct. Some low cost ammeters/multimeters don’t read right in the presence of near-field RF.

 

[majoco]

Thanks, techman. This is the only 2m transmitter I have, so no other source of RF power on frequency - of course the SWR meter sensitivity is frequency dependant so I can't set up an HF frequency on a 'scope for 5watts into a 50ohm load and then attach the 2m rig and adjust for the same reading. The voltage at the SWR detector is a function of the length of the trough line in wavelengths so it's much more sensitive at VHF then HF.
I think I'll experiment with a diode detector across a dummy load and measure the DC with a digital voltmeter and see what I get.

 

[majoco]

Well, I think it worked! Connected a BNC tee to the antenna socket, a 50 ohm 12 watt dummy load to one leg and a modified Heathkit RF probe to the other leg. The probe was modified by removing the pointed tip and replacing it with a short length of RG58 coax with a BNC plug on the end - the output of the probe was plugged into a Fluke 77 DVM switched to read DC. Switched the Tx to low power, pressed the PTT and the Fluke read just over 14 volts. Given that P = V²/R that gave me a bit over 4 watts. I'll have a go at adjusting the low power adjustment to give me 15.8 volts which should be about 5 watts. This was on 146.645MHz - if I was being a perfectionist I should do a sweep across the band at 500kHz intervals but we'll see!.

 

[prcguy]

5 watts into 50 ohms would be 15.81 volts and that should be RMS. 15.81 X 1.414 = 22.36 volts peak to peak. That's what you should read on a 200MHz or greater oscilloscope using good VHF RF practice to do the measurement. Then you can stick two 50 ohm loads on a T adapter to get a 2:1 VSWR for calibrating an SWR meter.

 

[majoco]

Agreed - but my scope only goes to 40MHz Anyway, my experiment seemed to work well enough - by twiddling the tiny little trimmer potentiometer I achieved 16 volts on the Fluke which is just a smidgeon over 5 watts which is what the instructions say and on high power I adjusted the other pot to give 12 watts for a very short time into the 12 watt load. A sweep across the band gave a slightly higher output at the low frequency end but not enough to worry about. As long as I can hit the local repeaters all is good. Thanks everyone for your hints!

 

[majoco]

Ha! Out of interest, I scrutinised the schematic diagram of the 2030 - the output power is controlled by a feedback system which is almost like the method I used to calculate the voltage required! A diode from the antenna is used to rectify the output and the DC produced is used to control the gain of the RF pre-amplifier before the power amplifier block - negative feedback in effect. If I had spotted that beforehand I would have just tapped into the loop and done the adjustments. Hindsight is a wonderful thing!