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RF getting into my test equipment?
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captainvi...@gmail.com
Jun 2, 2015, 2:34:11 PM6/2/15
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I presently have two old, (PRC77's for whoever may be familiar with these), ex military portable FM back pack radios on the bench that I'm checking out. These units tune 950 channels between 30 and 75 MHZ and output between one and two watts depending on frequency. Admittedly these circa 1968 radios are no longer very practical, but fun to put on six meters and bring to Ham fests. The radios operate on 14.4 volt battery packs.
The old magnesium battery packs were dead so I removed the special connectors from them, soldered 9.0 foot 14 AWG. leads on them and connected these to two bench power supplies. I was trying to measure the voltage and note the current during transmit. I had my 265 set to the 60.0 volt range and connected at the power supply. The power supplies were both set to 14.0 volts as noted on my meter. Every time I keyed the transmitter the meter reading dropped to slightly below zero. At first I thought that the power supplies might be current limiting but that didn't make sense because the squelch remained open, clearly indicating the presence of a carrier on the scanner.
I tried a portable digital as well as my bench Fluke. The portable dropped to 4.80 volts and the fluke's power supply, (apparently) for whatever reason broke the squelch on my scanner, so I wasn't able to use that meter at all. Interestingly the only meter that was "almost" impervious to the anomaly was my old RCA Senior Voltohmyst. The meter fluttered very slightly but remained pretty steady. I seem to recall in the specs for that meter that the upper limit was something like 3.50MHZ.
Finally in desperation I connected my scope using a X1 probe up in place of the meter and confirmed the presence of an RF carrier superimposed on the DC level every time I keyed. I tried using "HF REJ" on the scope hoping that would eliminate the carrier but surprisingly it made no difference.
So I built a small PI filter comprised of two small chokes and a small capacitor, values unknown at this point. The configuration is as follows: Each lead of the scope is in series with a choke, then the cap is across the other side of the two chokes, and the line then goes to the radio, and then the power supply.
This filter decreased the RF displayed on the scope considerably but when switched over to the 265, not enough to effect the problem with it very much. However when this filter was placed ahead of the Senior Voltohmyst there were no anomalies present on it. Yay for old technology!
So this is not a huge problem. I don't mind using my RCA meter with my make shift filter to read this voltage. But then in theory to use my other equipment such as the scope it would seem like I need to design a filter having a resonant frequency equal to every frequency I want to check. That's not very practical.
I woud reall appreciate any further thoughts on this. Thanks, Lenny
The old magnesium battery packs were dead so I removed the special connectors from them, soldered 9.0 foot 14 AWG. leads on them and connected these to two bench power supplies. I was trying to measure the voltage and note the current during transmit. I had my 265 set to the 60.0 volt range and connected at the power supply. The power supplies were both set to 14.0 volts as noted on my meter. Every time I keyed the transmitter the meter reading dropped to slightly below zero. At first I thought that the power supplies might be current limiting but that didn't make sense because the squelch remained open, clearly indicating the presence of a carrier on the scanner.
I tried a portable digital as well as my bench Fluke. The portable dropped to 4.80 volts and the fluke's power supply, (apparently) for whatever reason broke the squelch on my scanner, so I wasn't able to use that meter at all. Interestingly the only meter that was "almost" impervious to the anomaly was my old RCA Senior Voltohmyst. The meter fluttered very slightly but remained pretty steady. I seem to recall in the specs for that meter that the upper limit was something like 3.50MHZ.
Finally in desperation I connected my scope using a X1 probe up in place of the meter and confirmed the presence of an RF carrier superimposed on the DC level every time I keyed. I tried using "HF REJ" on the scope hoping that would eliminate the carrier but surprisingly it made no difference.
So I built a small PI filter comprised of two small chokes and a small capacitor, values unknown at this point. The configuration is as follows: Each lead of the scope is in series with a choke, then the cap is across the other side of the two chokes, and the line then goes to the radio, and then the power supply.
This filter decreased the RF displayed on the scope considerably but when switched over to the 265, not enough to effect the problem with it very much. However when this filter was placed ahead of the Senior Voltohmyst there were no anomalies present on it. Yay for old technology!
So this is not a huge problem. I don't mind using my RCA meter with my make shift filter to read this voltage. But then in theory to use my other equipment such as the scope it would seem like I need to design a filter having a resonant frequency equal to every frequency I want to check. That's not very practical.
I woud reall appreciate any further thoughts on this. Thanks, Lenny
Dave Platt
Jun 2, 2015, 3:42:39 PM6/2/15
to
In article <b8ba0a30-f890-41f8...@googlegroups.com>,
tests?
If so, does the problem recur if you install a proper RF dummy load in
place of the whip antenna?
My guess is that when you transmit through a whip, the whole chassis
is going to be RF-hot (it's the "ground" or "counterpoise" side of the
antenna system), and a lot of the RF is going to propagate back down
the power cables. There's likely to be enough RF getting into the
metering electronics (whether digital or analog) to cause
rectification in diodes/transistors in the measurement
circuitry... hence the rather bogus readings you're getting.
An old-style "hot wire" ammeter might be your best bet for evaluating
DC current draw in this situation.
<captainvi...@gmail.com> wrote:
>I woud reall appreciate any further thoughts on this. Thanks, Lenny
Were you actually transmitting (through an antenna) during these
>I woud reall appreciate any further thoughts on this. Thanks, Lenny
tests?
If so, does the problem recur if you install a proper RF dummy load in
place of the whip antenna?
My guess is that when you transmit through a whip, the whole chassis
is going to be RF-hot (it's the "ground" or "counterpoise" side of the
antenna system), and a lot of the RF is going to propagate back down
the power cables. There's likely to be enough RF getting into the
metering electronics (whether digital or analog) to cause
rectification in diodes/transistors in the measurement
circuitry... hence the rather bogus readings you're getting.
An old-style "hot wire" ammeter might be your best bet for evaluating
DC current draw in this situation.
captainvi...@gmail.com
Jun 2, 2015, 6:02:11 PM6/2/15
to
https://www.google.com/search?q=prc77&tbm=isch&tbo=u&source=univ&sa=X&ei=lyZuVdmdGY-1sQTbjYGoBA&ved=0CB4QsAQ&biw=1024&bih=615#imgrc=53bT9hnw-U_VPM%253A%3BWVuMYSHaxTgw2M%3Bhttp%253A%252F%252Fwww.prc68.com%252FI%252FImages%252FPRC77-02b.jpg%3Bhttp%253A%252F%252Fwww.prc68.com%252FI%252FPRC77.shtml%3B1280%3B557
Michael A. Terrell
Jun 4, 2015, 9:17:19 PM6/4/15
to
after I got out of the US Army. The bench supplies we used for final
testing only had 18" leads, and used remote sensing. Try wrapping the
wire through some large ferrite torroids at the power supply, and at the
radio. We had to do most of the testing in well shielded, RF tight
screen rooms. That was a 1970 RCA design, a so called improvement on the
PRC-25. It was built with obsolete Germanium transistors that we had to
have Motorola make in large batches, and buy untested. They sent what
they hoped were enough extras, for the fallout at incoming inspection.
The subassembly work was done in Mexico, and required a lot of rework to
meet specifications. I had real fun, reassembling one of the tuning
gearboxes that an assembly worked took apart by mistake, as well as
teaching them how to solder the audio output transformers to the circuit
boards, for rework.
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