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Author Topic: Simpson 142 VU Meter RFI Problem  (Read 1912 times)
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AJ1G
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« on: April 19, 2020, 07:54:12 AM »

I recently broke out a new in box big old Simpson Model 142 VU meter and set  it up for use in my external audio chain, worked great, until I started transmitting.  The meter is EXTREMELY sensitive to RF energy, and deflects way up scale whenever I am radiating from an antenna.  There is not much stray RF in the shack, and I suspect it is responding to near field radiation directly from the antennas.  On 75 meters, I've been able to knock down most of the RF related deflection using a 600 ohm line to line audio isolation transformer between the meter and the line level output of audio chain.  It is not responding to RF on the audio chain, in fact, it will deflect nearly full scale during operation on 40 meters with it just sitting on the operating table not connected to anything at all, even with a shorting jumper connected across the meter circuit terminals.  In order for it not to deflect during 40 meter operation, I have to short out the meter terminals to each other, and also to the terminals for the internal light bulb, and the 4 panel mounting threaded screws.  Up to this point, I set just set it "naked" it on the operating table without a shielded enclosure, have tried adding grounding leads to the signal low, the at present unused lamp terminals and the mounting screws to no avail, also various series RF chokes and bypass caps on the meter terminals.  Any ideas on how to do any better?  For all I know the damn thing has been bouncing around in its box for all the years it has been sitting on the shelf above the work bench whenever I have put a signal on the air! According to info on the meter face card, it has a VERY sensitive 200 microamp meter movement.  It also has AMPEX stamped on the meter scale card, was apparently made for use on a 1950s - 1960s vintage Ampex tape recorder.
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Chris, AJ1G
Stonington, CT
AJ1G
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« Reply #1 on: April 19, 2020, 02:59:49 PM »

Had a discussion on the Simpson 142 VU meters with Nicky Badwires this morning after the AM Carrier Net. They apparently use a 20 uA DC meter movement and a copper oxide rectifier.  So unless properly bypassed and choked off for RF they can be very sensitive RF field strength meters.  Or as we learned on the Paranormal Net this past Halloween  night, excellent paranormal energy field detectors.  Also need a pad with a series nominal 6.3 k series input resistor feeding a shunt 600 ohm resistor across the meter terminals,.  Hopefully the pad, along with some effective RF bypassing and choke decoupling will tame it.
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Chris, AJ1G
Stonington, CT
W7TFO
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« Reply #2 on: April 19, 2020, 11:42:39 PM »

Standard VU meters have a 200uA movement.  They also have an internal copper oxide FW rectifier.

73DG
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