Message 24183 in TECH.WW
From: G8MNY
Date: Sun, 14 Jun 26 08:33:00 Z
Newsgroups: TECH.WW
Subject: DYMAR Mod Meter Type 1785
Message-ID: <62403_GB7CIP>
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From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (Updated Mar 08)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I have one of these old VHF-UHF AM/FM Modulation meters it is similar to the
Marconi TF2300 type. It has a blue grey metal case plastic case 42 x 17 x 20cm
and has a large 13cm mirror scaled meter on the left and slow motion Rx
converter tuning knob & dial on the right.
Large Meter Mode Fine Band
On Bat Chk IN Attn Set Range Comms IF
SCHEMATIC
I have now reverse engineered the whole thing and have all the circuits. I have
done lots of modifications, so it now fully calibrated with much improved spec.
MODS
1/ Modulation detector rebuilt, now true very fast peak & mean reading.
2/ FM squelch (fast shut, slow open).
3/ Internal monitor amp & LS added.
4/ Rebuilt PSU, it now copes with external 12V operation as well.
5/ Broadband mode added, FM & AM detectors widened.
6/ HiFi filter set to 30Hz-15kHz @-3dB (sharp M derived HF cut off)
7/ Comms filter set to 300Hz-3KHz @-3dB
8/ Used it for FM deviation up to 900MHz (e.g. UHF radio mics) OK.
NEW SPECIFICATION
Power : 180 - 250V AC, 50 - 100Hz (90 - 125V internal straps)
12 - 24V DC @ 200mA
Internal battery option (not fitted).
With external/internal battery charging & battery check.
Frequency range : 27 - 480MHz (2 x 2 BANDS) (900MHz possible on harmonics)
Modulation Detector
Peak Hang time: > 1 Sec to -10%
Peak Rise time: < 50uS to -10%
AF Bandwidths: Wide Mode 10Hz - 50kHz @ -3dB
HiFi Mode 30Hz - 15kHz @ -3dB
Comms Switch 300Hz - 3kHz @ -3dB
AF Scope output follows detector bandwidth & range levels.
Monitor LS " " " " " " " &
has volume control & 50uS de-emphasis. Output Power 500mW.
Meter Scales
Set Level MODE : dB scale -12dB to +2dB, cal to 100%
AM Modulation RANGE : 3 - 10 - 30 - 100% Modulation depth.
Anyone want further information on it?
See my buls on "RTT Mod Meter (100)", "Simple Carrier Squelch", "Low Voltage
Drop PS", "FM Deviation Calibration", "FM Stereo Radio Principles",
"AM Broadcast Radio Principles", & "Meter Damping & Speed Up".
Why don't U send an interesting bul?
73 de John, G8MNY @ GB7CIP
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