• RTT Comms Test Set

    From G8MNY@21:5/101 to TECH on Fri Jul 12 09:20:09 2019
    R:190712/0747Z @:LW3DBH.1744.BA.ARG.SA #:39204 [MORENO] FBB7.00i $:12116_GB7CIP R:190712/0730Z @:LU4ECL.LP.BA.ARG.SOAM #:5028 [La Plata] FBB7.0.8-beta2 R:190712/0725Z @:GB7CIP.#32.GBR.EURO #:12116 [Caterham Surrey GBR]

    From: G8MNY@GB7CIP.#32.GBR.EURO
    To : TECH@WW

    By G8MNY (Updated Apr 19)
    (8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

    I bought this large test set in it's flight case, at a club junk sale. It looked very smart & could do quite a lot. "Radio Telephony Test Systems" set was made in Feltham UK in 1989. It uses PLL oscillators for the RF generator & CTCSS tones. It came with an Operating Guide, but no service information. (nothing on internet either) So I did a lot of reverse engineering of it.
    ___________________________________________________________
    / /|
    / / |
    ▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄/ |
    ▐ /__________________________________________________________/▌ |
    ▐│┌─────────────────────────────────────────────────────────┐│▌ |
    ▐││┌───────┐┌───────┐┌───────────────────┐┌───────┐┌───────┐││▌ |
    ▐│││ \ AF ││ \SINAD││ 5 6 0.0 0 0 0 0 0 ││ \MOD ││ \POWER│││▌ |
    ▐│││__\____││__\____││___________________││__\____││__\ ___│││▌ |
    ┌┴││o _ _ _ _ _ ││┴┐ |
    │_││ (_)_______(_) (·) (_) o ° ! ! ! (_) o ││_│ |
    AF││o _ │________│ _ _ _ _ ││ |
    in││o (_) o o (_) (_) ! ! =(_) =(_) (_) o ( · ) ││RF /
    ││CTCSS SELCAL MOD _ GEN FREQ ·_· _ _ _ ││in/
    ││1234 123456 12 (·) 1 2 3 4 5 6 7 (·) (·) (·) (·)││ /
    │└─────────────────────────────────────────────────────────┘│/
    ~~▐▓▌~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~▐▓▌~~

    SO WHAT IS IT?
    It is VHF/UHF RF Transceiver test set, with all of this in one Box...
    Dummy Load & attenuator.
    Power meter (Peak reading).
    Frequency Counter (9 decimal places, very accurate).
    PLL RF Signal Generator (7 Decimal Thumbwheels). CW AM FM & PM
    PLL CTCSS tone Generator (4 Decimal Thumbwheels).
    Selcall ID Generator & Decoder (6 Hex Thumbwheels, but no PCB, so removed this) AF Oscillator (Low distortion Wayne type, controlled by 2 Decimal Thumbwheels) AM/FM/PM Rx (Automatic Scanning AM/FM Rx).
    Modulation Meter, +/- Peak reading.
    SINAD Meter (AGC & 1kHz Notch, for Rx Sensitivity & Distortion measurement/adj) AF Volt & dB meter (level & frequency response checks).
    IF Xtal Oscillator. (Xtal tester & IF reference Output)

    SPECIFICATION
    Power AC IEC Mains input 230V/115 @ 60W, or XLR for 12V DC @ 4A.
    RF Input N socket 50Ω 40dB Load atten. 1kW peak, 50W continuous.
    Power meter 1 3 10 30 100 & 300W (RF detection for Counter & Rx switch) Frequency Counter 9 digit 1Hz-560MHz in 3 gate times & averaging to 1Hz res. Freq Reference Accuracy < 1/10^7/month, internal Oven Xtal reference.
    Signal Generator 100Hz steps 60MHz-560MHz, PLL type (OK from 10MHz). Attenuator 0-9dB +7x10dB steps gives 1.8uV-50mV before 40dB Load atten. Pilot tone(CTCSS) PLL 10Hz-999.9Hz Mod Level adjustable & Counter Decoder. Selcall 6 char, Mod Level adjustable & Decoder.(No PCB, so removed) Modulation AF Osc 100Hz-9.9kHz or external, Level adj.(Mod to 15.9kHz) AF Meter Bal/floating 3,10,30,100,300mV,1,3,10V & dBs, 11Hz-27kHz -3dB SINAD meter Auto level, 1kHz Notch, 0-20dB/0-100% Noise & Distortion. Modulation meter Auto Rx. +/- peak, AM 50% 100%, FM 5kHz 20kHz, PM 2Rad 5Rad. Rx AF Filters Comms 300-3kHz or HiFi 50-15kHz -3dB.

    SCHEMAIIC (reverse engineered)
    _______ ________ ________ _______________________ AF:──┤Bal Amp├─┬>─┤Range SW├─┤AF Meter│ Mains─┤PSU +26,12,10,5,-12,-20│
    IN └───────┘ │ └────────┘ └────────┘ 12V__/└───────────────────────┘
    _______ │ _______ ________ ___________
    :─│IF Xtal│ └>─┤AGC Amp├─┤Rej 1kHz├─┤Sinad Meter│
    o─┴───────┘ └───────┘ └────────┘ └───────────┘ oExt Mod
    __________ ________ link ___________ _________ │ ____ RF │Dummy Load│ │ Relay ├o--o─<─┤ Switched ├<─┤ PLL Sig │_/ ────────┤ AF │ IN o┤Attenuator├─>┤Splitter├───┐ │Attenuators│ │Generator├─────<──┐ │Gen │ OUT └──────────┘ └─┬────┬─┴─┐ │ └───────────┘ └┬──────┬─┘ mod│ └────┘
    RF│ rly│ │ │ ________│_ _│_ │
    ___________ _│____│_ │ └──>───────┤1/÷10 Freq├<─┤Ref│ _____ │ _______
    │Power Meter├─<┤ RF │ │ Ex in o┤ Counter │ │Osc├>┤CTCSS├┴┤Selcall│
    │ & Range SW│ │Detector│ │ └─────┬────┘ └───┘ └─────┘ ├───────┤
    └───────────┘ └────────┘ │ ctcss│ │Display│
    _________ _______ └────>──────┐ _____│_____ └──┬────┘
    │Mod Meter├─<┤ AF │ ____________ └┤ Automatic ├────────>─────────┘
    │+/-Switch│ │Filters├─<┤Range & Mode├<┤Scanning Rx├>─oRx Mod
    └─────────┘ └───────┘ └────────────┘ └─┬───────┬─┘
    Ext RF In o─>┘ └──>─oIF Out

    Auto RF switching changes the Rx & Counter over & disconnects the Signal Generator, when the Power Meter detector sees Tx RF, so only the Tx signal is measured. Useful, but confusing when I was chasing faults! (see /E)

    FAULTS
    Going through the maze of functions, I found several faults....

    1/ Mains switch. The long push rod had come off the PSU switch deep inside, & the button top had swollen & jammed in the front panel. I sanded the button down & heat glued the push rod on the switch, & a rubber sleeve behind the front panel captured it so it won't fall out again.

    2/ Tx Load 40dB attenuator, presented 147Ω not 50Ω! It looked clean & not smelling, so not burnt out? I stripped it down, to find it was all quite loose. It had screw on ends that held large ally washers with spring washers in the tube surrounding the RF resistors. There were no open circuit Rs or burnt bits. Put backs all together & did up tight, all OK now. (previous owner's been playing?)

    3 Small rod type Rs ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐
    / | \ ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐
    (o)─8Ω─┬─ ──47Ω────┬─39Ω─<SMA __┌▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀┐
    N│ │ \loose │ │ N│ │▄._____▒_____▒_____.│─┐SMA
    │ 139Ω here 14Ω │ │__│▀'~~~~~▒~~~~~▒~~~~~'│─┘
    └────┴───────────┴─────┘ └▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄▄┘
    \ / ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐
    2 Disc type Rs ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐ ▐

    3/ FM deviation (Rx) range not right/un-calibratable. Stripping down the "not easy to work on" Rx unit, I found a broken 10k preset. Replaced, but needed to be 15kΩ to calibrate the 5kHz range! Also the 20kHz range needed a series preset on the range switch to get that right too. Inside the Rx box there are also discriminator adjustments that affect calibration & scanning stop ability! (previous owner's been playing?)

    4/ 12V DC input, uses a 2 transistor inverter had a NPN collector framing (OK on isolated battery).

    IEC 0.25A
    L────o-o───────/o────────────────────────┐
    N──────────────/o────────────┐ Mains │ Transformer +12V──o-o─┤>├─┬─/o──────────┐ (((((((((((()
    5A │ ===========│====================
    120R ()))) ()))))))))) ()))) ())) ())
    │ │ │ │ │ 8-0-8v │ Other
    INVERTOR├──(─┘ │ │ │ Secondaries
    BIAS │ │101└─)─────┐ │
    │ └─────)───┐ │ │ The cure was to file some metal case away.
    22R \│ │ │ │/ This may have been from a bad upgraded
    │ ├─┘ └─┤ from small to large Tab transistors?
    │ e/│ │\e
    0V──────────┴────────┴─────────┘ I also added 4700uF across 12V input.

    On off switch stops inverter OK, but leaves bias load on the battery! Mains lead must not be connected when the 12V is used !!

    5/ PM un-calibratable. Something is wrong, had to change op amp gain R in the Rx as a get around it. Same as FM for inside the Rx box. But I did learn that, "1 Radian of PM occurs when FM AF Mod Freq = Deviation."

    6/ Push button thumbwheel BCD selected AF Modulation frequencies were not very accurate. I adjusted one of the Wayne C values (room for 4 caps on PCB for it). The 2 BCD thumbwheels operated 8 relays to controlled the osc Rs. I also put a CR (220 & n82) across the 150R (Thermistor to earth R) to frequency calibrate the extended upper frequencies after my modification. (See G/).

    7/ CTCSS Pilot tone poor shape @ LF. The output device is ICL 8038BC, so added some LF frequency correction bias to the Set square 1:1 preset, as I have done on these AF osc ICs before, but this correction DC had to come from decoded BCD data line with Rs adding together this time.

    MODIFCATIONS
    A/ Made a set of test leads for it, that fit in the flight case (small bits in a plastic box not to scatch the meters).

    13A Plug≡====================> IEC right angle Mains

    DC + >─fuse┬=================> XLR 12V
    - >─────┘
    3.5mm ┌─> Balanced AF & SINAD
    Rig LS·-===================┴─> Meters. 4mm plugs

    PL259 -Θ]===================N> N Plug RF Dummy Load in
    Both UR67
    N Plug <N===================N> N Plug RF Dummy Load in

    BNC <========================> BNC general patch lead

    B/ I changed the Ovened Reference Oscillator's fine tune 10kΩ preset (hidden) inside the counter box, for a multi turn version, & drilled a hole to access it with the top cover just slid off. This permitted the finest of tweaks when hot without modifying it for an external off air Reference. I also made polystyrene cover for the patruding hot oscillator box. This reduces the oven power & makes the temp more stable (& hence Frequency) more quickly.

    Warm up time.. Counter error of an air locked 500MHz reference.
    Time min 0 1 5 10 15 30 40 50 60
    Hz off -30K -1K +261 +122 +95 +69 +50 +42 +31
    Error -6/10^5 -2/10^6 5/10^7 3/10^7 2/10^7 <2/10^7 1/10^7 8/10^8 6/10^8

    C/ Max deviation measuring range changed from 20kHz to 100kHz. (E.g. now does 5kHz & 100kHz FSD), I put in a 10kΩ series preset on the range switch. But although usefull indication for broadcast work, the Rx is AF is asymetric @ >±50kHz dev!

    D/ Added Meter illumination LEDs. Ground down 4 white LEDs to a thin square sliver 4x4x2mm.
    __
    Original (__│== Ground []== ┌──┤<├──┤<├──┤<├──┤<├───220R─<+12V
    LED Down LED ┴ 4 under run white LEDS

    Wired up all in series & glued on top of meters, making sure the top fascia bezel did not foul, & feed them via a 220Ω from a +12V rail plug nearby Rx.

    E/ Added a Rx mode LED, near the Zero power pot, on the Tx/Rx relay's +8V via 470Ω, so you know which mode it is in.

    F/ Stripped out the Selcall thumbwheel bank from front panel & replaced it with a small LS, +Amp, +Vol control, & a Source switch. So AF in, Notched SINAD AF, Mod, or Rx AF, can be heard.

    +12V──10─┬───┐+
    SOURCE │ ===470u
    AF to SINAD───────o<──┐ │\ │6 ┴
    u1 │ 2│ \
    SINAD Meter AF─┤├─o 1M<──────┤ LM \ 5 +470u
    Log │ 386 >─────┬─┤├─┐
    MOD Switch────────o Vol ┌───┤ /┐7 10 │
    Pot ┴ 3│ / │+ │ │_/│8
    Rx Mod BNC────────o │ │/ │ === === |_ │LS
    │ 4│ │10u │u1 │ \│
    0V┴───────────┴──┴──────┴────┴

    G/ I changed AF modulation BCD controlled Wayne Osc's "kHz thumbwheel", from Decade type to Hex (from Selcall bank). This adds 6kHz more range, in 100Hz it now steps from 100Hz-15.9kHz. But the thumbwheel types were not the same, the data 1 2 4 8 & C lines were rewired on ribbon to the PCB to make it work. The Wayne Osc HF freq & gain were trimmed, so the AF osc is accurate (±2%) see/6.

    IN USE
    With all my fixes, calibration, & mods, it is now a very usefull test set. The occasionally slow broadband auto Rx lock settling, is a bit of a problem, but an off/on usually sorts it. The Signal Gen attenuator switches are a bit poor, (but would need a major strip down to service!) With an external spectrum analyser the 40dB high power attenuator works well too, but power meter diode does increase some harmonics!


    See my tech Buls on "Analyser Takeda Riken TR4122B", "AF Waveform Generator (RS)", "RTT Mod Meter (100)", "Marconi Distortion Meter TF2337", "Marconi 2019A Sig Gen"," Systron Donner 1702 Sig Gen", "198kHz Off Air Standard", "Comparing Off Air Freq Standards", "Off Air Lock for Ref Osc", & "Scope RF Trick".

    Why don't U send an interesting bul?

    73 de John, G8MNY @ GB7CIP

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    * Origin: HAMRADIO telnet lu9dce.dynu.com (21:5/101)