This came from the same stall as the two Racal 9917A frequency counters. I didn't pay much for any of them, as they were all in a sad state and looked as if they'd been stored in a leaky shed for a time. They were covered in cobwebs and there were signs of corrosion on the aluminium. Some of the screws were rusty. The bloke said he was selling them for the cases, which would have been a reasonable deal. This was the worst of the lot. It had a sticker saying "Dead" and the top and bottom covers were reversed, so if it was put on a bench on its feet, the instrument would be upside down suggesting something was wrong. I didn't have much hope for it.
I opened it and there were signs that there had been water pooled on the main PCB at some time. There was corrosion on the metal surfaces. It didn't look as if component leads had been eaten through. There were no exploded or blackened tants. They had a NiCd battery pack option but mercifully that was missing and was either not fitted or removed before it leaked.
I went through the test procedure in the manual. The first part is to measure the resistance between the line and neutral pins on the IEC connector. It's supposed to be around 300R, which it was. The next step was to check the stabilised power supply, which involves removing an internal fuse, putting an ammeter in its place, then powering on the system and setting the single supply line to 10V. I checked the fuse in place and found it was open circuit. Ominous. It didn't look blown. I removed it and the end caps fell off. It looks as if it had been destroyed by damp. I went through the procedure. The current draw was as it should be and it was easy to set the supply to 10V. I put in a new 500mA fuse.
I tried it with a signal it should be able to handle, 10MHz @ 100mV RMS AM modulated to 90%. There was no reading on the meter. There's an extensive circuit description, calibration and fault finding notes in the manual. The circuit diagrams give the waveforms you should see on a scope at various points, and this instrument has a lot of test points. I decided to check through with a scope to see if the circuit was behaving according to the pictures, starting with the sampling mixer and working through. Everything seemed pretty much as it should be up to the circuitry feeding the meter. I looked at the meter and it was reading around 90%. I suppose it could have been a bad soldered joint which I disturbed, but some of the switches were in a dodgy state through damp.
I dosed the switches with contact cleaner and exercised them. I cleaned off the PCB and metalwork with white vinegar, then distilled water. I went through the calibration procedure using a new Siglent function generator and a Marconi 2019.
It adjusted to agree with them to within the limits set in the manual. It also works as described with an external oscillator (the internal oscillator gives a reduced frequency range). There a test for distortion of the modulated waveform, but it looked OK on a scope, and I couldn't be bothered to dig out a distortion meter. The audio filter doesn't seem quite right. Filter in is supposed to agree with filter out at 1kHz and it doesn't really. I don't altogether see the point of the audio filter. It's supposed to help with noisy signals.
It's been an interesting thing to play with and it's always satisfying to get these things working, but I don't have much use for a modulation meter.
This one was owned by British Gas Southern Region. It had a lot of calibration stickers, indicating a working life of many years. I'd guess it is from the time when they had UHF telemetry links from their distribution hubs.
The sampling mixer receives fast pulses from a pulse generator based on a step recovery diode, which are rare beasts these days. I suppose if it had been impossible to get working, the step recovery diode would have been worth the price of admission.
Racal 9009 modulation meter
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Racal 9009 modulation meter
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Re: Racal 9009 modulation meter
Modulation meters are one thing that I've never had a use for, either personally or professionally. I'm not a ham (by deliberate choice) and the only modulations I've been interested in are purely digital (GFSK, COFDM) which makes an AM or FM modulation meter not very useful.
Contrariwise, it always fun to play with something new, and to make it work.
Contrariwise, it always fun to play with something new, and to make it work.
Re: Racal 9009 modulation meter
Such devices were an everyday part of my life for decades, although for TV sound & FM Broadcasting we used the large Marconi units.tggzzz wrote: ↑Thu Dec 14, 2023 8:03 pm Modulation meters are one thing that I've never had a use for, either personally or professionally. I'm not a ham (by deliberate choice) and the only modulations I've been interested in are purely digital (GFSK, COFDM) which makes an AM or FM modulation meter not very useful.
Contrariwise, it always fun to play with something new, and to make it work.
The Marconis were normally rack units & used as general purpose demods as well as testing.
From time to time, they were called upon for testing high band commercial mobile radios, & of course, ("nudge, nudge, wink, wink"), FM ham rigs, but that was "once in a blue moon."
They also had an AM range, but I can't remember ever seeing one used "in anger" for AM.
The MF & a few HF AM broadcasting sites normally used dedicated Modulation Monitors installed as part of the station.
These were simpler, & usefully, included much bigger meter scales than that of the Marconis.
They were also cheap enough that each transmitter in a multi transmitter site could have its own Mod Mon.