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Old 7th Apr 2020, 12:46 am   #1
Julesomega
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Default GEC-Marconi single-frequency HF Rx

eBay know what I like, and a recent suggestion led to me buying an "HF Module (non-synth) type J87-0063-01".

Mine has a channel crystal at 26.8125MHz and an IF filter at 10.725MHz and is marked "Ch.250". The big RF coils give a sharp dip at 16.5MHz so the rx channel is 16.0875MHz. It is dual-conversion and though I can't see any markings the monolithic crystal roofing filter must be at 37.5375MHz. The signal-frequency amplifier is a TO39 device marked IT1001A which must be a 2N5109-ish custom device.

The IF filter is followed by a HA12411 FM demodulator IC. This will have a roughly logarithmic RSSI detector which could be used for a very non-linear AM signal o-p but a quadrature coil is fitted so it is obviously intended for NBFM reception.

What system would have required tens or hundreds of NBFM HF rx? I can only surmise they may have been for maritime meteorological buoys or for hydrographic echo sounding. Perhaps these could be re-purposed for a favourite Broadcast channel or for Shannon Meteo
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Old 7th Apr 2020, 4:19 am   #2
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Default Re: GEC-Marconi single-frequency HF Rx

Shanwick is SSB, so that would need a change to a linear IF plus product detector and CIO. The multiplier in the quadrature detector would make a product detector, but it's embedded with a limiting IF, I think.

I don't understand the 37.5375MHz bit.

A 26.8125 LO will mix 16.0875 RF into a 10.725MHz IF

37.5375MHz would be the image and wouldn't need a filter to pass it. A band stop might make more sense but if needed would be commercially disastrous. The receiver would need a different design of that filter for every different channel.

For NBFM, the difficulty arises of making a narrow enough discriminator. For NBFM in a 10.7ish IF either a crystal or ceramic resonator would be used for the quadrature detector, or a further conversion to a lower IF would be used. The latter is more popular because it allows better/cheaper selectivity on the lower IF.

HA12411 is just a limiting amp (3 stages) into a quadrature detector. It isn't one of the NBFM specific chips with an IF downconverter at its front.


Could those coils at 16MHz be an image trap and the 37MHz frequency be the wanted channel?


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Old 7th Apr 2020, 9:49 am   #3
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Default Re: GEC-Marconi single-frequency HF Rx

Thanks David, I hadn't thought through the frequency relations to include the 1st filter. At first I wondered if it was low VHF despite the "HF Module" name.

The stages sequence seems to be bandpass tuned RF amp with capacitor top-coupling, followed by a Minicircuits mixer, VHF crystal filter, mixing back down to 10.7. The front end is designed for a high TOI level, maybe intended for use on a survey vessel. Looks to be a single device mixer after the 1st filter. We won't have much more to go on until I unsolder the 1st filter unit for a look. I'll see if I can work out the supply lines.

You are right that the demodulator output would be low with NBFM, maybe that's why there is an OP82 nearby.
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Old 7th Apr 2020, 10:24 am   #4
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Default Re: GEC-Marconi single-frequency HF Rx

Some sets fitted an FM IC to an AM or NBFM set just for implementing the signal metering.
They didn't do anything with demod audio. Pye definitely did it in the MX290 series.
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Old 7th Apr 2020, 10:46 am   #5
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Default Re: GEC-Marconi single-frequency HF Rx

Sticking an FM IF in as a side-chain was also a way of implementing noise-operated squelch.

The up-conversion to 35-ish MHz suggests that the modules are intended to be made for anywhere up to 30MHz. If thet's the order of IFs then I'd be looking for a conversion oscillator as the LO for the mixdown.

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Old 7th Apr 2020, 2:34 pm   #6
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Default Re: GEC-Marconi single-frequency HF Rx

D'oh! I pulled the little xtal filter and it's marked - guess what - 10.7MHz
So it's single conversion, explains everything. I guess the out of band response complements that of the multiple crystal unit and the slight difference of centre freq does not matter.
Re the FM detector, it is almost certainly used for the mute function
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Old 7th Apr 2020, 2:37 pm   #7
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Default Re: GEC-Marconi single-frequency HF Rx

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Old 7th Apr 2020, 3:00 pm   #8
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Default Re: GEC-Marconi single-frequency HF Rx

The little crystal filter is in the HC25 can between the two If transformers? If it's got three legs, then it's a common two-pole resonator quartz unit. Single conversion is a lot simler and makes more sense.

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Old 7th Apr 2020, 11:13 pm   #9
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Default Re: GEC-Marconi single-frequency HF Rx

Just to put this to bed, this version of the HF Module can be set in the range 12.5-18.5MHz. The Minicircuits TAK-1H mixer requires +17dBm LO drive which would account for the two tuned transformers and amplifier devices after the xtal osc. The dynamic range of the front end must be rather impressive. Whatever sort of noise it delivers is output via a balanced line transformer.

Just leaves the question, what application requires banks of high performance single-channel receivers in this frequency range.
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Old 7th Apr 2020, 11:49 pm   #10
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Default Re: GEC-Marconi single-frequency HF Rx

Have you determined what modulation type it receives? That may be a big clue as to its use.

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Old 8th Apr 2020, 9:42 pm   #11
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Default Re: GEC-Marconi single-frequency HF Rx

Hi Julian

The GEC Marconi HF receive module you have is what it says.
It was part of the receive section of the GEC Marconi designed MTP1327 Band 3 trunked radio system from the 1990's.

GEC Marconi chose to design their hill top base station node with 2, main & standby, receive down converters which took the received mobile tx signal into a down converter giving an IF output around 15 to 18 Mhz.
This was the feed to a bank of HF modules which would be fitted with a channel crystal for the channels required receive frequency.
Modules would normally be 1 control channel & X number of traffic channels. This was to try and reduce IP's generated on then busy comm's sites. There was a RSSI function which would be used to select the best site for the system to select

There was a synth version of the HF module looking the same except for 3 thumbwheel switches and an extra lock Led , based around the Motorola MC145152 chip

Phillips and Storno also had versions of the MTP3127 systems. Trunking is still around but is now down at Band 3 Sub Band 1 to allow for the expansion of DAB radio

Paul
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Old 9th Apr 2020, 8:53 am   #12
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Default Re: GEC-Marconi single-frequency HF Rx

Ha-ha that's a mystery solved!
I had forgotten the GEC B3 system worked like that.

I was involved with the Pye/Philips one doing the antenna system multicouplers.
1987 maybe?
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Old 9th Apr 2020, 3:44 pm   #13
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Default Re: GEC-Marconi single-frequency HF Rx

Thanks so much Paul. That perfectly explains the high dynamic range and NBFM detector
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Old 10th Apr 2020, 12:52 am   #14
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Default Re: GEC-Marconi single-frequency HF Rx

This is a bit of a digression but I am reminded that years ago I was in Marconi's head office in Chelmsford, and for some reason was studying a map of the hilltop sites which Marconi's either rented or owned. They were all over the country and the number was very large.
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Old 10th Apr 2020, 9:51 am   #15
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Default Re: GEC-Marconi single-frequency HF Rx

Band III was (is) allocated primarily for "Broadcasting" but "the band 223-230MHz is allocated to the land mobile service on a permitted basis"
That makes 7MHz
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Old 10th Apr 2020, 9:56 am   #16
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Default Re: GEC-Marconi single-frequency HF Rx

It wasn't that high. More like 206MHz BaseTX with mobile TX 8MHz lower around 198MHz I believe. Now whats left is down around 179MHz.
Don't listen there with the family in the room... It's a bit like modern day fishfone.

The site at Highgate in North London when first commissioned had a decent range.
I was using it up to Letchworth on the A1(M) when heading back to Cambridge.
I was quite pleased with that.
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Old 10th Apr 2020, 1:10 pm   #17
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Default Re: GEC-Marconi single-frequency HF Rx

That fits in with the UK frequency allocations. From 1995 the broadcast band was extended to 217.5-230MHz but the secondary PMR allocation included the 7MHz sub-bands 176.5-183.5, 200.5-207.5, 208.5-215.5 for base stations, with the mobile channels offset by -8MHz
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Old 10th Apr 2020, 3:31 pm   #18
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Default Re: GEC-Marconi single-frequency HF Rx

198 to 206 Mhz with 8 Mhz off set was where National Band 3 lived along with other user of band 3 trunking systems. I was interesting the coverage that band 3 gave.
When working Storno / Motorola i helped look after many trunked system on Low Band , High band VHF & UHF , it seem there demise is due to company's opting for mobile phone's
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