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Old 15th Jul 2019, 8:54 am   #1
AlanC
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Default Plessey/Multitone Pager

Just wonder if anyone had any info on, or knew about this little device that has piqued my curiosity....

While having a bit of a declutter recently I came across an old pager (please see the images) that has Multitone embossed on the case body, but Plessey on the belt clip.

As pagers go, it's definitely of the 'hospital bleep' variety, and I would have said late 60's/early 70's looking at the components; silicon transistors in epoxy cases (except one in a metal TO-18 can); both bead and axial tantalum caps. There's 10 transistors in total. Not an IC in sight!

The circuit appears to be a TRF receiver consisting of a tiny ferrite rod tuned with 3300pF and feeding a 4-stage untuned RC coupled amplifier, which feeds a voltage doubler type detector with 2 germanium diodes. The recovered audio is amplified in a single stage whose collector load is a winding on the pot core inductor, which is marked '720'. This same number also appears on a label in the battery compartment and I wonder if it is the single audio tone required to trigger it?

The tuned winding on the pot core feeds what appears to be a clever 'infinite impedance' detector using two transistors, the output of the second would go low and likely trigger the tone generator- I haven't traced this bit out yet.

The speaker looks like the same type of transducer as used in the Pye PF1.

There are unpopulated holes in the PCB and space for a second pot core so maybe twin tone was an option on these.

The battery type is unknown. Looking at component values I hazarded a guess at about 6 volts and with this applied and switched on it bleeps after a strangled parrot kind of fashion; this can be cancelled by pushing the contacts on the side. Holding the contacts down produces receiver noise from the speaker so presumably once paged you could listen for a message. There is nothing to be heard other than noise; it does crackle if you short the meter probes on ohms range nearby, but not much else.

I would love to know what RF band or frequency it is listening on, the ferrite antenna, large value of tuning capacitor and TRF configuration suggest a low frequency and I seem to remember having heard that in the early days some paging systems used wire loop antennas to radiate low frequency signals; though all the pagers I ever saw in my professional capacity were VHF and above and all used much more sophisticated crystal controlled receivers not to mention complex tone signalling.

There seems to be almost nothing on the web about early paging systems.

Any info welcome!

Kind regards,
Alan
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Old 15th Jul 2019, 10:25 am   #2
Jon_G4MDC
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Default Re: Plessey/Multitone Pager

Looks like it might be an idea to try a GDO on the ferrite antenna coil?
I'm wondering about the ISM frequencies at around 13.5 or 27MHz.

Lots of site and hospital pagers in the 60s were at 27MHz.
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Old 15th Jul 2019, 10:53 am   #3
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Default Re: Plessey/Multitone Pager

It is a Multitone pager. Looks like an inductive loop receiver for a small system. The extra core would be for two digit addressing (10-99 pagers). Supplied by one of the many permutations of our old telecommunications companies, Plessey Communications Systems.
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Old 15th Jul 2019, 12:00 pm   #4
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Default Re: Plessey/Multitone Pager

These were a service offered by Telephone Rentals. We used to service them but not to any great depth. I remember them as Multitone rather than Plessey. They were inductive loop but we had no info as to their frequency or internal workings. (They had transistors, at the time TR were still into mechanical exhanges and PA systems driven bt 807s and EL34s in parallel push pull)
The most common fault was that the clips either broke or just fell off. If they failed totally, which they did frequently, we returned them to the factory. The customers would have had a few spares, which usually ended up being broken as well.
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Old 15th Jul 2019, 5:13 pm   #5
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Default Re: Plessey/Multitone Pager

35Khz suppressed carrier. I have a Multitone product document covering VHF and looping, but it's too big to post, even zipped. If anyone would like a copy, PM your email.
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Old 17th Jul 2019, 7:40 am   #6
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Default Re: Plessey/Multitone Pager

Many thanks for all your replies everyone, all very interesting. 35kHz certainly makes sense in terms of the antenna. Another thing I noted was that the antenna ferrite rod is vertical when the pager is worn on the belt; this would result in minimum pickup of a 'normal' radio wave; but maximum coupling from a horizontal loop. Thanks Bill, PM sent

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Old 17th Jul 2019, 11:56 am   #7
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Default Re: Plessey/Multitone Pager

Quote:
Originally Posted by AC/HL View Post
35Khz suppressed carrier. I have a Multitone product document covering VHF and looping, but it's too big to post, even zipped. If anyone would like a copy, PM your email.
Interesting that it's suppressed carrier. Without a local BFO, how would the RX recover the modulating tone(s)?
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Old 17th Jul 2019, 1:22 pm   #8
AlanC
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Default Re: Plessey/Multitone Pager

Very good point Andy, I suppose that if you modulated the suppressed-carrier transmitter with a tone- say 1kHz as an easy example, there would be sidebands at +1 and -1 kHz. These would beat together in the detector to produce a 2kHz audio tone. So maybe for my unit, which I imagine (not tested yet) is expecting a 720Hz audio tone, you'd modulate the TX with 360Hz. Pure speculation of course, but I can't see how else it could work without some way to re-insert the carrier and clearly there isn't that- it is a very simple circuit.

The info that Bill kindly emailed to me doesn't expand much on the modulation scheme; but for info it says that the TX output of the standard TC3 loop transmitter is 10 watts and that the total maximum loop length is 1600 yards; which does not have to be a single loop- two loops of 800 yards is acceptable. The manual warns against using more than one TC3; if more than 1600 yards of loop needs to be driven, then a master transmitter type TC1 was available which could operate with an unlimited number of slave units- presumably the master generated the carrier.

Loops should not exceed 20 yards width. When this is unavoidable two loops in a figure-of-eight pattern can be used. One horizontal loop should cover 5 floors of a building if the said building is of light construction; 3 floors if medium construction and if constructed of heavy reinforced concrete then one loop per floor should be used.

The manual also mentions that the receivers (with the exception of one model) use a single mercury cell; I remember that the Pye PG1's that I used to get on my bench from time to time used these- we supplied Mallory ones. They are 1.35V per cell so my estimate was way too high! It also says the battery life was 12 weeks!

Cheers
Alan
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Old 18th Jul 2019, 7:34 am   #9
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Default Re: Plessey/Multitone Pager

My pager is a model RC24.
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Old 18th Jul 2019, 11:04 am   #10
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Default Re: Plessey/Multitone Pager

Found a compression tool: https://smallpdf.com/
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