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Old 11th Sep 2017, 6:53 pm   #21
SiriusHardware
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Default Re: Memories and other stories from Pye etc PMR engineers.

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Originally Posted by G6Tanuki View Post
There was a brief bit of potential amateur interest in (Band III MX295s)
There was a conversion for the MX295 from Band III to 2m published in one of the amateur magazines during the 90s, but, while the receiver mod (which wasn't just a retune and involved increasing the number of turns on receiver front end coils) worked really well, the transmitter modification as published proved very difficult to get working reliably.

The two or three I tried converting verbatim were really unstable and I eventually stopped trying to move them to 2m and just bought them for spares common to all MX series radios, plus their interesting TSM30D control boards.

The guy I worked with at the time was a rail fanatic / trainspotter extraordinare so I programmed one of my 295s up with BR band III frequencies of his choice and fitted a basic channel selector so he could listen around to his heart's content.
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Old 11th Sep 2017, 7:18 pm   #22
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Default Re: Memories and other stories from Pye etc PMR engineers.

Also, early A200 had a tantalum rated at 16v across the dc supply line on the pcb somewhere. (Top edge, if the PA o/p pair were to the right with lid off). Expected burnt blob often found inside. On sight, I changed it to a 25V. Also, the tx/rx relay often had intermittent contacts, just like the Olympic and reporter.
Rob
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Old 11th Sep 2017, 7:26 pm   #23
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The burnt out 24-12V dropper is a familiar story. We used to look after NB3 mobiles fitted to a haulage company and we had a lot of trouble with linear droppers failing. Same story, after hours, the drivers would tap into the handy 12V from the dropper and plug in everything from the TV to a travel kettle. In the end I think we went over to using switch mode droppers made by Alfatronix and these would current limit when overloaded. The Motorola T310 mobiles used to develop an interesting fault. The volume pot was mounted so that it allowed slight movement when in use. This led to the earth end of the pot's wire breaking so the radio was at maximum volume all the time. Even when the driver wasn't on a call, the NB3 system cell hand over/acquisition tones used to sound with an earsplitting bloop. You could diagnose the fault from about half a mile away. The fist mics used to go faulty with no mod. Easy, the driver had dropped the mic onto the hard floor and the little gain adjust cermet pot inside had cracked. The standard Motorola fist mic was the best design I have ever seen, and could take some abuse after the pot was linked out. I remember having to install a radio in a massive mobile crane. Nowhere to mount the aerial to prevent fouling except the mudguard. Great. Then after I started drilling realised the steel was about half an inch thick. A Panorama M8A base was juuusst long enough.
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Old 11th Sep 2017, 7:49 pm   #24
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Default Re: Memories and other stories from Pye etc PMR engineers.

Drilling vehicles.....yes!
I was installing a pmr into a Renault tractor. This was my first as all others were John Deere. Looking for a convenient place for the mic clip, I kept returning to either side of the dash panel, on a curved sloping surface. Having decided it was the best place and was about to drill, I looked forward out of the windscreen and to my horror saw that the area I was about to drill was just a continuation of the external diesel tank but inside the cab!
French vehicles! (BTW, I own a Peugeot 307!)
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Old 11th Sep 2017, 8:31 pm   #25
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After one particularly troublesome install with funny vehicle wiring colours (don't you just love 'em?), I went away quite pleased that I had done a good job. However I got a call the next day saying that all was well until it got dark and the lights were switched on. The radio went off. I went back to fix it and found that the "earth" on the fuseboard was in fact the positive supply to the headlights. It gave a nice zero ohms to earth through all the lights in the vehicle and I think? may have been brown in colour. Anyway, it went to +24V when the lights were on, and the isolated dropper/radio then had zero volts across it, and promptly went off. I have also had colleagues drill through fuel and water tanks, through spare tyres, and all the rest, thankfully it hasn't happened to me, although it has been a near miss on more than one occasion. We once had a customer who ran a large arable farm in the borders and I was asked to sort out his PMR system. He was impressed when I happily set up a little temporary workbench with my Marconi 2955 in the corner of a dusty shed and tested all his even dustier radios. I was tired out after jumping into all the vehicles, including a combine harvester. That was a first for me.
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Old 13th Sep 2017, 10:03 am   #26
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Default Re: Memories and other stories from Pye etc PMR engineers.

One of my colleagues spent a morning fitting a boot-mount 1st-generation analog cellphone to a client's Mercedes.

Only problem: there were several similar Mercedes in the car-park and my colleague had installed the phone in the wrong one! Turned out that the owner of the now-unexpectedly-phone-equipped Merc had been thinking about getting a carphone anyway, and so to pacify him he got the kit at trade-price and we didn't charge him for the 'fitting'.
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Old 13th Sep 2017, 5:41 pm   #27
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Default Re: Memories and other stories from Pye etc PMR engineers.

Somewhere out there is my MX295 converted to 50MHz.. about 1998 I reckon.
Worked all right in the early days of 6m FM, hopefully it still does.

One bit of fun was "High Power Amplifier for PYE L150" (used on long police links).

Also in the stores "LPFs for high power amplifier - issue one with each L150 High Power Amplifier."

Sadly no mention on L150 High Power Amplifier to issue one Low Pass Filter with each.

Result? QQVO6-40 Anode Tank Circuit link coupled direct to Antenna.
Intermod city.

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Old 13th Sep 2017, 6:05 pm   #28
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Default Re: Memories and other stories from Pye etc PMR engineers.

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Result? QQVO6-40 Anode Tank Circuit link coupled direct to Antenna.
Intermod city.
Definitely not-fun when you were on a shared mast.

I had bad experiences with intermod when one of my 'rather-healthy' transmitters intermodulated with another transmitter to drop a 'sprog' on the input of an emergency-services transmitter.

After much investigation [including my use of an amateur-band transceiver that was rather naughtily broadbanded in order to provide a local carrier] the offender was identified as the unbiased TX/RX switching-diodes in a Fire Brigade repeater.

**Schadenfreude**

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Old 13th Sep 2017, 6:47 pm   #29
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Default Re: Memories and other stories from Pye etc PMR engineers.

You set me in mind of another one....

In the days of building Canary Wharf there were many high cranes who needed to speak with their banksmen down below... One site was getting lots of grief from something called "Sierra Control" and they reckoned it unsafe to continue. The wooden dollars (cross charges for delay) are bleeding away - fix it - now! now!

The end result was rather nice... There was a 3rd order intermod at RF which meant Ford Car Works Dagenham and something else hit their high RX, ordinarily they wouldn't care because CTCSS (Tone Lock) were different. But not this time.

The Car Works (Sierra Control - Doh!! - lots of DF to discover that) and the 3rd party TX (I don't remember who) had CTCSS which were a 3rd order too. Put it all together as an RF and AF mix and Voila! RF1+/- RF2 +/- CTCSS1+/-CTCSS2 = CRUD!
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Old 13th Sep 2017, 10:31 pm   #30
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We had problems at one of our main-scheme sites in Lancashire with a low band transmission breaking through onto our main receivers. A couple of day's work with a spectrum analyser and a lot of waiting around for the offending transmitters to flash up (why do they remain eerily quiet when you are on site?) proved one to be on exactly half our receive frequency with a lovely second harmonic splattering everywhere. A quick call to the DTI or whoever it was before Ofcom and job sorted. Co-sited high power paging transmitters always seemed to find a way into your receivers without having to use extra bandpass filters in line too. We had a strange one up in north Northumberland with one of our microwave link PSU's alarming randomly. I had the task of sitting watching the thing one day after checking everything was fine and normal on the PSU. After a while, on the hour, every hour, an alarm came up just long enough to trigger the supervisory. A little bit of poking around discovered some form of data transmitter lashed inside a cabinet (why did people always leave the cabinet key on top of the rack?) with a rubber duck whip antenna on it. The RF was breaking into our alarm circuitry and causing spurious triggers. Another call to the DTI soon had that one stopped too.
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Old 14th Sep 2017, 4:14 pm   #31
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I remember a lowband fm talkthrough system and office controller of about 35 years ago. Complaint was tx from office ok, rx was weak and talkthrough had weak rx as well. It was a farmer. Testing tx rx performance on base and a couple of mobiles, all appeared ok. So, with no mobiles more than a short distance away, it was nff. A couple of days later, same complaint. Again, all tested ok and with no distant mobiles available I swapped over the tx and rx feeders and said "let me know if improved". BTW, the 2 dipoles were at about 90ft and spaced correctly. A bit longer before the next complaint of same poor rx on the base. This was my only lowband fm scheme so I had no base station gear I could swap out. Knowing talkthrough rx and base rx were both at fault and thinking it could only be an intermittent fault with the R17fm receiver, I started a complete system check to prove everything else was ok. The rx mute was at about 0.5uV with a direct signal into the socket, but when I checked the tx/rx antenna isolation by reconnecting the rx aerial and feeding sig gen into the tx aerial, it needed a far larger signal to open the 0.5uV mute than what I expected. This set me thinking. Interference maybe?
To summarise, it turned out that a local 11kV powerline insulator was arcing and hitting the rx with wideband noise. Being fm, it used a noise mute circuit. This also made the rx deaf when connected to the aerial. Like I said, it was my only lowband fm system as around 10 other local lowband systems were AM. If it had been the usual AM the fault would have been seen immediately. When the insulator was replaced, one happy customer.
Unusual fault that I never forgot.
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Old 14th Sep 2017, 5:21 pm   #32
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Default Re: Memories and other stories from Pye etc PMR engineers.

GB3AM on 6m gets this even now. I reckon a very old and decrepit power line has insulators which arc in poor conditions. Closest approach is ~1mile out.
Can't hear at ground level on site. Up above the 50/100++ Hz AM crud makes a total mess of FM signals with 77Hz CTCSS unless they are strong enough to overcome it.
Not many are on a bad day....

The nice new overhead power line at only 1/4 mile away - longer glass insulators so I guess higher Voltage, and with pylons which are not rusty, is silent by comparison.

It must need a Rain Static Board fitted - mmm? (just testing...)

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Old 14th Sep 2017, 6:31 pm   #33
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Can't comment on GB3AM.
I have another 2 stories to relate soon. One is a W15FM that had a deafness fault I had never seen before nor since.
The second is I feel rather spooky.....but true. Maybe nearer Halloween?
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Old 14th Sep 2017, 6:55 pm   #34
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Ah yes, rain static on AM systems. This bizarre and annoying condition used to plague our main scheme receivers. A sort of clattering sound in the operators headsets. When we used high band links a couple of times they used to somehow get into a kind of feedback loop caused I think by long distance co channel interference on reverse frequencies when there was a lift on. I seem to remember there was a well known problem with our UHF repeaters which had the same channel frequencies but reversed Tx and Rx in some other countries. If the conditions were right apparently a loop was formed, causing lock ups. When there was a lift on, we often had long range interference coming into the control rooms, but this was worse on our FM systems. The control room operators used to say "we can hear the trawlers again". I don't know who they actually were but were quite often foreign speaking. This was bad for the day job but good news for my radio hobby, so it was down to the car for a quick listen around while "trying to locate the source of the interference" for them.
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Old 15th Sep 2017, 12:06 pm   #35
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Default Re: Memories and other stories from Pye etc PMR engineers.

I'm not a PMR man myself, but I believe police forces' UHF systems along the East coast of the UK suffered interference from the Dutch Police when a lift was on. Finally solved I think by using CTCSS with non-standard tones.
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Old 15th Sep 2017, 1:37 pm   #36
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Default Re: Memories and other stories from Pye etc PMR engineers.

An interesting one that led me a merry song and dance was when, in my former life I was responsible for blue light comms in a part of northern Germany.

All blue light organisations used specific kit and frequencies exclusive to the emergency services. For our 4m Band scheme we had a standard duplex split of 9.8 MHz. In this incident, the local fire service control room complained of sporadic music on one of their channels, this only happening at certain times of the day. The TX/RX talkthrough system (we always had talk through active) was co-located at a main and remote site. Most control rooms were not usually tied to the hilltop RX via leased lines or radio links (these came later) but received the repeater off air as did the mobiles.

Ah, I thought probably interchannel modulation with one of the TXs on the local radio broadcast mast, however when listening on the output frequency of the fire service repeater I could hear that speech associated with the music was French. Listening to the music I noticed fading associated more with short-wave propagation. Then the penny dropped. When the TX came up the product of its frequency with a 9.8 MHz "interfering" signal fell exactly into the passband of the fire service RX.

Looking on Frequency tables I found that Radio France International transmitted on a frequency of 9.805 MHz. But where did this mixing take place. After many tests it was found to be a corroded antenna connector. While listening to the French station on the fire channel, I found that giving the base of the antenna (98m above ground) a hefty whack with a large hammer "solved" the problem.

Antenna and connector replaced. Problem gone. This was my first and only experience of secondary modulation and the "rusty bolt" effect. Happy days.

Although not connected to PYE etc. I hope this is on topic enough.

I find this thread most entertaining, it brings back many memories.
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Old 15th Sep 2017, 4:12 pm   #37
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A long time ago in a previous job we used to look after radio deaf-aids for school children. The teacher wore a little low power FM transmitter, and the hearing impaired pupil had a receiver fitted with an earpiece based on one of the commonly available radio chips at the time (I think it was an MC3361). This worked fine most of the time, but on occasion we had problems other than the usual dropped/got wet scenarios. One complaint was from a pupil who kept hearing strange voices in their earpiece. Every time we checked it out it in the workshop it was fine and this went back and forward a couple of times until we did a site visit and saw that the school was near a mast. We worked out the frequencies and found that the deaf aid receiver was on a direct image frequency for what I think was a local taxi firm. We gave the school a different channel colour, and all was well! The receivers weren't the most selective but most of the time we got away with it. I just kept imagining the poor kid trying desperately to convince his teacher that they were hearing strange voices.
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Old 15th Sep 2017, 4:32 pm   #38
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Default Re: Memories and other stories from Pye etc PMR engineers.

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Co-sited high power paging transmitters always seemed to find a way into your receivers without having to use extra bandpass filters in line too.
The sheer screaming brute-force of POCSAG transmitters around 150-155MHz was a bane for anyone who shared a mast with them.

There was another early digital data system I remember from the late-1980s which worked around 164MHz and would happily mess things up. It produced a constant 'growl' like some sort of large engine running. Several of my receivers needed seriously-narrowband copper-and-silver-plate cavity filters [everyone called them 'barrels' because they looked like beer-kegs] to keep this noise out but these were imported from the USA and very expensive!
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Old 15th Sep 2017, 4:43 pm   #39
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In the 1980s I was regularly involved in providing communications for the Lombard RAC Rally: the normal "marshal frequency" - licence held by the RAC Motorsports Association - was 86.4375MHz AM; there was another FM frequency around 169MHz used for medics. There were a number of other frequencies licensed by local support/4x4/marshal/rescue-groups, mainly low-band but a few were on high-band.

On one occasion a marshal-group from the South West of England were needed to support a rally-stage in the North-East, so they headed off to Teesside. Only to find 'their' frequency occupied by the radios of the local steelworks.

Both sides accused the other of 'stealing' their frequency. The slanging-match went on for hours: an interesting mix of Devon/Dorset and Geordie-esque accents!
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Old 15th Sep 2017, 6:06 pm   #40
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Default Re: Memories and other stories from Pye etc PMR engineers.

Biggles. "receiver was on a direct image frequency"
Can sometimes have fun!
My local county ambulance system that I maintained had one channel on 166.4MHz mobile rx using Europa MF5FM transceivers with a 10.7MHz IF.
I recall the time I was stuck at the traffic lights stuck behind an ambulance with "mad Philip" in the driving seat. I switched my 2M rig to 145MHz simplex (The image of 166.4MHz) and sent various messages. "Blue light on". He obliged. "Two tones" again he obliged. Thinking this was the end of MY fun, he then proceeded to reverse into my transit van and start pushing me backwards! That was HIS fun! With all my brakes on, I resisted him!
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