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Old 15th Sep 2017, 6:18 pm   #41
robinshack
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Default Re: Memories and other stories from Pye etc PMR engineers.

Ther highband W15FM was a mobile on a farmer's system with a complaint of rx not as good as other mobiles. Removing the set from the vehicle and onto my transit van workbench, i confirmed it was down by about 15-20dB. Fine tuning of front end showed little improvement still.
Every check gave normal voltage readings. IF at 10.7 was very sensitive indeed. First mixer injection was fine, despite those notorious BF152 in the Osc/multiplier strip. Problem seemed around the front end on the FET RF RX module. SO, I replaced the TR1 and tested again, no difference. Adjusting the 2 ferrite cores around TR1, but still no difference. Repeating these adjustments several times, all to no avail. I eventually snapped one core and then broke the second one. Oh well, take it to bits next? But no, I managed to rotate the 2 cores downwards and out of the coil former.
Upon obtaining 2 cores from my spares box and refitting and tuning, i was amazed that I could now get performance exceeding the factory spec!
Just what was wrong with the original cores i will never know.
Other radios on the scheme never had this problem.
Rob
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Old 15th Sep 2017, 6:40 pm   #42
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Default Re: Memories and other stories from Pye etc PMR engineers.

The system might have been Pacnet? This was on around 164 if I remember correctly and just permanently transmitted (packet?) data which did sound like a big diesel running around 1000 RPM. I haven't checked to see if it's still active recently but it was prolific in the nineties. It only occasionally broke through to our receivers probably when there was a bit of rectification in the metal joints on the masts more than anything else. I suppose GPRS and the likes have taken over nowadays. The other paging system was ERMES if I remember correctly, but these didn't seem to cause trouble. I think it was the sheer Tx power that made breakthrough more likely with POCSAG.
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Old 15th Sep 2017, 6:46 pm   #43
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Default Re: Memories and other stories from Pye etc PMR engineers.

I vaguely remember Pacnet being mentioned: though we all knew it as "the Tractor" because thats what it sounded like when it broke through.

[c.f. "the Russian Woodpecker" - https://en.wikipedia.org/wiki/Duga_radar - which was active on the HF bands in the same timescale, and whose pokka-pokka-pokka pulses could occasionally be heard as breakthrough in the background of VHF comms when it transiently chose 10.7MHz on which to radiate its hundreds-of-Megawatts]

I never bothered hacking the likes of Pacnet: though some of my colleagues built a really rather simple 3-transistor interface that would take the audio from a scanner and, with the help of an original IBM-PC, decode POCSAG into plaintext.

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Old 15th Sep 2017, 6:56 pm   #44
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Default Re: Memories and other stories from Pye etc PMR engineers.

Rob, the stopped at traffic lights thing has reminded me about a story I once heard that apparently someone was monitoring on his scanner when stopped at temporary lights and suddenly heard a very cheeky conversation about his car/appearance or whatever taking place between two contractors using handportable radios at the roadworks. He got out to angrily challenge them about it. They still never twigged how he had "overheard" their conversation.
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Old 15th Sep 2017, 9:08 pm   #45
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Default Re: Memories and other stories from Pye etc PMR engineers.

I hope this is not too far off-topic .....

A friend of my Dad was a keen electronics enthusiast and a musician in a band; and one day, he built a simple low-power radio transmitter to plug into his guitar allowing it to be played through a multi-band "worldwide" portable radio. (My Dad didn't remember which band it used.) Anyway, he adjusted it to a quiet frequency, plugged the earphone socket of his radio into his amplifier -- and was now able to wander freely around the stage with his guitar, unrestricted by a lead.

The band were Quite Good, and soon they managed to obtain a spot on the bill at the Queen's Hall, Derby. (If you remember anything about the geography of bygone Derby, you'll be nodding about now, having an inkling where this is going .....) The Queen's Hall is on London Road, opposite where the Derbyshire Royal Infirmary used to stand.

In the quiet Amber valley pit villages, there were few ambulances. But here they were, playing a gig in a hall directly opposite the only hospital with an emergency department for miles around. With the lead guitarist playing over a radio link ..... on a frequency that just happened to belong to the ambulance service. Due to the limited range of the transmitter, he did not cause ambulance drivers to jolt the patients as they tapped their foot on the gas pedal in time with his infectious rhythms. However, every ambulance leaving or arriving broke through his amplifier at full volume .....
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Old 15th Sep 2017, 9:55 pm   #46
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Default Re: Memories and other stories from Pye etc PMR engineers.

On a similar theme, we once had someone in from a telephone company who were in the process of replacing the telephone system in one of our control rooms. It was in the days when it was common to provide an tele/radio interconnect facility. The idea was that the radio network could interface to an incoming PSTN line and so enable an emergency vehicle radio to connect to a hospital or whatever by telephone. It was never used in anger and employed some kind of vox system which is very off putting to anyone used to having a full duplex conversation. Anyway, to cut to the chase the telephone switch had the music on hold facility. I had to quickly explain to the rep for the company that we didn't want that enabled for the radio interconnect. I can just imagine now every emergency vehicle radio in the county receiving a long rendition of Greensleeves or whatever was the "tune of the day".
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Old 16th Sep 2017, 7:28 am   #47
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Default Re: Memories and other stories from Pye etc PMR engineers.

Robin, re post #41

I used to come across quite a few 2m-Band (168-174 MHz) handhelds which were quite insensitive. Try as I might, I could not get the front end aligned to spec. Luckily I found the cause after an unmarked police vehicle turned up and the officer duly opened the boot to take out a combination of handheld with a big mag-mount quarterwave which was stored on top of the aluminium case of the radio in its storage box.


After much faffing around, I found that replacing the four tuning slugs in the front end and subsequent realignment brought the front end up to or better than manufacturers spec. Since then I kept a considerable stock of cores as this fault presented itself on serveral more occasions. No one would believe that it was not a good idea to store the mag-mount base on top of the radio because it magnetized the tuning slugs in the radio.
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Old 16th Sep 2017, 7:53 am   #48
robinshack
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Default Re: Memories and other stories from Pye etc PMR engineers.

Eddie, that certainly makes sense. It seems about the only answer. However, back when this fault appeared mag-mounts were rare as they were positively discouraged. Not to say that a large cb mount didn't get placed close too though! Strange it was only 2 cores out of 5 if I remember correctly.
Rob
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Old 16th Sep 2017, 9:28 am   #49
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Default Re: Memories and other stories from Pye etc PMR engineers.

Neosid F29 ferrite. Weller TCP Soldering Iron magnets could kill them. Generally only used in the VHF tuned ccts I think.
The cores in low freq coils in mutlipliers etc were not the same stuff and was why it never worked to pinch those from scrap boards as replacements.
A DC spike in the winding caused by accidentally grounding the collector/drain end could kill them too.

First time I have heard the magmount story - nice one.

Just found this - didn't know about shock, don't drop them on the floor!

Note: Perminvar ferrites undergo irreversible
changes of characteristics if subject to strong
magnetic fields or mechanical shock.

Last edited by Jon_G4MDC; 16th Sep 2017 at 9:39 am.
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Old 16th Sep 2017, 11:31 am   #50
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Default Re: Memories and other stories from Pye etc PMR engineers.

I never knew that about the cores. Back when we used remote boot mount Motorola MC Micro high band sets, the usual place to stick a magnetic blue rotating beacon when not in use was, yes you've guessed, on that lovely flat piece of metal called a radio. It didn't seem to affect the performance though, as when I first saw it being done, we did some checks and couldn't find a problem.
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Old 16th Sep 2017, 11:39 am   #51
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Default Re: Memories and other stories from Pye etc PMR engineers.

Did you ever try demagnetising the cores with a suitably-hefty AC field?
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Old 16th Sep 2017, 12:47 pm   #52
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Default Re: Memories and other stories from Pye etc PMR engineers.

No, at the time the idea to try to de-magnetize the cores didn't cross my mind. It was always a case of getting kit sorted and back on the road. The replacement cores cost the equivalent of about 5p each and came in packets of I think 10.

Incidentally, I only experienced this this phenomenon with BOSCH hand helds and not on later Motorola kit.
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Old 17th Sep 2017, 8:50 am   #53
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Default Re: Memories and other stories from Pye etc PMR engineers.

Although it seems like yesterday when it happened, anyone have memories, good or bad, of the infamous Millenium Bug? We had panic going on in the extreme among the upper management and of course the "engineers" were left to sort it out by predicting worse case scenarios and planning how to deal with them. Emergency services control rooms and IT systems supporting them were the main worry, but it even filtered down to suspicions of even domestic appliances and vehicles playing up at the critical hour. Every engineer was forced to be on call for the event, so I, like many others, spent the early hours of the New Year babysitting in a control room in Newcastle. The firework display was good, and I was paid well, and thankfully we didn't have to revert to "plan B" because everything just ran as it had done for many years before. Happy days.
Alan.
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Old 17th Sep 2017, 11:15 am   #54
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Default Re: Memories and other stories from Pye etc PMR engineers.

Another story about the VHF based warden-alarm system I used to look after, although this pre-dates my time working on it. In later years the system worked on VHF high band, as mentioned in an earlier post.

But for some time before that it had originally used a single frequency in 'M' band, which was usually a split-frequency setup using frequencies around 109Mhz / 139 Mhz.

While it was still on that band a crisis blew up when the main station receiver on a tower block in the west end was jammed for hours on end by a constant, very distorted transmission, usually during the day or evening, never during the night. The radio interference service were called in, and in remarkably short order they seized an innocuous little transistor radio from a hapless tenant somewhere down in the estate below the main station receiver.

The local oscillator of the radio - tuned to an FM broadcast station and sitting exactly 10.7Mhz above the frequency of that station - had been radiating such a strong signal that the base station was picking it up to the exclusion of all else.

I had to feel a bit sorry for the tenant. Imagine trying to explain to them that their radio receiver was being taken off them because it was -transmitting- a signal.
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Old 17th Sep 2017, 5:27 pm   #55
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Default Re: Memories and other stories from Pye etc PMR engineers.

Skipping back to the cores, it is not a problem of (de) magnetisation, more that the whole structure of the ferrite itself is ruined.

Apparently it has name which is "disaccommodation" and,

"The relative inductance drop in the period 1 to 10 hours after the shock is the same as the in the period 1 to 10 years, so that the long-term instability of the inductance can be predicted".

I wouldn't mind having a good stock of them. Bargain at 5p each but find some now...
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Old 17th Sep 2017, 6:34 pm   #56
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Default Re: Memories and other stories from Pye etc PMR engineers.

Disaccommodation in ferrite cores is quite normal and has to be handled in designing precision ferrite inductors for filters. It can be reset thermally, though the temperatures needed can be somewhat above the capabilities of wire insulation. I'm a bit surprised at a material with a disaccommodation offset trigger-able by the field from a good old TCP1 iron.

The full story is in "Soft Ferrites" by Snelling (Iliffe) which is pretty much THE book on ferrites

David
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Old Yesterday, 10:29 am   #57
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Default Re: Memories and other stories from Pye etc PMR engineers.

That's interesting David, I have never thought to see if they can be improved at all by a thermal cycle. I'm not sure I still have any to try out the idea.
Not having that book - what temperature would be required to have an effect?

I do remember that F29 grade was really sensitive to it and it was often best to replace those cores "on sus" before starting any serious fault finding.
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Old Today, 7:58 pm   #58
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Default Re: Memories and other stories from Pye etc PMR engineers.

Another comedy situation that I suffered as a young rookie engineer occurred when it was decided by the authorities that the PA speakers on fire appliance radios had to be disconnected as it breached confidentiality. It was usual to switch the appliance radio through to an external PA speaker when on fire-ground ops to enable the crew to hear Fire Control messages while outside the vehicle. Anyway, the speakers had to be disconnected on every appliance in the county so a senior engineer and yours truly had the job of doing that. It was normal practice to ring Fire Control to take the vehicle off the run while maintenance work was being carried out, but as it was a two minute job it was decided it wasn't worth the bother of doing that so I was duly sent up on top of the vehicle with a screwdriver to disconnect the PA speaker. Sod's law prevailed. As soon as I had climbed on top, a fire call came in, the bells went off and the crews came running. It was the fastest I had moved for quite a while. I think I made it down off that appliance in about two seconds flat.
Alan.
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