Modifying Vintage PMR Coils

[John G8MWF]

I am 'considering' modifying a 40+ years old Burndept BE439 UHF PMR handset for 70cm.

Luckily I was given diagrams of the set with notes that suggest an extra turn is required on the first local oscillator. Details enclosed.

This sounds simple but . . .
1. The board has to be removed from the mother board and the through plating for the earth connection has a nasty habit of lifting - but it can be done.
2. I have NO idea what gauge the coil wire is and I may cause more damage in carrying out this process.

I wondered if it would be much easier to simply tag another capacitor across the coil and bring the resonance down that way?
It sounds too easy to me and I suppose there would be some kind of trade off to the Q of the circuit and cause other issues?

[Biggles]

You may not need much extra capacitance to shift the frequency, with it being UHF. Why not try a few pFs to start off with, or if you are feeling brave a very small ceramic trimmer cap to see what happens? That's the approach I would use. Easier than adjusting or rewinding coils. You might just get away with it. From the original capacitor value you may be able to work out the inductor value with a bit of maths. From this you could possibly work out the approximate value of extra capacitance needed to adjust the tuning. Needs a bit of experimentation I think. Something in the back of my mind is telling me that to get the correct deviation on those sets you had to select a crystal from a few before you got within the correct range on Tx. I may be mistaken though...
Alan.

[SiriusHardware]

There is usually a reason why the originator of a mod / conversion has elected to do something the difficult way rather than the easy way - I remember that in Chris Lorek G4HCL's conversion of the band III Philips MX295 to 2m, he specifically concluded that the receiver front end 'tanks' had to be modified by adding turns rather than by just adding capacitance and I remember he added, rather testily 'Yes, I did try that!".

That said, one of my favourite sayings is 'always try the easy thing first', so why not see how it goes?

[g4aaw pete]

Hello John

I've not seen this particular piece of equipment.
Is the coil in question air-spaced? If so, perhaps squeeze the turns together slightly, to increase the inductance.

[G6Tanuki]

How much does the resonance need to be shifted?

In times-past I've tweaked various PMR stuff originally designed for 148-174MHz coverage down to 145MHz either by screwing the original tuning-slug a bit deeper into the coil than gives the resonance then screwing-in a second slug, or - for UHF stuff - soldring a couple of short wire 'tails' to the ends of the coil and bending the outer ends of these close together to provide a squib of capacitance; this was enough to drag a 440-470MHz General Electric radio down to 432MHz, and it still met the original sensitivity-specs!

[John G8MWF]

All points noted and thank you.

I went online and used a tool that calculates the component values dependent on the frequency using the formula 2*pi*F= 1/sqrt(L*C) by simply putting values into boxes.

My 'current' high band crystal is nominally 52.3Mhz tuned with a 27pF capacitor which makes L = to 343nH.

So if L is constant and we want a crystal frequency of 49.52 Mhz then C=30.4pF. a none preferred value.

So for a difference of just 3.4pF or an increase of around 12.6% it 'should' be possible to pull that circuit into range.

The BIG question is why did the manufacture change the coil values and not the capacitance?
Purely for keeping the parts list simple? Or getting a capacitor with this value is difficult - using a 33pF throws things right out!
Or is there a technical reason?

[G6Tanuki]

Historically, the "default" capacitance most crystals are supplied to work with was 30pF.

The difference between 27pF andf 30pF is around what you'd expect as a margin-of-error for generic 10%-tolerance commercial-grade capacitors.

Which makes me think you should just plug your crystal in amd see what frequency it comes up on.

Try it - you might be lucky!

[John G8MWF]

I agree - I will give it a go without modifications when the new crystal arrives in the next day or so

[Jon_G4MDC]

"L1 tuned by C9 select the 9th harmonic".

52.3x9 = 470.7? A 52.3 MHz crystal will be a 3rd overtone and series resonant.

That suggests LO is high side. I wonder if the need to change L1 was because LO was being put low side instead.
A common trick in conversions. What is your new crystal frequency?

[John G8MWF]

Test xtal = 49.30556Mhz

L1 appears to be 'tuned' by C3 in the 1st stage and the 3rd harmonic selected by C9 in the second stage.

The radio has crystals fitted by the manufacturer and the RX Xtal is given by

RX Freq + 10.7 / 9.
So 49.30556Mhz x 9 = 443.75 - 10.7 = 433.05Mhz.

Therefore they seem to be using the 'high' side technique.

[Jon_G4MDC]

Apologies you are right, I hadn't tried hard enough with the faint writing.
It is TR2 creating x9 when I had assumed it was amplifying x9 already selected at the oscillator.

I reckon you could do this either way. Adding some padding C is sure to be easiest.
Modern surface mount parts could be perfect for it. An 0805 would probably fit best across the pads if they use the capacitor series Pye were using in the same era.

[Biggles]

You could also try the "two bits of insulated wire twisted together to make a capacitor" technique to experiment. The extra lead lengths may add problems of their own of course, instability or otherwise confusing/unexpected outcomes. Been there done that, found out the hard way. I dare say it's all part of the fun. It all depends on how much spare time you have. I used to spend too many hours trying to get things to work, because of my determination to win over a bit of awkward kit.
Alan.

[John G8MWF]

Thank you all for the input and good ideas.

I made some progress today getting the 'scrapper' partially working on the rx side.
I have two chassis and the scrapper is the chassis that I am using to try out different options.
It has been heavily modified in the past and some of the tx modules are missing whereas the other chassis is still very close to original.

One of the rx xtals, around 51.4Mhz, from the 'good' set was placed in the scrapper and tuned up. This was achieved by listening on another handheld tuned to the local oscillator frequency which is 10.7Mhz above the rx frequency - a surprisingly quick and practical method!!

Interestingly the tuning slug in the 1st local oscillator was quite a way out of the holder which hopefully means that it will tune down to a lower xtal frequency, 49.30556Mhz, for the 70cm band.
It could just be that this module has already been modified in the past but I don't want to pull it apart and risk damaging it.

Hopefully this will be successful and it would be very nice to get to use a proper signal generator to line it up as best as possible.
All of the factory colour codes show that it is/was a high band set and it may be a bit deaf when using it on the lo band frequencies.

[John G8MWF]

The crystal for 433.05, that only cost me £5, arrived this morning and tuned up in no time. The only surprise was how little the core of L1 had to be adjusted into the coil.
This leads me to believe that this part of the radio may have been modified, very very nicely, in an earlier life?

I looked for other peaks in the coil but there was only the one.

Naturally the set needs tuning up properly with a signal generator as my only TX source is a handheld set using a dummy load. Not the ideal source for setting up with!

So it looks like I have a fighting chance of getting onto 70cm but I will have to source the proper test kit to see just how well the set will work.

Thank you all for your excellent advice - much appreciated.

[Biggles]

Ah yes, those photos are very familiar. A common problem was definitely the tiny screws coming loose and going missing resulting in a loose head, and possibly causing dry joints or bad connections to the main pcb? We also had our fair share of phantom tinkerers (bored bobbies on night shift out in the sticks) spraying them with WD40, trying to insert large self tappers into screw holes, cracked cases (some must have been run over by vehicles or thrown long distances onto hard surfaces etc etc). Bringing back some good and bad memories, and hours spent tuning them up on the calibrator. I seem to remember one of the spec checks was current taken on Tx. It had to be below a certain value, presumably because they were effectively powered by PP3 nicads.

[John G8MWF]

Originally Posted by Biggles

Ah yes, those photos are very familiar. A common problem was definitely the tiny screws coming loose and going missing resulting in a loose head, and possibly causing dry joints or bad connections to the main pcb? We also had our fair share of phantom tinkerers (bored bobbies on night shift out in the sticks) spraying them with WD40, trying to insert large self tappers into screw holes, cracked cases (some must have been run over by vehicles or thrown long distances onto hard surfaces etc etc). Bringing back some good and bad memories, and hours spent tuning them up on the calibrator. I seem to remember one of the spec checks was current taken on Tx. It had to be below a certain value, presumably because they were effectively powered by PP3 nicads.

It was only by chance that I found out one of the people at work is a retired police office who used to train officers in the use of this type of radio equipment.
The set he remembers best was the Storno - 'useless radio but due to its heavy metal case was very useful in a punch up!'

My 439 was covered in nicotine on the inside and took forever to clean.
I can only assume, in part of its life, that it spent many hours in a smoke filled car and/or rooms in a station somewhere way back in time which would probably not be politically correct nowadays.

[G6Tanuki]

Originally Posted by John G8MWF

It was only by chance that I found out one of the people at work is a retired police office who used to train officers in the use of this type of radio equipment.
The set he remembers best was the Storno - 'useless radio but due to its heavy metal case was very useful in a punch up!'

Reminds me of a comment from when I contracted in Dallas/Fort-Worth and one of their ex-police types said of a 1970s Motorola walkie-talkie that it was "Tough enough to beat a mugger unconscious and it still worked so you could use it to call for backup".

[The radio-in-question worked well on the 144-148MHz amateur band and a lot of the cops in the area had an 'unofficial' position on their channel-select switch to they could listen on the local repeater]

[Biggles]

The Motorola HT600 and 800's were very sturdily built. They were pretty much unbreakable apart from the extreme cases where one had been under the wheels of a vehicle. A few went missing also. I seem to remember doing a training course on new generation Burndept handportables that were launched around about the time when the emergency services went high band after the WARC band changes. Maybe a Burndept 600? This had a very robust outer casing made from die cast aluminium? There was also a Burndept mobile around the same time but we didn't have many. Most were Marconi 690's. There was a low band version of the 690 for the Fire Service also. We used a mix of PF85, later PFX, and of course the HT600E. I think I may have a couple of UHF PF2/PF70 handportables somewhere also. They were an odd beast, again with modules maybe. It's a long time ago.