PYE FM10 Cambridge - RX troubles

[Jon_G4MDC]

Hi, I have had one of these in the pile for a few years now.
It has a strange problem which I cannot seem to solve so I wondered if anyone here has come across it and knows a solution.

It is an FM set for G Band (50MHz) and by memory I think first IF is ~6MHz with second IF of 455kHz in the usual way. Sets for higher frequencies were 10.7MHz first IF.

The RX tunes up and works quite well so long as the 455kHz IF board is hinged out of the chassis. (The 2nd IF and Audio Boards on Cambridges were hinged to allow easy servicing). For some reason when the 2nd IF board is hinged down and locked in the normal position the set has some kind of regeneration where the normal FM no signal white noise takes on hollow sound - sensitivity is very poor when it is like this.

My reckoning is coupling occurs between the 2nd IF Board and the 6>455kHz mixer board just below. The resulting feedback is responsible for the regeneration effect. I can't find a cure for it. I cant see any issue with earths, cable screens, decoupling capacitors. I'm stumped.

Has anyone come across this before please? Thanks.

[Peter.N.]

I wonder if there are decoupling capacitors on the 12v and AGC lines, they might be small ceramics, other than that I don't know them well enough to be useful.

I had a PMR set up using these from the mid '60s until we moved down here in 1970 only mine were AM low band, we rented them from Pye initially but bought secondhand Westminsters some years after we moved here.

Peter

[Jon_G4MDC]

Thanks Peter, I have several AM ones and never had a spot of trouble with them.

This is my first FM. I guess there is quite a high level of 455 happening at the limiter before the discriminator. Maybe the 455 board should have a cover screen for FM? It doesn't have one and they never had them in AM versions.

There is a screen over the 2nd mixer board.

[mike g3zii]

Hello, I have somewhere, the remains of a HI Band FM Cambridge, I seem to recall that there is a screen plate fitted on the top of the 455 IF unit.
it's just a flat plate fitted with the usual 6ba screws. with hole to suit adjustments to the ifts etc.
it would probably cure it if you made up a plate to fit or I could send you the one off mine but it may take some excavation to find the rig.regards Mike.

[Biggles]

The Whitehalls I used to work on had an isolated negative rail, decoupled to the chassis/tinware with a rake of capacitors. They used to go leaky and cause all sorts of weird effects. If your set is similar (can't be sure from memory) it may be the act of connecting the board to chassis with screws that is causing problems. May be worth a check.
Alan.

[G6Tanuki]

If it uses any AF11x/OC17x transistors... tin-whiskerism can cause all sorts of odd effects.

[I recall some time sweating over a NukePol 30-50MHz Pye Vanguard FM100 that had whiskery trannies in the front-end; I'd 'fix' it then a day later it'd go dumb again. This was before the whiskery-issues were fully understood - just removing the radio from the car would give enough vibration to disconnect embryonic whiskers - for a week or so. ].

[Jon_G4MDC]

Thanks to all for replies.

Thanks Mike for your memory of a screen plate. Be good to confirm that for certain. I wonder what the material was. Plated mild steel, Aluminium or something more exotic...?

I have been around the floating earth sets. Olympics had about a hundred 4n7 ceramics in parallel - which one is leaky? Remove all the modules and apply plenty of Volts usually finds the one. Also have Whitehalls - I like them very much for the AM/FM capability.

It does of course have Tin Whisker prone transistors but the effect in question does not have sudden onset. The position of the 455 board almost works like a reaction control. It doesn't seem to matter the orientation of the set -just whether the 455 board is swung out or in.

When I get to it down the pile looks like a screening lid is the way to go.

[mike g3zii]

Hello Jon, I have done a 'time team' dig under the bench in the shack and have found the FM Cambridge.
there is a screening plate on top of it, the IF unit is type no 276254
the plate measures 4.5 x 2.5 inches it has 4 holes to take 6BA screws,
these locate in the tapped fixings in the screening box round the edge of the board. If that seems to be what you need I will gladly post it to you.
It will go as a standard letter so I am not worried about the cost.73 Mike.

[Jon_G4MDC]

Many thanks for taking all that trouble Mike!
I will PM you.

[Jon_G4MDC]

The plate from Mike arrived today - thanks Mike!

It makes a big difference - it's certain that one needs to be fitted, as well as the one over the 2nd Mixer Board and squelch.

The effect was not quite gone - there was still a slight hollow sound to the FM white noise with the 455 board fully home. I set about trying to find and eliminate the last of it.

A strap between the can of T303, the driver transformer for the discriminator (a modified form of Foster-Seeley I think) and the adjacent ground track killed it completely. I would think there is top track that should make the same connection. Maybe my transformer has been out at some time and the through holes are damaged? I don't know but it is gone and good riddance to it.

I now have a happy receiver which it turns out is on 10m not 6m. This is an H band Cambridge - 1st IF was 6MHz as I originally thought. It can hear 0.3uV perfectly well - wasted at that frequency most likely. Now to find a useful crystal - it's on 29.13MHz just now because of what was in the junk box. 29.6MHz would be better.

[G3VKM_Roger]

What actual crystal freqs do you need for 29.6? I acquired several 100s of rocks earlier this year, the bulk of them are HC6/U.

73

Roger

[Jon_G4MDC]

Hello Roger, thanks for the reply.The RX needs 35.6MHz in HC6U preferably but I'm not fussy - I'll bodge HC18 or HC25 or try some 11MHz fundamental at a pinch.

If you look in the components and circuits section I have started a discussion about exchanging crystals / rocks. We all have everything we don't need and would gladly exchange any/several for what we do need.

It's been this way for years - back in the day we would ask down the local club. Today we can do better perhaps?

It is on 29.13MHz because I had a 35.13MHz in HC6/U which x2 is 70.26MHz. Crystals are a treasure trove...

[G3VKM_Roger]

OK John,

Nothing in the 35MHz region I'm afraid - 36MHz I have dozens! Just confirm the TX crystal frequency, is it around 7 MHz?

73

Roger

[Jon_G4MDC]

Thanks for looking Roger. I have 35.13 (70.26/2) and thats it for 35MHz.
Some 36MHz and loads of 37MHz, some 38 and 39MHz.

The TX mult is x12 so would be looking for 2.4666MHz.

On Low Band AM Cambridges I did perfectly well using the RX Osc as a free running VFO for 70.26MHz and +/-. It wasn't perfect but it was good enough.

On 10m I suppose that trick would be about twice as good since no multiplier. I have IC502s for 6m which use a 36MHz free running VFO - and they are all right - even for SSB/CW.

For this Cambridge I don't know what is best. Originally I thought it was 6m, in which case I would have put crystals in it for the local repeater and used it as an antique trigger set. That would need CTCSS adding which it would have never seen in it's PMR days.

As it's a 10m set it could sit on 29.6MHz never doing anything except in good band conditions. Wait another 6-7 years before it hears anything, though there's a bit left in this cycle yet I suppose. It would really need to be able to tune around a bit to be interesting - the odd DX repeater etc. 10m FM hasn't been used around here for local chat since the £23 CB27/81 rig conversion era. They all got bored with it. I still have mine!

I can build it a PLL synthesiser. I have a PIC processor board and Si570 freq gen chip that would get it going for sure. But am I that much of a heretic to marry the two?

Thinking it over.

Edit: And I remembered something Radio Control crystals! I have bought 35MHz RC crystals for use on 4m. They were TX crystals and 17.5MHz for doubling to final frequency. 35.1 (70.2/2) was an RC channel for sure. Supposing there is some RC RX with 455kHz IF and high side injection.
Then 35.555MHz etc would exist for that market. Just my luck if they use low side and so RX crystals are 34.645MHz.

[G3VKM_Roger]

Hi John,

The nearest TX rock I have is 2472.5 in HC6/U, which would give you 29.670 MHz, a useful working freq. My crystal mound does have a box of AM CB and possibly R/C frequencies but nothing I could see would be of use for your FM-10.

Pye gear is becoming collectible, particularly the Pocktfone era and Whitehalls and Wesseys. It seems that police re-enactors like these sets for their restored Zodiacs and other classic jam-sandwiches, usually a non-working set is sufficient as long as it has the gizmos like the control box, etc.

P/M me if you want that TX rock.

73

Roger

[G6Tanuki]

Originally Posted by G3VKM_Roger

Pye gear is becoming collectible, particularly the Pocktfone era and Whitehalls and Wesseys. It seems that police re-enactors like these sets for their restored Zodiacs and other classic jam-sandwiches, usually a non-working set is sufficient as long as it has the gizmos like the control box, etc.

And for some time the "PF8" Pocketfone has been sought-after by those who want to recreate the "Bodie and Doyle" era of "The Professionals".

If Wessies are becoming collectible I'll hang on to the couple of dozen I've got stashed in my attic [all sorts: dash- and remote-mount, FM and AM, mostly VHF but there's a UHF one and a couple of "long" Wessies - from the old Post Office mobile-phone era. I used to have a lot of fun running an M30AM Wessie on 86.4375MHz as one of the RAC Rally safety-marshals'.

[Jon_G4MDC]

Now the Rx works thoughts have turned towards the TX. It has taken 6 weeks to cogitate it. I found a TX crystal which comes out about 29.4, close enough for a first tune up.

I went to give that a first go this evening. Then I hit the fact my manual is for a boot mount FM10B but it is a dash mount FM10D. The STBY/TX (heater) power and PTT switching is different with the control cable and box - so I chickened out.

Thanks to the forum I found a link to an AM dash mount schematic and that will be good enough to sort it out by reading across. Also in another thread there was some excellent info on the 455kHz filters - this one is (V) 25kHz -I suspect it will be too wide for 10m FM but something to worry about later. I was most pleased with the info about low and high impedance versions which happened with Valve In/Transistor Out (Vanguard), Transistor In/Transistor out (Cambridge). You learn many many things on this forum.
https://www.vintage-radio.net/forum/...d.php?t=105199
That thread is a gem. It also made me remember what I was told by an old boy in early days at Pye
N-50kHz - Narrow (VHF only needed barn door selectivity before I suppose)
V-25kHz - Very Narrow
S-12.5 kHz - Super Narrow
All this because Ofcom's predecessors kept halving the channel spacing to get more licencees in the bands!

On order are a few Si5351 synthesiser boards. Once it is working on internal xtals I'm going to give it a pseudo-VFO tuned by a multi-turn pot with a 4digit mini LED display (like Kojak's 1970s watch). That bit won't be original (could appear somewhat vintage) but at least it will be useful and perhaps one of a kind. Tuning over 29.2-29.7 MHz is planned - I might even give it a switch for -100kHz repeater shift on TX. Optimist!

[Wendymott]

Hi Jon. I came across the AD9850 DDS VFO Sinewave chip a while ago, which generates frequencies from say.... 100 Khz to 55Mhz.... No birdies or squarks at other frequencies as long as the output is filtered. To control the DDS a 16F628 PIC is used and a 16X2 LCD display.
My friend in Aussie wrote the software a while ago, and has modified it for me.... it will do IF offsets, TX/RX frequencies.............. I know its not in the spirit of "Vintage" but it helps if crystals are not readily available.
I am using one of these DDS chips as the 2nd Oscillator to drive my SSB project.

[Jon_G4MDC]

Thanks Wendy, I'm still waiting for the Si5351 parts to arrive. The only good thing about that is it puts off the evil day when I have to start writing PIC code, I don't enjoy that much.

The TX tuned up easily all Voltages OK to handbook and 16Watts out. Can now look for split frequency contacts on 29.13R / 29.38T so I guess it will be a fairly long wait for one.

I was surprised there appears to be no adjustment possible of the 2.5MHz coil in the phase modulator - modulation seems OK but that surprised me. Are there any FM10 experts reading who know about this please?