AM Cambridge FM conversion. - UK Vintage Radio Repair and Restoration Discussion Forum

sentinel040 · 4th Mar 2018, 5:27 pm

I think This ought to go down, in part at least, as a case of admitting past sins. I can also hear people wondering why on earth I am still messing around with stuff like this. The simple answer to that is "because I want to"....

Back in 1980 having just graduated I installed a modified AM10B in my newly acquired second hand half timbered Morris Minor. I actually still have both, although the AM10B in no longer installed. At the time the AM10 RX was converted to FM by the use of a a suitable Garex board, however the TX conversion was, shall we say, slightly more Heath Robinson. Following the vogue among my peers the Modulation transformer was disconnected from the PA feed and the PA fed directly. A wirewound resistor of indeterminate value attached across the mod transformers secondary along with a pot and one end on the secondary winding earthed. The wiper of the pot went via 220nF cap to the oscillator valves screen grid, which was decoupled by a 2.2nF cap instead of the original 1nF, for reasons I can no longer fathom. The result was TX audio which was characteristically thin and with a noticeable whine, courtesy of the DC-DC converter. In an attampt to increase this; the audio and not the whine, although both tended to go together; the resistors in series with the TX audio amp and Mod driver transistors Emitter decoupling caps were shorted out. This had the side effect of also increasing the RX audio level as well requiring a mod to reduce that!

Not a particularly elegant solution; but it sort of worked for a number of years.

I am tempted to but the Cambridge back in the Morris and every now and again I wonder if there was a better way of converting the TX to FM, other than going the whole hog and modifying it as per the FM10 with the additional stage, but apart from requiring a coil of unknown characteristics it has the unwanted side effect of requiring new crystals and that is further than the budget will stretch!

I realise that starting at 8mHz, as per the AM10 as opposed to the 4mhz (if I remember correctly) of the FM10 will limit the available deviation, due to the lower multiplication factor.

So, is there anyone out there who has had more success than me in this matter and has a more elegant solution? Or has any other useful mods (I am considering replacing the diode 1st mixer with a FET as per the Garex Twomobile).

Regards

Ian (G8KSZ)

Biggles · 4th Mar 2018, 5:35 pm

Sorry can't help with the mod but another thing to consider during modification is the requirement to transmit CTCSS tones alongside the audio if you are intending to access 2M repeaters.
Alan.

Bazz4CQJ · 4th Mar 2018, 6:02 pm

Quote:

Originally Posted by sentinel040

realise that starting at 8mHz, as per the AM10 as opposed to the 4mhz (if I remember correctly) of the FM10 will limit the available deviation, due to the lower multiplication factor.

I don't think that is an issue; I seem to recall many people with 2m rigs based on 8MHz crystals easily achieving more than enough deviation. Unfortunately, most of my stash of mags of that period was lost, but I'm sure there were lots of designs around. I think that one option was to use a varicap diode alongside the crystal, this requiring very little AF to drive it.

B

G6Tanuki · 4th Mar 2018, 7:12 pm

Given that there was a FM10B Cambridge which was widely used, I really think it would be more-sane to get one of those rather than trying to coerce an AM version on to FM.

Why beat yourself senseless messing-around with a conversion when Pye did it properly? Go to a few radio-rallies and I'm sure a FM10D Cambridge will appear, either on one of the stalls or the local radio-club's "Bring-and-buy".

[Sidenote: I've got a loft-full of FM15/FM30 highband Pye Westminsters and also half-dozen of the low-band AM versions on the old 86.4375MHz rally-radio channel which will easily retune to 70MHz if you have the crystalsl]

Jon_G4MDC · 4th Mar 2018, 8:08 pm

I can relate to OP's sentiments.
An AM10 with Garex board for FM RX worked surprisingly well in my experience.

The TX modifications were never quite so successful. Inverter whine was the bane of all of them. I am not sure how they would cope with CTCSS for modern times.

I'll be controversial and put up the idea that a few channels of crystal control made 2m a better band than full VFO/Synthesised rigs and everyone hiding on their secret channels.

It was never the same around here after S0 or 145.0 became a repeater input - it marked the beginning of the end. I count myself lucky to have been licensed before that happened. I had a few good years out of it. Cross mode with AM etc was all part of the fun.

G6Tanuki · 4th Mar 2018, 8:19 pm

If you're convering old PMR radios to 144MHz, remember that a couple of decades back we switched to 12.5KHz channel-spacing in the 145MHz-and-higher FM/Repeater slice of the band.

Which means that these days you need to throttle-back the deviation on the transmitter, and also need narower-bandwidth filters in the receiver.

Running legacy 25KHz FM deviation on your transmitter will not make you popular! Most repeaters will refuse to repeat overdeviated signals.

And of course these days you need CTCSS not the old 1750Hz "whistle-up" to get through a repeater.

Jon_G4MDC · 4th Mar 2018, 10:27 pm

heheh I am looking for FM contacts on a Yaesu FT202 - 1978 vintage, a TR2200GX maybe 1977 vintage (my first rig!), and some Pye Bantams. The trouble is not so many heard.
Many of those who are heard are working their own nets and break in does not appear welcome.

That is my point about 145.0/S0. If there was anyone there they would bust a gut to work you. Today - sri OM your deviation is low - yes I know- hopefully none at all - because I am running AM!

sentinel040 · 4th Mar 2018, 10:37 pm

Thank you all.

As I said, this is not something I intend to start, I did that circa 1980, rather something I intend to finish, well try to find slightly more elegant way, if there is one.

The problem with finding and using an FM10 (I have a low band one on 70.26) is that I would have to buy new TX crystals, which makes it financially untenable. And the whole point of this is to try to improve what I have lurking in a dark corner. Operating is not my main thing, I am a dabbler, its the challenge that interests me, providing of course it not too much of a challenge.

I suppose I also align myself with the comments made by Jon.

CTCSS, well that's not something I was contemplating, 'CF still plays with 1750 and once that goes I will give up on that front. Also, the multiplicity of frequencies and the associated switching makes it impractical. Hmm Ledex switched CTCSS, now there's an idea.

Or maybe or not....

Deviation wise, well I would probably hook it up to the 2950 test set, mind you the dev meter on that is still playing up; but a good quick test is to listen on one of my collection of Pye MX294's, the filters on those are nice and tight for 12.5 and if you have too much dev the resultant noise closes the squelch. Actually, I think too much dev would be a nice problem to have!

As I have some 12.5 block filters for the Cambridge series receivers (the S ones) the receive side is not a problem in that regard, if I found I needed a tighter filter. Actually with regard to the switch to 12.5 spacing, the shack is also my office and when I am here I often listen on 145.5 and very few people seem to use the 12.5 channels and most have their radios programmed to the "wide" setting because they think their transmission sounds better, or that's what their mates tell them...

But I digress slightly. The varicap route is perhaps the way to go, rather than using the 6BH6 oscillator valve, but rather than re-invent the wheel I was looking to see if anyone can remember how to do it.

All the best

Ian

CambridgeWorks · 4th Mar 2018, 10:42 pm

I no longer have it, but I seem to remember circuits in general in the RSGB NBFM manual from the 1970s I think?
Maybe someone can help?
I must say, you do have enthusiasm for this AM10!
I did a 6bh6 screen grid mod back about 1975, but then soon progressed to a proper Pye LC10FM boot mount, 10 channel bootmount, ex. very early radiophone system used in London I think.
Rob

Jon_G4MDC · 4th Mar 2018, 11:05 pm

Well I reckon this needs to be done!

Full credit to you Ian. I will recall dear old Peter G3THS who stirred it up very well on GB3PI - back in the day - he could not stand "Rice-boxes" or their operators in the main.

We used to discuss AutoWestEuros and HyCoMinsters...superb amalgamations of what was around at the time. They were bitsers I suppose - made from bits of this and bits of that. Great days.

And the rig used to make the main subject of the QSO. Now it is FTXXX+wah, ICXXX+wah, with a bought in aerial/feeder +++ wah/wah/wah.

I for one would rather struggle to make the QSO through some inverter whine.
More power to you.

Bazz4CQJ · 5th Mar 2018, 2:59 am

Attached is a varicap circuit from the 1971 RSGB Handbook; it's applied to a VFO but I'm sure it can be adapted for a crystal. I suspect that you could get some results with almost any modern Si diode(?).

As per the comments above, unless you have some friends who want to get involved in this project, I'm not sure exactly where this venture will take you (in terms of working other amateurs) in the modern 2m era

B

G3VKM_Roger · 5th Mar 2018, 8:18 am

Garex did the job for you, if you can find one! It was called a "Twomobile". I bought the one pictured for 50p at a rally but long since parted with it.

73

Roger

sentinel040 · 5th Mar 2018, 12:51 pm

Once again thanks for the comments.

Roger; I have a Twomobile looking at me from one of the shelves in the office / shack, it was the second rig I bought (bought in 1975), the first being a HW17A, which being AM only lasted a couple of months after being licensed. The Garex is still used from time to time, mainly for AM QSO's. The big difference is that being based on the LC10FM (in part) it has the transistorised phase modulator / multiplier board before driver and PA. It is a bit difficult to use mobile though, not only mounting wise but being VFO on RX has it challenges when mobile: I last tried that in the late 70's and it was not easy then. With the boot mount Cambridge the control box is an easy fix, its been there before.

Bazz; thank you for the circuit, I suppose the critical(ish) bit is finding the optimal bias point for the diode. Then its a matter of the AF signal level required. Where will the venture take me? Well hopefully it will provide a mobile rig that suits that particular car. At the end of the day 7W of RF is 7W of RF no matter where it comes from, although half decent audio helps! And it is that latter matter that is the point of the excercise, as in the form it was last used it was not particularly pleasant.

Regards

Ian

Julesomega · 5th Mar 2018, 1:05 pm

I'm sure it will come through on every TV and audio system within 100yds, just as it did in the 70s

Jon_G4MDC · 5th Mar 2018, 2:41 pm

What about building the Osc and Phase Modulator from a Pye Bantam?
That is just 2 transistors. You could feed that to the 6BH6 as a buffer and put the screen grid circuit back as it was before.

The Bantam phase mod was for ~4MHz crystals so it will need some changes for 8MHz. There was mention of adjusting it at 10kHz deviation so if it only manages 1/2 that (likely) it is enough for the 5kHz max used on 2 meters. Actually these days 3kHz is more normal.

While you are at it you might as well build a mic amp, limiter and filter using opamps so that the modulator can go back to the standard circuit and do duty just on RX. Hopefully the supply and mic cables can be sufficiently filtered and screened to keep out the dreaded inverter!

Herald1360 · 5th Mar 2018, 2:46 pm

Quote:

Originally Posted by G6Tanuki

Running legacy 25KHz FM deviation on your transmitter will not make you popular! Most repeaters will refuse to repeat overdeviated signals

Apart from repeaters, is there enough activity on two these days for anyone to care how wide an FM signal is?

G8HQP Dave · 5th Mar 2018, 4:42 pm

Perhaps a part of the all mode section of 2m could be put aside for cross-mode working and split-frequency working? To aid CQs, perhaps it should be divided on geographic lines too. Now where have I come across this idea before?

unixmanuk · 5th Mar 2018, 5:27 pm

My first QSO in 1979 was on an AM PYE Ranger, modified to FM using just a few components in a simple arrangement to apply direct FM to the 8 MHz crystal. Worked pretty well at the time, deviation was adequate for the local repeater, which i whistled up. Circuit came from the RSGB handbook (not the circuit described further up this thread). I actually still have the rig after all this time. I can dig out the circuit if you're interested.

Bazz4CQJ · 5th Mar 2018, 7:41 pm

It's curious, and quite sad, how activity on 2m surged in to life in the late 60's as the mountain of Pye gear became available, and then progressively died away with the arrival of the various commercial 'black-boxes', first on SSB then on FM.
I don't recall ever causing TVI with my 2m rigs, but I found out indirectly that I did have a local "listener" who could pick me up on his electronic organ; a bit of what we we would now call a "stalker"

B

Jon_G4MDC · 5th Mar 2018, 7:55 pm

It has certainly changed very much.

I would prefer to use a phase modulator over direct FM if more than one channel is involved. Pye used it in the PF85 and it meant each channel had a pulling coil to set frequency and a separate deviation control to cope with different crystal characteristics - what a pain.

Like your idea Dave HQP, brilliant.

4th Mar 2018, 5:27 pm	#1
sentinel040 Hexode Join Date: Feb 2007 Location: Near Lichfield, Staffordshire, UK, most of the time and Crystal Palace, S London, some of the time.. Posts: 330	AM Cambridge FM conversion. I think This ought to go down, in part at least, as a case of admitting past sins. I can also hear people wondering why on earth I am still messing around with stuff like this. The simple answer to that is "because I want to".... Back in 1980 having just graduated I installed a modified AM10B in my newly acquired second hand half timbered Morris Minor. I actually still have both, although the AM10B in no longer installed. At the time the AM10 RX was converted to FM by the use of a a suitable Garex board, however the TX conversion was, shall we say, slightly more Heath Robinson. Following the vogue among my peers the Modulation transformer was disconnected from the PA feed and the PA fed directly. A wirewound resistor of indeterminate value attached across the mod transformers secondary along with a pot and one end on the secondary winding earthed. The wiper of the pot went via 220nF cap to the oscillator valves screen grid, which was decoupled by a 2.2nF cap instead of the original 1nF, for reasons I can no longer fathom. The result was TX audio which was characteristically thin and with a noticeable whine, courtesy of the DC-DC converter. In an attampt to increase this; the audio and not the whine, although both tended to go together; the resistors in series with the TX audio amp and Mod driver transistors Emitter decoupling caps were shorted out. This had the side effect of also increasing the RX audio level as well requiring a mod to reduce that! Not a particularly elegant solution; but it sort of worked for a number of years. I am tempted to but the Cambridge back in the Morris and every now and again I wonder if there was a better way of converting the TX to FM, other than going the whole hog and modifying it as per the FM10 with the additional stage, but apart from requiring a coil of unknown characteristics it has the unwanted side effect of requiring new crystals and that is further than the budget will stretch! I realise that starting at 8mHz, as per the AM10 as opposed to the 4mhz (if I remember correctly) of the FM10 will limit the available deviation, due to the lower multiplication factor. So, is there anyone out there who has had more success than me in this matter and has a more elegant solution? Or has any other useful mods (I am considering replacing the diode 1st mixer with a FET as per the Garex Twomobile). Regards Ian (G8KSZ)

4th Mar 2018, 5:35 pm	#2
Biggles Rest in Peace Join Date: Feb 2004 Location: Hexham, Northumberland, UK. Posts: 2,234	Re: AM Cambridge FM conversion. Sorry can't help with the mod but another thing to consider during modification is the requirement to transmit CTCSS tones alongside the audio if you are intending to access 2M repeaters. Alan.

4th Mar 2018, 7:12 pm	#4
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: AM Cambridge FM conversion. Given that there was a FM10B Cambridge which was widely used, I really think it would be more-sane to get one of those rather than trying to coerce an AM version on to FM. Why beat yourself senseless messing-around with a conversion when Pye did it properly? Go to a few radio-rallies and I'm sure a FM10D Cambridge will appear, either on one of the stalls or the local radio-club's "Bring-and-buy". [Sidenote: I've got a loft-full of FM15/FM30 highband Pye Westminsters and also half-dozen of the low-band AM versions on the old 86.4375MHz rally-radio channel which will easily retune to 70MHz if you have the crystalsl]

4th Mar 2018, 8:08 pm	#5
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: AM Cambridge FM conversion. I can relate to OP's sentiments. An AM10 with Garex board for FM RX worked surprisingly well in my experience. The TX modifications were never quite so successful. Inverter whine was the bane of all of them. I am not sure how they would cope with CTCSS for modern times. I'll be controversial and put up the idea that a few channels of crystal control made 2m a better band than full VFO/Synthesised rigs and everyone hiding on their secret channels. It was never the same around here after S0 or 145.0 became a repeater input - it marked the beginning of the end. I count myself lucky to have been licensed before that happened. I had a few good years out of it. Cross mode with AM etc was all part of the fun.

4th Mar 2018, 8:19 pm	#6
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 13,998	Re: AM Cambridge FM conversion. If you're convering old PMR radios to 144MHz, remember that a couple of decades back we switched to 12.5KHz channel-spacing in the 145MHz-and-higher FM/Repeater slice of the band. Which means that these days you need to throttle-back the deviation on the transmitter, and also need narower-bandwidth filters in the receiver. Running legacy 25KHz FM deviation on your transmitter will not make you popular! Most repeaters will refuse to repeat overdeviated signals. And of course these days you need CTCSS not the old 1750Hz "whistle-up" to get through a repeater.

4th Mar 2018, 10:27 pm	#7
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: AM Cambridge FM conversion. heheh I am looking for FM contacts on a Yaesu FT202 - 1978 vintage, a TR2200GX maybe 1977 vintage (my first rig!), and some Pye Bantams. The trouble is not so many heard. Many of those who are heard are working their own nets and break in does not appear welcome. That is my point about 145.0/S0. If there was anyone there they would bust a gut to work you. Today - sri OM your deviation is low - yes I know- hopefully none at all - because I am running AM!

4th Mar 2018, 10:37 pm	#8
sentinel040 Hexode Join Date: Feb 2007 Location: Near Lichfield, Staffordshire, UK, most of the time and Crystal Palace, S London, some of the time.. Posts: 330	Re: AM Cambridge FM conversion. Thank you all. As I said, this is not something I intend to start, I did that circa 1980, rather something I intend to finish, well try to find slightly more elegant way, if there is one. The problem with finding and using an FM10 (I have a low band one on 70.26) is that I would have to buy new TX crystals, which makes it financially untenable. And the whole point of this is to try to improve what I have lurking in a dark corner. Operating is not my main thing, I am a dabbler, its the challenge that interests me, providing of course it not too much of a challenge. I suppose I also align myself with the comments made by Jon. CTCSS, well that's not something I was contemplating, 'CF still plays with 1750 and once that goes I will give up on that front. Also, the multiplicity of frequencies and the associated switching makes it impractical. Hmm Ledex switched CTCSS, now there's an idea. Or maybe or not.... Deviation wise, well I would probably hook it up to the 2950 test set, mind you the dev meter on that is still playing up; but a good quick test is to listen on one of my collection of Pye MX294's, the filters on those are nice and tight for 12.5 and if you have too much dev the resultant noise closes the squelch. Actually, I think too much dev would be a nice problem to have! As I have some 12.5 block filters for the Cambridge series receivers (the S ones) the receive side is not a problem in that regard, if I found I needed a tighter filter. Actually with regard to the switch to 12.5 spacing, the shack is also my office and when I am here I often listen on 145.5 and very few people seem to use the 12.5 channels and most have their radios programmed to the "wide" setting because they think their transmission sounds better, or that's what their mates tell them... But I digress slightly. The varicap route is perhaps the way to go, rather than using the 6BH6 oscillator valve, but rather than re-invent the wheel I was looking to see if anyone can remember how to do it. All the best Ian

4th Mar 2018, 10:42 pm	#9
CambridgeWorks Nonode Join Date: Jun 2010 Location: Spalding, Peterborough, Cambridgeshire, UK. Posts: 2,859	Re: AM Cambridge FM conversion. I no longer have it, but I seem to remember circuits in general in the RSGB NBFM manual from the 1970s I think? Maybe someone can help? I must say, you do have enthusiasm for this AM10! I did a 6bh6 screen grid mod back about 1975, but then soon progressed to a proper Pye LC10FM boot mount, 10 channel bootmount, ex. very early radiophone system used in London I think. Rob __________________ Apprehension creeping like a tube train up your spine - Cymbaline. Film More soundtrack - Pink Floyd

4th Mar 2018, 11:05 pm	#10
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: AM Cambridge FM conversion. Well I reckon this needs to be done! Full credit to you Ian. I will recall dear old Peter G3THS who stirred it up very well on GB3PI - back in the day - he could not stand "Rice-boxes" or their operators in the main. We used to discuss AutoWestEuros and HyCoMinsters...superb amalgamations of what was around at the time. They were bitsers I suppose - made from bits of this and bits of that. Great days. And the rig used to make the main subject of the QSO. Now it is FTXXX+wah, ICXXX+wah, with a bought in aerial/feeder +++ wah/wah/wah. I for one would rather struggle to make the QSO through some inverter whine. More power to you. Last edited by Jon_G4MDC; 4th Mar 2018 at 11:21 pm.

5th Mar 2018, 8:18 am	#12
G3VKM_Roger Heptode Join Date: Jun 2012 Location: Southeast Norfolk, UK. Posts: 773	Re: AM Cambridge FM conversion. Garex did the job for you, if you can find one! It was called a "Twomobile". I bought the one pictured for 50p at a rally but long since parted with it. 73 Roger Attached Thumbnails

5th Mar 2018, 12:51 pm	#13
sentinel040 Hexode Join Date: Feb 2007 Location: Near Lichfield, Staffordshire, UK, most of the time and Crystal Palace, S London, some of the time.. Posts: 330	Re: AM Cambridge FM conversion. Once again thanks for the comments. Roger; I have a Twomobile looking at me from one of the shelves in the office / shack, it was the second rig I bought (bought in 1975), the first being a HW17A, which being AM only lasted a couple of months after being licensed. The Garex is still used from time to time, mainly for AM QSO's. The big difference is that being based on the LC10FM (in part) it has the transistorised phase modulator / multiplier board before driver and PA. It is a bit difficult to use mobile though, not only mounting wise but being VFO on RX has it challenges when mobile: I last tried that in the late 70's and it was not easy then. With the boot mount Cambridge the control box is an easy fix, its been there before. Bazz; thank you for the circuit, I suppose the critical(ish) bit is finding the optimal bias point for the diode. Then its a matter of the AF signal level required. Where will the venture take me? Well hopefully it will provide a mobile rig that suits that particular car. At the end of the day 7W of RF is 7W of RF no matter where it comes from, although half decent audio helps! And it is that latter matter that is the point of the excercise, as in the form it was last used it was not particularly pleasant. Regards Ian

5th Mar 2018, 1:05 pm	#14
Julesomega Nonode Join Date: Dec 2012 Location: Stockport, Greater Manchester, UK. Posts: 2,106	Re: AM Cambridge FM conversion. I'm sure it will come through on every TV and audio system within 100yds, just as it did in the 70s __________________ - Julian It's good here

5th Mar 2018, 2:41 pm	#15
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: AM Cambridge FM conversion. What about building the Osc and Phase Modulator from a Pye Bantam? That is just 2 transistors. You could feed that to the 6BH6 as a buffer and put the screen grid circuit back as it was before. The Bantam phase mod was for ~4MHz crystals so it will need some changes for 8MHz. There was mention of adjusting it at 10kHz deviation so if it only manages 1/2 that (likely) it is enough for the 5kHz max used on 2 meters. Actually these days 3kHz is more normal. While you are at it you might as well build a mic amp, limiter and filter using opamps so that the modulator can go back to the standard circuit and do duty just on RX. Hopefully the supply and mic cables can be sufficiently filtered and screened to keep out the dreaded inverter!

5th Mar 2018, 4:42 pm	#17
G8HQP Dave Rest in Peace Join Date: Sep 2008 Location: Solihull, West Midlands, UK. Posts: 4,872	Re: AM Cambridge FM conversion. Perhaps a part of the all mode section of 2m could be put aside for cross-mode working and split-frequency working? To aid CQs, perhaps it should be divided on geographic lines too. Now where have I come across this idea before?

5th Mar 2018, 5:27 pm	#18
unixmanuk Pentode Join Date: Jan 2012 Location: Worcester, Worcs. UK. Posts: 112	Re: AM Cambridge FM conversion. My first QSO in 1979 was on an AM PYE Ranger, modified to FM using just a few components in a simple arrangement to apply direct FM to the 8 MHz crystal. Worked pretty well at the time, deviation was adequate for the local repeater, which i whistled up. Circuit came from the RSGB handbook (not the circuit described further up this thread). I actually still have the rig after all this time. I can dig out the circuit if you're interested. __________________ John

5th Mar 2018, 7:41 pm	#19
Bazz4CQJ Dekatron Join Date: Apr 2009 Location: Oxfordshire, UK. Posts: 4,934	Re: AM Cambridge FM conversion. It's curious, and quite sad, how activity on 2m surged in to life in the late 60's as the mountain of Pye gear became available, and then progressively died away with the arrival of the various commercial 'black-boxes', first on SSB then on FM. I don't recall ever causing TVI with my 2m rigs, but I found out indirectly that I did have a local "listener" who could pick me up on his electronic organ; a bit of what we we would now call a "stalker" B __________________ Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.

5th Mar 2018, 7:55 pm	#20
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: AM Cambridge FM conversion. It has certainly changed very much. I would prefer to use a phase modulator over direct FM if more than one channel is involved. Pye used it in the PF85 and it meant each channel had a pulling coil to set frequency and a separate deviation control to cope with different crystal characteristics - what a pain. Like your idea Dave HQP, brilliant.