Converting Crystal controlled radio to VFO

[mendipviews]

Having just obtained some nice valved muphy base station equipment I am wondering is it possible to convert it from a single channel crystal to a VFO so its not stuck on one frequency. Currently sitting on 165Mhz but I would like it to tune either the 2M or 4M bands

Does anybody know for any projects, circuit diagrams, pointers as I have googled it and doesn't see to be much about.

Many Thanks
Luke
2E0VHV

[mendipviews]

Hi Jamie,
an interesting little crystal radio but no good for I want to do.
Currently I have a Murphy Base Station Valved VHF Transmitter & Recevier in a Rack unfortunatly they only operation on one channel 166Mhz which I can't use.
So I was wondering if I could build a VFO which would tune over 144-146Mhz for the 2M Amature Band or the 4M Amature band and replace the crystal oscillator with it.
Seems a shame to not use it as its a nice beasty with 22 valves in it including 2 EL34s in the transmitter modulator unit.

Regards
Luke

[Skywave]

Conceptually, yes, it is possible. I did just that many, many years ago during the early, formative years of A.M. of 2m; 'tuning high to low', etc. I may well have the cct. of what I used somewhere, buried in the archive of archives!

Anyway, the leading initial question I have to ask is:
Does this Tx. use a scheme of frequency multipliers from the crystal osc. freq. up to the RFPA?
For example, with an 8 MHz crystal in a 144 MHz Tx., it was customary to use a series of freq. multipliers to produce an overall multiplication of 18, since 8 MHz X 18 = 144 MHz. So 8.111 MHz produced 146 MHz. Hence, a VFO tuning 8.000 to 8.111 MHz was all that was required.
Of course, issues of stability & drive level are relevant at this point - but that's another issue - for now, anyway.

Al.

[mendipviews]

Hi Al,
thanks for the reply, I have just located the service book for the unit so will have a look and report back.

Many thanks
Luke

[Yorkie]

Conceptually, yes, it is possible. I did just that many, many years ago during the early, formative years of A.M. of 2m; 'tuning high to low', etc. I may well have the cct. of what I used somewhere, buried in the archive of archives!
Al.

As Al says, in the 70s when lots of valved ex PMR AM equipment was available cheaply, it was customary to have a fixed crystal for a frequency on the 2M amateur band, and to have variable tuning on the receive side. You put out a call on that fixed frequency, and annouced that you would 'tune high to low' across the band, hoping to come across a like minded amateur who was transmitting on a different crystal controlled frequency. If you did make contact, your receiver would be tuned to his transmit frequency, and his to yours.

It's not just the technical aspects that are the limiting factor - it's low acitvty levels that have to be considered. There's little activity on any mode on 2/4 M these days, even in urban areas, though no doubt it varies. There are 22 amateurs listed in my village, but I only know of two others - I see no other evidence of antennas. There are at least 300 amateurs within a ten mile radius, but I got rid of my 2M gear a decade ago as there was no activity at all, apart from a few numpties on the lcoal repeater who thought it was safe and legal to give running commentairies about their near misses as they drove to work, dicing with death in heavy traffic with one hand on a mike, the other on the wheel, and their minds elsewhere. For a time, in the late 90s I used to take a 2M hand-held with me when I went up on the N Yorks Moors on my motorbike - I could access Tyne & Tees, Yorks across to Leeds, York, Humberside - Hull & Grimsby etc. Hopeless - rarely heard a thing, so stopped bothering.

But as you say, it's nice from a technical point of view to breathe new life into gear that was very robustly built for a hard life, and was, indeed still is, well up to the task. Pye Cambridges and the like were beautifully constructed, and the QQV06/40 transmitting valves were just about bombproof. In the early 90s we had happy times locally converting solid state ex PMR FM gear -Pye MX294s, Dymar Lynx etc, reprogramming e-proms. The Lynx was an AM rig primarily, but removing one link changed it to FM. A different world - people these days are cash rich/time poor. Too easy to flash the plastic at Martin Lynch et all, for the latest offerings from Keyasuicom.

There's bound to be info out ther in cyberspace about converting your Murphy, I'm sure. VMARS for example?

Lamented David.

[mendipviews]

Thanks for the replies Al & David,
upon looking at the manual for the Murphy it is indeed as Al said an AM unit and theres not a lot of activity on the 2M AM Band last time I checked so unfortunately probably not worth trying to convert the unit.This then leaves in a quandary of what to do with it as its a nice unit that also came with a mobile set and it seems a bit mean it break it for parts.

David I still have my ex-PMR rally bought 2M Radio which I had to convert and program eproms for and it still works and I use it most evenings and weekends.
I think it cost me a £10 along with the components for conversion and used to share the shack with a Pye Vanguard unit that I managed to source crystals for so I could work into the local repeaters.

Regards
Luke

[Skywave]

Conceptually, it should be possible to fit an external VFO and apply speech audio to produce FM; usually phase modulation with an audio freq. correcting network is a common method. Attempting to convert it to ssb would involve a complete re-design, so I don't think that that is a realistic option.
Not that I have listened much on 144 MHz recently, but apparently of the activity that is there, there is virtually zero AM - but I did read a report some time back of a some AM activity on 144 MHz on the South Coast and an enthusiastic group using AM on 70 MHz in South Wales.

So perhaps there is some hope & areas for exploration after all.

Al.