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Old 6th Mar 2018, 5:50 pm   #21
Jon_G4MDC
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Default Re: AM Cambridge FM conversion.

Just for fun I took a look at the original FM10 osc and phase mod.
It uses a 12AT7. One triode is the colpitts Osc and the other is the phase modulator.
Maybe your coil at L601 position in the original circuit could be used in the anode of the phase mod, L603 as it was in FM10?

Don't know the chances of changing the valve base to a B9A though.

Here is the circuit - I hope the quality is not too bad. It was taken with a
camera and compressed rather heavily.
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Old 6th Mar 2018, 9:50 pm   #22
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Default Re: AM Cambridge FM conversion.

Thanks again chaps.

Jon; I had not twigged that with an 8 meg rock the first 6BH6 stage would be superfluous. Actually V601 (12AT7) in the FM Cambridge sits in the came place as the Osc 6BH6 in the AM one and the chassis is punched for a B9A valve holder, the 7 pin fitted to the AM10 seems a special to fit. So it may be possible to simply(!) replace the valve holder and use the FM10's circuit as suggested, without the need for an additional tuned circuit. The heater circuit is going to need a ballast resistor though That has definitely given me food for thought though.

Uniixmauk; I used to have Ranger and regret selling it many years ago, I still have the manuals for dash and boot mount (AM and FM) versions. If it is not too much hassle I would be interested in how you arranged it.

I am off away for a week or so and when I come back this is probably going to be the next project to hit the bench. Well that's the plan at the moment. I might try a varicap (or other diode) first, probably arranged as per Bazz's circuit. This is least intrusive. If that does not succeed I will go for the FM10 osc/phase modulator arrangement, having seen the light, so to speak, on this.

A few people have commented on why, well in part I have explained this earlier but there is also the other factor; why do any us do any of this? in my case because of the challenge.

Thanks again

Ian (G8KSZ)
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Old 6th Mar 2018, 10:16 pm   #23
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Default Re: AM Cambridge FM conversion.

Ian, regarding "why?". This is a hobby. It keeps us occupied and gives us satisfaction. It helps us learn. We communicate with other like minded people. Hopefully we achieve a great satisfaction in what we achieve, either alone or with help. Most importantly it keeps the grey matter exercised!
Btw, your comment regarding valve bases explains to me why the b7g bases with b9a fixing centres were made. I have some...somewhere... I never thought of it the way you describe.
Rob
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Old 6th Mar 2018, 10:38 pm   #24
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Default Re: AM Cambridge FM conversion.

Hehehe - a good few days work here then.

We all learn something. Excellent. I have an FM10D for 10m (yes 10m) I must get back to sometime.

Maybe by the next sunspot cycle. Should be fun explaining the 50 yr old Rig to DX contacts?
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Old 7th Mar 2018, 3:14 pm   #25
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Default Re: AM Cambridge FM conversion.

Quote:
Originally Posted by sentinel040 View Post
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"....
Great. Why not go the whole hog and stick to AM?. Start a revival ( although it does still exist in certain areas, i believe ) I've always thought that the fun died when FM ( channelization and repeaters in particular ) came along.

73's

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Old 7th Mar 2018, 3:40 pm   #26
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Default Re: AM Cambridge FM conversion.

I'm in! My 2m AM rig hasn't been powered up for a decade or three, but I'll bring it up on the Variac and it will be OK because it's like that .

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Old 7th Mar 2018, 5:15 pm   #27
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Default Re: AM Cambridge FM conversion.

They tried that with the new band plan. Was it 144.550 for AM calling?
That was doomed to fail because who would buy new crystals?

I put a few of rigs on there, definitely a T4001, a P5001 and an MX293.
So far might have had 1 QSO out of any of them. I suppose they must still be around somewhere. Maybe I had a W15AM too but could be that donated it's crystals
to one of the others. I forget.

Ready to go 70.26AM though... FT221R, T30AM, AM10P, P5001,HP1AM and others
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Old 24th Mar 2018, 9:19 pm   #28
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Default Re: AM Cambridge FM conversion.

Sorry for the silence chaps, I did not get long at home (in Staffordshire). I did quickly try a varicap diode coupled through a 47pF cap to the junction of the two capacitors from grid to cathode to chassis. This produced some dev but was very distorted and varying the bias had no effect, actually applying any bias had no effect. Actually it sounded much worse than the audio fed into the screen grid, interestingly though there absolutely no trace of the inverter whine that is somewhat noticable with the original modification. I think the varicap I used was a BB139. Currently I am away again and am likely to be so until after Easter now.
73's
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Old 20th Jul 2018, 9:27 pm   #29
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Default Re: AM Cambridge FM conversion.

OK so after a slight delay I finally cracked on with this. Initially I removed the 6BH6 based oscillator and replaced it with the double triode based oscillator and reactance modulator as per the FM10. The result was that I could only struggle to get about 5W out rather than the original cloud burning 7-8W. I suspected that maybe there was a lack of drive and the 'scope did show this to be the case.
So, as per the FM10 I added another 6BH6, well if nothing else it would balance up the series / parallel heather arrangement! I used the same coil assembly as per the original AM10 osillator stage, just so it would act as a buffer with a bit of gain, rather than be a mutipler as per the original, but I am starting from 8 meg rocks rather than the 4 meg of the FM10. Resukt, 8W out.
Next audio, I lacked the proper FM audio board and the original arrangement of using audio from the secondary of the mod transformer seemed to be using a steam roller to crack a Walnut, so I inded up tapping it off the driver stage and this proved to be more than adequate, with the deviation level being easily enough for 12.5kHz channel spacing. And, the inverter whine is much lower.
Now quite what I will do with the radio now is another matter, probably put it back in my Morris Minor Traveller.
Thank you to all those who were kind enough to comment and help.
Regards and 73
Ian G8KSZ
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Old 20th Jul 2018, 10:34 pm   #30
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Default Re: AM Cambridge FM conversion.

Don't get rid of the inverter whine completely; that's an essential part of the Pye RT experience .

B
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Old 21st Jul 2018, 11:08 am   #31
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Default Re: AM Cambridge FM conversion.

I think that would be worthy of mission impossible...... I would not anyway, it is a badge of honour!
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Old 24th Jul 2018, 7:36 pm   #32
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Default Re: AM Cambridge FM conversion.

Amazed this thread came back to life and I was on holiday at the time, not reading the forum.

If this sort of thing was happening around my area I would be back in like a shot. It isn't.

The 1977/8 FT202 and TR2200GX (my first rig) still sit with no recent QSOs. The FT202 is CTCSS capable but no longer 1750Hz Tone access. (I stole the space for the encoder). It is also 12.5k RX filtered. The TR2200GX is standard except TX deviation has been reduced for 12.5k standards.

To my shame the 10m FM10D has not progressed much either and now the sunspots have gone. Maybe I will save it for the next cycle when it should have even deeper credentials!

The AM stuff is still there waiting for the day..
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Old 26th Jul 2018, 9:48 am   #33
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Default Re: AM Cambridge FM conversion.

Jon;

As I said sorry about the delay, family matters etc. got in the way; I have just discovered that it still has an AM mic insert fited so I need to look out for an FM one at some point. With regard to FM10's, I have long wished for one of those but doubt that I shall ever obtain one.

My second rig, bought after a 6 month struggle with a HW17A, is a Garex Twomobile and I stil have and use that, and there is a 12.5k block filter in the spares box to go in that one day; although around here many people drop the squelch on my MX294 as they are using the wide setting on their radio because it sounds louder.......

Ian
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Old 30th Jul 2018, 12:27 am   #34
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Default Re: AM Cambridge FM conversion.

There are some relatively uncommon 25Khz filtered versions of the VHF high band MX294 around, possibly originally intended for use on VHF marine band. These used to be the ones most ideal for 2m amateur use since the FM channels (when introduced) were 25Khz spaced.

Where found, they have the band / channel width identifier code 'AW' as part of their type number on the rear plate, whereas the much more common ones filtered for 12.5Khz channel spacing usually have the band / channel width ident of 'A0' or 'B0'.

These codes may possibly also apply to Pye radios of earlier vintage as well.
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Old 30th Jul 2018, 11:54 am   #35
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Default Re: AM Cambridge FM conversion.

I don't think there is much to worry about with the Mic inserts.
I like the AM ones.

Someone once said the difference was a fudge anyway. The type approval tests of the time measured frequency response with an AF generator at the mic socket not by sound pressure at the microphone.

The different hole sizes and patterns introduced a bit of lift or cut beyond what would get through the tests.
Could be a myth - or?

Look at the M/MX 293/4 mic (shaver type) and the Electret mic insert is identical but the RC circuit which follows is different.

As you don't have a type approval to go through you can do what you like within reason.

Last edited by Jon_G4MDC; 30th Jul 2018 at 11:59 am.
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Old 30th Jul 2018, 7:50 pm   #36
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Default Re: AM Cambridge FM conversion.

Jon;

Thank you for that. I have often wondered what the real difference is. I do have both types, I have a U10B up in the loft and if I was not so lazy / busy I would have dug that out and compared the mics!

I know about the M/MX series mics, these are my favourite *modern" radio (with the 'UJS TEDX mod).

Regards

Ian
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