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Old 9th Nov 2020, 5:30 pm   #1
G3PIJpeter
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Default SSM Sentinel 2m to MW converter

I recently retrieved this converter from a box at the back of the shed (where else?) but have since drawn a complete blank on the internet when searching for a circuit or any other technical details. I had the same problem with the SSM 28-144 QRO transverter a while ago but, needless to say, someone on this list came up with full technical spec. and a circuit.

The 'Sentinel MF' converter was designed largely for /M back in the amplitude modulated days of 2-metre mobile. It is specifically intended for tuning on a MW car radio 0.5-1.5 MHz (600-200 m) and has two separate local oscillators for inputs in two switched bands 144-145 and 145-146. A 1974 SWM advertised it for this purpose at a price of £20.25. I have a PCR receiver with added BFO and this little converter should work well with it.

Has anyone out there got any further details? I'll bet that someone does . . .

Peter G3PIJ
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Old 9th Nov 2020, 6:39 pm   #2
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Default Re: SSM Sentinel 2m to MW converter

Sentinel were based in Huddersfield. Longley, I think. Out of my price range at the time so I've no other knowledge of them even though I lived a few miles away.

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Old 10th Nov 2020, 12:43 am   #3
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Default Re: SSM Sentinel 2m to MW converter

What a thing! I vaguely remember them being advertised.
I wonder what the image rejection was? ~Zero?

I should think they bothered with 2 crystals only so the tuning worked in the right direction.
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Old 10th Nov 2020, 1:02 am   #4
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Default Re: SSM Sentinel 2m to MW converter

I recall SSM being very popular for some period of time. However, I also seem to recall that they used 40673 mosfets at the front and failure was rather common-place.

B
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Old 10th Nov 2020, 2:31 am   #5
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Default Re: SSM Sentinel 2m to MW converter

Two crystals make it look like a double conversion converter. One way to ease the image rejection problem.

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Old 10th Nov 2020, 11:27 am   #6
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Default Re: SSM Sentinel 2m to MW converter

Wouldn't that need 3 crystals to get 2x Switched Bands it clearly supports?
Both oscillators look identical apart from minor layout changes. I'm with single conversion.
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Old 10th Nov 2020, 11:33 am   #7
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Default Re: SSM Sentinel 2m to MW converter

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Originally Posted by Radio Wrangler View Post
Two crystals make it look like a double conversion converter. One way to ease the image rejection problem.
The crystals are 72250 and 71750 kHz; x2 = 144500 and 143500 kHz in separate oscillator circuits (one each end of the board) with what looks like switched +12VDC feed. A standard MW broadcast receiver tunes 200 - 600 metres (500 - 1500 kHz) so the 2-metre band has to be taken in two bites - hence the two switched local oscillator frequencies. I think this makes the converter a single-conversion job, especially considering there are just three trasistors fitted to the main part of the board.

The converter does not seem to have been 'got at' and is in a physically clean condition so I suppose the time has now come for 'suck it and see' - plug it in, see what happens and take it from there. One thought on the circuit is that it might just be the same as that fitted in the Europa TX/RX transverter - for which I have all docs.

Peter
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Old 10th Nov 2020, 12:18 pm   #8
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Default Re: SSM Sentinel 2m to MW converter

OK, so single conversion, two bands set by two LO frequency choices.

There can be very little image suppression, but back then the band wasn't much populated

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Old 10th Nov 2020, 1:59 pm   #9
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Default Re: SSM Sentinel 2m to MW converter

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... so single conversion ... There can be very little image suppression ...
When switched to the 144-145 MHz band, the LO is on 143500 kHz. By subtractive mixing, a signal at 144500 kHz will give an IF output of 144500 - 143500 = 1000 kHz (the IF runs from 500-1500 kHz). With the IF RX tuned to 1000 kHz, the image frequency will be at 142500 (again, by subtractive mixing 143500 - 142500 = 1000 kHz).

As usual, the image frequency is 2 x IF away from the desired frequency. Additive mixing does not come into the picture with these frequencies. The image response is set by the Q of the pre-mixer tuned circuits. It looks like there are two of these associated with the RF amp. Good enough to distinguish between two signals around 150 MHz that are 2 MHz apart? Not sure!

Peter G3PIJ
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Old 10th Nov 2020, 3:51 pm   #10
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Default Re: SSM Sentinel 2m to MW converter

There was a version published in USA which by choice of crystal put the Club Net and the Calling Frequency on each side at the same IF tuning.

The idea presumably being you had twice the chance of hearing something, anything, please!
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Old 10th Nov 2020, 10:49 pm   #11
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Default Re: SSM Sentinel 2m to MW converter

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Originally Posted by Bazz4CQJ View Post
I recall SSM being very popular for some period of time. However, I also seem to recall that they used 40673 mosfets at the front and failure was rather common-place. B
A hand lens tells me that this converter uses three 40670 mosfets (plus a couple of bipolars in the xtal oscillators). What was the problem with the 40673s? Are 40670s likely to have fared any better?

Peter G3PIJ
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Old 12th Nov 2020, 2:20 am   #12
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Default Re: SSM Sentinel 2m to MW converter

I have the 4-6MHz output version.
I think the crowbar diode was my addition.
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Old 12th Nov 2020, 6:56 pm   #13
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Default Re: SSM Sentinel 2m to MW converter

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Originally Posted by rambo1152 View Post
I have the 4-6MHz output version.
Looking at the photos of yours and mine, it seems to me that the physical layout of the RF amplifier sections are almost identical - but that's where the similarity ends. The two PCBs are different animals. I expect that any firm of this sort back then would repeat what works and adapt / develop the rest to suit the specific need. When I've got a spare five minutes, I'll plug in my MW BC RX version and see what happens!
Peter G3PIJ
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Old 16th Nov 2020, 5:33 pm   #14
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Default Re: SSM Sentinel 2m to MW converter

Well - surprise, surprise! I attached the converter to the much-modified PCR set (BFO plus internal AC PSU) and to a Marconi TF2008 signal generator. After replacing the wire from the mixer output to the selector switch that someone in their wisdom had removed, the thing burst into life. Ultimate sensitivity looks to be around 0.2 microvolts and tuning the 2-metre band in two bites across the whole of the medium waveband gives almost ridiculous bandspread.

Time to reassemble the J-Beam 5-over-5. I also have an SSM Europa 28-144 transverter with a dead RX side somewhere or other . . . who knows where this project will end?

Peter G3PIJ
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Old 16th Nov 2020, 5:38 pm   #15
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Default Re: SSM Sentinel 2m to MW converter

At least with your modified PCR you've got a BFO, so SSB and CW stations (and slope-detected FM) will be resolvable.

AM on 'two' is really rather uncommon these days, I think it was about 15 years ago I last heard it used.

"Tuning low to high".....
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Old 16th Nov 2020, 7:11 pm   #16
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Default Re: SSM Sentinel 2m to MW converter

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AM on 'two' is really rather uncommon these days ... "Tuning low to high".....
I was first licenced in 1961 (age 15) and, after 18 months or so on Top Band CW and AM, progressed to 2 metres with a Withers Nuvistor converter feeding an HRO-MX, with a Withers TW2 transmitter. The aerial was a J-Beam 6/6 (skeleton slot feed) at around 35 feet. The aerial mast was attached to the side of the house and cost my father a hernia. Various geographical areas in the UK were assigned various frequency bands by the RSGB and the Withers TX came with an appropriate FT243 crystal that was multiplied up 18 times. Beautifully built equipment that I sold later to finance my first car (a ghastly Fiat 600).

There is a modicum of activity around here on FM. This SSM MW converter will allow me to check around 144300 for SSB activity to be sure it will be worth while firing up the SSM Europa transverter. I also have a well-built GW3ZTH transverter using a QQVO3-10 balanced mixer into a QQVO6-40A - see SWM March 1973 pp. 29-33 at https://worldradiohistory.com/UK/Sho...WM-1973-03.pdf When last used around 20 years ago, there was precious little activity and so it fell into disuse. Possibly time for another outing? Or not?

Peter G3PIJ
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Old 16th Nov 2020, 7:36 pm   #17
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Default Re: SSM Sentinel 2m to MW converter

RSGB organise an Activity Contest every month on the 1st Tuesday

FM 7 to 7 55pm

then

Largely SSB 8 to 10.30pm local

There is also an activity group who use SSB on 144 265

Not that I do either

73 Fred
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Old 16th Nov 2020, 7:44 pm   #18
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Default Re: SSM Sentinel 2m to MW converter

There was a ramping-up of 2M activity round here [and also reported happening in other parts of the UK] when the first lockdown began - people dusting off their old radios and getting back on-air. It's tailed-off a bit since though: 2M activity has been in notable decline ever since us Class-B [I always think that sounds like we operate in zero-bias push-pull!] types got full HF privileges a couple of decades back.

There was some "vertically polarised SSB" activity on 144.250 a while back, with people using 'white stick' type collinear omnidirectional antennas. This was criticised in some circles but I'd rather have omnidirectionally-squirted signals on a bit of the band than the usual deafening silence.

For winter I'm looking at firing up one of my Pye Westminsters again.
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Old 16th Nov 2020, 8:48 pm   #19
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Default Re: SSM Sentinel 2m to MW converter

My memory has just popped up an address

SSM Solid State Modules Dalton Green Lane, Dalton, Huddersfield. I think I said Longley in error earlier. A bit of searching turned up number 14. It's probably changed hands a few times since. Despite living about a mile away, I never met the chap.

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Old 16th Nov 2020, 8:56 pm   #20
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Default Re: SSM Sentinel 2m to MW converter

There is a lot more activity on the V/UHF bands over the last couple of years, not so much on FM but quite a lot on SSB and the data modes which have shown what is actually possible on the bands. Paths of over 4000 kms are regularly worked on 2 and 70 from the U.K. and Eire, Regular transatlantic QSO’s from Cape Verdi (D4) to The Caribbean.
It’s only a matter of time before the Brendan trophy is claimed for the first trans Atlantic QSO from Europe to the US takes place.

Very interesting times
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