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Old 2nd Dec 2017, 3:08 pm   #1
runner_bean
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Default 4 Metre Converter - but which design?

Back in the mid 1960s I constructed a 70MHz/4 metre converter for use with my CR100, and I recall that it worked well. I am now trying to get it going again but have no circuit diagram. My guess is that that the design would have been published in an RSGB magazine (I was a member) but have no idea which one. I attach some photos below and would appreciate any suggestions as to what the converter might be. It uses three Nuvistor 6CW4 triodes for the VHF stuff and an ECC91 for the local oscillator.

Sadly the Nuvistors aren't too robust, with some of the pins having corroded through because of bimetallic corrosion with the cheap Cinch bases. But the internals are in fine condition safely inside an Eddystone die-cast box.

Any suggestions much appreciated!
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Old 2nd Dec 2017, 5:07 pm   #2
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Default Re: 4 Metre Converter - but which design?

There was a converter using the 6CW4 in 2m and 4m versions in the mid sixties RSGB handbook.

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Old 2nd Dec 2017, 5:44 pm   #3
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Default Re: 4 Metre Converter - but which design?

The layout of that converter looks very, very much like the one in the 4th Edition of the RSGB (first printed 1968, 3.75 !) but someone seems to have gone homebrew. The RSGB circuit uses a 6CW4 RF amp, but downstream of that is an ECC81 then two EF91's.

In fairness to Cinch, the topside of that unit does look like it has been in a 'fairly humid' environment for a wee while!

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Old 2nd Dec 2017, 5:47 pm   #4
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Default Re: 4 Metre Converter - but which design?

As here;
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Old 2nd Dec 2017, 6:48 pm   #5
John KC0G
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Default Re: 4 Metre Converter - but which design?

The design in the 4th edition of the RSGB handbook originally appeared in the October 1962, December 1962 and February 1963 issues of the RSGB Bulletin. The design came from the RSGB Technical Development Sub-Committee.

There was "A Low Noise Convertor for 70 Mc/s" by Paul Harris, G3GFN, in the April 1964 issue of the RSGB Bulletin. it used 2 * 6CW4, 1 * EF91 and 1 * ECC81 valves. The layout was different from the picture above. There were additional notes in the May 1965 issue.

There was "A High Performance Nuvistor Convertor for 70Mc/s, by M. Gibbings, G3FDW in the June 1966 issue of the RSGB Bulletin. This is not the one.

I looked in the indexes up to December 1970. I wonder if it could have appeared in the "Four Metres and Down" or "Technical Topics" columns.

I also looked at the Short Wave Magazine indexes from Mar. 1959 to Feb 1965, and found nothing appropriate.
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Old 2nd Dec 2017, 9:09 pm   #6
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Default Re: 4 Metre Converter - but which design?

Maybe a hybrid of a number of published designs?
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Old 2nd Dec 2017, 9:34 pm   #7
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Default Re: 4 Metre Converter - but which design?

The central inductor set looks like a direct hit.... the resonated phase inverter in the 6CW4 anode used to drive the neutralising capacitor.

Aye, it looks to have taken inspiration from a few places. One down, a few still to go

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Old 2nd Dec 2017, 11:02 pm   #8
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Default Re: 4 Metre Converter - but which design?

I too have a valve 4m converter; the front end of a lo-band Pye Vanguard, literally severed from the rest of the set with a hacksaw . Perhaps it's time to let such stuff go ?

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Old 3rd Dec 2017, 12:49 pm   #9
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Default Re: 4 Metre Converter - but which design?

Thanks for so much interesting feedback! According to my log book I was using this converter logging contests in 1963/64 so it would make sense that I had taken ideas from the Bulletin around the back end of 1962. I only have the 3rd addition Handbook. There was a lot of interest in 4 metres in the Worthing area at the time and I picked up on that when I was at High School. I clearly need to trace the circuit which I have already found to be very simplistic. But it did work.

Sadly the unit was kept in an unheated garage over decades and has suffered externally, but is like new inside! Fortunately my CR100 has also survived and is working as well as ever.

My biggest challenge is finding a power supply for it as all my old stuff went for scrap when I left home and it's not that easy generating a smooth 150v dc and 6.3v ac. Is there a popular solution to this? I haven't found much on eBay.
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Old 3rd Dec 2017, 4:00 pm   #10
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Default Re: 4 Metre Converter - but which design?

Quote:
Originally Posted by runner_bean View Post
My biggest challenge is finding a power supply for it as all my old stuff went for scrap when I left home and it's not that easy generating a smooth 150v dc and Is there a popular solution to this? I haven't found much on eBay.
Well, the classic one is a simple half-wave rectifier and OA150 voltage regulator for 150 plus or minus 5V odd.

That was fine for my converter.

In terms of vintage designs, you could easily mod the design that I posted for 4 metres instead of 2... take a look if you like? Very sensitive and selective little thing...

IF you want a more flexible power supply, try one like the one I've uploaded. I have used a very similar one in the past for a home-made Nixie tube clock. You can run it from a bench power supply or a battery.
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Old 3rd Dec 2017, 5:29 pm   #11
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Default Re: 4 Metre Converter - but which design?

In terms of a PSU, you could put a "wanted" request on to this forum, either for a complete one or the transformer.

Al has suggested one circuit, but an alternative is to use the cheap little LR8 regulator chip, so you could have a 250V transformer and get out stabilised 150 at the turn of a knob. There is a detailed thread somewhere on the forum (a number of us built them), but I didn't it find at first look.

I'm not sure if much goes on on 4m these days. If I recall, we were given the 5m (or was it 6m ?) band a while ago and that band is shared by many countries (including the US) whereas I think 4m was allocated in just a few countries? For anyone interested in those sorts of frequencies, it could be that the "new" band is the place to be now?

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Old 3rd Dec 2017, 6:02 pm   #12
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Default Re: 4 Metre Converter - but which design?

There's actually quite a bit of activity on 70MHz: see http://www.70mhz.org/

It tends to divide into local/semi-local chat-nets on 70.26/70.45MHz using AM or FM and vertical omnidirectional antennas (where it's good for covering 100 miles or so if the terrain's not limiting) and the hardcore DX-types who run horizontally-polarised multi-element Yagis and use SSB/CW where they can work well into central-Europe when the weather's in their favour.

My only serious foray onto 70MHz used the repurposed front-end of a Dymar "Lynx 2000" lowband radiotelephone, feeding the 10.7MHz +/- 200KHz IF into an AR88. We used to listen on 70.31MHz for the Gdansk high-power FM broadcast transmitter to tell us if there was an 'opening' - that was back in the days when Soviet-controlled countries used something like 70-85MHz for their FM-broadcast band.
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Old 3rd Dec 2017, 6:47 pm   #13
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Default Re: 4 Metre Converter - but which design?

Quote:
Originally Posted by G6Tanuki View Post
There's actually quite a bit of activity on 70MHz: see http://www.70mhz.org/
Do you know if the new 6m band has attracted much interest?

I cannot find the LR8 psu design discussed on this forum about 5 years ago, but some notes I made of my own project are attached

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Old 3rd Dec 2017, 7:35 pm   #14
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Default Re: 4 Metre Converter - but which design?

Is 1983 still considered new? I think that is when 6m was introduced special permits only.
It has been generally available since about 1995 I think.

4 and 6 have the same characteristics for activity, a few who use the band for the love of it are on most of the time, but sadly these are few and far between.

At the first sign of a lift or a contest it's "where have that lot been??"
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Old 3rd Dec 2017, 7:48 pm   #15
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Default Re: 4 Metre Converter - but which design?

Thanks everyone for the tips. I will try a "wanted" ad on the forum for a PSU.

It turns out that the converter design is a cascode one very similar to a 144MHz converter design in the ARRL 1963 year book. But there are so many detail changes that I think I must have copied it from somewhere else. But having got this far I now have the confidence to get it going again (after buying some new 6CW4s!).
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Old 3rd Dec 2017, 7:56 pm   #16
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Default Re: 4 Metre Converter - but which design?

Go for it. It could be most interesting if you have good paths to the East.
Holland have had the band for a few years and the Dutch are always keen.

They are just beyond my range for the most part but from where you are it could
be very good!

Just a question - what is your LO crystal and IF?
The 68.4MHz shown in one of the articles above surprised me slightly.

A good choice is 49MHz and 21MHz IF. Most designs for 4m use a 28MHz IF which is a particularly bad choice.

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Old 4th Dec 2017, 3:02 am   #17
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Default Re: 4 Metre Converter - but which design?

The design in the 1963 ARRL Handbook was also in the 1962 and 1964 editions, and possibly later ones. The design was originally published in "A Complete Two Band Station for the V.H.F. beginner, Part IV - Crystal-Controlled Converters for 50 and 144 Mc", by Ed Tilton, W1HDQ, QST, October 1961, pp 28-33, 164 and 166.
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Old 4th Dec 2017, 12:34 pm   #18
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Default Re: 4 Metre Converter - but which design?

Well I've traced the whole of the 4m converter circuit now, and it all makes sense, and hopefully will work again, subject to me replacing at least one of the Nuvistor valves, which has lost a leg, and rigging up a power supply.

The local oscillator is based on an FT-243 7.550MHz crystal in a Squier overtone circuit using one half of a CV4031/ECC91 double triode (producing 22.65MHz) with the other half wired as a multiplier (presumably 2x). So the LO frequency is 45.3MHz and the IF 25MHz. But this is a bit speculative as I don't have the coil design details and I don't want to dismantle the screening cans as they break easily.
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Old 4th Dec 2017, 4:12 pm   #19
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Default Re: 4 Metre Converter - but which design?

That seems a pretty good choice, quite a decent gap in the mixing spurs there.
Good luck finding the Nuvistor.

I had a couple of 2CW4 here somewhere but even if I could find them probably not much use. Heater 2.1V 450mA.
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Old 19th Dec 2017, 6:22 pm   #20
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Default Re: 4 Metre Converter - but which design?

Thanks for all the feedback and advice. I have now made quite good progress. My CR100/B28 is working pretty well after selectively replacing various Rs, Cs and perished wiring and it is mostly lined up, except for Band 6 which seems to have a few LO problems.

I have also got the 4 metre converter going after replacing the dead Nuvistor. It turns out that the design LO frequency is 7.550Mhz x3 and then x3 again = 67.95MHz, giving an IF of about 2.25 Mhz on the 4 metre band. This works nicely with my CR100. The design seems to be a personal mash up of various designs including the October 1962 RSGB one (which uses a 1.6 - 1.8 MHz IF) and the 1963 ARRL one (which uses 6CW4 Nuvistors in cascode). I think the design needs refining as there is a tendency to instability and I'm not convinced that some of the coils are right, but there's loads of gain and with a reasonable aerial I'm hoping I will be able to pick something up.

Is it worth me getting one of the used grid dip meters I see advertised? It's all a bit hit and miss at the moment trying to get the coils right as I only have a 10MHz scope!
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