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Old 22nd Oct 2012, 2:34 pm   #1
G6Tanuki
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Default GEX66 VHF mixer diode.

I'm re-activating an old 1960s-era Pye Cambridge AM10 radiophone; the GEX66 crystal-mixer diode seems to have lost its diodeiness. What's the best replacement? Given that the GEX66 is Germanium, it'll have a forward breakdown voltage of around 0.1v - if I put in something like a silicon 1N4148 that would have a forward breakdown of 0.6v or so - and I'm not sure if there would be enough local-oscillator drive available to get proper mixing action.

Suggestions please!

--G6Tanuki.
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Old 22nd Oct 2012, 3:28 pm   #2
WME_bill
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Default Re: GEX66 VHF mixer diode.

GEX66 alternative.
I did a lot of research upon suitable replacements for this GEC mixer diode some years ago. It is a point contact germanium, with particularly low capacitance. It was introduced about the time that GEC and Mullard combined their manufacturing, disappeared and then seemed to re-appear later as AAZ13. Ratings: 5v, 50ma, very low stored charge, capacitance 3.0pf at 0v.
The last germanium gold bonded were as good: eg Mullard OA47, Hughes HD1841, HD 1871.
These were also used as modulators in vhf/uhf signal generators such as the CT378 and the Advance SG69.
As your use is LW/MW/SW, probably not particularly critical. Try the OA47 (or even AAZ17) which is still around I think. But I'd suggest keep to germanium, for the very reasons you give.

Otherwise a Schottky type, which has a low switch on voltage: eg Motorola MDB101, HP 5080-2800, Mullard BA481,BAT81, or surface mount BAT17 etc. Lots to choose from.
I may have a few of these Ge types somewhere if your junk box hasn't got anything. wme_bill m0wpn

Last edited by WME_bill; 22nd Oct 2012 at 3:33 pm.
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Old 22nd Oct 2012, 3:49 pm   #3
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Default Re: GEX66 VHF mixer diode.

Cheers for the info - the use is actually upper-end-VHF: the RF signal input is at 145MHz, mixing in the GEX66 with the crystal-controlled local-oscillator to generate a 10.7MHz first-IF. After filtering and amplification it's converted to 455Khz for more filtering, more amplification and detection (and yes, there are several OC171s involved in these stages so I'm expecting tin-whisker problems there too).

Schematics are at: http://www.vmarsmanuals.co.uk/archiv..._Cambridge.pdf

I think I've got some gold-bonded diodes somewhere - had been keeping them for crystal-set use ... will give one of them a try. Thanks for the suggestion!

--G6Tanuki

Quote:
Originally Posted by WME_bill View Post
GEX66 alternative.
I did a lot of research upon suitable replacements for this GEC mixer diode some years ago. It is a point contact germanium, with particularly low capacitance. It was introduced about the time that GEC and Mullard combined their manufacturing, disappeared and then seemed to re-appear later as AAZ13. Ratings: 5v, 50ma, very low stored charge, capacitance 3.0pf at 0v.
The last germanium gold bonded were as good: eg Mullard OA47, Hughes HD1841, HD 1871.
These were also used as modulators in vhf/uhf signal generators such as the CT378 and the Advance SG69.
As your use is LW/MW/SW, probably not particularly critical. Try the OA47 (or even AAZ17) which is still around I think. But I'd suggest keep to germanium, for the very reasons you give.

Otherwise a Schottky type, which has a low switch on voltage: eg Motorola MDB101, HP 5080-2800, Mullard BA481,BAT81, or surface mount BAT17 etc. Lots to choose from.
I may have a few of these Ge types somewhere if your junk box hasn't got anything. wme_bill m0wpn
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Old 23rd Oct 2012, 12:32 am   #4
m0cemdave
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Default Re: GEX66 VHF mixer diode.

I have found a used GEX66 in one of my junk boxes.
A quick test on the "diode" range of my Fluke shows a forward voltage of 213mV and OC in the other direction, so it looks OK.
However the leads are only about 20mm long and very stiff, might break if you try to straighten them out.
You are welcome to it for the cost of postage. Send me a PM if interested.
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Old 24th Oct 2012, 2:31 pm   #5
G6Tanuki
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Default Re: GEX66 VHF mixer diode.

Thanks for the offer; I've found that vintage-parts.com has NOS GEX66 diodes available for a reasonable price:

http://markhindes.easywebstore.co.uk/GEX66_A3YV0.aspx

so a couple will be being ordered later this week!

[In the meantime I tried an OA91 and got some sort of mixing-action going on, proved by 'sniffing' the 10.7MHz first-IF with a little coil hooked to the input of my AR88-D. The OC171 first-IF amps are dead-as-dodos though, no doubt due to tin-whisker syndrome]

--G6Tanuki.



Quote:
Originally Posted by m0cemdave View Post
I have found a used GEX66 in one of my junk boxes.
A quick test on the "diode" range of my Fluke shows a forward voltage of 213mV and OC in the other direction, so it looks OK.
However the leads are only about 20mm long and very stiff, might break if you try to straighten them out.
You are welcome to it for the cost of postage. Send me a PM if interested.
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Old 24th Oct 2012, 4:14 pm   #6
jim_jobe
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Default Re: GEX66 VHF mixer diode.

You may be able to zap the whiskers. Connect a 9V battery momentarily between the case and each electrode in turn - plus to the electrodes and negative to the case. This usually works.
Jim
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Old 24th Oct 2012, 4:33 pm   #7
G6Tanuki
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Default Re: GEX66 VHF mixer diode.

Quote:
Originally Posted by jim_jobe View Post
You may be able to zap the whiskers. Connect a 9V battery momentarily between the case and each electrode in turn - plus to the electrodes and negative to the case. This usually works.
Jim
From what I've read though, the whiskers often grow back! I've got a few spare AF12x-type transistors that are easy enough to stick in to replace the snarky OC171s.
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Old 24th Oct 2012, 9:16 pm   #8
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Default Re: GEX66 VHF mixer diode.

As a bit of lateral thinking; from having worked on a number of "Ambridges" over the years I would recommend modifying the front end board to use a fet mixer as per the Garex Twomobile which is based on Cambridge parts (mainly the LC10 variant with solid state oscillator/early multiplier stages and quick heat valve driver and PA), I still use one of these....

This mod gives gives a noticeable improvement over the original diode mixer. The circuit can be found on Steve Morton's web site.

I hope this helps.

Regards

Ian (G8KSZ)
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