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Old 29th Jan 2023, 9:07 am   #1
suebutcher
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Default Vacuum tube crystal for Trio 9R-59DS

I recently bought a couple of Cold War surplus vacuum tube crystals from Eastern Europe in order to add a calibrator to my Trio communications receiver. I wanted 1MHz and 200KHz for calibration on high bands and low bands, switched by plugging the crystals into a seven-pin socket mounted on the back where the coaxial connector was. The 1MHz unit was fine, but I had a lot of trouble getting the other one to resonate at 200kHz with a FET oscillator. I'd either get 3,2890kHz or 498kHz out of it. But I kept making guesswork alterations to the components until 200kHz appeared at the output. Here's the crystal, and the circuit. The oscillator runs off the 6.3V filament line, via the switch on the back of the RF gain control. I think the 330pF running from the drain to earth is inelegant, but the thing needed a lot of damping down to run properly. Just adding a cap across the gate and earth wasn't enough.
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Old 29th Jan 2023, 9:54 am   #2
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Default Re: Vacuum tube crystal for Trio 9R-59DS

It's not too surprising. All crystals have a full set of overtone modes and several spurious modes. The number printed on the envelope just means that there is one resonance mode at or close to that value. Manufacturers do their best to make the intended mode as active as they can, but that still doesn't guarantee that that one is the most active out of all of them. Sometimes it is, but with unknown crystals, you get pot luck.

So, sure-fire crystal oscillators usually include an L-C tank circuit to try to force the wanted mode.

There are several other circuit details which get varied to suit individual crystal designs, whether the intended frequency is a series mode resonance or a parralel mode resonance (all crystals have both sorts, on close-together pairs, parallel being the higher of the two)

So with a 5:1 difference in crystal frequency, you might need different oscillator circuits for the two of them. It's a pain, but you might have no choice. Crystal losses will certainly be different and so what is enough gain for one will run the other into the end-stops, or just not start.

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Old 29th Jan 2023, 10:24 am   #3
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Default Re: Vacuum tube crystal for Trio 9R-59DS

That makes sense. The 1MHz was fine with any oscillator I tried it with, which fortunately includes the makeshift design above, and it was expensive. It has the frequency marked on it, and a test date (Feb 1968). The 200kHz was cheap, has no markings, and I guess it may have been military reject. They both work in the Trio.
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Old 29th Jan 2023, 10:35 am   #4
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Default Re: Vacuum tube crystal for Trio 9R-59DS

Some photos of the plug-in arrangement, the oscillator (built on tag strip), and the power switch, which takes 6.3V from the dial lamps.
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Old 29th Jan 2023, 11:01 am   #5
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Default Re: Vacuum tube crystal for Trio 9R-59DS

Well done on that Sue.

It’s been mentioned before, but might be worth repeating, that there was a series of excellent articles in Radio Constructor Magazine on improvements and modification to the 9R59DE. I had a 9R59DS back in 1970, which, as far as I could tell, was identical to the ‘DE’. I carried out most of the mods, providing worthwhile improvements to the power supply, and additions such as an OA2 voltage stabiliser, and 6BA6 crystal calibrator, for which I used a 100kHz crystal, which worked fine. Seemed odd that Trio punch the chassis for a valve-holder and crystal, but didn't install one, presumably to shave a bit off the price.

Lovely set, for which I had the matching speaker.

Happy days!

Like all single superhets it had the limitation of image reception, so at higher frequencies you received the same stations twice. Makes the bands sound busy!

The first Radio Constructor article was in October 1970 and covered mods to the PSU, screen grid resistors, RF gain control, oscillator stability, and addition of the crystal calibrator. This issue of Radio Constructor can be downloaded free from here:

http://www.americanradiohistory.com/...RC-1970-10.pdf

There were two further articles in March 1971 and April 1971, which covered a further eight modifications and alignment information, which can be found here:

http://www.americanradiohistory.com/...RC-1971-03.pdf

http://www.americanradiohistory.com/...RC-1971-04.pdf

Really, it’s quite a large set with not a great deal inside it, so there’s lots of space to work under the chassis. It’s much discussed and there are numerous youtube videos on internet which are helpful and instructive. They crop up fairly often at reasonable prices and in good condition. Each time I see one, I’m sorely tempted, but so far, common sense has over-ruled nostalgia! The matching speakers are quite rare.

Some seem to have become problematical due to the coils becoming de-tuned with degraded wax or some such problem.

To be expected I guess, after half a century.

Manual attached.
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Old 29th Jan 2023, 12:27 pm   #6
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Default Re: Vacuum tube crystal for Trio 9R-59DS

Thanks David, now I've got all those articles in one place! I used the oscillator circuit in the Trio manual as the starting point for the mod, but I couldn't get it to work, possibly because I didn't have the right transistor. My FET oscillator circuit is a cross between an Electronics Today FET crystal oscillator (which didn't quite work) and the typical circa 1970 Electronics Australia FET RF amplifier (which always works). Built on a tag strip because that's the way E.A. did it in those days. The 2N5459 is one of my favourites, it's a bit easy to burn out, but it's simple to use and versatile.

I tried the Radio Constructor power supply modification, but adding a smoothing choke caused a squealing instability somewhere. I solved the audio hum problem and the RF gain/BFO pitch interaction with more smoothing capacitance and slightly less smoothing resistance, and the 0A2 of course. I'll describe that once I've drawn up the modification.

I'm happy with the Trio, the drift isn't serious once it's warmed up, and it's no trouble listening to SSB on 80 or 40 meters. 20 metres is near the edge of usability, and above 14MHz it starts to become deaf. But it's a valve set, and it's fairly easy to fix.
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Old 9th Feb 2023, 7:00 am   #7
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Default Re: Vacuum tube crystal for Trio 9R-59DS

Here's the power supply anti-hum mod. I padded R37 down to 1.1kohm, and increased C40 to 100uF and C42 to 240uF. I happened to have a Rubycon 200/60uF that solved the problem neatly without the need to drill extra holes. (Five stars for old Rubycons!) There's now no audible hum in the phones at normal listening levels, and the RF gain has only a slight effect on the tuning, rather less than the normal overall drift.
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Old 9th Feb 2023, 8:20 pm   #8
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Default Re: Vacuum tube crystal for Trio 9R-59DS

Hi Sue, it is now common practice to use UF series diodes as the rectifiers.

Best practice when snubber caps were used across the slow diodes was to add about 100R in series with the cap to dissipate the stored energy

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