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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 19th Sep 2018, 5:44 pm   #1
Jon_G4MDC
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Default Trio/Kenwood QR666

I have never had one but I have just had my first encounter of an example which is owned by a work colleague.

It has 1970s Japan standard mains wiring - to be treated very carefully - but once you have got past that.

It appears that the main circuitry works for the most part. It is suffering from serious problems in the band switching of RF stage/mixer (2 tuned circuits) and LO. Instead of the wafer switches used in earlier generations, such as 9R-59 series, it uses plastic cogs on the bandswitch shaft to move multiple pole slide switches mounted on the pcb.

These must be custom creations dreamed up by companies who were losing out on selling CB walky talky TX/RX changeover switches!

So far switch cleaner has helped, for a few hours, then they resumed their old ways. Trying to dismantle them for manual cleaning seems a nightmare. I wonder if anyone has trodden this path before me? I might be calling a Taxi for a quick exit.

I rather like the fact that it is dual conversion on the high range with ~4MHz IF instead of 455kHz. In that much it might have an advantage on the 9R-59s on image rejection but we don't know the extra sprogs generated yet.

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Old 19th Sep 2018, 10:44 pm   #2
SiriusHardware
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Default Re: Trio/Kenwood QR666

Sounds like a bit of a beast

Sadly, if these are the sort of switches I'm thinking of then the only good cure is to desolder them cleanly, take them out, bend back the folded over metal tabs which hold the covers on, remove the covers and clean the moving and fixed contacts manually, just as you feared.

I did once do an emergency get-it going repair to a cassette recorder with a similar type of long end-actuated PCB mounted sliding switch by going to the track / solder side of the PCB heating each pin of the switch in turn - hot enough to melt the solder - and wiggling the switch back and forward a few dozen times while keeping the pin hot. The idea was to soften the muck enough to allow the sliding contacts to plough a clean track through it. I had to try this because there was no switch cleaner within miles. Last I heard, it was still working.

However, this approach requires that you have good access to the print side of every PCB with a switch on it and I bet that isn't the case here.
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Old 19th Sep 2018, 11:35 pm   #3
Synchrodyne
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Default Re: Trio/Kenwood QR666

Quote:
Originally Posted by Jon_G4MDC View Post
I rather like the fact that it is dual conversion on the high range with ~4MHz IF instead of 455kHz. In that much it might have an advantage on the 9R-59s on image rejection but we don't know the extra sprogs generated yet.
To some extent the QR-666 (1974, I think) looks to have been a clone of the 1969 Heathkit GR-78. Band coverage is similar, both had dual conversion on the 18 to 30 MHz band with a 4.034 MHz 1st IF, and both used dual-gate mosfets in the signal path from RF to 1st IF/2nd mixer.

The 4.034 MHz 1st IF on the highest band might have been chosen to allow the use of a 3.58 MHz crystal for the second oscillator. Such would have been readily available and cheap due to their widespread used in NTSC colour TV decoders.

3.58 MHz also meant that oscillator harmonics in the 18 to 30 MHz band were 6th and higher, probably not too harmful as compared with their lower brethren. Given that the 4.034 MHz went into a nominally square-law mixer, maybe its harmonics were not too problematical, unlike the case where it was subject to vicious detection. On the other hand, one might deduce that dual-conversion was not used for the next band down (7.5 to 18 MHz), even though it would have been otherwise advantageous, because then there could well have been significant sprogs.


Cheers,
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Old 20th Sep 2018, 12:57 am   #4
Damien VK3RX
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Default Re: Trio/Kenwood QR666

I had one of those, my second receiver after a Realistic DX150A.

I recall the 666 drifted like crazy, tolerable with SW broadcast stations but frustrating on SSB/CW. There was a service bulletin published by Kenwood to tighten some screws on a PCB, but that didn't do anything to fix the problem.

Ended up moving it on fairly quickly and replacing it with a 9R59DS which also drifted, but not as bad.
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Old 20th Sep 2018, 6:26 pm   #5
Jon_G4MDC
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Default Re: Trio/Kenwood QR666

Thank you all for those replies.
Only today did I cotton on to 3.578 crystal (HC6 or 18) in the 4MHz IF.
Oooh I know what that is!

We can't change history and make it work better than it was when new but the suggestion from SiriusHardware is a new take on the problem for me.

No way would I even think of trying to dismantle that bandswitch into component parts but some "hot" cleaning is a sort of half way house. It could be done by one of the brave - I begin to like it quite a lot.

The owner might not be doing it but I might. He was quite pleased with my results on a Fender Strat last year and a Yamaha Acoustic after that.

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