AR88 BFO injection mod?

[dodgy-dxer]

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Originally Posted by Martin G7MRV

Thanks everyone
I had it tuned to Shannon volmet on 8MHz last night and with it's rather weak signal I was able to successfully resolve it. Shannon on 5MHz though when strong wouldn't resolve.
I might try at first just linking both unused pins on the BFO valve to see if that provides sufficient extra level as in the CR88

Other than this, and the noise limiter 6H6 being dead I've few issues with the set. Selectively setting 4 isn't working, that may just be a switch contact, and an odd popping sound on some bands which I'll do some substitution on valves to see if that changes. Unfortunately I cant do this with all as I don't yet have spares for the 6SA7 or 6SJ7

[Aub]

I'm home now. I looked for the article I mentioned earlier. It's actually from Short Wave Magazine, October 1965. Hopefully it's on the World Radio History site
The article is by Harry Leeming, who became renowned for his expertise with Yeaseu equipment. The article does contain details of the twisted pair of wires to increase the amount of BFO injection.

Hope this helps.

Aub

[Martin G7MRV]

Thanks - yes ive found it on WRH.

Re: the selectivity switch setting 4 not working, well it is but just extremely low level compared to the others. So it doesnt seem to be the switch thats at fault. Trying to get my head around the switching of the various parts of the filtering now to work out which section is amiss

[Martin G7MRV]

ok so thanks to Aub and the SWM article, it all makes sense with the BFO now. The "gimmick" or trimmer goes between the Anode pin 3 and the unused pin 4 on the BFO valve V12, which is where the current connection is for the stray capacitance. I suppose i should have guessed at that, but my knowledge and understanding of valve circuits is not as good as i'd like.

I have already made the link between both unused pins 4 and 6 as per the CR88, but ive only a short length of wire indoors for the antenna at the moment and not much SSB has been forthcoming this evening for me to try it out. I did briefly locate Shannon volmet on 3MHz and it seemed to be working, but conditions are not great so further testing needed

[Radio Wrangler]

Originally Posted by Martin G7MRV

Re: the selectivity switch setting 4 not working, well it is but just extremely low level compared to the others. So it doesnt seem to be the switch thats at fault. Trying to get my head around the switching of the various parts of the filtering now to work out which section is amiss

All AR88s are old enough that their crystals have drifted a fair bit and it's likely the high-Q setting of the LC selectivity no longer agrees with the peak of the crystal. Sensitivity suffers a lot.

David

[Martin G7MRV]

Yes that makes sense. Having had a brief read of the alignment procedure I might leave it until I'm in a more can do mood!

[Radio Wrangler]

Swept alignment is a lot easier, especially to get the wider bandwidths set to spec. Start looking for a wobbulator !


Davjd

[Martin G7MRV]

I have a spectrum analyser, sweep generator, a marconi 2955B (although this is wearing out) etc plus other kit

[Radio Wrangler]

That should do nicely, then.

I've just been given a very nice 6GHz macro-VNA, complete with s-parameter box. I already had a set of APC 3.5 calibration standards for it. Overkill-R-Us

David

[Martin G7MRV]

My problem is trying to understand the switching to work out which parts are actually unique to selectively setting 4!

[Radio Wrangler]

Take multiple photocopies of the larger format EMER schematic, just the early IF section, and highlight the connections via the contatcts, one diagram for each bandwidth position.

It's suspected that the diagrams in the AR88 manual were done that way to sow confusion and brain damage should they fall into enemy hands.

David

[ms660]

The Xtal filter circuit switching in the schematic, S20....contact 1 is the wiper the wiper is shown in the broadest position (1)...four clicks clockwise to contact 4 is the sharpest position (5) The first two positions shorts out the Xtal.

Lawrence.

[mikezero]

Hi
I came across this document a few months ago. It dates back to 1964. It has useful info on alignment as well as a paragraph on how to strengthen BFO injection.
John

[Alan Bain]

A previous owner had added one of those beehive trimmer caps between the two "air spaced" valveholder pins in my AR88. Optimum setting was as I recall fairly close to min capacitance.

Alan