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Old 17th Nov 2017, 9:19 am   #1
Keith
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Default R209 - BFO injection

Does anyone have experience using this receiver? A recently acquired example seems to work fine on AM, but when switched to CW the BFO injection is very weak, requiring the gain to be turned down significantly. This results in a low level of audio output. It would be useful to know if this was a design "feature" or whether there is a fault present.
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Old 17th Nov 2017, 9:24 am   #2
Andrew2
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Default Re: R209 - BFO injection

I don't know much about the architecture of the 209, but some receivers of that era did not have a product detector and just squirted a bit of BFO into the final IF stage or the AM detector. Obviously this would activate the AGC and depress the audio. The recommended procedure was to turn up the AF gain and control the volume with the manual RF gain control. Never very satisfactory IMO.
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Old 17th Nov 2017, 11:52 am   #3
vosperd
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Default Re: R209 - BFO injection

Hi Keith,
I have a mkII R209 and, from memory, there appears to be plenty of bfo injection. The problem with my set is that it has been converted to mains. The cw tone sounds more like a buzzer and lots of drift on cw. I can resolve ssb as well but a bit restricted due to the audio filter.
I'll set it up out of interest this afternoon and see what it's like.
Don m5aky
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Old 17th Nov 2017, 3:43 pm   #4
G6Tanuki
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Default Re: R209 - BFO injection

The R209 is a bit strange: there's no AGC operating on CW; instead a squidge of the BFO's output is tapped off, fed to the diode in the 1S5 BFO-valve and rectified - this then provides the negative bias-voltage to feed [via the second gang of the 'gain' control] to the grids of the RF/IF amps to provide manual gain control.

If you have what sounds to me like low BFO-injection and gain-control issues I'd check the BFO valve, as if this isn't producing enough output you'll lack BFO injection as well as adequate bias.

Another thing to check: the R209s of my acquaintance make use of some strange decoupling capacitors with a grey rubbery outer-jacket. These are replace-on-sight, in the same spirit as Hunts and Waxies. Leakage here can cause significant issues.
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Old 17th Nov 2017, 6:47 pm   #5
Keith
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Default Re: R209 - BFO injection

Thanks for all the replies:

Andrew2 - Yes it is basic injection into the last IF. No AGC in CW mode - the "Volume" control becomes a manual RF/IF gain control.

Don - Mine is also a MkII. I'd be interested in how yours performs on SSB.

G6Tanuki - Yes, it took me a while to figure out the gain control rationale. Then I realised that the bog standard method of lifting up the cathodes won't work - cos there aren't any! The valve tests OK but its anode feed had drifted up from 33K to over 50K. Replacing that helped a bit. I shall have a look at the decouplers.

Keith
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Old 18th Nov 2017, 11:52 am   #6
Peter.G0DRT
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Default Re: R209 - BFO injection

I have an R209 Mk2 and the b.f.o works fine.

Peter.
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Old 18th Nov 2017, 3:47 pm   #7
vosperd
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Default Re: R209 - BFO injection

Hi Keith,
I have just checked the bfo signal via the IF socket on the front panel and I'm getting about 500mV p-p on a scope. Hope that is of some help.
Ssb can be resolved but it's not very good. It would be nice to sort out my bfo buzz one day but I only really use this set for AM.
Cheers
Don m5aky
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Old 18th Nov 2017, 5:55 pm   #8
Keith
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Default Re: R209 - BFO injection

Hi Peter/Don,

The BFO does work but requires the gain to be drastically reduced to resolve SSB. I've also measured the BFO level at the IF output. I get almost 2Vpp. I've now paralled C88 with 33pF and this brings it up to ~4Vpp, which improves things somewhat. I've also (reversibly) disconnected the Scott T CW filter which gives a better sounding result on SSB (it didn't actually seem that sharp when it was in circuit so presumably the high value Rs have drifted somewhat).

The IF output is completely one sided due to it being taken from the anode (grid in this case) of the detector. Presumably that was good enough for the FSK detector.

All in all, not a bad RX for AM although the audio from the waterproof speaker is very boxy - much better on phones.
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