AR88 BFO injection mod?

[Martin G7MRV]

Probably me being useless, but I cant seem to find details of the mod to increase the BFO injection level to improve SSB. I understand it's just adding a "gimmick" capacitor but not sure exactly what connections

Can anyone either explain or direct me to the details please?

[Silicon]

Forum member turretslug commented on this in a post dated 17th August 2021.

https://www.vintage-radio.net/forum/...d.php?t=182706

https://www.vintage-radio.net/forum/...7&postcount=16

He said that the later variant CR88 has a 10pF capacitor to increase the amount of BFO injection.
A comparison of the schematics might provide the answer.

[Martin G7MRV]

I've had a look at that thread, it actually states the SC88 which I can't find circuit diagrams for.
There is a difference between the AR and CR in this respect according to the diagrams - the CR has TWO unused pins connected for the stray capacitance rather than just one in the AR. Both then go to an unused connection on the IF transformer

I'm thinking surely if at least one end went to an actual active connection that would work better anyway?

Does anyone have circuit diagrams for the SC88?

[Aub]

I might have some notes at home, which a previous owner had made for the AR88 I used to have. I think it's just a pair of wires twisted together. I'm away at the moment but I'll look tomorrow or Wednesday.

Cheers

Aub

[Martin G7MRV]

Aub,
Yes I believe that to be the case, I just don't know where to connect them!

[Silicon]

I have just downloaded the manual for the RCA CR91.

It shows a small alteration to the BFO wiring.
The wire going to the IF was originally connected to the unused pin 4 of the 6J5 BFO valve holder.
Pin 4 is between the grid and the anode.

The wire was moved to unused pin 6.

I don't know the reason for this but I suspect that it may increase the signal going to the IF stage.

[Silicon]

The RCA AR88D schematic shows that the BFO signal is 'injected' at the junction of IF secondary coil L44 and capacitor C105 which takes the signal to the grid of V7.

The valve holder for V7 is designated X7 on the photographs.
There is not enough resolution to see how it is connected.

L44 is the secondary winding on IF transformer T8.

[Martin G7MRV]

But at the IF transformer, the connection is to pin A, which doesn't actually go anywhere. The diagram shows a sort of "ghost" capacitor I guess to indicate the stray coupling. I wonder if connecting directly to that junction would be ok? It's still to an unused pin on V7. Seems odd to essentially have a length of wire that doesn't actually connect with anything at either end other than unused pins!

[Martin G7MRV]

Ah! The CR88 does show an actual physical capacitor to that junction, and also the connection at V7 going to TWO unused pins. It seems to indicate this is a mod included in later versions. I can see how That would increase the injection Level

I wonder if that came along around the time ssb started to gain popularity?

A 2pF capacitor is shown between the IF can pins A and F where C105 connects. I can easily add that I think if I've a suitable part. I can't imagine theres much voltage at that point

[Radio Wrangler]

Put too much injection in and the sensitivity with BFO on begins to drop (RF gain has to be manual in AR88 with BFO on because detector rectifies BFO and dominates AGC, even unmodified sets)

So there is an optimum point to find.

Back in the day there were magazine articles on adding product detectors. But then you still had to shield the early IF stages from BFO pick-up.

David

[Martin G7MRV]

Yes, this is why ideally I'd prefer to fit a trimmer cap or gimmick that I can adjust.
I plan on using a volmet station to monitor while adjusting the level

[G6Tanuki]

One thing to be wary of is if you end up with several different paths by which the BFO signal gets into the IF chain; if this happens you may well end up with the different BFO components arriving at the detector with different phase-relationships, and when they mix together with a SSB signal in the less-than-perfectly-predictable environment of the detector diode, the result can be a horrible-sounding mess!

A slight whiff of BFO sneaking into the first IF amp [by way of poorly decoupled heater wiring, or along the HT supply] and the intended higher-level BFO injection direct to the detector-diode, for example. When you're trying to set the signal-level via the manual RF/IF gain control, having some stray BFO signal riding along with the signal through the IF stages can be deeply frustrating.

[thoyer454]

Originally Posted by Martin G7MRV

Does anyone have circuit diagrams for the SC88?

Attached. I have a couple SC-88's - nice radios.

Tom
W3TA

[Radio Wrangler]

Originally Posted by G6Tanuki

When you're trying to set the signal-level via the manual RF/IF gain control, having some stray BFO signal riding along with the signal through the IF stages can be deeply frustrating.

This is putting it mildly It can also pull the BFO

David

[turretslug]

I'm sure that I recall a post by sadly departed member Skywave where he mentioned using a trimmer capacitor- perhaps one of the ubiquitous 4-20pF surplus air-spaced jobs- and tweaking it for optimum compromise between good injection level and minimum AGC action. As others say, consider the IF strip as a high-gain 3-stage TRF that will sniff and amplify anything around 455kHz in its vicinity, plus the way that it's all wrapped around rather closely together in the AR88 rather than a long strip as in many other comms receivers.

[thoyer454]

Here's a link to the sC-88 manual

http://armyradio.wiki/lib/exe/fetch....iver_r-320.pdf

Tom
W3TA

[Silicon]

Thanks Tom, that manual has a thorough description of the theory of operation and a schematic which is easier to understand.

[Radio Wrangler]

Originally Posted by turretslug

I'm sure that I recall a post by sadly departed member Skywave where he mentioned using a trimmer capacitor- perhaps one of the ubiquitous 4-20pF surplus air-spaced jobs- and tweaking it for optimum compromise between good injection level and minimum AGC action.

I have a bagful of tubular trimmers in that sort of range if anyone wants one to play with. Quite small and would solder to a valve base tag.

SC88s are rather rare on this side of the Atlantic, I think we got all the AR88s. A friend had an AR8516L, But it's the CRM-R6A I'd really like.

David

[G3PIJpeter]

Originally Posted by thoyer454

Here's a link to the sC-88 manual - Tom W3TA

The cathode of the BFO is coupled via C-175 (10 pF) to Point C on the second IF amplifier IFT. Point C is shunted to earth by C-157C (0.1 microfarads). This capacitor has a reactance of 3.5 ohms at 455 kHz - so how much injection from the BFO will actually make it into the signal path via this route?
Peter
G3PIJ

[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