Codar BFO circuit.

[gezza123]

Guys.need some info please.
This is a circiut of my Codar IF/BFO.The problem I have is after I realign the set to 470kc I get all bands/stations ok,except that I can not get the BFO to Oscillate I turn up the volume control and use the IF Gain but still no oscillation.
If I realign to 455kc I can get some donald duck squeals when using the same procedure,but the set sounds terrible -would be ideal for walt disney ,can anyone tell me how this circuit is supposed to work.
I have changed all components in the circuit except the ebf89 valve,as I have not got a valve tester to test them ,I am reluctant to buy and change them willy nilly,so realy need some advice thank you in advance,Gezza

[Station X]

Is there a BFO pitch control? If there is then this should vary the BFO frequency on either side of the IF frequency (sic). If the BFO is set at the IF frequency then it will zero beat with AM stations. Reception of CW and SSB stations will be poor though as you'll have to tune them OUTSIDE the passband of the receiver to get satisfactory audio note on CW or resolve an SSB signal.

Fixed frequency BFOs aren't unknown, but their frequency is offset from the IF.

The procedure for tuning an SSB signal is to switch off the BFO and tune for maximum Donald Duck noise. The BFO is then switched on and its frequency adjusted until the signal is resolved. Apologies if you already know this.

[G8HQP Dave]

It looks like it hasn't actually got a BFO, but just gets the IF amp itself to oscillate. No obvious feedback path, so it must be relying on strays. Not the best way to design a reliable circuit! I would check and double-check all the components you have replaced, as it is easy to introduce another fault. Then change the EBF89 and EF85 - both are quite cheap.

I assume this is the CR66 - you didn't say.

[Skywave]

First observation: the sec'y. of L3 states "To pin 2 CV140" - what / where is this? Is this some BFO source? Feed to the AF demod. filter / gain control? We need to see a bit more circuitry.

Notwithstanding that . . .

Second observation: the screen grid voltage for the EF85 and the EBF89 are ajustable by a pot. divider; the value of the pot. itself is not stated. But it is labelled "BFO GAIN". Huh . . . "IF gain" I could understand; "BFO freq": ditto. But "BFO gain?"

Anyway, despite the fact that the common screen-grid rail is decoupled, I can only assume that if this common screen-grid voltage is taken low enough, then both stages go unstable on account of electron-coupling inside the valves. Hence, the whole IF amp will oscillate at the nominal IF.

I'm sorry to sound so critical & negative about this, but that circuit looks like a really cheap 'n' nasty way to go about fitting an 'ssb detector'. And upon inspection, the AM detector doesn't look too clever, either. If it were mine, I'd seriously think about a re-design at this point - starting with a discrete transistor oscillator as a BFO. I suppose such a decision hinges on what you want to do with this Item.

Al. / Skywave.

[gezza123]

HI:Guys,thank you for the quick responses.
This set has not got a seperate BFO Dave, you can only vari the pitch with the gain control.
I know what you mean when you say about adjusting the Osc so that a zero beat is heard and minimum reading on the ac voltmeter.
As being new to radio .I tried and tested on my Lafayette BFO and it works OK.thanks again Graham.

Yes Dave it is a CR-66 sorry for that,it has only got what you see on the IF circuit and thats all you get,and a volume control.
I was wandering how the manufactures set up this kind of set and is there some special way to do it.
You say they may be relying on strays,but where would you get them from,I do believe that some sets got a pickup by putting a wire into the IF,don't like that idea.
I also took out the Ef85 circuit completely and used no second IF this was how the set was originally made,the only differance was that the gain had to be turned full up to get the weaker stations,so I think maybe the Ef85 is ok,so will as you say try and get a Ebf89 on the wanted pages,if you have any more info on how this works would appreciate it thanks to you both again.Gezza

[gezza123]

HI.AL,thanks for your comments,but it states IF/GAIN/BFO ON THE FRONT OF THE SET,the CV140 was an adiction to the set I think it is for noise reduction and will work witout it in circuit like the ef85.
The front panel view is my interpretation,disregard the vco should read ANL
I am trying to put this set back as was when made ,still trying to sus out the rest of drawing,here is some info on the set,thanks Gezza

[G8HQP Dave]

CV140 is EB91 - a noise limiter?

This type of crude BFO feeding a diode detector will never be particularly good for SSB. You need to keep signal levels low at the detector, then make up for it by turning up the volume. I would just use this set for AM. You really need a product detector for SSB.

[G8HQP Dave]

So you can't keep signals low, as IF gain doubles as BFO on! I don't think there is really a solution to this. Sorry!

[gezza123]

Thanks dave ,I want to put it back to its original no ef85/cv140.have seen this set working perfectly on youtube in its original form an sounds great.

[Skywave]

So you've heard / seen it working - and were suitably impressed: O.K. - but was that on SSB reception - or AM ? That's kinda relevant here

Al.

[gezza123]

hi al ,goto youtube-militaryradiomuseums-channel.you will see it working
40Mtr ssb
40mtr cw
volmet report
shortwave,etc,etc
thanks Gezza

[Phil G4SPZ]

I have a valved Codar CR-70A, which has no BFO and uses IF self-oscillation to resolve SSB and CW, not very effectively The frequency of oscillation depends on signal strength, among other factors, and tends to change as fading occurs.

[gezza123]

HI:Phil.I have the manuall for that I tried to get one of these but they want a lott.there is a sepperate BFO in these and they switch out the AVC and the meter when in opperation,I think the osc coil is in the cathode of the ecc81 .Gezza

[gezza123]

HI: Guy's Back again,update on the Codar cr-66.
I had a good day in workshop wednesday all day went to bed happy.

Tried a Ef85 and a new Ebf89,still had same problem,so sat down and took out the second IF and Ef85 and put set back as manufacture spec.I then swapped the second IF for the third IF and realigned ,and to my suprize I could tune in for the first time a couple of hams from from scotland and the york area and also listen to morse.
I have done some tests by measuring the inductance on the IFt3,one test with the cores removed and the other with the cores present.
IF's are 470kc/s
With cores present inductance = 1.92 mh and peaks to 450.518 kc/s.
With cores removed inductance = 1.02mh and measures 618kc/s resonance.
This was done with the capacitors still in circuit,so need to take out caps an then do another test,the caps in these IF'S are 65pf,so some more help would be appreciated on how to really make tests on IF'S.
Like using a generator and scope to test peaks before replacing,how do you guy's go about it.

No formula's please. as they do my head in .thanks for your support.Gezza

[gezza123]

HI:Guy's back again an update,
As in my other post I have put set back to original,and aligned the IFs 470kc/s.Also I have put a gimmicon the last IF control grid to get it to oscillate as a BFO.
After removing all the modifications I now have a set working on all bands to satisfaction,
I have been listening to Freddy,in silvania nr Venice.Sugar-five.eight.foxtrot.alfa.on 40Mtr,also Simom,in Venice on 500w,India,Whiskey.Three,plus various in england .Reception is great on a Dicone Aerial
There was a couple of faults on the set due to wrong wireing.
So now I am in the middle of drawing the circuit out again,the front end is
simular to this one below.The reciever is 450 kc to 30 mc.
What I want to know now is how critical are the three condensers I have marked on the circuit, and if I short down the (Grid 1) and measure the frequency of the oscillator ,should the frequency shown match that shown on the dial.
Thanks for all the input again Gezza.

[G8HQP Dave]

None of those three capacitors is critical, although I would not change their value too much as it could upset the alignment. They are coupling capacitors, but as they are electrically near the tuned circuits they will have some effect on tuning.

The oscillator frequency will not match the dial. It will normally be 470kHz higher than the dial marking. Shorting grid 1 will not change this.

[gezza123]

HI: Thanks dave I understand now ,I wondered where the 470kc/s came from
din't realise it was part of the oscillator.Dial tracking fine now except on high
band ,goes off after 20Mhz so it is acceptible now.

I wanted to use my counter so could measure the actual frequency the station was on ,but I can now subtract the 470Kc/s from the figure.
thanks again dave for your input,prob have some more question as the old grey matter is not so good these day'sGezza