Hallicrafters Sky Champion S20

[LyntonP]

Originally Posted by Keith

As you may be aware, the BFO injection on most of these vintage receivers is quite low. It is usually necessary to turn down the RF gain by quite a lot to bring the signal level down sufficiently to hear the beat note or resolve SSB.

Hi Keith
Well the re-twisting of the wires had no effect. I followed your advice and reduced the RF gain but also no effect.
I am beginning to doubt the connection between the BFO and the IF transformer. The other transformer in the set had problems with broken wires between the coils and capacitors. I wonder if it has suffered the same fate?
Having said that when I aligned the IF frequency both transformers did respond well to the adjustments so maybe not?
I wonder if I could inject a signal somewhere just to check the operation?
Getting a bit beyond my limited knowledge!
Lynton

[Keith]

Hi again,

From your previous post it did appear that you had detected oscillation from the BFO as it varied in frequency with the operation of the control. If this is the case then either the amplitude is too low or the frequency is not on 455kHz. Make a note of the present position of the core in the BFO coil and then try screwing it in and out slightly whilst the set is tuned to an AM signal (but with the RF gain turned down so that it is just audible). If the BFO is working you should hear the beat note sweep through the signal. I'm assuming that you've checked/replaced the anode and grid resistors in the BFO circuit.

[usradcoll1]

Originally Posted by LyntonP

That’s the one I have Dave thanks

That's the same one the is available from Nostalgia Air. All kinds of information but the voltages on the valve sockets.
BTW, a great restoration, considering what you had to work with.
Dave, USradcoll1.

[Jon_G4MDC]

Always wanted one of these. Around age 14 a school friend had one.
His logbook contained much better DX than mine and I'm still jealous!

[LyntonP]

Originally Posted by Keith

I'm assuming that you've checked/replaced the anode and grid resistors in the BFO circuit.

Hi Keith
Tried on a weak MW station but with no success.
Managed to get the valve support box loose from the chassis giving access to the valve base of the 6J5. There was 0.01 cap and a 100K resistor (and a mica cap) as shown on the diagram.
I replaced the 0.01 and the 100k (not too far out of spec) put everything back together, but unfortunately no effect.
Strangely the wax cap was a completely different brand to all the other caps I have changed? May have been changed in the past?
Lynton

[LyntonP]

Originally Posted by usradcoll1

BTW, a great restoration, considering what you had to work with.

Thanks Dave
Just need to sort out the BFO and the stripped gears.
Lynton

[Keith]

Have you checked the value of R20 (25K)? if this has gone high it will result in low BFO output. This is a Hartley oscillator. Maybe the best place to connect your oscilloscope would be the cathode .

[LyntonP]

Hi Keith
R20 tests fine almost dead on 25K.
I had an idea last night to try and trace the wire from the IF transformer down to the twisted pair capacitor. Will have a look when my domestic duties are completed.
I will also try again with the scope on the cathode of the 6J5.
Lynton

[LyntonP]

Moe experimentation. I looked again for the BFO oscillator measuring on the anode of the 6J5. The result is in the first photo. Frequency alters when the control is rotated. Not a huge difference but I would have thought enough.
Next I dismantled the second IF can to see if any wires had come adrift. None had but I reffloed the connections. It was very difficult to remove the can, and even more difficult to put it back!
Both coils inside showed a resistance of around 10 ohms.
Everything back in place but no different outcome.
I tried wrapping more coils into the “twisted pair” capacitor but again no difference.
There are two 6J5’s in the set but swapping them made no difference.
Found a station using SSB but could not clarify the voice.
Tried with low RF gain, no difference.
Tried tweaking the BFO transformer, no difference.
Could I use a low value capacitor instead of the twisted pair?
Unsure which way to go!
Lynton

[ms660]

So what's the frequency of that waveform then?

Lawrence.

[LyntonP]

My guess is 50Hz Lawrence!
Lynton

[ms660]

Not BFO then...!

Check at cathode, X1 probe via a small pF and crank the 'scope gain up if needed.

The BFO's in the Hallicrafters I had worked quite well, I slightly increased the coupling for better SSB but nothing too drastic that I can remember.

Lawrence.

[Keith]

Lynton,
What is the scope timebase setting in you last picture (can't quite make it out)? Also is the fine control in the CAL position and is the X5 button pushed in?

[LyntonP]

Gentlemen
Scope connected to cathode via small capacitor.
On channel 2
Volts/div 0.1
Secs/div 1 microsecond
Peak to peak voltage 0.3 volts
Frequency 3 microseconds
BFO control does vary frequency a little
Hope you can see details on photo
Lynton

[Keith]

3uS corresponds to 330kHz which is too low. To be on the IF (assuming it is on 455kHz) it needs to be approx. 2.2uS. Can you get near this by adjusting the core?

[LyntonP]

Hi Keith
Tried to adjust down to 2.2uS but the capacitor is wide open before it gets there. I can now hear a hetrodyne when I operate the BFO control.
If I tune the radio with BFO turned off to find a station, then turn on BFO the signal disappears and if the volume or gain is too high I get acoustic feedback. The whole of the radio is very microphonic!
I did with some fiddling get an SSB signal and it was possible to clarify it.
Getting closer.
Thanks for all the help.
Lynton

[cathoderay57]

Hi Lynton, the only component you don't appear to have checked is C32, 250pF grid coupler. It's silver mica so not usually a cause of trouble, but they do drift in value and if it's a "micamold" type they leak. If it has gone leaky it would certainly adversely affect osc operation so worth a check or replacement. Either a 220pF or 270pF should work OK. Cheers, Jerry

[Station X]

What about the undesignated capacitor in parallel with L1? Presumably it's inside can?

To raise the BFO frequency will require less inductance and/or less capacitance.

Ideally zero beat should be achieved with the variable cap's plates half meshed.

[LyntonP]

Originally Posted by cathoderay57

Hi Lynton, the only component you don't appear to have checked is C32, 250pF grid coupler. It's silver mica so not usually a cause of trouble, but they do drift in value and if it's a "micamold" type they leak. If it has gone leaky it would certainly adversely affect osc operation so worth a check or replacement. Either a 220pF or 270pF should work OK. Cheers, Jerry

Hi Jerry
Yes have seen the capacitor in question, and I have seen comments that they are reliable but it is worth a try. It is a 'domino' type cap and can be seen in one of my earlier photos (when I lifted the mounting can for the BFO valve). I certainly don't have a replacement to hand so will have to search for one on line.
Cheers
Lynton

[LyntonP]

I'm struggling to find a replacement for this capacitor. do they have to be silver mica?
Would a ceramic disc work?
Lynton