Hallicrafters Sky Champion S20

[LyntonP]

Started a new restoration project. The Sky Champion S20.
Made in the late 30’s but only produced for a couple of years being superseded by the S20R.
It is in poor condition having spent the last 30 years forgotten in the back of a shed.
I do remember it working but was put away after power supply problems.
On examination it has been ‘modified’ in several areas.
The power supply had been modified to solid state.
The original speaker with its chokes and coils has gone and been replaced with a very large choke on the HT, and an audio output transformer with a permanent magnet speaker.
Some capacitors had been replaced with ceramic types, but the majority of the old wax capacitors are still in place.
The main transformer seems to be of the correct type with supplies for the valve heaters and HT (including a separate supply for the rectifier valve). It can be simply adjusted for different mains voltages.
The tuning capacitor cage is very rusty, but the vanes seem to be in reasonable condition.
The external tuning scale is not very readable due to corrosion.
I downloaded a users manual which has a circuit diagram and parts list but not much more. Searching for more information has been fruitless. (Plenty of info on the S20 R however).
I have traced as much of the various circuits following the diagram and nothing seem amiss.
I am now in the process of Changing out the 80 year old wax capacitors and converting the PSU back to valve operation.
It will certainly be a challenge and I suppose lots of people would have delivered to the tip but I would like to get it going again in memory of a good friend of mine who passed away on Boxing Day last year. He was a very good radio and TV engineer who without doubt would have had it working in a trice!
If anyone has any more information or bits and pieces from a similar set I would be happy to hear from them.
Cheers
Lynton

[Dave757]

Hi Lynton,

The Manual for the S20 is available at BAMA Archive, - I don't
know if this is the one you've got already.

It is a bit sketchy, but it may help.

Kind regards
Dave

[LyntonP]

That’s the one I have Dave thanks
Lynton

[LyntonP]

A few photos of the Hallicrafters

[Silicon]

Are you going to use a frying pan splatter guard mesh to protect the loudspeaker?

[Dave757]

Hi Lynton,

Nostalgiaair has 3 Rider sheets for the S20.

2 are much as the manual you have, but the third gives
Alignment instructions.

Kind regards
Dave

[LyntonP]

Originally Posted by Silicon

Are you going to use a frying pan splatter guard mesh to protect the loudspeaker?

That’s a good idea. Been wondering where I could get such fine mesh from!
Lynton

Originally Posted by Dave757

Nostalgiaair has 3 Rider sheets for the S20.

Thanks Dave. I have heard mention of Ryder’s manuals on Shango 66 Utube channel but never seen them before.
Lynton

[usradcoll1]

Nostalgia Air has all the scans of the Riders manuals. I have the large manuals that take a lot of space. Two large shelves worth.
Dave, USradcoll1

[lesmw0sec]

Interesting to see that, as it was the model issued to my grandfather during WW2 for Voluntary interception. I can just remember it.

[usradcoll1]

Originally Posted by Silicon

Are you going to use a frying pan splatter guard mesh to protect the loudspeaker?

Brilliant idea!
USradcoll1, Dave

[turretslug]

I haven't looked at frying pan splatter guard prices recently- I like the idea, but you can get stainless steel mesh sizes pretty inexpensively from eBay (and surely other suppliers) in a wide range of gauges. In particular, SS mosquito mesh has specifically higher light (and presumably sound!) transmission than standard mesh, whilst splatter guard will be designed to be somewhat opaque to tiny hot droplets. Being flat, they suit standard letter post charging rates.

[LyntonP]

The radio is now restored and working very well. I do have a problem with two stripped gears in the tuning mechanism. I can still tune the radio but the scale disc does not turn as it should. Having no luck finding spares as yet.
The BFO doesn’t seem to work so more investigation is required there.
The work was mostly straightforward but I did discover broken wires in a IF can which had to be repaired.
Thanks to a fellow forum member who tested the valves and provided me with the correct final output valve.
Glad that it is now ok and a fitting tribute to my old friend.
Cheers
Lynton

[G6Tanuki]

You say "The BFO doesn’t seem to work" - due to aged components it could be off-frequency sufficiently that its effect is irrelevant.

Or it might not actually be oscillating. Check the voltages around the 6J5GT - if it's oscillating the grid should - when probed with a high-impedance/low-capacitance VTVM - show a negative voltage.

[Station X]

Regarding the stripped gears. If it's the double gears which are stripped, could you reassemble them so that the stripped teeth in each half no longer coincide?

[LyntonP]

Hi Graham unfortunately not. The split gear in question has the outermost part fastened to the shaft, the brass bush is pressed onto it (square hole in gear) and the innermost gear is loose on the shaft. If I try to remove them I’m sure they would be damaged. There are also not that many teeth to play with due to the small diameter. The gears are only around 1.5mm thick. I have considered trying to make new ones from scratch but if it doesn’t work then the tuning scale would not rotate at all.
Cheers
Lynton

[LyntonP]

Originally Posted by G6Tanuki

You say "The BFO doesn’t seem to work" - due to aged components it could be off-frequency sufficiently that its effect is irrelevant.

Or it might not actually be oscillating. Check the voltages around the 6J5GT - if it's oscillating the grid should - when probed with a high-impedance/low-capacitance VTVM - show a negative voltage.

The valve in question is mounted on the chassis on top of a small enclosure which also has what looks like an IF type can sitting on it which I have not adjusted.
Although I have the alignment instructions which I successfully followed, there are no instructions for the BFO. Or is it a case of just setting the zero beat to a given frequency?
Unfortunately I don’t have a VTVM only a multimeter.
I have traced the Ht to the anode, and the cathode is connected to ground. When the BFO is switched on/off their is a slight ’pop’ in the loudspeaker but I suppose that could just be a dirty switch.
Lynton

[LyntonP]

Today I connected a scope to the grid of the BFO valve and the results are in the attached photos.
The first picture shows the circuit diagram. The highlights show what I have traced with a multimeter, resistor values I have checked and the voltage I found on the anode. It also shows the ’twisted pair’ capacitor but I have been unable to trace this to the IF transformer (non highlighted vertical line).
The second shows the 6J5 valve in question which has been tested and is ok. This also shows the mounting arrangement where the valve and ‘tuning’ section are sat on an enclosure on top of the chassis.
The third photo shows the screen display, but part of the waveform is missing due to the camera shutter effect.
The last two photos show the scope settings. I am no expert in operating a scope.
Please advise if I am on the right track.
Thanks
Lynton

[Keith]

Lynton,
Looking at the scope settings, I think what you are seeing is just mains hum. It may be that the scope probe on the grid is killing the oscillation. Try increasing the sensitivity (X1 probe setting) and just holding the probe near the grid or anode of the valve. The scope timebase needs to be set at around 2uS/div.

[LyntonP]

Hi Keith I did as you suggested.
Trying the grid first I could not detect any oscillation.
Trying the anode holding the probe close produced no result. Connecting to the anode produced what looked like several identical regular waveforms (much closer to a sine wave) happening at the same time. I played around with the scope settings but could not isolate just one.
Turning the BFO control on the front of the radio (small air spaced variable capacitor) did seem to change the frequency of oscillation but only by a small amount.
I adjusted the transformer adjacent to the valve by a very small amount and again this seemed to vary the frequency of the waveform.
Turning off the BFO with the front panel switch made the waveform disappear.
I found some CW around 7MHz but operation of the BFO made no difference
Further examination of the wiring has I think given me the location of the ‘twisted pair’ capacitor and it looks as if the wires are a little ‘untwisted’. This is in a hard to access place but I will try later today to re-twist the wires and see if this has any effect.
I also discovered a further wax cap that I had missed, again in a very difficult location, under a bank of coils.
Thanks for your help.
The investigation continues.
Lynton

[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. The alternative is to move the injection to an earlier IF stage but this can cause the gain to be reduced by AGC action so amounts to the same thing really.