Sky Baron

[Mike Watterson]

I got a Sky Baron today.
It was initially "dead", but seemed to be dodgy pot/switch and also the LW switch. A few operations and it was OK. The crackles in tuning knob went with a good wiggle and the volume got a small dose of switch cleaner.
Seems very sensitive on MW, not so great on LW (no doubt because there is no LW coil, the MW coil is padded with 1250 pF). It had already been tweaked to 198KHz rather than 200KHz, though you'll still get R4 if it hadn't.

If it was even a battery valve set I'd be replacing the paper caps, but with only 9V (about 6V EOL) and low impedance transistors it's probably not an issue?

Bright was missing off centre of tuning knob. Cheapskates, no slow motion drive. I replaced it with one off a broken knob. Someone fitted a 1/4" stereo jack socket, but used the tip and ring. I suppose works for mono and out-of-phase both ears for a stereo headset, which oddly does sound better than in phase on headphones. Strictly you need a Stereo to Binaural processor to listen to stereo properly on headphones.

The R&TVS instructions are wrong re: C9 LW LO trimmer. Easiest way is to remove tuning knob and tuning scale and there is an access hole. It's not adjustable from the rear, though the IFTs are. Removal is via 3 x brass nuts, not the screws on the PCB. It must have had a TERRIFIC bash or been assembled bad as the Ferrite slab was resting on a wood barrier and stressing PCB. Tracks wrinkled but not actually cracked. I took out the two PCB screws and fixed that and then added solder on the tracks just in case.

I should check no-signal is 10.5mA. Leaky electrolyic caps are possible. As are dried out ones. Anyone any experience of this model?
Electrolytics C21 (+9V), C22 (driver emitter decouple), C11 (AGC decouple @ IF1, 8uF!), C20 (volume coupling to driver base).
Probably six paper caps. C4, C6, C14, C18, C19, C24, maybe C23?. Perhaps only leakage in C4 might be an issue, but it would need to be really bad, 39K from secondary HT and 10K in parallel. So likely any dodgy electrolytic is more an issue.

Someone swapped the original two pin PP10 plug for a pair of PP9 snaps. I've made a PP10 box and think I can fit 6 x Alkaline D cells in it. Looks about x3 capacity of a PP9.

Anyone got a spare PP10 plug or the dimensions?

[Mike Watterson]

Plug for PP10
Research suggests it's the same as the common battery valve radio 7.5V plug. So anyone got a spare one?
The socket is the same dimensions but without the centre "pilot" hole.

So probably:
13.5mm = 0.531" centre spacing
-Ve 1/8" dia (3.2mm approx)
+Ve 5/32" dia (a smidgen under 4mm)

I have a 7.5V socket and it matches a scan of PP10 that seems to be correct size as the made up box fits nicely between the three guides and under the speaker. It seems very close to three PP9 in length and similar cross-section, so I wonder was it 3 off PP9 innards in parallel, or layer biscuits about three times thicker? In any case I think D cells diagonally size by side fit, and three in line giving six. Capacity of six Alkaline D cells is likely similar to a PP10 as that's about three times PP9 capacity at 10mA quiescent, better at loud volume.

I can't check capacitors or quiescent current right now as I've resprayed the top after cleaning and filled hole drilled for the 1/4" stereo jack socket. There is a big overlap of the 'Vinylair' fabric at the volume pot, so I cut off a square there to fill the smaller hole at the side. The inside was drilled larger so the socket could be inset and then chiselled to accommodate two of the socket connections. Some Sky Baron models had an external aerial socket, but I've never seen that, more likely a pair of sockets at the rear rather than the car aerial type socket as it's a table model.

[Mike Watterson]

I found a phono socket mounded on a round paxolin disk, a 1/8" pin on a small paxolin disk and an old 4mm banana plug part with no cover. So I've assembled them to make a plug that probably fits a PP10 and looks like the photo in the Ever Ready Operating Instructions for the Sky Baron. I've mounted the socket that certainly came out of a 7.5V pack into the top of the PP10 box with two wires for the custom 6 x D cell holder yet to be assembled. I'll put Velcro patches to close the bottom of the PP10 pack.

I know that in theory it's possible to make the LW tunable and that often people adapted 9V sets to use PP3 instead of PP7 or PP9 instead of PP10. Also add BT adaptors, earphone sockets etc. I've no shortage of "better" radio sets. So I think it's best to keep the original design and restore / undo what might have been reasonable changes in late 1960s to early 1980s when it was the only LW/MW table model someone had.

I'd adapt old test gear that I'm going to use. I can't see the point of adapting or updating a collectable item. Not even as a BT speaker / WiFi etc. You can buy decent 6" speakers, amps and make or buy decent wooden baffle cabinets. Which I must do with a little MP3/BT/FM radio I got for €12 that's a bare chassis. It works as handsfree and uses USB stick or SD card. Possibly can record microphone and/or radio. I'll maybe make a "boombox" style cabinet with a 12V gel cell.

[Mike Watterson]

Found two bits of wood to make a 4 piece frame inside the PP10 box to take 6 x "D" Cells.
Quite easy to fill with pairs of cells as you slide frame into the outer box.
I used pop rivets for the two + connections and same pop rivets to fasten the two battery springs saved from the top of a dead 996 6V lantern battery. Trapped the +, - and mid point jumper wires under the rivets in the two end pieces of wood. Frame slides in from the bottom. Should give similar or better run time to the PP10.
I've removed the 3 pole 1/4" jack that was wired tip & ring only and filled hole. The piece of fabric from inside case at volume control plate almost invisibly mends the hole.

Next is to see if leakage is an issue on any caps or if electrolytics have dried out. The very long switch on delay is puzzling.

[Mike Watterson]

Anyone any idea why the AGC capacitor on 1st IF is 8uF rather than the usual 0.1uF?
I'm presuming this is why there is almost 8 sec delay on turning on.
Either there was an AGC loop stability issue or they wanted to simulate a mains valve radio? Except Battery valve sets are pretty instant compared to parallel mains. It's the series filament mains sets that are slowest.

[pwdrive]

8uF sounds about right to me given the filter resistors value.

https://www.doctsf.com/documents/schemat.../TR82B.PDF

Lawrence.

[Mike Watterson]

Of course you can't have 1 to 1M Ohm from diode to grid (base).
8.2 K & 8uF doesn't give 8 second delay? Closer to a 1/10th of a second, which isn't unreasonable for AGC.

I can't see anything to give 8 seconds power on delay.

[Mike Watterson]

Schematic from R&TVS
   

[Mike Watterson]

The quiescent current is supposed to be 10mA. It started at 8.5mA and very gradually rose to about 13mA.
So I examined the caps.
2 x 50uF were only rated 6V. Leaking about 2mA and almost no capacitance. Replaced with 47uF
I forget what the 8uF were. Not good. Replaced with 10uF that measured 10.1uF. The original were +80 / -20 % tolerance.
There were 4x 0.1uF paper caps. Even at only 8V the leakage varied intermittently from 20uA to 50uA approx. Replaced with ceramic.
There were 2x 0.01uF (10nF) black crumbling Hunts. One is emitter resistor RF decoupling on the mixer/osc VT1. The other is IF decoupling junction of D1 and R12 volume. I probably could have left them, but I replaced them with small ceramic discs.
Oddly the PCB didn't seem designed for the fitted paper caps, the ceramic ones fitted perfectly. Those paper caps were obsolete rubbish a few years earlier than the radio production, but UK makers continued to use the obsolete UK paper caps. Partly because ceramic and polyester were more expensive and also tariffs on imported components. Even Eddystones of then and later used a lot of paper dielectric caps though later ones had a higher quality encapsulation than the crumbly black & brown Hunts and the card + wax cases.

The quiescent current is now 10mA and the set comes on instantly. At least unlike a valve set (battery or mains) the rubbish caps weren't going to do harm. Performance & volume is slightly better. I made a note of the connections of volume control/power and the two sets of transformer wires and disconnected them all from PCB for ease of work after the first two caps.

Photos soon.

[Mike Watterson]

Frankly I was surprised that 10 capacitors needed replaced in a transistor set.