Sky chief USA radio - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

newlite4 · 12th Jan 2022, 6:10 pm

I have two of the later Sky Chief radios dating from 1941, they are both rebadged "Paramount" (others were badged as "Mayfair"). These sets use a "Canadohm" dropper under the chassis rather than a barretter. The front end valve is a 12K7 as opposed to a 12SK7 however the layout is otherwise identical. I have measured the volume control in situ (from the antenna coil to the 12K7 cathode) and am measuring 2Kohm.
Neil

Goldieoldie · 12th Jan 2022, 6:14 pm

Thanks Neil I’ll look into that
Do you have the circuits please ?

newlite4 · 12th Jan 2022, 9:05 pm

No circuits available for this one, not even Nostalgia Air. It is a more or less generic American TRF midget, the hand traced circuit from the other thread is suitable for both versions.
Neil

PJL · 12th Jan 2022, 11:03 pm

Have you checked the heater voltage as the 49A is definitely 49V at 300mA and that includes the dial lamp.

The TRF I have here that uses a 12K7GT/12J7GT setup uses 35K with a fixed 200 ohm resistor added in the cathode path.

Goldieoldie · 13th Jan 2022, 12:27 am

Quote:

Originally Posted by newlite4

I have two of the later Sky Chief radios dating from 1941, they are both rebadged "Paramount" (others were badged as "Mayfair"). These sets use a "Canadohm" dropper under the chassis rather than a barretter. The front end valve is a 12K7 as opposed to a 12SK7 however the layout is otherwise identical. I have measured the volume control in situ (from the antenna coil to the 12K7 cathode) and am measuring 2Kohm.
Neil

Is the pot log or lin Neil ?

Goldieoldie · 13th Jan 2022, 12:02 pm

Quote:

Originally Posted by newlite4

I have two of the later Sky Chief radios dating from 1941, they are both rebadged "Paramount" (others were badged as "Mayfair"). These sets use a "Canadohm" dropper under the chassis rather than a barretter. The front end valve is a 12K7 as opposed to a 12SK7 however the layout is otherwise identical. I have measured the volume control in situ (from the antenna coil to the 12K7 cathode) and am measuring 2Kohm.
Neil

The reading of 2 k,was that with the pot at min because the wiper goes to chassis so will effect the reading if taken with the pot at max

newlite4 · 13th Jan 2022, 12:21 pm

The 2K reading was across the pot ie the resistance of the track. A volume pot should be log but lin would still work if a switched log pot is unavailable.
N

Goldieoldie · 13th Jan 2022, 4:10 pm

Yes but unless you disconnect the wiper or the aerial coil the reading would be dependant on where the wiper is set

G6Tanuki · 13th Jan 2022, 5:57 pm

The 'traditional' volume-control pots [which live at the input of the AF amplification stages] are logarithmic-law, but in this case the pot is acting as a combined aerial-attenuator and RF-stage cathode-bias control, so it's a bit different....

Some communications-receivers I have come across use an anti-log variable resistor as their RF/IF cathode-bias controls; others a simple linear-law resistor. Yet others use a dual control with 2-tracks but one spindle - one track for the RF stages the other for the IF stages!

In your case I think the manufacturer would probably have gone for whatever was the cheapest option, even if this might have led to an apparently-strange relationship between the control-knob's rotation and perceived volume.

Goldieoldie · 14th Jan 2022, 1:32 pm

I’ve tried a 2k pot between the aerial coil and cathode with the wiper to chassis .
It works but with very little gain .
I’ve found the best result is obtained when I connect a 150 ohm resistor between cathode and chassis and the aerial coil connection disconnected .
I don’t really understand the circuit .Is the connection between the aerial coil and the cathode meant to produce extra gain by feedback ( regeneration ? )

G6Tanuki · 14th Jan 2022, 4:01 pm

The potentiometer is wired so that at one end of travel the resistance between antenna-and-earth is maximised and the resistance in the cathode of the RF amp is minimised; this gives the highest gain.

At the other end of travel the antenna is largely grounded and he resistance in the cathode line is highest; this corresponds to minimum gain.

Considering that these radios were made for the US market, where [at least in urban areas] there were a lot more stations than in the UK, it's quite a cunning way to provide gain-control for a TRF and avoiding the poor thing being swamped by that 50-Kilowatt transmitter on top of the Empire State Building a couple of miles away.

[TRF receivers generally have poor selectivity and swamping/cross-modulation was a big problem].

12th Jan 2022, 6:10 pm	#41
newlite4 Octode Join Date: Nov 2003 Location: Bath, Somerset, UK. Posts: 1,806	Re: Sky chief USA radio I have two of the later Sky Chief radios dating from 1941, they are both rebadged "Paramount" (others were badged as "Mayfair"). These sets use a "Canadohm" dropper under the chassis rather than a barretter. The front end valve is a 12K7 as opposed to a 12SK7 however the layout is otherwise identical. I have measured the volume control in situ (from the antenna coil to the 12K7 cathode) and am measuring 2Kohm. Neil __________________ preserving the recent past, for the distant future.

12th Jan 2022, 6:14 pm	#42
Goldieoldie Octode Join Date: Jul 2014 Location: Petersfield, Hampshire, UK. Posts: 1,043	Re: Sky chief USA radio Thanks Neil I’ll look into that Do you have the circuits please ?

12th Jan 2022, 9:05 pm	#43
newlite4 Octode Join Date: Nov 2003 Location: Bath, Somerset, UK. Posts: 1,806	Re: Sky chief USA radio No circuits available for this one, not even Nostalgia Air. It is a more or less generic American TRF midget, the hand traced circuit from the other thread is suitable for both versions. Neil __________________ preserving the recent past, for the distant future.

12th Jan 2022, 11:03 pm	#44
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Sky chief USA radio Have you checked the heater voltage as the 49A is definitely 49V at 300mA and that includes the dial lamp. The TRF I have here that uses a 12K7GT/12J7GT setup uses 35K with a fixed 200 ohm resistor added in the cathode path.

13th Jan 2022, 12:21 pm	#47
newlite4 Octode Join Date: Nov 2003 Location: Bath, Somerset, UK. Posts: 1,806	Re: Sky chief USA radio The 2K reading was across the pot ie the resistance of the track. A volume pot should be log but lin would still work if a switched log pot is unavailable. N __________________ preserving the recent past, for the distant future.

13th Jan 2022, 4:10 pm	#48
Goldieoldie Octode Join Date: Jul 2014 Location: Petersfield, Hampshire, UK. Posts: 1,043	Re: Sky chief USA radio Yes but unless you disconnect the wiper or the aerial coil the reading would be dependant on where the wiper is set

13th Jan 2022, 5:57 pm	#49
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Sky chief USA radio The 'traditional' volume-control pots [which live at the input of the AF amplification stages] are logarithmic-law, but in this case the pot is acting as a combined aerial-attenuator and RF-stage cathode-bias control, so it's a bit different.... Some communications-receivers I have come across use an anti-log variable resistor as their RF/IF cathode-bias controls; others a simple linear-law resistor. Yet others use a dual control with 2-tracks but one spindle - one track for the RF stages the other for the IF stages! In your case I think the manufacturer would probably have gone for whatever was the cheapest option, even if this might have led to an apparently-strange relationship between the control-knob's rotation and perceived volume. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.

14th Jan 2022, 1:32 pm	#50
Goldieoldie Octode Join Date: Jul 2014 Location: Petersfield, Hampshire, UK. Posts: 1,043	Re: Sky chief USA radio I’ve tried a 2k pot between the aerial coil and cathode with the wiper to chassis . It works but with very little gain . I’ve found the best result is obtained when I connect a 150 ohm resistor between cathode and chassis and the aerial coil connection disconnected . I don’t really understand the circuit .Is the connection between the aerial coil and the cathode meant to produce extra gain by feedback ( regeneration ? )

14th Jan 2022, 4:01 pm	#51
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Sky chief USA radio The potentiometer is wired so that at one end of travel the resistance between antenna-and-earth is maximised and the resistance in the cathode of the RF amp is minimised; this gives the highest gain. At the other end of travel the antenna is largely grounded and he resistance in the cathode line is highest; this corresponds to minimum gain. Considering that these radios were made for the US market, where [at least in urban areas] there were a lot more stations than in the UK, it's quite a cunning way to provide gain-control for a TRF and avoiding the poor thing being swamped by that 50-Kilowatt transmitter on top of the Empire State Building a couple of miles away. [TRF receivers generally have poor selectivity and swamping/cross-modulation was a big problem]. __________________ I'm the Operator of my Pocket Calculator. -Kraftwerk.