Bestone model 20? - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

egerton · 14th Jul 2018, 8:11 am

Quote:

Originally Posted by ms660

If an HT choke is fitted there would normally be another capacitor (usually an electrolytic) connected between the cathode of the rectifier and chassis so as to make a CLC (Pi) filter arrangement, eg: Reservoir capacitor...choke inductor...filter capacitor (aka smoothing capacitor) the filter capacitor can be a larger value than the reservoir capacitor.

So far as I can make out the 650 ohm resistor is the cathode bias resistor for the 25L6, it's bypassed by an electrolytic (Micamold ?) The 500 kohm resistor is the detector valve's anode load and the 2 megohm resistor is the detector valve's screen grid feed, the 470 kohm resistor is probably the 25L6's grid resistor, the detector looks to be of the anode bend type, can't quite make out the value of it's cathode resistor (can't see the colour dot) but it might be 25 kohm, it appears to be bypassed by the 20uF (if that's what it is) capacitor.

Most single ended output transformers would work as a replacement for the one that's open circuit, typical impedance (not DC resistance) being 5 kohm as presented to the anode.

Lawrence.

DOH! yes of course the 'new' RS cap. is after the choke, in fact it's all falling into place and making sense now. Thanks for the info. on the resistors too and putting me on the right track.

egerton · 14th Jul 2018, 8:15 am

Quote:

Originally Posted by Boater Sam

red body, black tip, green spot = 1M ohm

The 20MF cap is an oddity, looks not to be an electrolytic and too small as you say, what value does it read if you measure it? Where in the circuit is it?

Lamps in series with the filaments? The valves are 0.3 A heaters. What does the chain add up to?

Thanks. Will have a go at measuring on my new DMM with a cap. range but doubt it's very accurate and so far doesn't seem very repeatable measurements....

PJL · 14th Jul 2018, 6:31 pm

There is a circuit for the Detrola 162 bottom right that has a choke and field coil on HT. Amongst other changes, Bestone would have replaced the coils for LW reception. http://www.nostalgiaair.org/Resource...=\M0003900.pdf

egerton · 15th Jul 2018, 6:49 pm

Many thanks. Actually the circuit of mine is very similar to the Emerson BA201 you provided the link for me in your earlier post, but mine has the speaker field coil across the H.T and is higher impedance. I have drawn out the circuit and attached it here. There could be a few errors so would value your opinion - does it look right? Surprised to see the volume control on the RF stage. How does the detector work, is there some background reading you know of? The component count is even lower than the Emerson and this looks a very simple set.
Paul

Boater Sam · 15th Jul 2018, 7:08 pm

I stand corrected, finger and proof reading errors. 2M ohm of course. Thank you Chris.

Has the "20M.F. 50V" cap got any + or - markings on the ends? It looks to have a line on one end which would normally mark the outer foil if indeed it were a foil/paper cap.
But its far too small for a paper cap of that capacity. Can you measure its remaining value?

egerton · 15th Jul 2018, 8:17 pm

Hi! I tried to measure it and it seems to have a high resistance value around 20M ohms. Strange because it's in parallel with the cap. and resistor already in the cathode. It does have a splash of red inside one end. The artwork on the body has what looks like a 'lightning' flash symbol so I wondered if it was some sort of surge protector but that can't be the place in the circuit for that sort of thing. All a bit mysterious.

ms660 · 15th Jul 2018, 8:31 pm

A brief explanation...The detector (6C6) works on the anode bend principle, the grid is biased near to cut off in the region of the bottom bend of valves Vg/Ia curve so that the upper half of the RF signal and the modulation it contains is amplified, the amplified signal is developed across the anode load resistor, the anode is decoupled with a small value capacitor this has the effect of removing the RF, that just leaves the audio signal which is what is required.

The bulk of the lower half of the RF signal and the modulation it contains gets cut off due to the bottom bend of the curve, however, a small proportion of the lower half will get through because that small portion will be between the valves cut off point and the valves operating point that's set by the bias, the bias voltage is the voltage developed across cathode resistor, the reason the valve isn't biased to cut off is to ensure that weak signals can be detected with less distortion as would occur if the valve was biased to cut off.

Here's a link with a bit more detail:

https://www.radiomuseum.org/forum/sc...standard2.html

The volume control in the cathode of the RF amp is not unusual for a receiver of this type, the RF amp is a 6D6 which is a vari mu valve, that in effect means that the mutual conductance of the valve can be easily varied over a wide range by altering the grid bias voltage and in this receiver that voltage is developed across the cathode resistor (the volume control) it has the benefit of being easily able to reduce very strong signals that might otherwise overload the detector and a non vari mu RF amp. The greatest attenuation of the signal happens when the shunting effect of the volume control takes effect across the antenna input coil.

There appears to be an error with the RF amps screen/suppressor grid connections in the schematic, giving it a quick glance the rest of the schematic looks plausible.

Lawrence.

egerton · 15th Jul 2018, 11:43 pm

Well spotted with the grid connection errors, I have corrected that and have also added the band change switch which seems to just switch in / out an additional coil. I have uploaded to one drive and shared a link which I can update here:

https://1drv.ms/u/s!AlDn_84okuoJr0yDZUOIlbWixKhu

I have some good news; the o/p transformer is not open circuit, there was a dirty conductor and when
scrapped clean it measures around 1.5K on primary and crackles can be heard from the speaker when a few volts are applied to primary - even without the field coil in use, so I should be good to with it

ms660 · 16th Jul 2018, 9:56 am

Are you sure you've got the wave change switch and coils correct in your schematic ? I would have expected different, a common trick for the tuned windings was to have two windings in series for LW and short one of them out for MW.

Lawrence.

PJL · 16th Jul 2018, 1:59 pm

There is no decoupling to the 6D6 cathode which I guess is not an issue as the cathode is grounded at maximum gain anyway.

C9, the 6C6 screen decoupling, is odd as I would not expect it to be wired to the the 6D6 cathode? It would seem to inject RF into the 6D6 screen that bypasses the tuned circuit and would do odd things when the volume control is twiddled with.

egerton · 16th Jul 2018, 3:00 pm

Quote:

Originally Posted by ms660

Are you sure you've got the wave change switch and coils correct in your schematic ? I would have expected different, a common trick for the tuned windings was to have two windings in series for LW and short one of them out for MW.

Thanks! I think that's it, I have corrected the schematic. Looking forward I need to find a way to get 120V into the set, first step, I plan on testing the rectifier & heater chain in isolation and checking the HT. I had thought I could use an external dropper as a temp. measure but estimating the current as around 400mA ( 60mA field coil + 300mA heaters + 50mA HT?) gives around 50 Watts for a dropper which is a bit large isn't it?

Boater Sam · 16th Jul 2018, 3:11 pm

Series 60 or 100w light bulb?

So where in the circuit is the mystery 20 MF cap?

ms660 · 16th Jul 2018, 3:11 pm

Re: The tuning coils....It still looks wrong on your schematic, the shorting out of one of the two windings in series trick was done like it is with the tuning coils in this link:

https://vintage-radio.net/forum/atta...0&d=1144184530

Double check all the windings and their connections.

Lawrence.

egerton · 16th Jul 2018, 4:00 pm

Quote:

Originally Posted by PJL

There is no decoupling to the 6D6 cathode which I guess is not an issue as the cathode is grounded at maximum gain anyway.

C9, the 6C6 screen decoupling, is odd as I would not expect it to be wired to the the 6D6 cathode? It would seem to inject RF into the 6D6 screen that bypasses the tuned circuit and would do odd things when the volume control is twiddled with.

Your expert eye spotted that one for me, many thanks! The 6C6 is bridged cathode to screen but the 6D6 is not I drew it wrong I think a copy and paste of the valve symbol when I first drew it. whoops. I will correct the schematic online. Paul

ms660 · 16th Jul 2018, 4:45 pm

Yes you need to correct the schematic, you have pin 4 as the screen grid and pin 3 as the suppressor grid, it should be the other way round, you've shown the screen grid of the RF amp with no HT feed to it and the suppressor grid connected to HT which seems wrong to me.... double check the reverse engineering.

Lawrence.

egerton · 16th Jul 2018, 6:54 pm

Quote:

Originally Posted by ms660

Double check all the windings and their connections.

Yes it does look like it is connected to the tuning cap. like that so I need to redraw yet again. I don't have the second switch gang though, so there is no band switching on the coil feeding the detector stage. Many thanks. Paul

ms660 · 16th Jul 2018, 7:32 pm

Still looks wrong unless it's a circuit configuration I've never come across before, no screen grid HT feeds to either valve and HT feed to suppressor grids, one direct and the other via a 2 meg resistor....?

Lawrence.

egerton · 16th Jul 2018, 8:06 pm

Quote:

Originally Posted by Boater Sam

Series 60 or 100w light bulb?

So where in the circuit is the mystery 20 MF cap?

Good idea as I have a lamp limiter so perhaps I can try a 60W bulb.
The mystery component is in the cathode of the 6C6 across C7 & R5 makes no sense to me and measure as high resistance 20 M so not sure what the hell it is...

Quote:

Originally Posted by ms660

Still looks wrong unless it's a circuit configuration I've never come across before, no screen grid HT feeds to either valve and HT feed to suppressor grids, one direct and the other via a 2 meg resistor....?

I have redrawn it since.

ms660 · 16th Jul 2018, 8:12 pm

Still looks wrong to me.

Lawrence.

egerton · 16th Jul 2018, 11:56 pm

I hope to get there in the end. Can't see the wood for the trees. I had another go. The 6D6 RF stage screen goes to HT, sup. grid coupled by C9 to 6C6 detector screen and pulled up to HT by 2M resistor. 6C6 Sup. grid tied to cathode. Definitely is connected like that and think I've drawn that now.

What is the purpose of C9? coupling what signal? I can understand the anode bend detector but not C9 function and it is connected like that I have gone over it many times now. I suppose it could have been got at...

14th Jul 2018, 6:31 pm	#43
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Bestone model 20? There is a circuit for the Detrola 162 bottom right that has a choke and field coil on HT. Amongst other changes, Bestone would have replaced the coils for LW reception. http://www.nostalgiaair.org/Resource...=\M0003900.pdf

15th Jul 2018, 6:49 pm	#44
egerton Pentode Join Date: May 2018 Location: Beverley, East Riding Posts: 110	Re: Bestone model 20? Many thanks. Actually the circuit of mine is very similar to the Emerson BA201 you provided the link for me in your earlier post, but mine has the speaker field coil across the H.T and is higher impedance. I have drawn out the circuit and attached it here. There could be a few errors so would value your opinion - does it look right? Surprised to see the volume control on the RF stage. How does the detector work, is there some background reading you know of? The component count is even lower than the Emerson and this looks a very simple set. Paul Attached Thumbnails

15th Jul 2018, 7:08 pm	#45
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bestone model 20? I stand corrected, finger and proof reading errors. 2M ohm of course. Thank you Chris. Has the "20M.F. 50V" cap got any + or - markings on the ends? It looks to have a line on one end which would normally mark the outer foil if indeed it were a foil/paper cap. But its far too small for a paper cap of that capacity. Can you measure its remaining value?

15th Jul 2018, 8:17 pm	#46
egerton Pentode Join Date: May 2018 Location: Beverley, East Riding Posts: 110	Re: Bestone model 20? Hi! I tried to measure it and it seems to have a high resistance value around 20M ohms. Strange because it's in parallel with the cap. and resistor already in the cathode. It does have a splash of red inside one end. The artwork on the body has what looks like a 'lightning' flash symbol so I wondered if it was some sort of surge protector but that can't be the place in the circuit for that sort of thing. All a bit mysterious.

15th Jul 2018, 8:31 pm	#47
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bestone model 20? A brief explanation...The detector (6C6) works on the anode bend principle, the grid is biased near to cut off in the region of the bottom bend of valves Vg/Ia curve so that the upper half of the RF signal and the modulation it contains is amplified, the amplified signal is developed across the anode load resistor, the anode is decoupled with a small value capacitor this has the effect of removing the RF, that just leaves the audio signal which is what is required. The bulk of the lower half of the RF signal and the modulation it contains gets cut off due to the bottom bend of the curve, however, a small proportion of the lower half will get through because that small portion will be between the valves cut off point and the valves operating point that's set by the bias, the bias voltage is the voltage developed across cathode resistor, the reason the valve isn't biased to cut off is to ensure that weak signals can be detected with less distortion as would occur if the valve was biased to cut off. Here's a link with a bit more detail: https://www.radiomuseum.org/forum/sc...standard2.html The volume control in the cathode of the RF amp is not unusual for a receiver of this type, the RF amp is a 6D6 which is a vari mu valve, that in effect means that the mutual conductance of the valve can be easily varied over a wide range by altering the grid bias voltage and in this receiver that voltage is developed across the cathode resistor (the volume control) it has the benefit of being easily able to reduce very strong signals that might otherwise overload the detector and a non vari mu RF amp. The greatest attenuation of the signal happens when the shunting effect of the volume control takes effect across the antenna input coil. There appears to be an error with the RF amps screen/suppressor grid connections in the schematic, giving it a quick glance the rest of the schematic looks plausible. Lawrence.

15th Jul 2018, 11:43 pm	#48
egerton Pentode Join Date: May 2018 Location: Beverley, East Riding Posts: 110	Re: Bestone model 20? Well spotted with the grid connection errors, I have corrected that and have also added the band change switch which seems to just switch in / out an additional coil. I have uploaded to one drive and shared a link which I can update here: https://1drv.ms/u/s!AlDn_84okuoJr0yDZUOIlbWixKhu I have some good news; the o/p transformer is not open circuit, there was a dirty conductor and when scrapped clean it measures around 1.5K on primary and crackles can be heard from the speaker when a few volts are applied to primary - even without the field coil in use, so I should be good to with it

16th Jul 2018, 9:56 am	#49
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bestone model 20? Are you sure you've got the wave change switch and coils correct in your schematic ? I would have expected different, a common trick for the tuned windings was to have two windings in series for LW and short one of them out for MW. Lawrence.

16th Jul 2018, 1:59 pm	#50
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Bestone model 20? There is no decoupling to the 6D6 cathode which I guess is not an issue as the cathode is grounded at maximum gain anyway. C9, the 6C6 screen decoupling, is odd as I would not expect it to be wired to the the 6D6 cathode? It would seem to inject RF into the 6D6 screen that bypasses the tuned circuit and would do odd things when the volume control is twiddled with.

16th Jul 2018, 3:11 pm	#52
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Bestone model 20? Series 60 or 100w light bulb? So where in the circuit is the mystery 20 MF cap?

16th Jul 2018, 3:11 pm	#53
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bestone model 20? Re: The tuning coils....It still looks wrong on your schematic, the shorting out of one of the two windings in series trick was done like it is with the tuning coils in this link: https://vintage-radio.net/forum/atta...0&d=1144184530 Double check all the windings and their connections. Lawrence. Last edited by ms660; 16th Jul 2018 at 3:18 pm.

16th Jul 2018, 4:45 pm	#55
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bestone model 20? Yes you need to correct the schematic, you have pin 4 as the screen grid and pin 3 as the suppressor grid, it should be the other way round, you've shown the screen grid of the RF amp with no HT feed to it and the suppressor grid connected to HT which seems wrong to me.... double check the reverse engineering. Lawrence.

16th Jul 2018, 7:32 pm	#57
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bestone model 20? Still looks wrong unless it's a circuit configuration I've never come across before, no screen grid HT feeds to either valve and HT feed to suppressor grids, one direct and the other via a 2 meg resistor....? Lawrence.

16th Jul 2018, 8:12 pm	#59
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Bestone model 20? Still looks wrong to me. Lawrence.

16th Jul 2018, 11:56 pm	#60
egerton Pentode Join Date: May 2018 Location: Beverley, East Riding Posts: 110	Re: Bestone model 20? I hope to get there in the end. Can't see the wood for the trees. I had another go. The 6D6 RF stage screen goes to HT, sup. grid coupled by C9 to 6C6 detector screen and pulled up to HT by 2M resistor. 6C6 Sup. grid tied to cathode. Definitely is connected like that and think I've drawn that now. What is the purpose of C9? coupling what signal? I can understand the anode bend detector but not C9 function and it is connected like that I have gone over it many times now. I suppose it could have been got at...