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Forum 141

[Closed] Recreating the GEC 8161 Mirror Lid TV

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Anonymous
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An AVO valve tester must be a contender

 
Posted : 02/10/2012 9:42 am
Till Eulenspiegel
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More thoughts about the GEC BT8161 TV and radio receiver. The vision IF in the TV only BT9121 is 6Mc/s, the sound is 2.5Mc/s.
However I am becoming of the opinion the the vision IF in the BT8161 is even lower, as low as 4Mc/s. That's very low indeed but is worth bearing in mind that early Murphy TVs used a very low vision IF, in the V58A it was 4.25Mc/s. The sound was 0.75Mc/s.
I have no information for the front end of the radio, but it is most likely that the IF is about 500Kc/s, a bit higher than than the standard frequency of 465Kc/s.
Operating 405 television on a low carrier frequency was used by Rediffusion on the TDUK2 two channel wired system. One channel was 4.95Mc/s and the other 8.45Mc/s. The system frequency receiver used a full wave vision detector to reduce the posibillity of carrier dot pattern being apparant when the lower frequency was selected.
The GEC BT8161 has a full wave vision detector.

Till Eulenspiegel.

 
Posted : 03/10/2012 11:55 pm
Till Eulenspiegel
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News Flash: Found the circuit diagram of the radio part. The so called A chassis.

Till Eulenspiegel.

 
Posted : 04/10/2012 12:12 am
Refugee
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Would this be a possible temporary chassis while the bits are being collected for a full reproduction.
http://www.vintage-radio.net/forum/show ... hp?t=88718
It would appear to be an AC/DC set that has been converted with an isolating transformer for use with a radio and gram.
They are doing the same as you on a slightly more modern set.

 
Posted : 04/10/2012 12:37 am
crustytv
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Would this be a possible temporary chassis while the bits are being collected for a full reproduction.
http://www.vintage-radio.net/forum/show ... hp?t=88718

That PAM Chassis would be perfect assuming it's 14" and more suited to restoring my empty PAM case here than in this pre-war set. I would certainly give him more than the suggested tenner

Chris

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Posted : 04/10/2012 7:20 am
Brian Cuff
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I see no point in putting anything other than original or reproduction chassis in the cabinet. It will not bring anything positive as the cabinet will be in a room with about 30 other TVs, most of which could be used. In my opinion, it would be a waste of my time and I could damage the cabinet while doing it. - Sorry, refugee, but thanks for the suggestion!

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Posted : 04/10/2012 1:24 pm
Refugee
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It was just another option and the owner was thinking of selling it.
Someone else may well go for it. The information could be useful to others.

 
Posted : 04/10/2012 1:53 pm
Anonymous
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Like Chris :)

 
Posted : 04/10/2012 2:08 pm
Till Eulenspiegel
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Found three model numbers of GEC Fidelity Five radio receivers: BC3650, BC3651and BC3658.
Unlike EMI, I don't think GEC made a radio receiver with TV sound.

Till Eulenspiegel.

 
Posted : 04/10/2012 2:34 pm
Refugee
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Chris does indeed have a good cabinet and we will have to wait and see if he can get it to fit.
It is always worth a look :thumbl:

 
Posted : 04/10/2012 3:39 pm
Brian Cuff
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I was going by the control knob/dial layout which, for the SW5 looks similar to the radio in the 8161 - if it's the radio on the LHS of the screen. Looking now at the circuit, it is very different (RF stage etc.) and I think that the Triode hexode is the frequency changer for both vision and sound. In the TV mode, the AGC is disabled, the oscillator becomes a Colpitts oscillator and the vision IF is tapped off in the anode circuit. I notice also, that there is a variable bandwith/tone control - there is a dotted line ganging a pot in the AF tone control with a pot across the 1st sound IFT and the primary of the 2nd IFT. I would suggest that this latter bit is a mechanical device which varies the mechanical position of the primary winding wrt the secondary. That's going to be tricky!!!

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Posted : 04/10/2012 6:34 pm
Till Eulenspiegel
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In 1937 GEC made the Allwave Super Six radio receiver model BC3860.
This six valve LMS receiver has an RF amplifier stage employing a W63/6K7G vari-mu pentode. In the BT8161 radio the RF amplifier is a VMP4G. The frequency changer in both receivers is an X41, followed by W63, DH63/6Q7G and KT63/6F6G. U50 rectifier.
Info for the BC3860 can be found here: http://www.radiomuseum.org/r/gec_ac_all ... _radi.html
Also: http://www.radiomuseum.org/r/gec_ac_all ... x_bc3.html

Till Eulenspiegel.

 
Posted : 04/10/2012 10:14 pm
Brian Cuff
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Well, I have the cabinet at home at last. It's not in bad condition - no woodworm or large chunks of veneer missing. Badly scratched and the top especially will have to be re-finished some time.
It hasn't been too badly messed about with although there are some witness marks where blocks have been removed. There are 2 mounting blocks fixed to the outside of the two chassis on each side of the tube but no corresponding pairs on the inside of the apertures. The lid is counterbalanced by two pulley systems, attached to the stays, which lift counterweights as the lid is raised. However, the weights are missing.
One mystery is solved - the knob in front of the CRT screen is the magic eye for the radio (Y63 introduced in 1937). There are small threaded angle brackets which support the wooden escutcheons covering the two chassis so they will be easy to make once the layout of the knobs has been determined.
I have been thinking about the variable bandwidth mechanism and, assuming that the radio chassis is mounted vertically, there will have to be some sort of cam system operating either a system of levers or two cams acting directly on the IF transformers - how exciting!!!

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Posted : 06/10/2012 8:41 pm
Brian Cuff
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I found the switching in the A Chassis schematic a trifle confusing so I photoshopped it to remove the switching and set it permanently in the TV mode. For anyone interested, here's the result and it does make it much easier to follow!
Sorry about R39, part of the radio AGC system - I missed it during the process. It will be exterminated :twisted: in a later version.

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Posted : 06/10/2012 9:06 pm
Anonymous
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The Saba is the only Radio I know with mechanically varied IF bandwidth and that is a factory adjustment!

I understand the theory though.

 
Posted : 06/10/2012 9:17 pm
Brian Cuff
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The HMV 800 has a mechanical system for moving one coil in the IF transformers as does the tuner in the Decca Decola. In the 800, the control is linked with a switch which alters feedback components in the audio stages. Both are user controls.

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Posted : 06/10/2012 10:49 pm
Till Eulenspiegel
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Hi Brian,
I don't have a Decola but I do have the Beau Decca television. This set has a radio unit that is similar to the one fitted in the record reproducer of the same name. I'm not sure if it has a mechnical IF bandwidth mechanism. I'll take a look at it later today.
The variable IF bandwidth is a good idea for the BT8161. Max bandwidth is desirable on television to take advantage of the high quality sound that was available from AP and also to accomodate some local oscillator drift
Till Eulenspiegel.

 
Posted : 07/10/2012 2:04 pm
Brian Cuff
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Does the tuner in the BeauDecca have the horizontal drum type dial Till? If so, it's probably the same tuner unit as mine was - it was in a Decola Kensington which I have since sold.

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Posted : 07/10/2012 2:16 pm
Till Eulenspiegel
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Hi Brian,
Here's some pictures of the radio chassis of the Beau Decca television receiver. Pictures 1 and 2 show the selectivity control.
The third picture shows the cams that control the coupling in the first two IF transformers. This chassis has two stages of IF amplication.
Picture four shows the dial stringing arrangement and the positions of IF transformers 1 and 2.
Valve line up in this chassis is: ECH35 FC. EF39 1st IF amp. 6B8G 2nd IF amp. EB34 detector and AVC.
EM34 tuning eye.

Till Eulenspiegel.

 
Posted : 07/10/2012 10:38 pm
Till Eulenspiegel
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Hi Trevor,
It was generally up-market sets that had the variable selectivity facility although I can't think of any EMI models that had it.
I've attempted to take a close up picture of the cam mechanism but those tag strips which I think are not original get in the way. The dark object just to the right of the control shaft is the cam operated plunger to give it a name.
My Beau Decca television receiver was found with all the TV parts missing. Someone had converted it into a radiogram. The audio amplifier and power supply unit, radio receiver and the gram preamplifier were still in the cabinet.
The television parts came from a 12" Defiant console model of 1948 vintage. I've still got that set.

Till Eulenspiegel.

 
Posted : 07/10/2012 11:21 pm
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