Recreating the GEC 8161 Mirror Lid TV

[Till Eulenspiegel]

Hi Brian,
I've checked through all the previous posts to find out if I introduced my notion that the BT8161 and BT9121 receivers had in fact employed similar coils and transformer in the IF amplifiers.
In my sketch of the response curves of the IFs it can be seen that the designers might have got away with using the same vision IF passband characteristic in both receivers. The BT8161 vision IF is 4Mhz and operates on the upper sideband with a very wide "trough" between the vision and sound IFs. No need for sound rejection traps needed here.
In the BT9121 the vision IF is 6Mhz and therefore it is the lower sideband that is used. The gap between the 2.5Mhz sound IF and the limit of the vision IF passband is much closer compared with the BT8161 but no sound rejector will be needed.

Till Eulenspiegel.

[dtvmcdonald]

I was going to ask about this project this morning ... and what do I find? New posts.

But I'd like to hear how its coming along. Recreating old electronic TVs is not
something we do much in the USA. Recreating old mechanical ones, yes, at least a little.

As to IF frequencies, my Franken-TRK-12 (a Frankenradio or TV is made by combining
parts from different sets) of 1939/1940 uses 8 MHz or so and upper sideband. Thus,
since the US always used upper VSB, the local oscillator is lower in frequency than the
signal. The IF strip is very complicated with stagger tuning, with no alignment instructions!
However, it seems very stable ... mine was absolutely, completely, text-book perfect as-was.
It was, in fact, absolutely as good as the response of my 1954 CTC-100 color TV, which
tried for the very best possible response in the color-carrier region. The TRK-12
does use a full wave demodulator.

[Brianc]

I see what you are getting at, David. In the description of the BT9121, The vision IF is stated to be 6MHz ±2.5MHz giving the LO frequency at 39MHz (as the sound IF is 2.5MHz. With the BT8161, the LO frequency is 41MHz with the IF at 4MHz. The band limits of the 6MHz IF strip are 3.5MHz and 8.5MHz with the BT8161 IF strip’s being 1.5MHz and 6.5MHz. Thus if everything else was the same (valves etc.) then the same IFTs would cater for both as you stated, David. However, both the valves and the IFTs are different!

Firstly, the valves:

Valve	Cg pf	Ca pf	Valve	Cg pf	Ca pf
VMP4G	14	8.7	X41	21.5	7
MSP41	17.2	10	KTZ41	14	10.5
N43	15.5	16.5
D42	4	4	KTW63	5	10

Secondly, the IFT DC resistances:

1

	BT8161 Manual		BT9121 Manual
IFT	Prim.Ω	Sec.Ω	Prim.Ω	Sec.Ω
1	1.17	3.4	n/a	n/a
2	11.9	8.7	3.8	4
3	11.9	8.7	5.3	4
4	13.4	9	5.3	4
5	10	8.1	n/a	n/a
Detector	5.2	15.5+15.5	4	8 + 8

The difference in resistance is quite marked, and this is a bit surprising as, doing some 4/6MHz resonance calcs, with 25pf and formers similar to those used in the BT9122, gives a turns ratio between the same mechanical coil for the 4:6 frequency ratio is 2.1:1. This is about right for the secondaries but not the primaries. I obviously need to do a bit more work bringing in the valve capacities as well!
Thanks for the kick, David
Oh dear! I forgot about the table problems! I'll see if I can find Terry's table maker.
PS The W in the second table were Omegas in the original spreadsheet!!

[Till Eulenspiegel]

Hi Brian,
Your BT8161 has those monster N43 pentodes in the vision IF amplifier stages whereas my BT9121 employs the very high slope (for 1938) KTZ41 RF tetrodes. And because of the high slope, 12mAV, the input capacitance of the KTZ41 will be quite high. This valve will also have a low input resistance at very high frequencies so it comes as a surprise to find the BT9121 has two RF stages using KTZ41s.

Till Eulenspiegel.

[Cathovisor]

Oh dear! I forgot about the table problems! I'll see if I can find Terry's table maker.

It's here, for the next time you do this

[Brianc]

Thanks Cathy.

[Terrykc]

Brian,

Make sure you take note of this post in that thread:

http://www.forum.radios-tv.co.uk/viewtopic.php?f=17&t=5444&p=67772#p110765

and the post immediately after it which links to the 'new and improved' version 2!

[Brianc]

Thanks Terry. I have now downloaded version two.

[dtvmcdonald]

I just wonder .... how is this amazing project coming along? We here in the
"new world" just don't do such things!

[Brianc]

I just wonder .... how is this amazing project coming along? We here in the
"new world" just don't do such things!

I can understand that - you're all too bl**dy smart to get into such a project!

Having said that, I'm still working on it but not every day as I used to. I have made some progress with the IF amp and have got the last IF amp and the detector driver working reasonably well having built another test jig. The first jig was very poorly designed mechanically as it was too difficult to access both the top and bottom of the chassis at the same time, important if there are valves with top-caps!
I'll post some pictures of the jig and some of the results soon.

[Brianc]

As promised, although belatedly, here are a couple of pics of the replacement experimental RF chassis. As you can see, it gives very good access to all components and the coils are easy to install and remove.

Access to top of coils, valve top-caps etc. much easier now.

Coils removed undoing one 6BA nut on thread captive o the chassis.

The chassis carries the complete RF/Video except for the frequency changer, one IF stage (of two identical stages) and the sync separator. It is powered from a bench supply of 4VAC and 275VDC. First trials are promising!

[PYE625]

It is an unfolding miracle...... I simply cannot comprehend the true genius here of this whole re-creation.

Changing a few caps and cleaning a radio chassis is one thing.
This is light-years beyond anything like that.

[Katie Bush]

It is an unfolding miracle...... I simply cannot comprehend the true genius here of this whole re-creation.

Changing a few caps and cleaning a radio chassis is one thing.
This is light-years beyond anything like that.

Seconded!...

[peter scott]

This is recreating GEC R&D lab.

A truly great thread.

Peter

[Till Eulenspiegel]

I'd imagine the 1930s GEC development engineers employed a similar test jig for the original receiver. Dyed in the wool engineers love these projects.
Out of interest when servicing the Ekco T221 it was noticed that the bandpass coils in the channel 1 tuner biscuits have a resistance of 5 ohms, that's a high resistance for coils having only twelve turns. The oscillator coil has a low resistance, <1ohm.
In order to achieve a wide bandwidth with such a low IF, 4Mc/s in the BT8161 and 6Mc/s in the BT9121 it would appear the choice of IF transformer wire is crucial to flattening the response curve.

Till Eulenspiegel.

[Brianc]

Hi Till
If you remember, we talked about the designers using resistance wire to reduce Q for the lower frequency coils nad Steve McVoy of the ETF mentioned high resistance coils in the IFTs of his RCA prototype receiver. The difference between Bl and Blll signal strength at the receiver aerial socket in the early days of ITV must have been a worry to the manufacturers and the eagerness of the RF valves to oscillate at the lower frequencies must also of had a bearing on the design.

[Brianc]

Hi All.
You may have noticed that actual progress on this project has all but stopped with the building of the new experimental chassis. This is because of my on-going health problems. I have Metastatic Prostate Cancer and after Radiation and Chemo treatment , I am now in respite care with a lot less energy and to a certain extent, less concentration.

Due to the circumstances, I have decided that I do not have the necessary application to continue the more theoretical side of the project so I have decided to hand it over, lock stock and 12” radar tubes, to a fellow forum member well known to all of us as an all-round good egg not to mention him being a very good engineer (well, the BBC thinks so). I am, of course, talking about Bader Meinhoff Wheels, AKA Mike Izycky.

Most of the Mechanical and electronic bits have been sorted leaving the RF side for fettling and Mike has very kindly agreed to take the project on – I didn't want the project to be scrapped when I pop my clogs so I thought that this is the best way to go. I cannot thank you forum members enough for the encouragement and praise which kept me going through some of the harder bits and that really helped.

All in all guys, a very absorbing project and one that I wish Mike lots of success with his continuation of the GEC BT8161. He will be picking the kit up from me in the near future and will be taking this thread over with me poking my nose in occasionally.

MANY, MANY THANKS FOR FOLLOWING THE FIRST PART OF THE THREAD AND HERE’S TO PART TWO – OVER TO YOU MIKE

[Jac Janssen]

Hello Brian,

That is bad news!
I most sincerely hope that your health improves and that you get over this!
All the best and all encouragements I can give you from the Netherlands!

Jac

[peter scott]

Like Jac I too want to wish you all the best for a full recovery Brian. I'm sorry that you feel the need to pass the GEC on but I know that you have lots of other wonderful projects that are just waiting like the rest of us for your recovery.

Very best wishes and kind regards,

Peter

[Lloyd]

Hi Brian,

Really sorry to hear of your health, I hope you make a full and speedy recovery! Shame your passing the set on, but its great that it's going to another forum member and we will all get to see what comes next for it, you have done an amazing job on it so far

All the best,
Lloyd.

[marc]

Brian, like everyone else on here I'm very sorry your health problems mean you have to move this wonderful project on but at the same time I can't help thinking that you couldn't have put it into better hands of our very own 'Catho' I'm sure Mike will do you proud.

I wish you all the best and hope your health improves.

Take care,
Marc.

[Katie Bush]

Oh my God, Brian,

I had no idea... Did I miss a page somewhere?

As others have said above, and I will echo now, I offer you my best wishes for the future and a hope that this thing can be appropriately managed.. Here's to praying for as long as you can get.

Marion

[PYE625]

Oh my God, Brian,

I had no idea... Did I miss a page somewhere?

As others have said above, and I will echo now, I offer you my best wishes for the future and a hope that this thing can be appropriately managed.. Here's to praying for as long as you can get.

Marion

My turn to second Marion and all the above too.
Bless ya heart Brian.

[Brianc]

Thanks guys. I seem to be doing alright at the moment with little pain and sleeping reasonably well at night (thanks to Ora-Morph, an opiate swig which is fairly fast acting and providing small doses are taken (5ml) not addictive. I'm up at about 6-30 for a cereal breakfast and a read of the paper and am up and showered by 8.00. I have bought a special reclining chair in which I can rest very comfortably and that helps a lot.
The big thing today is that I have said "Goodbye" to my BT8161 project as Mike Izicky hired a van and drove down to collect the bits. Fortunately, Graham and Jeremy also came round for lunch and they were able to help load the van. I wish Mike all the luck in his endeavors to complete the job as I must admit, it was beginning to weigh somewhat on my mind. Hopefully, Mike will continue this thread and that Steve McVoy can start to update his precis of the project. Cheers everybody and thanks for the over 54,000 hits on the thread.

[Cathovisor]

Well... the Eagle has landed chez Cathovisor In fact two of them, as it came with a BT4542 thrown in!

I did remark that Brian has set the bar monumentally high with the workmanship on this set, and indeed I shall do my best to maintain that standard. For now, the set has gone into one of my storage units, well protected with blankets. The simple reason is not just that of space, but also time as I still work, and I have other projects/sets for other people that require my attention first.

The test chassis is accessible, and I am now looking at suitable sweep generators to tackle the problem of the IF strip. Somewhere is my copy of Langford Smith...

Watch this space, as they say.