I take me metaphorical hat off to you yet again..
And I bow low to you, Marion.
I know that this is supposed to be a replica but maybe it shouldn't be called that. I am very near to starting the wiring and, in the absence of the main power Xfmr, I have decided to start with the timebases which are fairly simple and use standard components (apart from the O/P transformers). Not having an original under-chassis picture, I have decided to build as much as possible on tagboards as being the best and neatest way to progress. I will document the layout for an extra page(s) in the manual and to get as logical a tagboard layout as possible. The pictures show some of the tagboards that I will use for the timebase circuitry together with the proposed layout - with the vertical tagboards laid out flat for clarity. What do you think? Should I take this route or will it be too drastic a change from the original (possibly)? Your comments will be appreciated
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One very odd thing about the set is the range of resistor values. Typically there are 125k, 99k, 55k, 990R, 440k, 400k and several others, some of which I have never met before!
... One very odd thing about the set is the range of resistor values. Typically there are 125k, 99k, 55k, 990R, 440k, 400k and several others, some of which I have never met before!
But what tolerance are they?
Modern 120k, 100k, 1k, 470k (or 430k if you prefer something closer) and 390k will be guaranteed to be more accurate than the originals of these precise values ever were ...!
I was always fascinated by circuits for fairly simple non critical designs in Practical Wireless and the like which always specified 5k1 rather than the normal 4k7.
The 5k figure almost certainly came from a rough back-of-a-fag-packet calculation and no doubt the designer of these circuits had a full range of high precision components at their fingertips, courtesy of the UK's electronic defence industry ...
We only ever stocked the E12 series* but, from 1Ω to 22M, that's 86 values! I saw no point in doubling up just to suit this practice but I did get rather fed up with explaining this to each new wave of schoolboy enthusiasts! Why, oh why couldn't they have specified 4k7 ...?
* Two exceptions: 43k used by Bush in the TV10x, TV11x and TV12x line timebase and, of course, 75Ω.
I take your point about tolerances and probably they were mostly 20%ers. I will use the nearest E12 values which either both higher or both lower in areas when the ratio of the components have an effect on gain.
The E24 series (4k3, 5k1 etc.) was used by the BBC in their in-house products and almost always Erie 2% HiStab which always struck me as a bit wasteful but, of course, it meant that only one range had to be bought/stored.
Ampex often used odd precision values in their designs and I remember one example where a 1% component was used in series with a variable resistor. While on the training course for this particular device (the VR3000, a 2" quad portable VTR), I asked the obvious question - "Why, when pot tracks are often 10% or worse?". The answer was that during the initial setup of the circuit, the pot was set to its minimum value so that the levels could be measured with a known value in circuit. If it measured correctly, the circuit was working properly.
One company I had dealing with (Snell and Wilcox) had an in-house value range of passive components and no design could use any outside this range. These SMD components were so cheap that should a non-stocked value be required, then two stocked values were used in series/parallel. This also saved set-up time for the pick and place machines and extreme miniaturisation was not an issue.
Brian
I like the tag boards. It is very much in keeping with the neatness in your overall construction. It is one thing having good tools (like your lathe etc.) but quite another to use them as proficiently as you obviously do. This will be a very fine set.
Cheers
Ian
Thanks Ian. They say that practice makes perfect but all practice does is to make it less imperfect 
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I have started the wiring in earnest now by wiring the heaters. There are 7 heater windings on the mains transformer, 4 for the timebases & signal valves, 1 for the sync separator, 1 for the CRT and, of course, 1 for the HT rectifier. The EHT transformer has just the high voltage winding and the heater winding for the EHT rectifier. Once the heaters are finished, I will start with the timebases and perhaps power them with bench PSUs to start with.
Just a quick note to say that I've been looking at Till's layout sketches of his BT1921 which is a console direct-view TV of the same vintage. I seem to have made the right decision about using small tag/group boards as the sketches show that this is what GEC used in the BT9121. One of the really useful drawings is the layout of the EHT bleeder and CRT resistor chain boards! As the circuit is almost identical, by checking with the photos I have, I have been able to tweak my layout so that it must be a reasonable facsimile of the original BT8161.
Thanks Till 
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Hi Brian,
I'll pull the chassis out and take some pictures of the underside.
There are some similarities in the design although we've established that the intermediate frequencies are different.
The BT8161 has the lowest possible vision IF of 4Mc/s. I think it's 6Mc/s in the BT9121.
Till Eulenspiegel.
Under chassis photos of the construction technique of the IF amplifiers in the GEC BT9121.
Till Eulenspiegel.
Thanks for those pictures, Till. I must say that the BT8161 chassis is a bit bigger than that and there's much more room, thank goodness. It does show that the form of construction I have decided could well be the correct one! There are not that many components in the wiring, mostly stoppers and biggish caps.
The timebases are almost complete now so hopefully, I'll be getting some sawteeth into dummy loads!
Hi Brian,
you are making amazing progress with the reconstruction project. Puts me to shame. to think I acquired that BT9121 way back in late 2033. Crivvens, that's almost ten years ago! OK, I rebuilt the cabinet but that's about all.
Till Eulenspiegel.
Hi Brian,
you are making amazing progress with the reconstruction project. Puts me to shame. to think I acquired that BT9121 way back in late 2033. Crivvens, that's almost ten years ago! OK, I rebuilt the cabinet but that's about all.Till Eulenspiegel.
Where did you get your time machine from?
Does it go back 20 years as well?
Hi Brian,
you are making amazing progress with the reconstruction project. Puts me to shame. to think I acquired that BT9121 way back in late 2003. Crivvens, that's almost ten years ago! OK, I rebuilt the cabinet but that's about all.Till Eulenspiegel.
Where did you get your time machine from?
Does it go back 20 years as well?
Hi Refugee,
Mea culpa. That should have read 2003. Not 2033. It does now.
Till Eulenspiegel.
30 years in reverse gear 
Don't forget, Till - that you are working on lots of different projects - the simple TV, for example. I seem to be concentrating on the BT8161 at the expense of others. I've got an Ekco TSC1113 chassis (and 17" CRT) on the other bench with a sound IF fault and I have abandoned that one!
Progress is steady on the 8161 however and the pictures show how the wiring is going. The long tagstrip will act as terminations for all the heater windings except the HT rectifier (I will make sure that the tags will not carry the full current for the two 6A circuits!) - otherwise, the heater wiring would be rather unwieldy.
By the way, Till, can your time machine go backwards - maybe you could pick up a chassis or two for me
Hi Brian,
I wish I could create a time machine. I'd not be greedy, just go back 40 years, simpler times.
Once ran a mile in just under five minutes. Can't do that now.
Till Eulenspiegel.
Hi Brian,
Two pictures of the two EHT bleeder panels to be found in the BT9121. Both panels are the same size. 4 inches wide and 3 3/4 inches high.The mounting "L" brackets are two inches wide and 3/8" X 3/8"
The fixing holes are 1 5/8" apart.
Till Eulenspiegel.
Look at this. Just cleaned the panels with Cillit Bang. Bang and all the stains have gone, well almost.
Till Eulenspiegel.
Thanks again, Till. Unfortunately, I have already made the boards and they are not as high as yours. Hopefully though, they'll be high enough to avoid arcing (at 6kV shouldn't be a problem).
I should be getting the power transformer tomorrow so that I can complete the PSU side of things as it makes things a lot easier when testing. One thing I will add is a separate mains switch for the EHT transformer so that it can be easily disabled!
I have been doing some mechanical work on the IF transformers for the B chassis. I have mentioned the electrical problems with this part of the circuit earlier in the thread but I would like to raise the question again. There are 7 tuned circuits (tuned by self and stray capacity only), and damped with reasonably low value resistors to form the 4MHz IF strip. The table below shows the winding resistances and the damping values. There are no capacitors except the one at the head of the list. This sits at the end of a long coaxial cable between the two chassis.
In order to achieve the bandwidth required, I assume that the transformers must be stagger tuned and be reasonably tightly coupled. The center dips caused by over-coupling perhaps overlap one another resulting in a fairly flat response! Is this possible? What do the experts think? All suggestions welcome.
