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Old 10th Sep 2019, 6:16 pm   #1
peter10tv
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Default Emiscope 3/16 Crt

I know that EMI gave conversion details for replacing an Emiscope 3/4 with a TA10. Conversion kits were available, but did they ever give details for replacing the TA10 with an Emiscope 3/16.. I have done so successfully, fitting a 3/16 into an 1805, but an official description of a conversion kit, if available at the time, would be interesting.

Thanks, Peter
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Old 10th Sep 2019, 8:53 pm   #2
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Default Re: Emiscope 3/16 Crt

Hello Peter, I don't think EMI would have suggested this. The 3/4 became obsolete, hence the kit to fit the TA10. The TA10 and 3/16 CRTs were available for quite a number of years
as replacements. The TA10 gives a startling picture in the 1805. The 3/16 would have been a bit disappointing compared with the TA10.
Heater voltage and EHT differ widely with the 3/16 and the TA10. The TA10 is a tetrode compared with the triode 3/16 requiring focus modifications. Regards, John.
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Old 10th Sep 2019, 8:57 pm   #3
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Default Re: Emiscope 3/16 Crt

Hi Peter, Back in the sixties advertisers in the Practical Television were offering the 10" Cossor 108K CRT for a few pounds. There was a 10" Emiscope tube which was a rebranded Cossor, it was fitted in some HMV 1807 sets.
Apart from fitting a heater transformer for the 3/16 tube did you need to do any modifications to the line and frame timebases for extra scanning power?
The modification notes issued by EMI service for fitting a TA10 in the HMV 1804 and Marconi VT50 gave details how to do modifications to the timebases.

DFWB.
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Old 10th Sep 2019, 9:43 pm   #4
peter10tv
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Default Re: Emiscope 3/16 Crt

Hi
I did not need to do any alteration to either the frame or line time bases, as the scan was adequate. The only electrical alterations were to the focus circuitry and to supply the 8.5v for the tube heater, this evaluated by the .3amp current drawn. The tube neck gantry was moved to its extreme adjustment to get the best results.
It may be the focus circuitry could be left original if the focus coil could be moved to a better place along the neck of the tube, or maybe a different value of winding? More experimentation on this may be required.
I look upon this as a temporary measure as I am persuaded by John's praise of the TA10 which I hope someday to find! Perhaps two....as I also have a fully working VRC52a which has as soft TA10.
Best wishes
Peter
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Old 11th Sep 2019, 12:24 pm   #5
peter10tv
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Default Re: Emiscope 3/16 Crt

Hi all,
Here is a pic of the 1805 with the fitted 3/16... John, apart from a different screen tint, how much better would a TA10 perform..
Best wishes
Peter
P.S. Hopefully I can attach the file...
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Old 11th Sep 2019, 3:04 pm   #6
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Default Re: Emiscope 3/16 Crt

Hello Peter. You must know how I like a challenge.. I hawked out two EMI receivers, A VC53DA, fitted with the 3/16 the Marconi version of the HMV1807 and the Marconi VRC52DA console fitted with the same television chassis as the HMV1805 employing the TA10.

To be fair it has been difficult to get a fair comparison from the pictures. The 1805 chassis is much easier to photograph especially displaying the considerable quality capable from the TA10. The BBC used them for monitors at AP.
The 1805 series takes a lot of beating from any angle and without doubt was EMI's best model by far.

The triode 3/16 fitted in the 1807 is a bit soft unfortunately and the other one I have fitted in a Marconi VRC54 has an O/C heater! Other than that the screen colour is similar but not quite the same. The 3/16s varied a lot and considering they were aluminised and operated at just under 5kv [for correct focus] it is not surprising they struggled. Some had a distinct sepia.
appearance.
It was EMI's first AC/DC tube and judging by the number of heaters that went O/C just outside the guarantee and the number of heater voltage changes during production, they were obviously having problems with it.
Some 1807 series were fitted with a modified COSSOR 108K 4 pin based tube that performed considerably better with a long life.
The TA10 is a tetrode and was fed with 7kv of mains derived EHT. It produced a pin sharp, black and white high contrast range picture.
Great to know you managed to get the 3/16 working in your 1805. It is probably being operated more closely to the requirements of an aluminised tube.
The pictures show the 1805, film and test card with the 3/16 test card at the end. John.
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Old 11th Sep 2019, 4:46 pm   #7
peter10tv
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Default Re: Emiscope 3/16 Crt

Hello John
I can certainly see the difference between the two pictures, the 3/16 does seem more woolley... I have the console version of the 1807 and must dig it out to make a comparison between the two..
My very first television as a boy was VRC52a, this when the set was only nine years old..1958.. It certainly gave a good picture. Bought for only pocket money from a local television dealer, it had been a part exchange and was thought to be a non worker. The fault turned out to be a faulty octal socket. My parents had no idea what danger I was playing with, neither did I!
Unfortunately that particular set is long gone..
Best wishes
Peter
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Old 11th Sep 2019, 4:53 pm   #8
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Default Re: Emiscope 3/16 Crt

I am interested in this discussion about the HMV 1805 because it was our first TV set.
I understand they were well regarded in the trade.

What I want to ask is this, if 1805 was the London model and the 2805 was for Birmingham, would our set, receiving Holme Moss, be a returned 1805?

Or to put it another way, was the front-end of the two models capable of being tuned to any BI channel and the 2805 was just a convenience to save the dealer some time.
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Old 11th Sep 2019, 6:16 pm   #9
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Default Re: Emiscope 3/16 Crt

All five versions of the X805 employed the TA10 tube. It's big brother 15" models employed the 15" TA15 that had identical characteristics to the Ta10 but appeared to have a shorter life probably due to pumping difficulties with such a massive bulb.
The front end unit in the 1805 London model is a high quality TRF unit, fix tuned of course to 41.5/45 mc/s
The remaining models were all superhets 2805 3805 4805 5805 used the B7G based X78 as a frequency changer. These again were fixed tuned but EMI did give data to tune then to alternative BBC transmitters should their owners move etc.

In is interesting to note that the London model, due to it's low frequency, could be realigned as an IF strip 34/38mc/s [with the local osc disconnected] so that a Band 3 14 channel tuner could be fitted. An interesting chassis and the only one designed for the TA10/15, not including the fitting of the TA10 with mod kits in the model 1804 and the pre war HMV 907. John.
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Old 11th Sep 2019, 8:15 pm   #10
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Default Re: Emiscope 3/16 Crt

Ah, the 1d finally had dropped!

It hadn't occurred to me that the first digit was channel number, I thought it was the order of Tx deployment, so I was wrong to say the 2802 was a Birmingham set, that must have been a 4802 which preceded the 2802.

Were they really in production long enough for all 5 channels to be in use?
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Old 12th Sep 2019, 9:28 am   #11
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Default Re: Emiscope 3/16 Crt

They certainly were. These were expensive models. The same chassis was fitted in the 1902, the magnificent 'Teleradiogram.' [We have one at the Dulwich museum together with a good example of the HMV 1851]This was still in production in 1953 and must have been the last to still obtain it's EHT from the mains supply until the end of production.
There was nothing to compare it with in the EMI sales booklets other than the AC/DC range that was miserable in comparison. The TA10/15 gave such good pictures that the range was still in demand later than it normally would have been.
Models that employed this chassis in five channel construction: HMV 1805 1806, 1851, 1902, Marconi VRC52A and there may have been others. The 10" models, TA10 were dropped from production around 1951 but the very expensive 15" models, in various guises continued. Due to the original high cost of receivers EMI continued to supply replacements for the TA10/15 well beyond the expected life of the receiver itself. They were very reliable and fitted with a very robust EHT transformer.
Just a thought.
If you have a problem focusing the 3/16 you will probably have to pass a little more current through the focus coil. Maybe a resistor mod is required to centralize the gantry.
Regards, John.
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Old 12th Sep 2019, 2:37 pm   #12
peter10tv
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Default Re: Emiscope 3/16 Crt

Hi John
I did alter the focus circuitry to get good focus within the middle of the control..
Some more experimentation of moving the tube gantry, more than the screw slots will allow, may make the focus circuit alteration unnecessary.
The focus field necessary for the two types of tube may be different.
Better brains than mine may expand the thoughts.
Best wishes
Peter
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Old 12th Sep 2019, 2:52 pm   #13
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Default Re: Emiscope 3/16 Crt

The TA10 is a tetrode with a certain degree of pre focus. It therefore requires less focusing field than the 3/16 triode.
My brains if I ever had any were eradiated in the 1960's repairing Philips G6 receivers with the screening /safety covers removed and a banana plug inserted in the interlock socket.
Oh well. I survived. John
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Old 12th Sep 2019, 5:58 pm   #14
peter10tv
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Default Re: Emiscope 3/16 Crt

I used one of those small grub screwdrivers which were everywhere at the time... Luvery blue glow!!
Peter
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Old 13th Sep 2019, 3:40 pm   #15
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Default Re: Emiscope 3/16 Crt

"Stead" 5000 volt?
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Old 13th Sep 2019, 8:05 pm   #16
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Default Re: Emiscope 3/16 Crt

Steadfast I think.
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