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Old 20th Apr 2017, 3:17 pm   #1
FERNSEH
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Default HMV 1804 Television.

Do you consider this rather sad looking HMV 1804 a viable restoration project? For starters the wood worms have a grand feast out of the cabinet and the electronics have got at.

DFWB.
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Old 20th Apr 2017, 4:20 pm   #2
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Default Re: Hmv 1804

Looks like a scene from psycho David. You only need Norman Bate's mother in the chair to complete the setting.

Difficult one this and that 1804 has had many unofficial modifications How did you keep it still enough to take that picture? The woodworm appear to have extensively damaged the cabinet.

I would be tempted to put it aside for a while and see what turns up. The EHT transformer will no doubt be useless and you don't know the condition of the 3/4 CRT or the later TA10 if it was replaced in the early 50's.

Life is too short. John.
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Old 20th Apr 2017, 5:08 pm   #3
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Default Re: Hmv 1804

Hi John,
The CRT is the original Emiscope 3/4. The replacement is the TA10 as is fitted in my electrically similar Marconi VT50.
As for the 1804, there has indeed been some strange modifications done to it.
The original Z66 pentodes replaced by EF50s. If that wasn't bad enough the X81 triode-hexode frequency changer valve has been replaced by another EF50 which functions as the mixer and an additional valve an EC52 triode does the job as the local oscillator. The D43 vision detector diode replaced by an EB34, why bother doing that?
All these adventurous modifications must have worked OK so why not just leave it as it is rather than rebuild the set back to it's original specification?
The sound IF amplifier should be a KTW61, replaced by a Mazda SP61, now that's going too far, a Mazda valve in an EMI set!
So what was the reason for doing all these modifications? Perhaps to make the set perform better which might make sense because the set was found in a farm house in Cambridgeshire, way outside the range of the Alexandra Palace transmitter.
To sum up, the Z66 pentodes were replaced because the person who did the mods believed the EF50 was a better valve. The pentode frequency changer valve possibly had a lower noise factor than the X81 hexode.
The KTW61 sound IF amplifier replaced by the SP61 to improve amplification of that part of the set.
Even the single diode interlace filter circuit was replaced by a Pye type of circuit using an EB91 double-diode.

Why not just plug the set in and see what happens?

DFWB.

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Old 20th Apr 2017, 6:13 pm   #4
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Default Re: Hmv 1804

You can do it David - but wear your Kevlar Y Fronts eh? !!
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Old 20th Apr 2017, 6:32 pm   #5
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Default Re: Hmv 1804

Its got to be worth a go just to see the upgrades at work.
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Old 20th Apr 2017, 6:41 pm   #6
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Default Re: Hmv 1804

Not my field, admittedly, but such a thorough-going contemporary rework deserves to be restored and preserved as-is, rather like a vintage racing car extensively modified in-period.
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Old 20th Apr 2017, 6:42 pm   #7
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Default Re: Hmv 1804

Quote:
Originally Posted by FERNSEH View Post
Why not just plug the set in and see what happens?
Give me a minute David, I'm just nipping round the back of my settee !

Marc.
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Old 20th Apr 2017, 9:04 pm   #8
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Default Re: HMV 1804 Television.

Hi all,
there seems to be a consensus that the modifications to the RF and IF amplifiers should stay in place, after all someone took a lot trouble to carry out the mods for some reason or other. However,that Mazda SP61 in the sound IF amplifier will be replaced with a KTW61 or 6K7G. And that interlace diode will be removed, it's just an untidy mess.
See attachment.

DFWB.
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Old 20th Apr 2017, 10:18 pm   #9
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Default Re: HMV 1804 Television.

What a lovely set! I like the look of it, and the fact it's full of mad fiddlings sounds even better. I'd definitely take it on, I love a project that's miles outside my comfort zone!

Oliver

Edit: Given the set came from Cambridgeshire, and seems to have a Pye whiff about the modifications, which clearly needed more-than-usual technical know how, maybe it was a Pye employee that modified it.. They would surely have considerable faith in the EF50..

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Old 20th Apr 2017, 11:12 pm   #10
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Default Re: HMV 1804 Television.

Hi Oliver,
the interlace filter does resemble the circuit in the Pye FV1, called "the positive interlace filter". A Pye engineer would be familiar with the EF50. According to the Valve Museum the Z66 wasn't a very good valve.
http://www.r-type.org/exhib/aaa0354.htm Characteristics of the Z66 are similar to those of the SP61.

DFWB.
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Old 21st Apr 2017, 7:12 am   #11
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Default Re: HMV 1804 Television.

That valve screening can also looks like a Pye product. The Z66 valve was without doubt one of the MOV's worst valves. LLJ said in Practical Television that he had experienced endless problems with it but once a good set of valves had been selected, they worked very well. He was referring to the slightly later 1805 series, a really excellent receiver. John
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Old 22nd Apr 2017, 5:29 pm   #12
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Default Re: HMV 1804 Television.

I'm working on the set in my luxurious garden workshop, you've all seen that chair. More evidence that the modifications were performed by a Pye engineer. The original frame blocking oscillator transformer had been replaced with a Pye component, the same one that is used in the D16. Now that good news for me because I been on the lookout for that particular part for years. After the connections have been traced out the transformer can be fitted into my D16. A normal two winding blocking transformer can be fitted in the HMV.
The interlace filter unit has been removed and the circuit will be returned to the original HMV circuit. A standard Plessey blocking oscillator transformer can be wired in.
Further examination of the modified RF and IF circuits shows that the work had been carried out in a competent manner.

DFWB.

Last edited by FERNSEH; 22nd Apr 2017 at 5:40 pm.
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Old 22nd Apr 2017, 6:31 pm   #13
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Default Re: HMV 1804 Television.

Hi David, I can easily reverse engineer the FBOT for you, I probably even have some spare cores.

Ed
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Old 22nd Apr 2017, 6:55 pm   #14
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Default Re: HMV 1804 Television.

Hi Ed,
that will be will be a great help, not only for me but for others who might be having problems with the D16 frame TB oscillator transformer.

DFWB.
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Old 22nd Apr 2017, 8:03 pm   #15
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Default Re: HMV 1804 Television.

David
I have a couple of spare 1804 chassis which I can help you with! I have one decent cabinet (very decent) and three or four chassis to suit.
I will check what I have tomorrow and let you know so that we can msort something out.
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Old 22nd Apr 2017, 9:00 pm   #16
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Default Re: HMV 1804 Television.

Hi Brian,
I think I'm gonna need your help with this one. There's some pretty weird things going on in this set.
Made some progress today.

DFWB.
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Old 22nd Apr 2017, 9:51 pm   #17
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Default Re: HMV 1804 Television.

Taken out the 1804 is the frame blocking transformer with the sync injection winding, it will go in my Pye D16.

DFWB.
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Old 23rd Apr 2017, 11:06 am   #18
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Default Re: HMV 1804 Television.

I think we're going to enjoy this thread! There's a piquancy in the outlandish and unusual. Are the modifications going to stay in situ, as previously discussed?

I'm always envious of how so many of you have managed to assemble collections of the older (and more interesting, in my opinion) sets and parts.

Oliver
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Old 23rd Apr 2017, 12:33 pm   #19
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Default Re: HMV 1804 Television.

Hi Oliver,
I'm still trying to fathom out these strange signal circuits. I just can't understand why some of the modifications were carried out in the first place, but who knows this set might be very sensitive and only required a few microvolts to show good pictures in it's remote location far away from the transmitter.
The attachment shows the original signal paths before the modifications were done.

DFWB.
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Old 23rd Apr 2017, 1:34 pm   #20
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Default Re: HMV 1804 Television.

The signal paths have been worked out. The receiver has two RF amplifiers employing EF50 pentodes. Another EF50 functions as the mixer and an EC52 triode as the local amplifier. So it follows that the receiver has only two IF vision amplifier stages. The 4.5Mhz sound IF is taken off from the anode of the mixer and supplied to a tuned circuit and then to the Mazda SP61 pentode amplifier. The sound demodulator and audio amplifier stages are unaltered.
The vision demodulator diode type D43 has been replaced with an EB34 double-diode, the spare section functions as a white spot clipper.
So we now know how this wonderful circuit works it's now time to find out if it works and preserve as much of it as possible.

DFWB.
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