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Old 16th Jun 2017, 1:12 pm   #101
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Default Re: HMV 1804 Television.

I'm surprised that [that] cap hasn't let go! You do like to live dangerously Young David!

But ..... the results are very encouraging - I'm hoping that this set will surprise us all [as opposed to shocking you] !!
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Old 16th Jun 2017, 5:00 pm   #102
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Default Re: HMV 1804 Television.

David, I have an old hat box and a transformer salvaged from a 1910 battery charger.
Any chance of building me a 405 receiver to fit an old wardrobe I pulled off the tip?

Really fantastic David. Once you get your thoughts into a project like this you cannot let go till the end. Your really lucky with that 3/4. Great work! John.
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Old 17th Jun 2017, 3:52 pm   #103
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Default Re: HMV 1804 Television.

"Any chance of building me a 405 receiver to fit an old wardrobe I pulled off the tip?"

Hi John,
the wood can be used for my next cabinet project.

An interlace filter circuit must be reinstated into the receiver. Rather than the Pye two diode circuit favoured by the engineer who did all those modifications I'm returning frame sync circuit back to the original EMI design consisting of one half of a D63/6H6 double-diode. In order to give the other diode section something to do a sync pulse clipper will be introduced into the anode circuit of the sync separator valve. This is part of the sync separation circuit of the later model 1806.

DFWB.

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Old 17th Jun 2017, 4:03 pm   #104
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Default Re: HMV 1804 Television.

The interlace displayed by the 1805/6 is truly excellent David. J.
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Old 17th Jun 2017, 6:45 pm   #105
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Default Re: HMV 1804 Television.

Circuit diagrams of the sync separator stages in the HMV 1804 and 1805/6.
The interlace filter circuit I've opted for is the one shown in the first attachment. The sync clipper as used in the 1805 will be employed in my 1804. Only two additional components are required, R48 (47Kohms) and C36 8microfarads) The third attachment shows the circuit of the two diode interlace filter in the Pilot CV34.

DFWB.
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Old 19th Jun 2017, 2:56 pm   #106
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Default Re: HMV 1804 Television.

The width control has an open circuit carbon track. What has happened the wiper has gouged out a groove in the track. It might be possible to reposition the wiper so that it comes into contact with the undamaged track.

DFWB.
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Old 19th Jun 2017, 3:10 pm   #107
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Default Re: HMV 1804 Television.

If the wiper is a pointed type end then i have used a tiny brass washer before, it must be perfectly flat to avoid further damage to the track and sits under the point.
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Old 19th Jun 2017, 3:23 pm   #108
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Default Re: HMV 1804 Television.

Hi Steven,
That's a good tip. Will report back later. The problem being that special slim potentiometers are required because of lack of space. Can you believe that in a chassis that is so big?

DFWB.
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Old 19th Jun 2017, 4:02 pm   #109
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Default Re: HMV 1804 Television.

Hi David,
it looks like the inside of the type of pot found in my HMV 905.
There should indeed be a polished washer.
Have a look at post 97 here:-

http://www.vintage-radio.net/forum/s...=hmv905&page=5

Let me know how many washers are missing, I have lots of spare pots to take them from and I might be able to supply you with replacement tracks if I happen to have the right values. Btw, having replaced the washer in one of the pots (I think it was the contrast control) in the 905 I found that the increase in value from 5K to 7K due to the scratch on the track did not make any real difference to the operation of the set.

Cheers
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Old 19th Jun 2017, 4:17 pm   #110
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Default Re: HMV 1804 Television.

Hi Andy,
Thanks for the link. The reference to the type of potentiometer in question is on post #97. First attachment.

DFWB.
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Old 19th Jun 2017, 5:18 pm   #111
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Default Re: HMV 1804 Television.

Hmmmm ..... in any event, that pot's seen many 360deg jamborees - I wonder why?
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Old 19th Jun 2017, 6:01 pm   #112
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Default Re: HMV 1804 Television.

Its quite silly the way they are made, if the washer sticks to the track for any reason, the pointed wiper jumps out and scores the track. My guess is it would become instantly useless and the owner would just twiddle it back and forth scoring the track in the process.
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Old 19th Jun 2017, 6:56 pm   #113
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Default Re: HMV 1804 Television.

Oh what nasty pots! EMI loved horrible presets. I have a large range of convergence presets. Quite high ratings and they may be suitable. John.
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Old 30th Jun 2017, 7:56 pm   #114
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Default Re: HMV 1804 Television.

A few metal parts have been painted, for example the rear control panel and the EHT capacitors. The set won't work any better but at least the chassis looks a lot better. An etch primer was used on the rusty metal parts.

DFWB.
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Old 30th Jun 2017, 10:00 pm   #115
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Default Re: HMV 1804 Television.

The first attachment shows the very crudely made aerial input coil. There is no proper input transformer to match the low impedance aerial to the grid of the first valve. As things are at the moment that's probably just as well because I'm having difficulties taming this high gain receiver. A correctly matched and tuned input transformer would increase the gain of this receiver even further.
The second attachment shows the very "soot and whitewash" picture displayed with both the contrast and sensitivity controls turned well down. To reduce the signal level even further an 18db attenuator is inserted in series with the aerial input.
The CRT is excellent, it's even becoming brighter with use. The burn mark is less noticeable.

DFWB.
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Old 1st Jul 2017, 7:32 am   #116
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Default Re: HMV 1804 Television.

I think you have done a great job on this 1804 David. It's very much a tribute to the guy that carried out all that work to improve the sensitivity of what is basically a 1939 chassis. I wonder what he would say he if knew his 1804 was once again showing a picture in 2017, seventy years after it was originally produced at Hayes.

I wonder how extreme fringe reception looked like back then on his much modified chassis?

I must have been about nine years old, on holiday with my Gran in Thornaby. We visited an uncle who had just purchased his first TV. Oddly for me I cannot remember the make of it but I remember him switching the channel selector from the local signal to another ITA channel that was very snowy indeed. He said it had alternative programmes that were sometimes better than the 'local'.

As a teenager my sister Yvonne had friends at Folkstone in Kent and used to visit them. She celebrated her 80th Birthday back in May. I remember her telling me that the picture was covered in snow and was unwatchable during certain weather conditions.
John.
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Old 1st Jul 2017, 11:20 am   #117
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Default Re: HMV 1804 Television.

Effected an improvement to the picture quality and stability by replacing the final IF amplifier valve, another Z66. There was a problem with sync crushing, a very unusual condition with positive modulation TV signals. Normally when the gain controls are turned up it's the video part of the waveform that's becomes crushed. Replacing that valve solved the problem. The Z66 is not a good valve and as a reliable replacement it might be a good idea fitting an international octal base on a Mazda SP61.
Despite the reintroduction of the original EMI diode interlace filter circuit the interlace is not good. Careful adjustment of the frame hold control is required for optimum results.
To improve matters I will employ the Pye double-diode interlace filter circuit as was used by the engineer who did the modifications but using the existing D63/6H6 valve rather than the separate subassembly.

DFWB.
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Old 1st Jul 2017, 2:20 pm   #118
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Default Re: HMV 1804 Television.

Like many MOV valves of the period the Z66 was a real horror. I remember LLJ in PT stating that he had experienced endless trouble with them but once you had managed to get a good set of Z66s the chassis settled down well. The valve itself like the B36 in the 1807 suffered from quality control problems. Many were off spec from new creating problems that should not have occurred from any manufacturer, least of all the Marconi Osram valve works.
Unfortunately there was no direct plug in equivalent to the high gm Z66 unlike the B36 that had a much more reliable replacement, the 12SN7.
It was well worth all the effort David. John.
PS Love the venting EHT cap. Just my sort of excitement!
If the EHT transformer gives up the ghost I have managed to find you a replacement. You may have to alter the value of the EHT bleeder to reduce the EHT a little. Let me know if you want it and I will bring it up on a low loader. J.
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Old 1st Jul 2017, 5:04 pm   #119
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Default Re: HMV 1804 Television.

That's not an equivalent, its a diode, er, single pole switch. Would last well though, can you megger it first?
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Old 1st Jul 2017, 7:56 pm   #120
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Default Re: HMV 1804 Television.

"If the EHT transformer gives up the ghost I have managed to find you a replacement. You may have to alter the value of the EHT bleeder to reduce the EHT a little. Let me know if you want it and I will bring it up on a low loader". J.

Hi John, It'll need a special adaptor plate. The EHT smoothing capacitor looks scary enough but it seems to be holding up, so far that is. Same goes for the EHT transformer.

DFWB.
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