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Old 2nd Jul 2017, 11:14 pm   #121
FERNSEH
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Default Re: HMV 1804 Television.

Been using a Mullard GZ34 for the HT rectifier. The reason for using this valve instead of the original U52/5U4 was because the GZ34 has an indirect heated cathode which means there is no HT surge so it is kinder to all those aged components. Also, it has a much more compact glass bulb.
This evening I decided to replace the GZ34 with an U52 or 5U4G. The 5U4G has a direct heated cathode which results the HT being present seconds after switch on. Until all the valves have warmed up the HT voltage can rise to >450 volts dropping back to the correct 380volts when all the valves are at full emission. Well, that HT surge took out two components in the video amplifier, the 0.047mfd decoupling capacitor C29 and HT feed resistor R29.
With the GZ34 the HT voltage was a bit on the high side at about 400 volts.
The 5U4 delivers the correct HT voltage which is 380V. The result of fitting the older valve is a reduction of width and height, no problem because the timebases have plenty spare power even with the reduced HT.

The second attachment shows snow on the screen when no signal are supplied to the receiver.

DFWB.
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Old 3rd Jul 2017, 11:34 am   #122
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Default Re: HMV 1804 Television.

As excellent as it is the Mullard GZ34 is not recommended for long term use in the HMV 1804 or any of the variants of this model such as the pre-war model 1800 and Marconi VT50.
It is the much lower forward voltage drop in the GZ34 that causes problems, with this valve the HT voltage can be over 400 volts. The 5U4/U52 has a forward voltage drop of 44V @ 225mA. It is obvious the designer had factored this in during the development of the chassis. If one can be found the best valve for the job is the Mullard GZ33 or Cossor 53KU.
http://www.radiomuseum.org/tubes/tube_gz33.html

I'm now using a Canadian made Westinghouse 5U4G in the set.

The attachment shows a comparison between the GZ34 and 5U4G.

DFWB.
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Old 3rd Jul 2017, 12:15 pm   #123
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Default Re: HMV 1804 Television.

DFWB,

Have you decided which TV channel the set was set up for? Reading the earlier (many!) posts it seems a little unclear to myself !

Is it possible that somebody attemted to retune the Set and gave up?

By the way: It was possible to receive Holme Moss just outside Cambridge (albeit with co-channel from the Cambridge B2 relay). Of course this set's operation may have predated the relay. My experience was that Crystal Palace on B1 was a stronger signal (which would also have been true of Ally Pally days, if you study the service area maps).

Either way, I'm pleased to read of EF54s and EF50s in fringe action !
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Old 3rd Jul 2017, 12:47 pm   #124
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Default Re: HMV 1804 Television.

Have you decided which TV channel the set was set up for? Reading the earlier (many!) posts it seems a little unclear to myself !

Is it possible that somebody attempted to retune the Set and gave up?
Hi Steve,
it's certain that the set was modified to receive the Holme Moss transmitter. I've tried retuning the set to channel 4 and also channel 1, the receiver will just not work properly on those channels. Everything in the RF circuits has been optimised for channel B2. And as has been stated earlier in this discussion the conversion work has been done by a very competent engineer, someone with a good knowledge of RF circuit design. It is evident by the choice of valves, the EF54 for superior HF performance and the EF50 for it's good gain control characteristics. But it's not only the extreme sensitivity but also the excellent picture quality, it's good for 3Mhz.

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Old 5th Jul 2017, 12:04 pm   #125
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Default Re: HMV 1804 Television.

DFWB,

... of course channel B3 was used for BBC East Anglian TV from Tacolneston, and B5 was used fom Peterborough... but perhaps this was later than the modification epoch of your telly ...
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Old 5th Jul 2017, 12:25 pm   #126
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Default Re: HMV 1804 Television.

I'd say that the modifications to this set were carried out well before the Tacolneston transmitter came into service.

Tacolneston timeline: https://www.thebigtower.com/live/Tac...2800500191.htm

DFWB.
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Old 5th Jul 2017, 4:23 pm   #127
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Default Re: HMV 1804 Television.

Unbelievable perseverance David .... and you survived it! I bet you're pawing the earth to get started on the cabinet eh?

Well done mate!
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Old 5th Jul 2017, 6:52 pm   #128
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Default Re: HMV 1804 Television.

"Unbelievable perseverance David .... and you survived it! I bet you're pawing the earth to get started on the cabinet eh?"
Can't wait to get on with the cabinet repairs. The HMV 1804 is a far betting looking set than it's EMI stablemate the Marconi VT50. Actually the cabinet repair is the last of my worries. After the set has been working for about 15 to 20 minutes the picture becomes dim and large. The measured EHT drops back to less than 3KV from the correct 4KV. It's most likely the EHT transformer is faulty. The two 0.03microfarad EHT smoothing capacitors were replaced with no improvement. That's two 1804/VT50 EHT transformers that now need rewinding.

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Old 5th Jul 2017, 7:40 pm   #129
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Default Re: HMV 1804 Television.

These things are sent to try us eh?
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Old 6th Jul 2017, 6:22 pm   #130
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Default Re: HMV 1804 Television.

Hi David,
The EHT transformer still lasted longer than I predicted.
Not bad for a set that had clearly been in the damp.

I gather you have other EMI transformers awaiting rewind. Nasty things potted in tar. I've never known a PYE D16T transformer fail though (wax potted).

I'll stop there I think...

Cheers
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Old 6th Jul 2017, 7:13 pm   #131
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Default Re: HMV 1804 Television.

I'm sure you could jury-rig a temporary blocking oscillator to give you a bit of less dangerous, energy-limited EHT in the meantime .....
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Old 6th Jul 2017, 7:17 pm   #132
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Default Re: HMV 1804 Television.

Hi Andy,
The EHT transformer in my Marconi VT50 has also failed, well I believe it has. There was a bang and smoke came out from the ventilation holes in the back cover. The VT50 is electrically identical to the 1804 but doesn't have a very nice looking cabinet.
Getting back to the 1804. This morning there was a distinct smell of hot pitch coming from the EHT transformer and the windings are quite warm. Reckon the transformer is faulty.

DFWB.
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Old 6th Jul 2017, 7:19 pm   #133
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Default Re: HMV 1804 Television.

Quote:
Originally Posted by julie_m View Post
I'm sure you could jury-rig a temporary blocking oscillator to give you a bit of less dangerous, energy-limited EHT in the meantime .....
I've been thinking about something like that.

DFWB.
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Old 8th Jul 2017, 5:50 pm   #134
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Default Re: HMV 1804 Television.

Fitted an international octal base to a Mazda SP61 so that it can function as a replacement for the Z66.
The second attachment shows the internals of a Z66. The old fashioned glass pinch electrode support limits the HF performance of this valve. The EF50 and EF54 was much better. The electrodes are mounted directly onto the base pins.
Instead of having separate RF gain and contrast controls I've rewired the cathode circuits of the second RF amplifier and first vision IF amplifier so the contrast control alters the bias on both valves. That's much the same way it was done in the original 1804 circuit except the contrast control adjusted the cathode bias of the frequency changer and first IF amplifier stage.

DFWB.
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Old 14th Jul 2017, 12:09 pm   #135
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Default Re: HMV 1804 Television.

Even with only 3.5KV EHT the picture still continues to get brighter. But after a few minutes or so the EHT transformer becomes very hot and the EHT drops to less tan 3KV. The transformer will have to be replaced.

DFWB.
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Old 13th Oct 2017, 11:35 am   #136
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Default Hmv1804.

Link to my previous thread about the HMV 1804.
http://www.vintage-radio.net/forum/s...=135873&page=7

Mike Barker has done a magnificent job rewinding the EHT transformer so now it's time to refit it.

DFWB.
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Old 27th Oct 2017, 10:07 am   #137
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Default Re: HMV 1804 Television.

The EHT transformer has been refitted. The anode of the CRT is now receiving the full 4KV, sufficient for the Emiscope 3/4 CRT to display a very bright picture. The ion burn doesn't appear to be so noticeable. But this is not the end of the restoration of this set. The two diode interlace filter will be reintroduced into the circuit. Instead of the separate interlace module made by the engineer who modified the set all those years ago I will use the existing D63/6H6 interlace diode. The original EMI circuit uses one section of this valve.
A replacement loudspeaker will have to be found to replace the badly damaged original. Finally the cabinet will need some attention. Hopefully the worms have vacated the cabinet.

DFWB.
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Old 27th Oct 2017, 10:38 am   #138
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Default Re: HMV 1804 Television.

That's a very bright picture indeed David!
Not much, almost no, ion burn visible now.

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Old 27th Oct 2017, 11:18 am   #139
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Default Re: HMV 1804 Television.

Quote:
Originally Posted by Jac View Post
Not much, almost no, ion burn visible now.
You haven't added a circuit to boost the video gain in mid screen have you?

Peter
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Old 27th Oct 2017, 11:34 am   #140
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Default Re: HMV 1804 Television.

"You haven't added a circuit to boost the video gain in mid screen have you?"
Hi Peter, Can you imagine the circuits needed to do that? And with valve technology!

DFWB.
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