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Old 23rd Apr 2017, 3:57 pm   #21
FERNSEH
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Default Re: HMV 1804 Television.

More strangenessness. The first RF amplifier stage employs an EF50 pentode, nothing wrong with that, correct valve for the job. However, the screen grid is connected to the signals tag of the interstage RF transformer and the anode is connected to HT and decoupled to chassis by a 0.0023mfd capacitor, so it can be said the valve is effectively operating as a triode. Seems the mystery engineer had got the screen and anode connections crossed when assembling the new RF amplifier. Look out for more funny circuits in this set.

DFWB.
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Old 23rd Apr 2017, 5:14 pm   #22
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Default Re: HMV 1804 Television.

In fact it would appear the RF amplifier is an EF54 so the valveholder is correctly wired. In this valve the anode is connected to pin 2 and the screen grid to pin 3. The cathode is connected to no less than four pins, the designer of this valve made sure that the cathode inductances were reduced to the minimum. Seems the guy who did the modifications knew what he was doing after all.
Info about the EF54: http://www.r-type.org/exhib/aaa1093.htm

DFWB.
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Old 23rd Apr 2017, 10:11 pm   #23
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Default Re: HMV 1804 Television.

Impressive- but is that a common valve in TV work? I'm not familiar with it, but my experience is extremely limited.

Is the chassis verdigris critical to fringe reception?

Oliver
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Old 23rd Apr 2017, 10:49 pm   #24
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Default Re: HMV 1804 Television.

Hi Oliver,
I don't think the EF54 was ever employed in any TV receiver, it certainly caught me out. I just assumed the RF amplifier valves were the common EF50. The EF54s used in this set are ex-services types, a VR136 and the other has finally been determined as a CV1136. The mixer is another ex-services type, a VR91 = EF50.
It's likely the chassis was in good condition at the time it taken out use possibly sometime in the mid 1950s. The underside of the chassis is in excellent condition.

DFWB.
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Old 26th Apr 2017, 9:53 am   #25
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Default Re: HMV 1804 Television.

News Flash! It turns out the set was retuned to the Birmingham transmitter.
The RF tuning cols have less turns than are required for 45Mc/s and the tuning cores are metal. Also, the big giveaway to confirm that the set is tuned to channel B4 is that local oscillator tuning capacitor has been changed from 40pF to 22pF. The local oscillator will be tuned to 53.75Mc/s instead of 37Mcs.

DFWB.
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Old 26th Apr 2017, 10:00 am   #26
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Default Re: HMV 1804 Television.

The EC52 and EF54 (VR136 and VR137, I think) were used in the ex-GEE RF units (RF26, RF27, R1355) which were popular for converting into TV receivers post-war. Perhaps the retuning to the higher frequency of the Birmingham transmitter needed the extra performance of these valves? I think the EF54 was an improved version of the EF50, able to deliver more gain at higher frequencies.

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Old 26th Apr 2017, 10:05 am   #27
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Default Re: HMV 1804 Television.

Hi Chris,
Whoever did the modifications to the set certainly knew what he was doing. The EF54 pentode was obviously chosen for it's better VHF performance compared with the earlier EF50 or the MO Z66. All the more reasons why the modifications must be stay in place.

DFWB.
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Old 28th Apr 2017, 10:58 am   #28
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Default Re: HMV 1804 Television.

Some progress with the HMV 1804. A suitable frame blocking oscillator transformer has been found to replace the Pye type transformer with three windings, that component is now fitted in my Pye D16T. The replacement transformer was removed from a scrap Ekco TMB272 and in order that it would fit in the HMV an adaptor plate has been made for it. The frame oscillator and output stage have been isolated from the rest of the circuit and supplied from a separate power source, and it works, there's a 50volt P - P sawtooth waveform present at the anode of the KTZ63 oscillator valve. The waveforms present at the anode of the KT61 output valve are bit strange though.
So later today I'll turn my attention to the line timebase blocking oscillator.

DFWB.
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Old 30th Apr 2017, 4:42 pm   #29
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Default Re: HMV 1804 Television.

The line oscillator is tested in the same manner as the frame oscillator. The LTB circuits are isolated from the main receiver and supplied from a separate power supply.
The line blocking oscillator is working OK. Easy to adjust for a 100microsecond repetition rate. Good 50volt p - p sawtooth waveform present at the control grid of the KT44 line output valve.
Before the mains is ever connected to this set what really is needed is a complete set of new HT supply electrolytics.
So now it could be said the timebases are serviceable. Now I'll turn my attention to the 4.5Mc/s sound IF amplifier. get rid of that SP61 valve, fit an international octal base valve as the replacement.

DFWB.
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Old 30th Apr 2017, 9:03 pm   #30
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Default Re: HMV 1804 Television.

Waveforms from the line timebase. The sawtooth drive to the KT44 grid and the pulse present at the screen grid of the KTZ63 oscillator. Oscillation takes place between the control and screen grids. The sawtooth waveform is developed on the anode. Repetition rates apart, it's a similar circuit arrangement for the frame timebase.

DFWB.
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Old 4th May 2017, 2:18 pm   #31
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Default Re: HMV 1804 Television.

Re post no. 26:

I agree. The EF54 was a 'better' valve for higher frequency work.
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Old 4th May 2017, 3:06 pm   #32
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Default Re: HMV 1804 Television.

Quote:
I agree. The EF54 was a 'better' valve for higher frequency work.
Strange that the second RF amplifier is an EF50 when as far as I can understand EF54 would have been entirely suitable for the task. The first RF amplifier stage employs an EF54 and is biased to operate at max gain. The second RF is gain controlled in the normal manner by a variable resistor in the cathode circuit. Perhaps the EF50 has a better gain control characteristic compared with the EF54.
Anyway, back to the work done on the set this week. The Mazda octal valve socket used for the sound IF amplifier valve has been replaced with an international octal socket. The Mazda SP61 pentode will be replaced by an Marconi-Osram Z66 or, if I can get away with it a vari-mu KTZ61 or W61.
The attachment shows that the vision IF transformer have received some modifications by the mystery engineer. I'm not going to make any attempt at returning the transformers back to the original EMI designers specification.
It seems unbelievable that someone has gone to all the bother of altering the receiver with such complex modifications when a simple external channel 1 to channel 4 frequency shifter unit would have done the job.
Some mornings I look at this set and think should I give up on this strange receiver? I've still got the mighty tools from my mine explorer days, those tools would see the set off alright.

DFWB.
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Old 4th May 2017, 6:47 pm   #33
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Default Re: HMV 1804 Television.

Yes I know the feeling David. Test the tube, if it is U/S this might decide it's outcome.
What a mess!
Mind you I need talk. I'm battling with an Ekco 15" T164 at the moment. Dozens of hard to get at torpedo shaped caps all O/C. Add the smoothing block and just about every other capacitor, six valves and a replacement CRM153 to replace the useless CRM152A. Making focus shunts to reduce the focus field for a tetrode tube. The picture is very good, no doubt about that but what an epic and still more to do!
It came from a customer during the shop days and has spent it's life under the workbench. The scope has been very useful tracing an obstinate sync fault so maybe it puts yours into perspective. John.
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Old 4th May 2017, 8:23 pm   #34
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Default Re: HMV 1804 Television.

Hi John,
good advice, I remember you saying that the Ekco gave you a bad time but in the end all the efforts proved worthwhile. So I will tough this one out to end, but it take a long time to finish the job. When the set is up and working we can compare it's performance against other "Birmingham" sets.
The replacement HT electrolytic capacitors were delivered today. 16 + 16mfd @ 500volts.

DFWB.
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Old 5th May 2017, 7:03 am   #35
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Default Re: HMV 1804 Television.

You could always cheat and fit the telly bit from a Fidelity AVS1600, that is if there is anything left of it after being lobbed from the 18th floor of a tower block. J.
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Old 6th May 2017, 9:25 am   #36
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Default Re: HMV 1804 Television.

Replacement HT smoothing capacitors were fitted yesterday. There was five 16mfd capacitors on the bracket, two were connected in parallel for 32mfd. The original capacitors were useless and have been replaced with three 16 +16mfd dual capacitors.

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Old 10th May 2017, 1:41 pm   #37
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Default Re: HMV 1804 Television.

Hi David,
I've left it so long to comment (no internet in the new home yet...).

Regarding the IF mods... I would try it as it is and then convert it back to the original design, but that is just my opinion.

The EHT transformer, er it might last 30 mins maybe?

I have a line blocking oscillator transformer, but the frame one I have is now in the 905, perhaps if I had the original 905 transformer rewound then you could have my 1804 spare.

The original Emiscope 3/4 CRT if it is any good is way better than the TA10 IMHO.

The 1804 is the only post war set where I could not get any of the electroytics to re-form. Mind you the set had hardly been used.

I look forward to the next installment.

Cheers
Andy
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Old 10th May 2017, 5:36 pm   #38
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Default Re: HMV 1804 Television.

Hi Andy,
It's all a bit of a dilemma, do I return the set back to it's original EMI design or tidy up the work that some unknown engineer had done more than sixty years ago? Why anyone should have gone to all the trouble of redesigning the signal circuits in such a manner is a mystery. As mentioned earlier, some sort of external frequency shifter device to receive the Birmingham TX would have been an easier solution. In 1982 I acquired an Ekco TS46 in which someone had fitted such a device in order this TRF receiver could receive a local transmitter in the West Country. Just one valve was employed, a 6A8GT pentagrid frequency changer valve.
There's no doubt the conversion work has been done in a competent manner, whoever done all this work knew what he doing.
Meanwhile work is progressing on the set. The resistance measured between the HT rail to chassis is now over 35Kohms. It was just a few Kohms.
I'm prepared for the EHT transformer to fail. The same component in my Marconi VT50 has done so already. The VT50 and 1804 are electrically identical, just differently styled cabinets.
Picture of the VT50.

DFWB.
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Old 10th May 2017, 6:56 pm   #39
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Default Re: HMV 1804 Television.

No, I wouldn't- if it had been messed up by a complete numpty, then yes, obviously- but that doesn't seem to have been the case. There are other factory-standard examples of this set out there, but this chap has only modified this one, and with an exceptional level of knowledge, too. Why? When? I think it's fascinating, and clearly a labour of love for someone. Whoever he was, (and he's almost certainly no longer with us) give him a massive pat on the back and keep it as it is, he worked hard on that set.

Oliver
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Old 10th May 2017, 7:41 pm   #40
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Default Re: HMV 1804 Television.

Hi Oliver,
That's right I don't think the HMV 1804 is all that rare, in fact it could be said the Marconi version far less common.
The big problem is that almost every IF transformer has received attention from the mystery engineer. Until a signal generator is hooked up we don't even know if the vision and sound IFs are the same as the original EMI values.
For safety the EHT transformer can be disconnected when performing adjustments on the signal circuits. The same transformer also supplies the timebase valve heaters and the CRT heater.
Very early production sets had a switch ganged to the brightness control which switched off the mains supply to the EHT transformer thus allowing the set to be operated as a sound only receiver. Evidently the BBC National radio service was at one time radiated from the AP transmitter. Can anyone confirm this?

DFWB.
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