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Old 10th May 2017, 8:05 pm   #41
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Default Re: HMV 1804 Television.

Hi David
I`m eagerly awaiting you to apply power to this set.
The 1804 is a real oldie and it would be lovely to see it working again.

Robin
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Old 15th May 2017, 4:22 pm   #42
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Default Re: HMV 1804 Television.

Hi Robin,
The restoration of the set is getting nearer to completion.
This morning it was the sound circuits that came under scrutiny.
The sound IF amplifier, demodulator and noise limiter circuit were modified by the unknown engineer.
The first attachment shows the original EMI sound circuits. The noise limiter operates by means of a cancelling circuit. The out phase noise spikes detected by the second section of the D63 double-diode are compared with those from the demodulator.
The second attachment shows the modified circuits. The "B" section of the D63/6H6 double diode functions as series type noise limiter, the type that was common use in 405 line TVs. The interference limiter diode is normally conducting. However, when a when a positive going spike comes along the time constant of the resistor and capacitor at the anode is long enough prevent the valve conducting.
An added refinement is AVC. One of the diodes in the DH63/6Q7G is used an AGC detector to control the gain of the sound IF amplifier valve. A degree of AVC delay is provided by the cathode bias volts of the DH63.
Once again, it's not clear why all these modifications were carried out, after all there wasn't anything wrong with the original EMI circuits.

DFWB.
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Old 16th May 2017, 8:05 pm   #43
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Default Re: HMV 1804 Television.

The screen grids of the Z66 video amplifier and the KT61 frame output valves are decoupled by two 4 microfarad electrolytic capacitors which are contained in a common can. To maintain the original appearance I've decided to restuff the dual capacitor.

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Old 18th May 2017, 11:09 am   #44
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Default Re: HMV 1804 Television.

Getting there, it can't be long before the set is fired up.

DFWB.
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Old 20th May 2017, 10:10 am   #45
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Default Re: HMV 1804 Television.

News Flash! At long last the set is connected to the mains. OK, it's not direct but through the variac. For starters the output is set to 100 volts just give the set a taste of electricity. An indirectly heated rectifier valve will be used for the initial tests and later on the correct directly heated U52/5U4 valve will be fitted. I'm presently concentrating my attention on the signal circuits and later I'll get brave and connect up the EHT transformer.
Attachment shows the mains plug. It's many years now since I've seen anything connected a plug like this.

DFWB.
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Old 20th May 2017, 10:17 am   #46
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Default Re: HMV 1804 Television.

People used to connect everything to the lighting circuit, you could even get double adapters for them. I remember the electric iron being used, light bulb in one socket iron in the other, swinging from the ceiling.
All good fun until it went dark when the fuse blew.
Frank
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Old 20th May 2017, 10:18 am   #47
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Default Re: HMV 1804 Television.

That plug reminds me of a Thurber cartoon titled "Electricity was leaking all over the house."

Peter
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Old 20th May 2017, 11:20 am   #48
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Default Re: HMV 1804 Television.

What's happened to the end of it? It looks like the contacts are drawn on a piece of card in pencil, which might give an unusually high input resistance. Or is this an ultra-cautious variation on a lamp limiter?

Oliver
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Old 20th May 2017, 11:43 am   #49
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Default Re: HMV 1804 Television.

Hi Oliver,
It's just dirty. I'll clean it up, you never know I might find some use for it. For the sake of nostalgia I could connect a TV to the light socket with it.
Could tell you some horror stories about domestic house wiring we'd often encounter in the fifties and early sixties.

DFWB.
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Old 20th May 2017, 8:08 pm   #50
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Default Re: HMV 1804 Television.

Quote:
Could tell you some horror stories about domestic house wiring we'd often encounter in the fifties and early sixties.
Sounds like a good idea for a whole new thread David !

Marc.
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Old 21st May 2017, 10:37 am   #51
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Default Re: HMV 1804 Television.

Quote:
Sounds like a good idea for a whole new thread David !
Hi Marc,
I'm sure there is many ex TV engineers in this forum who will be willing to tell us about the early days of TV servicing.

Getting back to the HMV 1804. Last night's servicing session ended with dramatic bangs and flashes, lots sparks inside the temporary 5Z4 rectifier. There is no fuses in this set but rest assured there will be after the restoration is completed. Fortunately the fuses in the variac blew so there was no serious damage. There doesn't appear to be any short circuits in the receiver, the resistance between the HT rails and chassis is >30Kohm. Replaced the 5Z4G with the correct 5U4G. This is a directly heated valve so the HT rises to 450 volts on switch on. All the HT electrolytics are rated at 500 volts so can withstand the switch-on surge.

DFWB.
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Old 21st May 2017, 2:10 pm   #52
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Default Re: HMV 1804 Television.

Never a dull moment ..... this is getting exciting David!! Sounds like asbestos gloves and Kevlar Y fronts time .......

Be careful Mate - and keep the dry powder extinguisher handy !!!!!!!
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Old 21st May 2017, 5:26 pm   #53
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Quote:
Originally Posted by Tractionist View Post
Never a dull moment .....
How about this? The severe motor boating noise has blown out the loudspeaker cone.

DFWB.
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Old 21st May 2017, 6:11 pm   #54
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Old 21st May 2017, 7:16 pm   #55
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Default Re: HMV 1804 Television.

Flipping 'eck ! That's certainly some motorboating !

Marc.
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Old 21st May 2017, 8:05 pm   #56
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Default Re: HMV 1804 Television.

I've never seen a loudspeaker cone moving in and out as much as this one was. Good fun while it lasted.
Good advice from Tractorist: Be careful Mate - and keep the dry powder extinguisher handy !!!!!!!
I'm prepared for the next drama with this set.


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Old 21st May 2017, 9:37 pm   #57
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Default Re: HMV 1804 Television.

Quote:
Originally Posted by FERNSEH View Post
How about this? The severe motor boating noise has blown out the loudspeaker cone.
Did I mention ear protectors and crash helmet? !!!!!!
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Old 21st May 2017, 10:28 pm   #58
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Default Re: HMV 1804 Television.

Did I mention ear protectors and crash helmet? !!!!!!
Things will get scary when the EHT transformer is reconnected. The EHT transformer carries the heater windings for the timebase valves and CRT. It's a legacy from when the early versions of the 1800 and 1804 had the facility to operate sound only. The HT supply transformer has the heater windings for all the valves in the signal circuits.

Connecting up the signal generator reveals that the sound IF is 11Mc/s, that's a long way from the original EMI frequency of 4Mc/s.
So what's the vision IF? We'll find out tomorrow.

DFWB.
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Old 22nd May 2017, 9:07 pm   #59
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Default Re: HMV 1804 Television.

The 33Kohm screen feed resistor of the Z66 final vision IF amplifier valve was found to be open circuit, likely the result of a leaky 0.047mfd decoupling capacitor.
The signal generator has been connected to the mixer grid. It turns out the vision IF is 14.5Mc/s. So it follows the local oscillator operates below the carrier frequencies. Tomorrow, I'll turn my attention to the RF and frequency changer stages.

DFWB.
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Old 23rd May 2017, 10:59 am   #60
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Default Re: HMV 1804 Television.

The local oscillator is working. This was proved by connecting a voltmeter via a 22Kohm resistor to the grid of the EC52 triode. A reading of 15 volts negative was noted.
This morning more exciting pyrotechnics. lots of smoke pouring out from the underside of the chassis. This time it is one of the IF transformers that has broken down. Later today the transformer will be removed from the chassis for examination.

DFWB.
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