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Old 9th Jun 2017, 9:32 am   #81
FERNSEH
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Default Re: HMV 1804 Television.

The first attachment shows the vision demodulator and amplifier stages of the HMV 1804 and Marconi VT50.
Negative grid bias for the video amplifier valve is derived from the voltage drop across potentiometer VR2 which is connected in series with the HT negative return. Now that the timebases are up and running it is now possible to check the performance of the post detector video amplifier stage. While the timebases were out of action it was not possible to test the video amplifier because insufficient negative bias was developed across video amplifier bias control VR2.
The maximum video developed across the video demodulator load resistor R25 should not be more than 3 volts P - P. As a result of the high sensitivity of the modified receiver an additional vision gain control is required and such a device was fitted in this receiver, the 5Kohm potentiometer was mounted on a bracket and was fitted on the underside of the set, see second attachment. This control will be relocated at the back of the set next to the sensitivity control.

DFWB.
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Old 10th Jun 2017, 8:21 pm   #82
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Default Re: HMV 1804 Television.

I reckon the CRT is good. Using the HKS method of testing a CRT, the heater is supplied with only 3.5 volts and the anode 2.5KV from the portable solid state EHT generator. See post #70. In fact the EHT generator produces nothing like 4KV and the highest potential is at present only 2.5KV. The solid state module is receiving only 7.5volts. I'm not sure what the maximum input voltage should be.
For the test procedure the grid and cathode are connected together and returned to the negative terminal of the EHT generator. +2.5KV is supplied to the anode.

DFWB.
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Old 10th Jun 2017, 8:59 pm   #83
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Default Re: HMV 1804 Television.

More progress. Following the successful CRT test procedure the receiver was switched on and we now know that the timbases are functioning. The focus control does not work however. It's possible that the wirewound focus pot is OC.

DFWB.
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Old 11th Jun 2017, 3:04 pm   #84
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Default Re: HMV 1804 Television.

The focus control is OK. However, the control has a 150 ohm series resistor, it's open circuit.

At present full HT current is flowing through the focus coil.

DFWB.
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Old 12th Jun 2017, 11:40 am   #85
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Default Re: HMV 1804 Television.

This morning the open circuit 150 ohm resistor was replaced so now the focus control is working as it should. Even with only 2KV there is a reasonably bright raster. There is a great big ion burn on the screen, this might not be as bad when a full 4KV of EHT is supplied to the CRT anode. Also worth bearing in mind the tube heater is under run, there's actually only 3.3 volts present on the heater connections.

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Old 12th Jun 2017, 12:56 pm   #86
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Default Re: HMV 1804 Television.

Fingers crossed David! I'm not sure whether I can survive the tension!! I know that this one has fought you all the way ..... but I can't wait to see a picture!
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Old 12th Jun 2017, 2:06 pm   #87
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Default Re: HMV 1804 Television.

Ah.. making good progress David.
It would be interesting to see how the visibility of the ion burn is dependent of the EHT value!

Jac
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Old 12th Jun 2017, 2:07 pm   #88
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Default Re: HMV 1804 Television.

Well there's just the video amplifier to sort out. The high gain RF and IF amplifiers have been tamed so now there is a sensible 3volts of +ve video on the grid connector of the Z66 video amplifier. The negative going video signal at the anode of the valve is AC coupled to the CRT cathode through a 2mfd electrolytic condenser, that part needs replacing. More serious is the fact the CRT connector has fallen into pieces so something will have made up there.

Let's see if some sort of picture can be displayed on the screen tonight.

DFWB.

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Old 13th Jun 2017, 10:54 am   #89
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Default Re: HMV 1804 Television.

At last something to show after all the time and effort it has taken to get this far. The sync separator is not working. It's rather difficult to take voltage and waveform measurements because the sound output transformer blocks access to the KTZ63 sync valveholder and other components.

DFWB.
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Old 13th Jun 2017, 2:37 pm   #90
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Default Re: HMV 1804 Television.

Getting there! Getting there! (Like British Rail .... !!!)
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Old 13th Jun 2017, 3:15 pm   #91
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Default Re: HMV 1804 Television.

I think it's time to take a chance and connect up the receiver's own EHT transformer. Also, the set is still running on reduced HT, a 10Kohm resistor was inserted in series with negative tag of the reservoir capacitor.

OK, maybe not, let's sort out the sync separator first.

DFWB.
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Old 13th Jun 2017, 6:33 pm   #92
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Default Re: HMV 1804 Television.

You've come so far - "measure twice - cut once mate!" I do hope that the ion burn fades out ..............
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Old 13th Jun 2017, 7:42 pm   #93
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Default Re: HMV 1804 Television.

The best part of 7000 views and all we have is a 'Blob'!

If your scared of the EHT David I can send you up a pack of surgical rubber gloves and a pair of rubber galoshes.
I could enclose my Neoprene diving suit but it's rather too hot for that at the moment and you might scare the locals.
If you are unfortunate enough to grab hold of the puny EHT hopefully you will come to no harm.

Seriously that must be a really good 3/4 tube to produce such a bright image with less than 3kv on it's anode. Go on, plug it in! Regards, John.
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Old 13th Jun 2017, 8:04 pm   #94
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Default Re: HMV 1804 Television.

Hi John,
"dangerous voltages exist in television receivers and the back cover should be only removed by competent personnel".

Alright then, tomorrow I'll go for broke and reconnect the EHT transformer.

DFWB.
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Old 14th Jun 2017, 12:11 am   #95
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Default Re: HMV 1804 Television.

Quote:
Originally Posted by Jac View Post
Ah.. making good progress David.
It would be interesting to see how the visibility of the ion burn is dependent of the EHT value!

Jac
Good progress. The sync separator fault has been fixed and now line timbase is locked to the sync pulses. It's not the same for the frame oscillator because the interlace circuits were removed during the early stages of the restoration process.
The ion burn: hopefully when the EHT is raised to the correct voltage the ion burn will become less visible.

DFWB.
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Old 15th Jun 2017, 11:19 am   #96
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Default Re: HMV 1804 Television.

The EHT transformer has been rewired into the receiver circuit. To be on the safe side the variac supplied the mains to the receiver. The U16 EHT rectifier was not used at this stage. Over an extended period of time the supply voltage was increased until the full 240 volts was attained. I can report there has been no problems. The timebase valves and CRT heaters are supplied by the EHT transformer and are glowing normally. It's safe to say that 4KV AC is present at the top cap connector of the U16.
So now it's time to find out if 4KV DC is available for the anode of the CRT. It all depends on the condition of the two 0.03mfd EHT smoothing capacitors. I'm advised that according to an EMI service bulletin these capacitors can be replaced by a single 0.1mfd capacitor.

DFWB.
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Old 15th Jun 2017, 1:11 pm   #97
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Default Re: HMV 1804 Television.

Hi David,

Looks like it is going to be a great set after all.

On my 1804 I did a trick which I often do on electromagnetically focussed sets, that is I reversed the focus pot. Ofted the sweet spot on the focus pot is worn, so reversing it gets round replacing the pot (assuming the focus point is not exactly in the mid way position of the pot).

Cheers
Andy
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Old 15th Jun 2017, 2:51 pm   #98
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Default Re: HMV 1804 Television.

Hi Andy,
Considering the decrepit state the set was in when it was first placed on the work bench it came as a surprise to find all the rear wirewound potentiometers are in good condition.
I'll return to set this evening and then we'll find out how good the CRT really is and hopefully if the EHT transformer lasts the course.

DFWB.
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Old 15th Jun 2017, 7:12 pm   #99
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Default Re: HMV 1804 Television.

The results of this evenings work. The CRT is receiving the full 4KV on it's anode. Video drive to the cathode is 35 volts P - P. The 2.5Mc/s definition bars are visible.
The ion burn doesn't look so bad now.

DFWB.
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Old 16th Jun 2017, 11:05 am   #100
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Default Re: HMV 1804 Television.

Might be a good idea to replace the EHT smoothing capacitor, it's starting to bulge.
Check out the gassy Z66 video amplifier valve. Despite the pretty glow inside the valve it still works in it's role as the video amplifier.

DFWB.
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