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Old 29th Aug 2017, 6:33 pm   #21
FERNSEH
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Default Re: Gec 12" bt5144.

Even with only 175 volts mains supply there is already a 2mm spark at the anode of the U37 EHT rectifier diode.
Let's wind up the variac and see what happens.

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Old 29th Aug 2017, 6:41 pm   #22
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Default Re: Gec 12" bt5144.

Full 240 volts mains is supplying the set and a 10mm spark can be drawn form the EHT rectifier.

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Old 29th Aug 2017, 10:55 pm   #23
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Default Re: Gec 12" bt5144.

Waveform present at the grid of the frame oscillator, V8B. Scope is set to 50volts/div. The negative going sawtooth waveform has an amplitude of 100 volts P - P. Note the amplitude of both the positive and negative spikes.

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Old 31st Aug 2017, 6:34 pm   #24
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Default Re: Gec 12" bt5144.

After the short stint as a test receiver to analyse the currents in the HT reservoir capacitor the restoration of the other circuits can continue.
Today my attention has been turned to the RF deck. It was noticed that the voltage of the decoupled HT supply would reduce when the volume control was turned up. Obviously a leaky coupling capacitor and it was soon found the capacitor C24 (0.02mfd) between the anode of the DH77/6AT6 audio amplifier and the grid of the N37 output valve was leaky. Two other capacitors in the audio signal path were found also to need replacement. This set has the later RF deck which employs a GEX34 germanium diode as the series sound noise limiter.
Later today I'll turn my attention to the vision circuits in the TRF deck.

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Old 31st Aug 2017, 7:37 pm   #25
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Default Re: Gec 12" bt5144.

Circuit diagram of the sound stages showing in dotted lines the later noise limiter employing a GEX34 germanium diode.

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Old 31st Aug 2017, 7:46 pm   #26
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Default Re: Gec 12" bt5144.

The attachments show the negative going video waveform present at the anode of the video amplifier valve. A full 40 volts P - P of video is available to supply the cathode of the CRT. Contrast control is fully clockwise and the RF gain near to the maximum setting. Signals supplied from an Aurora standards converter set to channel B1.

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Old 31st Aug 2017, 9:34 pm   #27
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Default Re: Gec 12" bt5144.

I could be mistaken but I can almost see a test card appearing in the first image of the 'scope.
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Old 31st Aug 2017, 9:49 pm   #28
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Default Re: Gec 12" bt5144.

Quote:
Originally Posted by FERNSEH View Post
The attachments show the negative going video waveform present at the anode of the video amplifier valve. A full 40 volts P - P of video is available to supply the cathode of the CRT.
That is a good looking line of video (on the first image), it is easy to see from the shape of the syncs and the rising and falling edges and the amount of overshoot that the frequency response is about right for a good picture. Your RF circuits must be in good alignment.It does look like a line of a testcard image as Dom says.
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Old 31st Aug 2017, 10:33 pm   #29
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Default Re: Gec 12" bt5144.

Hi David,
As has been said, that is a very smart video waveform. Lovely sharp sync pulses! So when do we get to see what it looks like on the screen??
Another great 'back from the brink' restoration I'm enjoying the progress so far!
All the best
Nick
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Old 1st Sep 2017, 12:14 am   #30
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Default Re: Gec 12" bt5144.

Dom wrote: "I could be mistaken but I can almost see a test card appearing in the first image of the 'scope"
Hi Dom,
I've seen that effect, apparently the trick is to invert the video waveform by turning up the vision gain control so that a negative picture appears on the screen.
Hi Argus25, the GEC BT5144 is a TRF receiver so it is much easier to achieve an vision excellent bandwidth characteristic in comparison with the superhet TV receivers that were made about the same time as the BT5144. Vision intermediate frequencies as low as 9 to 14 MHz was common in post-war receivers. 4Mhz in certain pre-war TV receivers such as the GEC BT8161. My GEC BT9121 has a vision IF of 6Mhz. Not easy to achieve full 2.7Mhz vision bandwidth in these receivers.
Hi Nick,
I'll reinstall the CRT tomorrow, I've a feeling the tube is a good one. There's a good spark at the anode of the EHT rectifier and I can hear the frame output transformer purring at 50Hz.
The CRT was made by GEC and has proven to be a very robust thing. Has a simple triode gun assembly, flat faceplate.

DFWB.
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Old 1st Sep 2017, 12:57 pm   #31
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Default Re: Gec 12" bt5144.

Results at last, but all the rear slider controls need repair, it was expected these would be damaged when the chassis got bashed up.

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Old 1st Sep 2017, 1:14 pm   #32
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Default Re: Gec 12" bt5144.

Looking really close now David, News at tea time ?

Marc.
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Old 1st Sep 2017, 1:35 pm   #33
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Default Re: Gec 12" bt5144.

"News at tea time ?" Hi Marc, it's News Flash time, just a few jobs to do around the frame timebase circuit. The vertical form control doesn't work and the sync lock is weak.

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Old 1st Sep 2017, 4:48 pm   #34
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Default Re: Gec 12" bt5144.

I love GEC! Well built with no gimmicks. That looks a cracker David.

Considering it was an unwanted puppy left on your doorstep you have done a great job bringing it back to life.
How many Bob Martins tablets did you have to ram down it's throat?

The GEC triode picture tubes [not the GEC badged Mazda ones]from 1951 to 1957 had a very good reputation. The chassis runs nice and cool and that alone must have guaranteed reliability. I think the GEC made the best use of the rather poor MOV valve range. John.
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Old 1st Sep 2017, 5:39 pm   #35
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Default Re: Gec 12" bt5144.

Hi John,
The CRT is absolutely brilliant, just the focus magnet to attend to, it's jammed and can't be adjusted.
The frame linearity fault has been sorted, 'fraid it's a case of mea culpa, the linearity feedback capacitor C58 had been soldered to the wrong tag. The capacitor had a 2megohm leak so it was replaced.
Definition is excellent, good for the 3meg bars.

DFWB.
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Old 1st Sep 2017, 5:49 pm   #36
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Default Re: Gec 12" bt5144.

Great work [and effort] David .... Bravo!
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Old 1st Sep 2017, 7:59 pm   #37
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Default Re: Gec 12" bt5144.

Hi David,

That has come on well hasn't it, mind you I do have a soft spot for GEC.
What time will Robert Dougall be on ?

Marc,
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Old 1st Sep 2017, 9:08 pm   #38
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Default Re: Gec 12" bt5144.

Marc wrote:
"What time will Robert Dougall be on?" 10 o'clock and the picture has started t roll. What could have gone wrong? Could be the B36, the search is on for a 12SN7GT.

DFWB.
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Old 3rd Sep 2017, 12:11 pm   #39
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Default Re: Gec 12" bt5144.

Two metal adaptor plates have been made to attach the tensioning springs for the webbing strap that will be used to secure the CRT in situ.

It'll be a lot better than the previous arrangement where a piece of co-ax cable was used to keep the tube in position.

DFWB.
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Old 5th Sep 2017, 8:31 pm   #40
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Default Re: Gec 12" bt5144.

Found two springs to tension the CRT securing strap. See first attachment.
The two 25ohm upper sections of the mains dropper have gone open circuit. There's absolutely no chance of finding an exact replacement but I reckon the one I've found will serve as an excellent replacement for use on the 240volt mains supply.
The three 17ohm HT sections add up to 51ohms and the heater sections add up to a value near enough as the original resistor. However, the original mains dropper is secured to the chassis by a capacitor clip, to use the replacement a special adaptor plate will be made and the dropper will held in position by a piece of threaded rod.

DFWB.
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