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Old 22nd Sep 2015, 5:11 pm   #21
FERNSEH
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Default Re: Ferguson 247T: huge console TV set.

Both timebase oscillators are now functioning. The frame oscillator delivers between 5 to 30 volts of sawtooth drive to the grid of the PCL83 output valve. First picture
The negative going drive to the line output valve is 150 volts P-P.

DFWB.
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Old 2nd Oct 2015, 10:39 pm   #22
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Default Re: Ferguson 247T: huge console TV set.

Time to perform more tests on the chassis. The line output transformer has been reconnected into the circuit and also the sawtooth drive to the PL81 control grid.
Even without the scan coils connected there is a healthy spark present at the anode of the EY86 EHT rectifier. The EY86 heater was unlit so in went a replacement.
I reckon it's time to refit the chassis and see what the results will be like on the screen of the CRT.

DFWB.
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Old 3rd Oct 2015, 11:39 am   #23
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Default Re: Ferguson 247T: huge console TV set.

The chassis has been reinstalled in the cabinet. The CRT socket and the scan coils plugged in, the results? No EHT. Unplug the scan coils and the EHT returns, it would seem like the scan coils are faulty or perhaps a component associated with them.

DFWB.
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Old 3rd Oct 2015, 3:57 pm   #24
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Default Re: Ferguson 247T: huge console TV set.

The line scan coils have a DC resistance of 2.9 ohms, this may the correct figure, the line scanning coils in the later model 308T have a resistance of 2 ohms.
Reference to post #10. John mentions the possibility of the insulation the tag strip on the scan coils breaking down.
The service notes on page 380 of the R & T servicing book inform us that the scan coil assembly can be removed without the need to withdraw the CRT from the cabinet.

DFWB.
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Old 3rd Oct 2015, 5:46 pm   #25
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Default Re: Ferguson 247T: huge console TV set.

Another monstrous horror that has found its way up North to David rather like an under fed cat...J.
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Old 3rd Oct 2015, 6:19 pm   #26
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Default Re: Ferguson 247T: huge console TV set.

I'm certain now that the scan coils are not faulty. I've hooked a another scan assembly, albeit not Ferguson, and when those coils are connected into the line circuit it's the same effect, the EHT dies.

In these line timebase circuits the stored energy in the scan coils is at the time of flyback contributes the first part of the scan. It could be possible that another component is breaking on account of the extra energy from scan cols.

DFWB.
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Old 3rd Oct 2015, 6:25 pm   #27
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Default Re: Ferguson 247T: huge console TV set.

I hope you changed the boost capacitor..Truth please. John.
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Old 3rd Oct 2015, 6:31 pm   #28
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Default Re: Ferguson 247T: huge console TV set.

Hi John,
I cannot tell a lie, I have not but rest assured I will.

DFWB.
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Old 3rd Oct 2015, 9:34 pm   #29
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Default Re: Ferguson 247T: huge console TV set.

It was the boost capacitor all right, open circuit! . Glasnost, mea culpa. Well to be honest it was disconnected.
Anyway, in with a new one.

DFWB.
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Old 3rd Oct 2015, 10:20 pm   #30
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Default Re: Ferguson 247T: huge console TV set.

That's more like it! EHT is present with or without the scan coils plugged in.
Strange that the boost voltage could be measured at pin 10 of the CRT without the boost capacitor in the circuit.

DFWB.
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Old 3rd Oct 2015, 11:42 pm   #31
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Default Re: Ferguson 247T: huge console TV set.

News Flash! Real results at last. A bright raster, seems that the CRT is good, plenty emission. The line hold control has no effect so it's likely the lack of width could be caused by the oscillator running at the wrong frequency.
The original PCF80 in the tuner had to be be replaced in order channels could be tuned in on band 1.
The contrast control doesn't work, it's part of a complex gated AGC system.

So there we have it, real progress today.

DFWB.
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Old 5th Oct 2015, 5:02 pm   #32
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Default Re: Ferguson 247T: huge console TV set.

The reason why the line hold control was inoperative was due a leaky 1 mfd Hunts make capacitor C161. Down to 20Kohms! This capacitor is part of the filter network which supplies the control voltage to the reactance valve V15, an EF80. C161 provides the long time delay necessary for the flywheel line sync system to function properly.
V15B, ECC81 is the line oscillator and the A section is a pulse shaper.
In fact all the waxies and Hunts capacitors in the line discriminator had various degrees of leakyness and so the whole lot have been replaced.

A number of capacitors in the vision AGC line had leaks and so those components have been replaced also.

The next stage of the restoration is to reinstall the chassis to find out if all this work has proven worthwhile.

DFWB.
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Old 5th Oct 2015, 7:06 pm   #33
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Default Re: Ferguson 247T: huge console TV set.

Nice bit of detective work there David. That line oscillator/flywheel sync circuit should be 100%.
The EF80 reactance valve used to go to sleep due to under use but would wake up if swapped with one of the IF valves.

If you don't get any video signal but all seems ok, remember the vision detector crystal inside the final vision IF can. These tend to become O/C after all these years. Changed a few recently.

Looks good so far. That monster started life as a little 203T 14" table model!
John.
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Old 5th Oct 2015, 9:53 pm   #34
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Default Re: Ferguson 247T: huge console TV set.

Quote:
Originally Posted by Heatercathodeshort View Post
That monster started life as a little 203T 14" table model!
Hi John,
That's right, in fact the main chassis is quite compact. Additional metalwork is added to accommodate the wider control panel used in the 21" models.
The OA60 vision detector checks OK. However, two additional germanium diodes are employed in the video amplifier, W106 (OA70) and W102 (OA61). A short circuit in the PCF80 video amplifier valve has resulted in the failure of W102 the OA61 vision interference limiter diode. It's gone short circuit. For the time being there is no need to replace this diode.
The other diode W106 functions as an extra white spot limiter and is employed only in the 21" models. An additional feature on schedule B 21" models is a picture quality control which takes the form of a variable resistor R233 (2Kohms) in series with the vision detector load resistor.

DFWB.

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Old 6th Oct 2015, 9:06 pm   #35
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Default Re: Ferguson 247T: huge console TV set.

After reinstalling the chassis into the cabinet I discovered that the line hold control still doesn't alter the frequency of the oscillator. So it's out with the chassis again and back on the bench.
The slider of the line hold control is connected to the line discriminator via a tag strip on a separate line oscillator module which is located above the main chassis. It was found that the screening plate was shorting line hold control to chassis. I've made a spacer to prevent that happening again.
Upon testing the chassis again I found the line hold control still doesn't work.
Another short circuit, this time metallic particles were found in the special line hold control. The control was removed and cleaned up. At last the line hold control works.
A video waveform can be monitored across the diode load of the video detector, but there was evidence of over loading from the signals from the Aurora and my own make standards converter. All the Hunts and waxie decoupling capacitors in the vision AGC system were as expected found to be leaky so the whole lot were replaced.
The result of this action now the overloading is even worse. Now I'll turn my attention to the gated AGC circuit. This consists of an EF80 pentode which functions as the line sync back porch sampler and amplifier. The amplitude of the negative going pulse present at the EF80 anode depends on vision signal amplitude, this is rectified by an EB91 diode and filtered and supplied to the AGC line.

DFWB.
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Old 7th Oct 2015, 12:15 pm   #36
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Default Re: Ferguson 247T: huge console TV set.

The circuit of the gated and amplified vision AGC system. The negative going line sync pulse present at the anode of the sync separator is differentiated by a low value capacitor and the positive going trailing edge which is co-incident with the back porch of the line sync allows the V7 to conduct only in that period. The resultant vision signal amplitude dependant pulse is rectified by the EB91 V8A. Delayed AGC is supplied to the tuner from diode W103.

The vision AGC system is now working correctly but there are still problems with the line flywheel discriminator to sort out.
Refer to the circuit on post 32. The cathodes of the EB91 are not receiving any sine wave drive from the discriminator transformer. The coils rsistances are correct, 66 + 66 ohms. It's possible the 0.01mfd capacitor C172 is OC.

DFWB.
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Old 7th Oct 2015, 1:00 pm   #37
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Default Re: Ferguson 247T: huge console TV set.

Well here's a surprise. I've opened up the line oscillator transformer can. I was expecting super quality silver mica tuning capacitors but just look at the nasty things found instead inside the can. Just ordinary Hunts 0.01mfd capacitors.
And both are leaky! Let's see if I can something better.

DFWB.
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Old 7th Oct 2015, 5:22 pm   #38
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Default Re: Ferguson 247T: huge console TV set.

'I've changed everything Captain Mainwaring and it still does not work.'
You missed these you stupid boy..

I must admit David I thought that you had changed those nasty brown cockroaches as they are clearly marked on the circuit diagram. Number 1 trouble spot even back in 1956!

You should see some real results now.
Now let's get this buttoned up, back on, cleaned up and soak tested. It will be a cracker by the look of it. John.
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Old 7th Oct 2015, 5:41 pm   #39
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Default Re: Ferguson 247T: huge console TV set.

As they used to say:
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Old 7th Oct 2015, 7:36 pm   #40
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Default Re: Ferguson 247T: huge console TV set.

Quote:
Originally Posted by Heatercathodeshort View Post
Now let's get this buttoned up, back on, cleaned up and soak tested. It will be a cracker by the look of it. John.
Hi John,
changing those capacitors has certainly improved matters, although the picture will need centering by adjusting the phasing coil on the transformer.
Now the bad news. The vision AGC system has packed in again, no control of contrast. So it'll be out with the chassis again.
Just wait one more week and it will really be a fine set again.

DFWB.
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