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Old 3rd Jun 2018, 3:53 pm   #1
FERNSEH
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Default Marconi VT158 Picture displacement.

No amount of adjustment of the picture shift magnets will centralise the picture on the CRT screen. The fault appears after fifteen minutes.
The cause of the problem lies in the line sync discriminator circuits.
The line oscillator and flywheel discriminator employed in the Marconi VT158 and Ferguson 405T is a development of the circuits used in the 1955 Ferguson model 247T.
Link to the restoration of the Ferguson 247T: https://www.vintage-radio.net/forum/...d.php?t=119613

When working correctly the line timebase circuits employed in these sets are very stable and adjustment of the line lock control is rarely required.

The first suspect components are C121 and C123, particularly the latter.
The capacitors in the Marconi VT158 are of Suflex make.

DFWB.
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Old 3rd Jun 2018, 6:59 pm   #2
Heatercathodeshort
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Default Re: Marconi VT158 Picture displacement.

Now that is a proper TV David. Those caps are the first stop then a check on the discriminator diodes, EB91?

I have the Ferguson version on the pile, the 405T. When time permits. John.
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Old 3rd Jun 2018, 7:02 pm   #3
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Default Re: Marconi VT158 Picture displacement.

Capacitors C121 and C123 are OK, no need to replace those. However, replacing C128 (0.005mfd) effected an improvement. C128 is connected between pin 2 of V19 and ground. Also, a further improvement was effected by replacing the discriminator diodes, V19 (EB91).
In order to adjust the line hold control the knob must be pushed in and turned to set the line oscillator as close to the correct frequency as possible. After the knob is released the picture should be correctly locked, if it is off-set then the core of L59 must be adjusted to position the picture in the raster.
This can't be done because the core has been crunched up and is locked solid in the coil former.
The vision AGC is now acting up. The line sync back porch gated AGC system can't handle the strong signal levels issuing from the Aurora, the RF gain control has to be turned well down. Possibly the decoupling capacitors in the AGC circuits are leaky.

DFWB.
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Old 3rd Jun 2018, 7:39 pm   #4
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Default Re: Marconi VT158 Picture displacement.

News Flash! The fault has returned, in fact the picture is even further off-set to the left.
The Ferguson 405T and the HMV and Marconi branded versions models 1871 and VT158 are part of a series of high performance fringe area receivers dating back to 1953. Starting with the models 990T and 991T.
The final model in the series was the Ferguson 505T.

DFWB.
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Old 3rd Jun 2018, 10:36 pm   #5
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Default Re: Marconi VT158 Picture displacement.

There are generally two reasons this picture shift can occur. If the phase shift in the signal fed back to the discriminator from the line output stage is altering (its done with RC filtering). Or the other way is excessive H oscillator drift, because any drift is attempted to be cancelled out by the flywheel discriminator (feedback loop) to keep the H osc in lock. This is why on these circuits of course the H hold control rotation causes the picture to move from side to side.

However also in this case every circuit component is suspect in the H osc & discriminator circuit, as they are all enclosed in the feedback loop. Any changes inside the loop will have this effect and the picture phase will shift in an attempt to neutralize the change, until that is, it goes too far and drops out of lock.

This can be one difficulty fault finding inside a feedback loop. One service technique is to break the loop (disconnect the discriminator's control voltage to the H osc, and substitute a DC voltage an look for what is changing then)

All the usual suspects, power supply stability to the stage, leaky capacitors, faulty resistors, the valves themselves all need to be checked. It will be interesting to see which part it turns out to be.
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Old 3rd Jun 2018, 11:50 pm   #6
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Default Re: Marconi VT158 Picture displacement.

..and of course the circuit relies on being fed a sensible & stable amplitude H sync pulse, so if there are problems in the AGC or signal overload issues in the RF/IF system that could affect the form (shape) of the sync, but the clue there is that the picture contrast (video signal amplitude at the detector & video opt) would likely change when the fault occurred, and then the fault would be outside the confines of the feedback loop. It might pay to hang the scope on the video detector output and then the sync sep outputs too.
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Old 3rd Jun 2018, 11:59 pm   #7
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Default Re: Marconi VT158 Picture displacement.

Argus25 wrote: "All the usual suspects, power supply stability to the stage, leaky capacitors, faulty resistors, the valves themselves all need to be checked. It will be interesting to see which part it turns out to be."

Two components in the feedback loop between the discriminator and reactance valve have been replaced. The 1microfarad capacitor C114 was found to have a 1Mohm leak. Also replaced C127 (0.1mfd). Another capacitor in the loop will be replaced tomorrow, C124 (0.005mfd).
So now the picture is slightly displaced to the right.

DFWB.
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Old 4th Jun 2018, 3:44 pm   #8
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Default Re: Marconi VT158 Picture displacement.

Replacing the capacitors in the feedback loop has effected a complete cure.
I'll turn my attention to the vision AGC system. More about that later today.

DFWB.
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Old 4th Jun 2018, 8:56 pm   #9
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Default Re: Marconi VT158 Picture displacement.

Circuit and description of the vision AGC circuit.
The AGC works in the sync cancelled principle. Negative going video is supplied to the grid of V7A. The sync pulses are cancelled out by the negative going sync pulses from the anode of the sync separator.
The other section of V7 functions as a pulse gated AGC amplifier.

DFWB.
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Old 5th Jun 2018, 8:47 am   #10
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Default Re: Marconi VT158 Picture displacement.

Hello David, These Thorn chassis are without doubt the best fringe models ever produced. There was a lot of snobbery directed at Ferguson from snooty Bush, Ekco, Murphy and other high end dealers but it is forgotten that Thorn led the way with development of television design and circuitry and they were incredibly simple to service both electrically and mechanically.
Their service manuals were one of the very best.

Pye became a close second but their chassis were often difficult to service and suffered from assembly line errors.

I must get that 405T out and give it justice! From the front it looks exactly like the common 406T that is, until you take the back off. There the similarity ends. [It's the bottom receiver] John.
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Old 5th Jun 2018, 10:33 am   #11
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Default Re: Marconi VT158 Picture displacement.

Quote:
Originally Posted by Heatercathodeshort View Post
Hello David, These Thorn chassis are without doubt the best fringe models ever produced. There was a lot of snobbery directed at Ferguson from snooty Bush, Ekco, Murphy and other high end dealers but it is forgotten that Thorn led the way with development of television design and circuitry.
I agree, “Fine sets these Fergusons”!

Last edited by dazzlevision; 5th Jun 2018 at 10:33 am. Reason: Extra text.
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Old 5th Jun 2018, 10:17 pm   #12
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Default Re: Marconi VT158 Picture displacement.

Hi!

Dick and Smithy in Radio Constructor's "In Your Workshop" did a full article on servicing this gated AGC Circuit, it's a design by Mullard and one of the most straightforward of all the horrible 405–line–only gated AGC systems!

A search on the Radio Constructor page should find it – try looking for "In Your Workshop Mullard AGC" on American Radio History!

I've not replied with a circuit – explanation in this case because Smithy & Dick in R.C. have already done so in as much detail as I could with explanatory waveforms, etc!

Chris Williams
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It's an enigma, that's what it is! This thing's not fixed because it doesn't want to be fixed!

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Old 6th Jun 2018, 11:28 am   #13
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Default Re: Marconi VT158 Picture displacement.

The vision AGC system is now working correctly, the local-distant control is set to the point when the picture becomes snowy, then turned back a few degrees to display a noise free picture.
The attachment shows that the picture is displaced slightly to the right. It is possible to centralise the picture but it would involve removing the line oscillator transformer in order that the crunched iron dust cores can be replaced.
Much better to leave things as they are.

DFWB.
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