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Old 7th Jul 2006, 7:44 pm   #1
Colourstar
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Default Ferguson 506T Quickie!

Evening all. Just a quickie! I acquired a Ferguson 506T recently, previously rescued from the basement of a house. The set was filthy inside and out (see photos for as-found condition!) There is a component listed as Z1 in the ERT service sheet, which has clearly lived a hot and hard life above the dropper. All that is left are it's two legs and a fragment of shattered carbon disc. I'm still a fumbling beginner in these matters and am a little embarassed to admit that I'm not entirely sure what component this was and indeed what would make a suitable modern replacement. Anyone care to assist?
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Old 7th Jul 2006, 8:06 pm   #2
ppppenguin
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Default Re: Ferguson 506T Quickie!

Z1 is a varistor (VDR) in the line timebase. I think it's to do with stabilisng the width. I doubt it will have overheated on its own, more likely due to the dropper nearby.
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Old 8th Jul 2006, 8:55 am   #3
Mike Phelan
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Default Re: Ferguson 506T Quickie!

The Fireball tuner will need a good clean, as well!
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Old 8th Jul 2006, 2:38 pm   #4
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Default Re: Ferguson 506T Quickie!

So it's a Voltage Dependent Resistor! Thanks Jeff. Are these still available in any guise suitable for this application? It doesn't seem to have any specific value stated on the circuit, so how are they rated?

Yes Mike, the tuner will get a good squirt of switch cleaner in due course!

Steve
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Old 8th Jul 2006, 2:41 pm   #5
tvden
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Default Re: Ferguson 506T Quickie!

looks like the set has a bit of worm that needs treating den
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Old 8th Jul 2006, 3:02 pm   #6
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Default Re: Ferguson 506T Quickie!

Hi,

Z1 is listed as a VV111 in the Newnes radio & TV Servicing book for this model. I've not heard of this type.

An E298ZZ/06, made by Mullard (Philips), will almost certainly replace it (or the PCB mounting version).

The most likely cause of Z1's demise is the line pulse coupling capacitor, C65 (shown as 1000pF). C65's replacement will need to be a high voltage ceramic type (I suggest 2kV).

The function of Z1 and the associated components is to keep the width constant with mains voltage variations and line output/boost diode valve ageing.

Regards,

Dazzlevision
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Old 8th Jul 2006, 5:35 pm   #7
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Default Re: Ferguson 506T Quickie!

Thanks Dazzlevision.

Dennis- Luckily the set has suffered no woodworm. With a wafer thin plywood case it wouldn't provide much of a meal anyway!

Steve
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Old 9th Jul 2006, 10:12 pm   #8
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Default Re: Ferguson 506T Quickie!

Hello,
I thought this was a bit odd when you said it was mounted ABOVE the mains dropper. I immediately thought that this was the FRAME stabilization VDR mounted in that way so that it would be deliberately effected by the said heat. [ As the scan coils warm up their resistance increases causing the height to drop slightly. The vdr in circuit across the primary of the frame output transformer compensates for this. There is a MISTAKE [printing error]on the circuit. The vdr mounted above the dropper is Z2 [frame] and the line timebase one is mounted on the lower P/C board just behind the stabilization pot at the front edge. They are not the same type. The later 850 chassis ones can be used, green spot frame, biege spot line. All this is from memory so take care with my advice.! Regards John.
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Old 10th Jul 2006, 7:13 am   #9
dazzlevision
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Default Re: Ferguson 506T Quickie!

Hello again.

If the VDR that has failed is mounted across the primary winding of the frame output transformer (Z2 on the official Ferguson circuit), it can be replaced by the "green spot" Mullard/Philips E298ED/A258 type. The function of this is to suppress high peak voltages in the primary circuit during frame flyback.

There is a THERMISTOR in series with the frame scan coils (actually mounted on the scan coils) that compensates for height reduction due to their resistance rising with temperature increases as the set warms up.

Regards,

Dazzlevision
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Old 10th Jul 2006, 7:47 am   #10
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Default Re: Ferguson 506T Quickie!

Hello,
Of course you are right about the thermistor in the scan coils. Most sets if not all from around 1958 had ome mounted deep in the scan coils.
The thermistor mounted above the dropper is Z2 and can be replaced with the green spot rod type, I well remember fitting them across the primary of the Philips 19TG170A frame output transformer to prevent premature failure. Regards John.
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