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Marconi 4701. 2000 series colour. My greatest restoration p

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Till Eulenspiegel
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Hi Chris,
Thanks for the kind offer. In fact T4 is OK in both chroma boards. As well as the 7.8Kc/s coil transformer T1 is missing in both spare boards. I'll have to check out what T1 does in the circuit, Again, the Mullard book might come to the rescue for coil winding details. There does appear to be a certain amount of similarity between the Mullard and BRC designs.
It's the same with the early CTVs from Pye and GEC.

Till Eulenspiegel.

 
Posted : 11/04/2016 11:39 pm
crustytv
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T1 on the Chroma board, that'll be this fella then :bba

Same offer but if you'd rather build, then hopefully these observations and photos may help.

It does look very simple to recreate, no fancy winding, just a few turns on a paxolin block with two holes and one ickle cap. The wires seem to have a different outer varnish, one is green.

Winding ohms reading

Posted : 12/04/2016 12:50 am
Till Eulenspiegel
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Hi Chris,
That's the one alright. What's more judging from the picture it is constructed in the same manner as the transformer in the Mullard PAL TV book. The type of core is not mentioned in the text. However, according to the circuit diagram the B-Y demodulator transformer T5 has 4 turns for the primary and 6 turns for the secondary.
The R-Y demodulator transformer T3 has 6 turns each for the primary and secondary windings.

Till EulenspiegelT

 
Posted : 12/04/2016 2:50 am
Till Eulenspiegel
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I have found another line timebase module and just like the rest of the Marconi it is all nice and corroded. One of the BC107 transistors had the rotted lead out wires so that one has been replaced with a BC547.
All the other semiconductor devices check OK. Again, like all the other 2000 series modules the carbon resistors are way out of tolerance.
The power supply module: The is the one which is located at the top left side of the chassis above the IF panel. I reckon this is possibly the only module for which a replica can be made. All the components can be mounted on a metal plate, there is only the PCB edge connector that will cause some concern. Modern electrolytic capacitors are much smaller than those made in 1968. An alternative to the fusible power resistors will be required, can't find those so possibly extra glass fuses? Most of the small components can be fitted on tag strips.

Till Eulenspiegel.

 
Posted : 01/06/2016 4:58 pm
malcscott
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Hi David, i have found the psu panel for this set, Malc.

 
Posted : 01/06/2016 7:18 pm
Till Eulenspiegel
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Hi Malc,
That's good news, I'll send you a PM.

Till Eulenspiegel.

 
Posted : 01/06/2016 10:48 pm
Till Eulenspiegel
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The power supply module employs two high wattage zener diodes to provide the stabilised 30 volt supplies to the decoder and IF board.
Each zener diode is supplied via a high wattage 180 ohm resistor from the regulator board which delivers 55 volts.
The power module I'm using is borrowed from the 25" Ferguson 3700. One of the 30 volt zener diodes has been replaced by very neatly made stabiliser circuit which employs a BD241A transistor and a few other components. I don't know if this is an official modification but rest assured I have no intention of returning the PSU module back to it's original state. The original 30 volt regulator method is rather crude, but that's how they did things in 1967. In the modified circuit the BD241A transistor functions as a series loss element, much better.

Till Eulenspiegel.

 
Posted : 02/06/2016 2:40 pm
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