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Old 31st Aug 2019, 12:11 am   #198
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Baird T5 restoration project.

Originally Posted by FERNSEH View Post
The sketch shows the DC restorer in the T23. There is something not right about this circuit. The positive going video is supplied to the CRT grid. The function of the DC restorer diode is to conduct on the sync pulse tips to the DC reference level set by the brightness control. Normally the DC reference will be a positive voltage but in the Baird T23 it is negative and in all accounts this is the wrong polarity for the circuit to work properly.
The circuit in your post might look odd, but is good with the diode shown as the correct polarity for a DC restorer.

The easiest way to understand these circuits is to imaging the diode is a little leaky or has a 1M resistor in parallel with it, and for a moment is not a diode.

This causes the coupling capacitor from the plate of the output valve to charge to a potential equal to the sum of the plate voltage and the absolute value of the negative voltage on the brightness pot's slider.

When an AC signal comes along it attempts to swing the voltage of the coupling capacitor's output terminal above and below the DC level of the brightness pot's slider. If there is the diode present, when the voltage attempts to fall below this value, on sync tips, the diode conducts, altering the average charge on the coupling capacitor, which tends to stabilize the sync tip voltage at the potential of the pot's slider. So as the brightness pot is rotated the sync tips follow that voltage.

It is not necessary to actually have a leaky diode or a theoretical resistor in parallel with the diode (that is when enough signal is present to drive the diode into conduction). However with no signal present, especially with a valve diode, there should be a resistor in parallel with it to allow for this condition. No doubt with the vintage semiconductor diode, there is more than enough leakage to allow for the no signal condition and no resistor is required.

Its sad that the CRT turned out to have leakage issues, if that is the case. Andy's suggestion in the post above looks very good.

Since the leakage resistance of the diode is ill defined, you could also try tacking a 250k to 1M resistor across it and check if that gains increased brightness control.

Last edited by Argus25; 31st Aug 2019 at 12:19 am.
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