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Old 5th Oct 2019, 2:10 pm   #129
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: Bush TV22 with TC184 B3 converter.

Quote:
Originally Posted by FERNSEH View Post
So ideally a buffer/inverter stage should be inserted between the frame sync integrators and the oscillator. A correctly shaped positive sync pulse will be supplied to the valve grid instead having been distorted by the oscillator transformer.
The transformer conveniently inverts the signal, there is no need for an extra active stage to do it.

The point is though, you are interested in the quality of the sync pulse as it appears at the grid, that it has been correctly filtered. If under filtered there are H serrations that cause poor interlace, if over filtered the amplitude is lower and the rising edge slower and more possibility for external interference H signals or power supply ripple to affect the interlace.

Some filtering will be achieved by the transformer (due to its limited high frequency response) most of the filtering by the integrator RC components.

However the RC filter-integrator loads into the anode circuit and is affected by the impedance there. So, when it comes to adjusting the filter (resistor or capacitor values) it is better not to alter the circuit by disconnecting the filter/ integrator output from the anode circuit, it is better to simply monitor the form of the vertical sync pulse on the valve grid, with the V osc disabled. This way the V sync waveform you see will be what is actually there in practice later when the oscillator is running/ enabled.

In summary, use the battery to disable the V osc (CRT brightness on zero), have the dual gang 50k pot or similar fitted in place of the 100k resistor & the diode. Adjust it so the sync pulse waveform at the grid on the scope is just free from H serrations and perhaps a tad higher resistance at that point, fit fixed resistors . And if still troubles, then make sure the HT supply to the V osc is free from interference by H pulses or 50Hz ripple and after that you will have the best interlace you can possibly get.
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