Need help with sawtooth vertical scan on homemade video camera

[old_tv_nut]

I missed the point about the saturation voltage of TR20, so, yes the peak sawtooth voltage should be about 9.7. I don't think you need to change the 4.7 microfarad, though, because this should be in the range of the rheostat. Setting the rheostat to max 5k should get you a sawtooth that is only 3.4 v peak.

So, I agree a shorted TR20 is most likely the cause, or maybe TR21.

Don't mean to talk down to you, but are you sure the transistors are the correct polarity (PNP/NPN) and the b-e-c connections are not interchanged? In the past I made a mistake like that and got some very strange results.

[Penthode]

Yes, I suppose that the capacitor change is not necessary. Perhaps the range of rheostat and limit resitor could be changed to prevent the ramp becoming non-linear? But then again, if the range is sufficient I guess there is no need to change anything there.

There is one otherthing that troubles me, however. That is the TR21 directly across the capacitor. Shorting the capacitor directly with TR21 is a bit vicious and might provide a bit of a strain on TR21.

I would put a resistor in series to limit the discharge current. with the ramp reaching about 10 volts, perhaps the peak discharge current could be limited to about 1 Ampere or so with a 10 ohm resistor.

I would be careful about inserting too high a value of resistance here. But the discharge time constant with 10 ohms is about 0.3ms compared with the ramp duration of 20ms. The capacitor must be sufficiently discharged during vertical retrace.

Yours conservatively,

Terry