Re: 7904 excessive shadow/flare

Roger Evans

My comments about cathode voltage regulation were based on seeing a similar problem (and fixing it) in a much lower bandwidth scope without PDA.

I hardly ever use my 7904 at low sweep speeds, but lo and behold I get a similar displaced, diffuse arc (almost a complete circle) at 200msec/div. Poor regulation of the cathode voltage gives a diagonal line (or just a bright spot and a weaker diffuse spot) pointing to the CRT centre. When using the 7904 normally. I am used to a little 'flare' if the brightness is too high but it is never a problem and never a clearly defined second trace.

For what it is worth the HV regulator test point is TP1635, connected to pin 3 of the op-amp U1635 (schematc <11> CRT Circuit). It should be very close to 0V DC, it is difficult to say what ripple is excessive unless you can find a reference in the manual. The AC divider ratio at TP 1635 is very different to the DC divider ratio because of R1642/C1642. A DC measurement of the cathode voltage may not show a problem, especially if you have a digital meter, it mostly samples the correct, plateau, voltage and occasionally catches the dropoff before the capacitors charge again. An analogue meter would show the average voltage to be slightly low.


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