Thierry Delaitre wrote in
:

I have a faulty Philips PM3240 oscilloscope. The problem is that the
spot is not showing up anymore. The spot is sometimes illuminating
very rapidily for one 10th of a second as a one off and then disapears
for 1 hour. I have checked the modulation signal applied to G1 and it
seems fine. The CRT is a D14-125 GM/08 but i do not have its
characteristics and it is difficult to find out if the voltage between
the wehnelt and cathode are correct or not. There is a ZA1004 neon
which is acting as a voltage stabilizer between the cathode and
wehnelt. This ZA1004 seems to stabilise 230V but i have no idea if
this is correct or not as i cannot find any information about this
component. The voltage values of the CRT are as follows:

G1: -1500V
G2/G4: +33.4
G3: from -950 to -1120 (focus)
G5/G6: ground
G7: +45V
G8: 8KV

I've also checked the filament and it's current is constant even when
the spot is not displayed.

Any help would be greatly appreciated !

Cheers,

Thierry.


If the neon between grid and cathode is lighting,that means the DC restorer
circuit/grid drive has problems.The neon is to LIMIT the G-K voltage in
case something in the grid drive circuit(DC restorer and Z-axis
amp)fails.The DC restorer shifts the Z-axis signal(unblanking signal) to
CRT potentials(HV). Possibly a diode breaking down under HV,or a bad HV
cap(less likely)

(My experience is with TEK scopes,though)

--
Jim Yanik
jyanik-at-kua.net