Thierry Delaitre wrote in
:

Hi,

Is there anyway i could replace the BAX17 with some kind of more
standard diodes for testing purposes ?

Sure,as long as the PIV is equal or better.Small-signal diodes have better
freq. response characteristics,which would affect the risetime of the
unblanking signal to the CRT grid.
You could use the ECG recommended substitute for the BAX17.
--
Jim Yanik
jyanik-at-kua.net


Thierry.

Jim Yanik wrote:
Thierry Delaitre wrote in
:

Jim Yanik wrote:
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


Hi,

Thanks for this very useful information. The neon is lighting up.
Note that the neon is connected as follows:
------
-| 2.2M |--
| ------ |
------ ------ ------ | ------ | ------
-1600V ---| 10.5 |---| neon |-----| 1.2M |----| 2.2M |-----| 5.6M
|--- 0V
------ | ------ | ------ ------ ------
| |-------- ^
| - | | |
| |1| | --- |
| |0| | /_\ |--||--0V
| |0| | | |
| |K| | | |
| - K G1 G3 focus
| |
| filament
|
|---------|-----
| --- |
--- 100pF--- | |
\ / | | |1M
--- | | |
| |\ | -
---||--------| |--------|----- 1M --- 511 ---- G1
470pF |/ |
BAX17 --- 470pF
---

I've checked the two BAX17 in the DC restorer unit. I've also
checked the resistors which are in series with the neon. I've tried
to disconnected the two 470pF interfacing with the DC restorer to no
avail. I've even tried to disconnect the plug at the back of CRT
with GT8 and the neon is still lighting up with 230V accross its
pins. Could it be that the neon is faulty ? What is the normal
voltage accross it ? What is meant to be the normal voltage accross
it ? Where could i get one if it is faulty ?

Cheers,

Thierry.


To check DC restorer diodes,you need a curve tracer to check them at
or near the circuit's operating voltages,at least 100V.They could
break down under HV.

If I read your ASCII circuit right,the neon being in series with the
focus string looks like it's supposed to act as some sort of V
regulator in the focus string.
For the neon to be a protector against too large a G-K voltage
difference,the neon would have to be across the G-K.
In my experience,the neons fire at around 60V,TEK uses two of them in
series across the G-K,to limit G-K voltage difference to no more than
120V.