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Thierry Delaitre
 
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Default dead Philips PM3240 oscilloscope

Hi,

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

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.