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Thierry Delaitre
 
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Default dead Philips PM3240 oscilloscope

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.