Hi,

I'm trying to repair an oscilloscope of type: pm3335/60Mhz.
If i switch the power on, only the lcd display shows settings.
crt screen stays dark.
I've tried to measure the power supply output.
All voltages seem ok, except for the EHT output.
Instad of -2,1 kV i measure -1,8 Kv dc. (10 resistors of 1Meg in series trick)
At the input of the voltage multiplier i measure 0 ac. (also before c6209)
Test point X49 (input of transformer T6201) shows correct voltage signal.
T6201 measures 13 ohms between 6 & 11, 43 ohms between 6 & 12.

Anyone an idea what to try next ?

Regards,

Hans

Hans wrote in :


Hi,

I'm trying to repair an oscilloscope of type: pm3335/60Mhz.
If i switch the power on, only the lcd display shows settings.
crt screen stays dark.
I've tried to measure the power supply output.
All voltages seem ok, except for the EHT output.
Instad of -2,1 kV i measure -1,8 Kv dc. (10 resistors of 1Meg in series
trick)

You are probably loading the HV supply;for a DMM with 10 megohm input Z,you
need a 90 Megohm probe resistance(9 10MegR's) in series with the DMM,for a
total of 100 Meg.Less will load the HV supply.
Or use a Fluke HV probe,a complete divider.

At the input of the voltage multiplier i measure 0 ac. (also before
c6209)
Test point X49 (input of transformer T6201) shows correct voltage
signal.
T6201 measures 13 ohms between 6 & 11, 43 ohms between 6 & 12.

Anyone an idea what to try next ?

Regards,

Hans



If LV supplies (#ONE check!)and HV are correct,then you need to check the
CRT filament supply and CRT filament resistance.(a good idea to use a
floating DMM for this;the CRT filament is elevated to cathode V)
Then you check(with a scope) for proper sweep on the horizontal CRT
plates(lets you know the trigger and sweep are working),check Z-axis amp
and DC restorer(if used in your model) for proper CRT unblanking,then
short the vertical CRT plates to eliminate vertical problems.

--
Jim Yanik
jyanik
at
kua.net

Jim,

Thanx for the tips. I've did some additional checking.
Yes there is high voltage present and also CRT filament resistance.
The're ok.

It pointed the problem to be in the CRT drive XYZ.
I just removed the PCB's and checked all diodes and transisitors.

Guess what: 1 diode was broken (BUV21) and one transistor dead (bc548)
After replacing these parts, i had to adjust the blanking level and BINGO!
It works again!

Ok: i bought this scope from my firm since it was too expensive to repair.
Repair costs estimated: Euro 875 incl. calibration certificate.
I payd..... Euro 10,- and i repaired it for: Euro 0.35
(ok, i had these parts on stock)


Regards,

HAPPY HANS