Help to repair oscilloscope Philips PM3207 (Algemeen)

door Maurice , Dordrecht, 04-11-2020, 00:29 (1482 dagen geleden) @ edison
Gewijzigd door Maurice, 04-11-2020, 00:30

Question.

Is the 24V light glowing? If so thats wrong.(a defect circuit that is)
It must be off or at least dimm.

V800 must be 2,8V across.

Than D801,
As seen in table by Onno, around the 0,4 there is a flip of the output.
That is the oscillation point were BC337 has to go oscillating.

That is only accomplished to subtract from that +12V power source.
That subtraction is done through R801 voltage.
+12V +(-R801 voltage) = voltage pin 3 D801, where R801 is a negative voltage divider from that -1500Vdc

As you measure +11,4V there is no negative voltage from resistor network
R801 etc.

The difficulty is , it is not a static power supply.
In that way you have to see if at start up for a short moment there'is a negative
peak or not.

The only way to see that is to manually start that BC337 and watch with a scope
that point 3 D801 if it briefly change, and so the output also.

To start the oscillation, at start BC337 conduct, a power peak provide charge the transformer, due to timing with capacitors the BC337 will close and the transformer will be discharge and That will make the first negative voltage pulse.
That must in fact be enough to flip D801 again.
After milli seconds it will stabilize in oscillation and minor changes in voltage will provide a longer or shorter pulse-lenght of conduction BC337.

DC measurements can't be done in running mode on that pulse network.
Only at the DC HV end of the circuit.

The problem., as you already checked the components there must be a problem
why at start the negative pulse is not ending up at the D801.

I think Pull that CRT plug first, and see what happens.
Than also check seperately those zeners D800 (2,4V) and D804 (39V)
On the internet is to find how to make e very simple test circuit.

Then also make with a potmeter to vary voltage at pin 3 D801.
Simply take a 50K or so potmeter, attache to +12V and -12V.
The middle should now be 0V, and by turning around that 0,4V it must flip the output.
Than see if Bc337 will close.

Pulse networks power supply's are not very easy to repair if there are big problems.
Its all about timing through coils and capacitors.