Help to repair oscilloscope Philips PM3207 (Algemeen)

door Onno-I , 03-11-2020, 23:04 (1273 dagen geleden) @ edison
Gewijzigd door Onno-I, 03-11-2020, 23:16

Hi Carlos
The HV on the cathode is -1500V according to the manual. Do not try to measure this with a DVM, most are built for 500 or 1000V maximum. You may get an internal flash-over which will badly damage your DVM.
You need to use a HV probe, to divide the high voltage.

But your scope already has a voltage divider for the HV voltage: R801/R802, it provides the input for the control circuit around D801.
It has a 82,5/10082,5=0,0082 ratio, so with 1500V input you will get -0,37V on the +input of D801 (pin3).
Resistors R806, R807, R803 define the amplification of D801 to about 181 times.
You can calculate the input and output voltage of D801 (pin 5) for different values of the HV.
I put a simple calculation in a spreadsheet to produce a table:
HV +in D801 Output D801
-1600 -1,19 -12
-1500 -0,37 -12
-1400 0,45 +2,5
-1300 1,26 +12
-1200 2,08 +12
-1100 2,90 +12
-1000 3,72 +12
-900 4,54 +12
-800 5,36 +12
-700 6,17 +12
...
-200 10,27 +12
-100 11,08 +12
0 11,90 +12

Of course, this is theoretical, the output will be less than +12V and higher than -12V.
From this simple model of the amplifier, it will stabilize the HV to about -1405V (where the output voltage is -10...-5V)
But there are small errors in the model, probably it will be -1500V as Philips says.

Anyway, your results are 11,4 V on pin 3, meaning HV must now be 0..-100V, so if it is so low for now you _can_ measure the HV with your DVM. Don't do that if you measure lower than 5 V on pin 3

The low value may have 1 of 2 causes:
- the 24V lamp is limiting the power to the transformer T801
- T801 has a short in one of the windings (or the rectifier circuit C804-806 V802-803 has a bad component)

(by the way: when we are talking about a shorted winding in a transformer or coil, this means two adjacent loops of the wire are shorted, not the whole coil. This will absorb a lot of energy from the transformer)

You may replace the lamp by a 12V 5W car lamp or 10 ohm resistor, then 4,7 ohm, then 2 ohm etc to see if you get HV .... if you don't, probably the transistor will get hot again, because there is still a fault in the circuit.