Svenska ElektronikForumet

Today I have removed the transformer and made some measurements on it with my LCR meter.
The high voltage part was: 270ohm, 14mH, and Q=0.107
I have also made a ring test with the method and device written here: https://danyk.cz/avr_ring_en.html
The different windings gave 0 rings, pictures on the scope here:
HV (1500V) winding: https://drive.google.com/file/d/1mB2LNF ... sp=sharing
2 other windings: https://drive.google.com/file/d/1q8ym9M ... sp=sharing, https://drive.google.com/file/d/12SYbTD ... sp=sharing
CH1 (transistor Gate swithcing 5V to the transformer, CH2 is the transformer windinng)

If I did not make the test wrong, then I think my transformer has a short circuit.

Anyone has idea how I can fix the power supply without a transformer like this? Or anyone has a donor for this transformer from another old philips scope?

Regards,
Laszlo

Hi!

I have fixed the power supply of my scope so now it works! (The HV winding had a short circuit as suspected.)
If anyone has similar issue or interested about the solution can contact me.

Regards,
Laszlo

Please just post a description of Your repair here and let us all share it.

All right. I just did not know if anyone is interested at all, cause no one reacted so far and a long time passed since the latest activities in this thread.
But then here comes my story.

The initial problem was no power led indicator, no ray on display, and a burnt BD237 transistor of the oscillator pair of the supply.

First I replaced the BD237 pair, replaced the electrolyt capacitors either on the power supply and also on the main board.
But no change blank screen, dim power on led and low voltages on power lines going to main board.

As I wrote before I have suspected that the HV winding of the power supply has a short. This was also described in a forum topic on eevblog as a typical problem of old Philips oscilloscopes this kind and age. I borrowed a high voltage scope probe and measured the -1500V line of the power supply and only -70V came out.

So took out the transformer. In my case the whole transformer was dipped during manufacturing into some insulating material (a rubber/plastic kind):


I had to scratch the insulation to remove the high voltage winding. Here you can see the HV winding end removed from the connector:


Then I simply teared out the winding. On the two sides it remained in place since I could not remove the insulating material below the ferrite.
I made the "ring test" again with removed HV coil and it presented much better oscillation than before:


Soldered back the transformer and the voltages became perfect. The signal on the oscillator transistors also became much better. The only issue is the frequency which is higher than it should be. This maybe because the load is different without high voltage part:


From the beginning I got a lot of help from a different forum member who also gave me a high voltage circuit, which currently generates the -1500V high voltage signal with 12V DC input:


Currently the HV circuit is driven from my bench power supply and a need to solve to feed by the scope by itself. I don't want to drive it from 12V of scope power supply because it draws 350mA when both channels are displayed on CRT. So I will probably feed it from the mains transformer secondary DC with converting it to 12V...

But at least the scope is in working condition:


Now comes the finalization of supply, cleaning of switches and calibration according to its service manual...

BTW here is a complete winding instruction for the transformer, if someone would like to go this way, but I did not take the risk to disassemble the whole transformer and break it accidentaly:
http://www.cfp-radio.com/realisations/r ... -2696.html

Bye!