Monitor Philips CM8500 (same 1081 Commodore)

[demonlg]

Hello at all,

another monitor with a strange issue, this monitor arrived to me totally dead, no power, no sound, no pitching sound, nothing, i have opended it and disconnected power supply board and tested out of the screen, no output voltage present.

I have found two component broken, a ceramic resistor 4,7ohm 5w and the BJT transistor on switching power..
The transistor presenti is a 2SD1575 i have new one, replaced it and resistor the monitor starting up and working perfectly for hours but............i have turned off, then again on, then again off......then the monitor going again to dead!
And again the 2SD1575 resulsts in short, but ceramic resistor are good.
Now i don't have same 2SD1575 and i have replaced it with a BU508AF, it have good characteristic to replace the original one, the board have a hole for pins with large space and predisponing for this type of transistors, i have replaced and turned on, again alla ok but i have missed to insert the main power camble and it going to power off and on and off and on three times and again transistors going in short!

Have replaced again three times the BU508 and again if i turn on and off and on and off the monitor the transistors going to short.
I have disconnetted again power supply from main board, replaced again the transistors and powered on without charge of main monitor circuit conneted, but i have three output wire lines on the schematics:

+125v
+25v
+16v

but if i measuring with multimeter i have

+250v
+50v
+32v

!!!!!

But if i connect the mainboard the monitor starts to working perfectly without any issue or high luminance or distortion, i dont' have measured voltage with all connected and powered on at the moment.
But now i have two question:

Why this transistor going to short when i turn off/on any time monitor?
And, the power supply without mainboard connected why give me the voltage doubled? with this voltage the output electrolytic caps with 25v, 35v and 200v max range for the three lines do not explode?


Emanuel

 

[demonlg]

This is a not good resolution schematics of power supply:

 

[demonlg]

This model of power supply, i don't know why, without charge on output have the output rail to high! Now, i have repaired power supply, replaced the SD1575 with a BU508D ( not 508AF) with a dumper diode integrated in a transistor and replaced the diode RGP10D with new RGP10M, and now the monitor starts correctly and the transistor not going to short, and i have recapped it.

Now i have another problem, monitor is working, color is ok, contrast and brightness is ok, but when cold and minus when hot i have a little vibration on the screen, little horizontal lines, similar to a noise in orizontal scan and I hear a slight whistle coming from the EAT.
It is likely that some other electrolytic on the main board is tired, or maybe the EAT headphone. I say the headphone because the rubber of the latter has hardened and deformed and to avoid electrical arcs I put some temporary silicone around it, this headphone has a resistor inside it and it is a removable and replaceable Philips headphone. Or could it be some other problem in the horizontal deflection?

 

[daver2]

I am sure (in another thread) we have covered a similar (if not the same) issue. If the original transistor has a "dumper diode" you MUST replace it with a similar transistor. This is where careful attention to the data sheets are a must...

Switch mode power supplies can give strange voltage readings without a load attached to them. Hence (probably) your strange readings...

Dave

 

[Hugo Holden]

You would have to post a video to work out what the little lines on the screen are.

One reason why an smps transistor or a horizontal output transistor can fail is excessive collector voltage. When the transistor switches off, the field in the transformer starts to collapse, and given that it is an inductor, it attempts to maintain a its current. This results in a positive going voltage on the transistor's collector. The value it gets to depends on the value of the tuning capacitor, the 2.2nF part (C118 I think hard to read on the diagram). This capacitor needs to be carefully checked and if defective replaced with a low loss type with welded to lead internal foils. If these capacitors lose value, or go open, the transistors collector voltage gets exceeded.

Also, as noted, the damper diode is very important. After 1/2 cycle of the transformer & tuning capacitor related oscillation noted above has occurred, the collector voltage attempts to swing negative with respect to the emitter for the next half cycle of oscillation, at that point the damper diode conducts and clamps the reverse collector voltage to a volt or less, protecting the transistor. So if the damper diode is not there, it usually destroys the transistor.

If the DC output of the power supply is too high, it can result in excessive EHT and more corona discharge around the Rubber EHT cap. Also, if the similar tuning capacitor on the collector of the horizontal output transistor, has lost value, this increases the amplitude of the flyback pulse on the collector (exactly the same situation as in the psu) and the EHT increases (unless regulated), so any tuning capacitors there need to be checked too.