Philips pm3231

[BeginusElectricus]

Hi guys, I bought broken philips pm3231 scope (just for fun, and trying repair them with your help) I try to figure out what is wrong but I defeated with it, I have measured voltages with service & manual pdf and that I have found is : voltages in power supply are little low than should be, I found one strange thing - there is a neon indicator (b401) za1004 (this is some stabiliser or? ) and... this indicator is lighting when power is on (I red that it shouldn't be light when all is good) and I can hear silent "squeak" and voltages are dropping down but when I unplugged -1100v cable from this neon indicator - all voltages are correct, Do you have some advice for me what should I do to repair it? thanks a lot.

 

[BradtheRad]

I can hear silent "squeak" and voltages are dropping down but when I unplugged -1100v cable from this neon indicator - all voltages are correct

'Squeak'... may be similar to my old Tektronix scope when the high voltage creates arcing in the 6.3V (crt heater) winding. The beam jitters in one spot or makes a row of commas across the screen. The arcing is audible as a squeal coming from the main power transformer.

My scope also has a glass tube about an inch long, which has to do with the high voltage. It's a diode, or a regulator, which is a connection node between high voltage and 6.3V somehow.

Does the squeaking stop when you disconnect high voltage? Then power supplies become normal.

I solved the arcing problem by installing a separate 6.3 V transformer for the crt heater. After a few years it began arcing. I believe I'm on my third transformer now.

 

[BeginusElectricus]

Hi there, sorry but I forgotten say that there is no any "main time base lines" on the screen, screen is blank.
Yes - squeking stop when I disconnected main cable of -1100v from pcb ( and also stop squeking when I disconnected this neon bulb from pcb) then voltages of power supply returns to normal values, but still not working. I try disconnect 6.3v~ and check what will happen?! (somewhere in web I found that instead of this ?neon bulb za1004? that is some kind of regulator or??? probably is possible to put zener diode of 82v value but I don't have them, if I buy it someday maybe I will try replace this strange neon regulator for zenere diode).

 

[BradtheRad]

Better to wait a while before you think about replacing components. Right now your next step is to diagnose which is missing, whether high voltage or heater voltage.

Same as my scope your schematic shows high voltage on the same wires with 6.3 V heater voltage. Both voltages are essential to generate the beam. I was baffled at the idea these are combined at a single filament in the CRT. Furthermore it implies over 1000 volts running within a secondary winding of the main power transformer. Eventually it broke through the insulation and began arcing. I don't know how it avoided blowing up. By good fortune my scope was fixable.

Since then I thought of constructing a substitute high voltage circuit. However I cannot figure out a foolproof method to combine these two:
a) The high voltage (3kV in my unit) is a few uA (I guess)
b) The 6.3 is a few hundred mA (I guess from seeing small transformers with 6.3V secondary rated 300mA).

You scope appears to be dual trace. Can you trace wiring to see if the two heater filaments get 6.3V from different supplies? Or in series via a 12.6 V supply? Does the CRT light up orange? Do other tubes light up? Can you measure -1000V anywhere in the unit?

 

[BeginusElectricus]

Yes it is dual trace. Hmm, I will try to find some other transformer with 6,3~V, as you said maybe there is some broken insulation beetwen -1100V and 6,3~V transformer?! I will try to unconnect this 6,3V transformer and check what will blow up ;-). Yes inside crt lamp I can see orange glowing so 6,3~V works. There isn't other tubes only this small za1004 and crt tubes, this scope is all semiconductors. Yes I measured -1100V with simple resistor divider 1/10 and my DMM and it was about -1100V when HV cable was unconnected, and about 2950v on 3kV point.

 

[BeginusElectricus]

Wow, first beam works! I used separated transformer 6,3V~. In original transformer there is some kind of short circuit to ground or something like that, because I measured resistance between 6,3V pins and ground and there is about 150ohm so it must be that. With second beam I will engage later. Bradtherad you helped me. but in the future I will try to post here what is with second beam and original transformer.

 

[BradtheRad]

Way to go! A major accomplishment.
Best of luck making further progress.