Philips pm 3110 10 mhz oscilloscope

[nucleopolymere]

Hi!
I have purchased this oscilloscope for 5 bucks at my school because they were throwing it away.The oscilloscope gives me a precise  reading when i put the probes on the probe adjustment(calibration), so the amplitude and horizontal deflectors are ok.I have an annoying but not major problem. When I turn on my oscilloscope and I put the intensity potentiometer at maximum, the trace is very bright as expected , but the trace is shading off slowly. It takes about 1h30 until the trace completely disappears. I've downloaded the service manual for the scope but I am rather new to electronics and I don't know what the problem could be. I ,after carefully observing the schematics, suppose the problem would come from the power supply of my oscilloscope but I cannot ,due to my little knowledge, determine what component could be defective I would suspect a bad high voltage capacitor .Why?... I don't know, it is just a feeling.

Service manual can be downloaded here ( wait 30 seconds then you can download it) http://elektrotanya.com/philips_pm3110_service_manual.pdf/download.html
The schematics are in the manual at page 85

hope you understood my problem

Thanks for your help 

[tautech]

Welcome to the forum.
We have some Philips experts here that should be able to steer you in the right direction. 
For additional reading get the Tek troubleshooting pdf in the first post on this board.
Normally the first things are to check ALL rails from the PSU are exactly to spec, in your case you need to ensure they are not sagging as they heat up.
I doubt they are, but it should be eliminated from all question.
PSU ripple can be checked with your meter on AC.

Wise to take notes of any repair or adjustments, before and after.
It will help if you give us an idea of how much gear you have to undertake this repair.

[tautech]

Search for grumpydoc and check his posts.
oldway also is very experienced with old scopes.

Listen to any advice they may offer.

[nucleopolymere]

Thanks tautech for the information.

I have an old Fluke 8010m to test my scope and a soldering station to remplace any defective component. I have another multimeter but I have some doubts of his precision I have a 2A/24V power supply. And obviously, my scope . After a quick view of the Tek troubleshouting manual , I will try to simulate with an hair dryer to see if any component shorts when operating at high temperature.

How do I test power rail? I take some mesurements on the outputs of the transformer and then I take other mesurements on the boards and compare to see if they are the same? Also the shading off of the trace is caused by a decreased current/voltage to the input of the cathode emmiter or it can be something else ?

I will certainely see posts of grumpydoc and oldway

 Many Thanks 

[tautech]

How do I test power rail?

Test points and measurement spec on P68 in manual.

Also check each resistor in the resistive divider chain in series with the Focus pot on PCB labeled UNIT 4 (PSU). Desolder as necessary. Be sure each resistor matches it's banded value and within it's banded tolerance.
Edit. And check R609
I have had to replace all these high value resistors in past repairs. (not Philips)

Take note of test requirements on P73.
A specified operating setup is usually required for measurements.

Often a "freeze" spray aerosol can be used for finding heat related problems, though I have no experience using it on HV.

Good you have a full manual with schematics and adjustment procedures.

[nucleopolymere]

Hello,

Now that i am definitively motivated to repair this oscilloscope, I've took some measurements and here are the results.
I have tested the power supply (unit 4) of my scope, I tested almost each components except the autotransformer (#751) and I've found 2 defective, which was a shorted diode(#405) and a open 5.6 Mohm resistor(#612). So now my Focus works better ! but i still have the fading problem
I've also measured the power rail that comes out of my power supply and they all seemed to be good to me, even after the trace fade away
At this time I wanted to mesure the voltage on the CRT but I didn't proceed to because my multimeter (Fluke 87-V) can only read up to 1000V.
So how can I mesure higher voltage (preferably safely  ). On the schematics it seems that I'll have to read like 2kV-2.5kV on the CRT.

So I'll put attachements to show you what measurements I've taken.
Here is the manual
http://elektrotanya.com/philips_pm3110_service_manual.pdf/download.html
the schematics are at PDF page 84/88

[tautech]

The DC voltages on the schematics might help you.
Refer to P73 and the setup required for the listed DC voltages to be valid.

I keep a old AVO meter for HV measurements, the right model will allow up to 3 kV to be measured.

As you probably suspect the fading trace is likely to be a faulty HV circuit.
You could try freeze spray but around the HV I think I'd used compressed air to find a heat related faulty component, particullary around the primary side of #751.
Diodes 402-404 are HV diodes and will have a high Vf, maybe 10V or more.

Fingers x'ed for you.

[GreyWoolfe]

nucleopolymere, here is how to measure high voltage with your fluke:
http://www.ebay.com/sch/i.html?_from=R40&_trksid=p2050601.m570.l1311.R1.TR2.TRC1.A0.H0.Xfluke+high+.TRS0&_nkw=fluke+high+voltage+probe&_sacat=0
I have measured up to 1800 VDC with mine.  Just keep one hand in your pocket so you stay safe.

[nucleopolymere]

GreyWoolfe :
Thank you for the info I'll definively get one !

I've took some measurements with the conditions specified at p73 of the manual and i have a almost the same voltage (G=17.57 F=14.15 E= -14.79 D=-17.6 A=152.3 B=431.0 C=141.5 instead of G=18.6 F=14.7 E= -15.2 D=-18.6 A=150 B=440 C=140) but then i bought some freeze spray and I've sprayed near HV diode and then the trace came back !!! after few other sprays i found that the #507 ceramic cap (1nF ; 3000V ) was making very tiny sparks. And each time i was spraying some freezer the trace came back (this spray is very handy !!) So I'll get another capacitor and see if it solve my problem !

Ty for your advice Tautech 

[tautech]

GreyWoolfe :
Thank you for the info I'll definively get one !

I've took some measurements with the conditions specified at p73 of the manual and i have a almost the same voltage (G=17.57 F=14.15 E= -14.79 D=-17.6 A=152.3 B=431.0 C=141.5 instead of G=18.6 F=14.7 E= -15.2 D=-18.6 A=150 B=440 C=140) but then i bought some freeze spray and I've sprayed near HV diode and then the trace came back !!! after few other sprays i found that the #507 ceramic cap (1nF ; 3000V ) was making very tiny sparks. And each time i was spraying some freezer the trace came back (this spray is very handy !!) So I'll get another capacitor and see if it solve my problem !

Ty for your advice Tautech 

Glad to see you making progress.

When I mentioned DC voltages I meant the few listed throughout the schematics, mostly with a line under then that slopes to the point on the schematic to test. Usually chassis or circuit Gnd referenced.
Some are also in boxes.
It seems it's not likely you'll need them but they can be handy for finding the area of fault.