PM3207 only shows half of the screen, help needed

[TASBALA]

Hello everyone,

I bought a Philips PM3207 dual channel 15 MHz scope for cheap, it was untested. It has a few problems, mainly that it does display the input waveform, but I can only move it down till it hits an invisible line at the 1/3 of the screen (pictures). If I adjust it all the way down, it shows a ripple-y line at the lower third of the screen. The ripple does change depending on the time base. I can adjust the waveform upwards well over the top of the screen, as if the screen was virtually shifted up.

I have checked all of the supply voltages, most were spot on, -100V was -115 and +100 was 120V. I have also checked all of the transistors that I could check in their place. The CRT and its connection to the main board is fine, so is the time base, amp/div settings and triggering. However, it can only show one trace at a time, but both channels work on their own. XY mode functions as it should, but it also can't draw anything under that 1/3 line.

I've also asked on another forum with not too much success, so I'm hopeful that someone can help me here.

Thanks in advance

Service manual: https://elektrotanya.com/philips_pm3207_service_manual.pdf/download.html

[shakalnokturn]

Your pictures didn't make it here.

CRT connection have been checked. Does that include the ones to the vertical deflection plates if they are separate from the rest?

With trace as centered as possible check voltages at the Y plates.

[TASBALA]

 There are pics but they were too large so I compacted them and attached them as a zip

[TASBALA]

I have swapped the polarity of the vertical deflection plates' connector and as expected, the waveform was now on the bottom of the screen

[shakalnokturn]

The waves in the trace could be bad electrolytics, you'd also have to check the ripple on various power rails.

I doubt the offset problem is related to capacitors, you could have a dead semiconductor overloading a rail and causing the waves though.

[TASBALA]

Is there any way I can measure ripple without another scope?

[TASBALA]

Voltages at the Y plates are: -3V and -5V

[shakalnokturn]

Is there any way I can measure ripple without another scope?

You can use a DMM on ACV.

Select and ground one channel, measure DCV (differential) across test points M5 and M6.
Can the voltage be brought to zero with position control about center position?

Considering the components used in the final Y stage, a very wild guess would be that one of the BF199 transistors is defective, more testing needed though...

[TASBALA]

Voltage between M5 and M6 can be adjusted between .528V and -0.513V, but by the time it reaches -0.13V, it turns into the ripply line and it can't be moved down.

I tried to measure ripple on the power rails with my DMM, but it shows 25VAC on the 12V rail so I'll borrow a friend's DSO

[shakalnokturn]

Set the differential voltage between M5 and M6 as close as possible as close as possible to 0V as previously then check between collectors of V507 and V512, it should also read close to zero.
If you don't read close to 0V check those two transistors.

Otherwise the problem is in the output stage (that's where I expect it to be by now).
Check R528, R532 high value resistors on high voltages sometimes go open.

You should get about the same voltages across pair V501, V502, pair V506, V508 (C-E) and so on.

[TASBALA]

The resistors tested fine (96K and 94K) but V501 only showed up as a diode on my multitester. I triple checked it, put it back and got the same results as before so I didn't overheat it with the soldering or anything like that. I'll replace it and report back.

[shakalnokturn]

If you're going to order some BF199's on purpose order a handful, they'll be useful for your oscilloscope sooner or later.


As a side note I was replacing some in a CRO recently and found that the originals (Philips late '80s early '90's) have a C-E diode while the new ones I received (Continental Device India Ltd.) do not.

[TASBALA]

After 2 weeks of shipping I finally got the new transistors, replaced the bad one and now the whole screen works. Thanks a lot for your help.

This isn't to say that it's perfect: trigger is sloppy on the ms setting (works perfectly on the uS setting), and it can only show one channel at a time, but both channels work fine on their own. It isn't tied to one channel either: sometimes A shows, and sometimes B. Also the problem is probably on the display side, because X-Y mode works, so the channels don't "block" eachother.

I know it doesn't matter much, but I recieved the DSO from my friend, the supply rails and slightly noisy but everything is on a DSO138. There is no ripple that it can pick up on.

The trigger does sometimes just starts to work when I flip the AC/TV selector around. It only works in TV, and only occasionally. I haven't been able to recognise any sort of pattern or correlation with anything I do and the trigger, I honestly have no idea of what could be causing this behaviour.

Once again, thank you very much for your help.

[shakalnokturn]

For your remaining problems in this scope I think a good starting point would be a spraying of contact cleaner on all the switches, connectors, socketed IC's.

I'd check the three capacitors C606, C608, C609 for the lame trigger. There are also a few BF199's in that section...

Then there are a few points to test to see if the alternate sweep (dual trace) logic is working.

[TASBALA]

Now that I've got the DSO I can do some more extensive testing of the measuring points that I couldn't do with the DMM. I've already got the switches covered and there aren't any sockets.

[WirelessESD]

I have a fully functional pm3206 they should be very similar inside. If you want me to measure something as reference let me know.

[shakalnokturn]

Check what's going on at D553 pins 8 and 9.

[TASBALA]

Today I had a bit more luck with the trigger: in uS it still works perfectly, in the ms setting, it does work, but only if the waveform fills the whole screen.

I tested all of the measuring points, everything is fine. C606, C608, C609 were all a bit high: 8uf instead of 4.7, 22uf instead of 15. I had replacements so I put them in. All of the transistors in this area are also good.

[TASBALA]

Pin 8 is at .1V, pin 9 is at 3.62, I don't see any activity on the dso. What should I be looking for?

The trigger problem seems to have solved itself: I just flipped the AC/TV to AC and it works perfectly. It was either the switch that I sprayed or the new capacitors, but it works now.

[shakalnokturn]

Pin 8 is at .1V, pin 9 is at 3.62, I don't see any activity on the dso. What should I be looking for?

The trigger problem seems to have solved itself: I just flipped the AC/TV to AC and it works perfectly. It was either the switch that I sprayed or the new capacitors, but it works now.

I think I was a little too concise there.
The point in checking there is to try and find out if the alternate sweep logic is working.
Before checking you'll want to feed the scope an input and make sure it is triggering, switch the display to dual.
Hope to see the outputs toggle.

[TASBALA]

I am sorry but I still don't quite understand what I should be doing. I have a signal connected to channel A, it's triggered, channel B is on and the display is set to A&B. D553 - that's in field D3, right? And pin 8 is connected to pin 11 of D551, and pin 9 is to 8, according to the manual. Measuring between either of these pins and ground doesn't show any activity on the dso, pin 8 is at 0V and pin9 is at 3.6.