Reviving a Phillips PM3212 Osciliscope

[Mediarocker]

So thanks to a generous fellow here (Thanks  ) I now am the proud owner of a Phillips PM3212 25MHz Analogue Osciliscope.



It used to work however the display faded out on him. So when it arrived, I decided to plug it in and see what's going wrong with it. I plugged it in, and nothing. Nothing happened....or so I thought. I proceed to leave it plugged in as I tried to source a service manual for the device online... I then started to smell magic smoke... and I immediately ran over to my bench and noticed it starting to puff from the back of the osciliscope! I immediately unplugged it, and now, it is time for a teardown to figure out what caused the problem with the display (and the magic smoke on my bench!). (Now just so everyone knows, I got the thing for the cost of shipping, and I consented to it knowing there was a problem with it. I figured it would be a great learning experience, and I could learn how oscilloscopes worked internally.)

So as a basis for comparison and proper laboratory style diagnosis, I denoted the variables in which I did my entry testing. I decided to do step by step diagnosis from the input to the tube, so I can systematically check off a list and determine the cause for failure.

Now please note. I am a beginner! And I am in no way a professional. I just know what I know and do what I know. If you have input, it would greatly be appreciated!

So let's begin!

I started with a visual inspection, albeit I had no idea what I was looking at to begin with, so I started by deciding to measure the power going into the device...

My home mains connection was measured and I came up with this:

Maximum Voltage: 123.4VAC @ 59.98Hz

Ok, thats normal. Mains around here run ~125VAC @  ~60Hz.

So now that those variables have been defined I can go further and measure the power supplies output voltage.



What I found interesting, is that the "power supply" is actually a stepdown transformer, which just takes the 110-125VAC mains and brings the voltage down. The rest of the power conversion is apparently handled on the main board which can also take DC power. Neato!

So I took measurements and notated it in my notebook:

PSU Output:

Maximum Voltage: 26.84 VAC @ 59.97Hz
Minimum Voltage:  26.71 VAC @ 59.97Hz
Mean average of fluctuation: 26.775 VAC   --- This seems nominal, the fluctuation wasn't too bad, but somewhat unsettling. I wonder if this means that the transformer is wearing out... I will take resistance measurements later.


Well as it seems that the power supply voltage is nominal, so I now take the scope apart and check the driver/power supply board (at least that's what I'm calling it since the electron guns are connected to this and the power transformer is directly connected to it...)



And upon further inspection... lookie what I found...




These... what I think are BJT transistors (I'm assuming such because of their interesting heatsinking) appear to have failed, somewhat spectacularly. I noted some other issues which I repaired and cleaned up.





They were located here:


So... it appears that I may have found something responsible for failure.... Orrrr a part that failed as a result of something else failing. Further testing is required.

However can anyone help point me in the right direction on these transistors? They appear to be BJT type, but the numbering on it is difficult to decipher. It looks like it says BD237 on it. Which makes it a NPN type BJT Transistor, 2A 25W.

One of them has a 50K printed on it as well.. Any ideas?

Thanks for reading!

Edit: Here's an idea, I'm thinking that the slight oscillation in the transformer maybe enough to have caused the transistors to fail... this is only a hypothesis, but it seems plausible.... As I mentioned before, further testing is required.

[Smokey]

Damn power supplies.....  its always the power supplies!!!

I have a similar Phillips scope that needs all the knobs replaced.  They don't make reliable contact anymore and it distorts the hell out of the signal.  Other than that it works fine.  No guarantees, but if I manage to break the scope fixing the knobs or just give up since it looks like a lot of work, we might be able to work something out where I could send you my working power supply.

If you want me to try to read part numbers off anything, its still all apart on the bench for now.

[grumpydoc]

One of them has a 50K printed on it as well.. Any ideas?

If you don't have the manual grab it off the 'net. Usually for the old Philips 'scope there isn't a separate service manual but the schematics etc are in the "User" manual. They were built in the days when anyone using a 'scope was likely to be able to fix it.

The marking certainly looks like BD237. I'd go so far as to say it is BD237  - grab a couple and replace the dead ones. If you can't find anywhere else I see there's an ebay seller who will do you 4 at $3.60 including postage.

[Mediarocker]

Damn power supplies.....  its always the power supplies!!!

I have a similar Phillips scope that needs all the knobs replaced.  They don't make reliable contact anymore and it distorts the hell out of the signal.  Other than that it works fine.  No guarantees, but if I manage to break the scope fixing the knobs or just give up since it looks like a lot of work, we might be able to work something out where I could send you my working power supply.

If you want me to try to read part numbers off anything, its still all apart on the bench for now.

I certainly appreciate it! But I'm in it to win it. I really want to see if I can bring this puppy back to life with some of my own skills. I do however will keep you in mind if you can't manage to get your scope working 

One of them has a 50K printed on it as well.. Any ideas?

If you don't have the manual grab it off the 'net. Usually for the old Philips 'scope there isn't a separate service manual but the schematics etc are in the "User" manual. They were built in the days when anyone using a 'scope was likely to be able to fix it.

The marking certainly looks like BD237. I'd go so far as to say it is BD237  - grab a couple and replace the dead ones. If you can't find anywhere else I see there's an ebay seller who will do you 4 at $3.60 including postage.

Thanks Smokey, You gave me the confidence I needed to move on those transistors. I am looking for the manual as I type this out. I appreciate the help everyone!

[Mediarocker]

Found the users manual. MAN is it a wealth of information!

Found the parts on the board schem and found the PN and it is IN FACT a BD237 on the dot!



Let's see if these fix 'er up!

[robrenz]

If you are a beginner then you need to familiarize yourself with all the safety precautions of working on a high voltage power supply. That thing can kill you. I didnt hear you mention any safety steps like discharging the caps before you started to work on it so that is why I am mentioning it.

This is an example https://www.eevblog.com/forum/general-chat/be-careful-with-those-teardowns-kids!/msg152513/#msg152513

If non of this applies to you, great, my appologies.

[Mediarocker]

If you are a beginner then you need to familiarize yourself with all the safety precautions of working on a high voltage power supply. That thing can kill you. I didnt hear you mention any safety steps like discharging the caps before you started to work on it so that is why I am mentioning it.

This is an example https://www.eevblog.com/forum/general-chat/be-careful-with-those-teardowns-kids!/msg152513/#msg152513

If non of this applies to you, great, my appologies.

I assure you, I took those steps before I even thought about touching the board. Matter of fact if you read through the posted thread a little you'll find my post showing how a simple cap can kill you in the right conditions 

While I am a beginner to oscilloscopes, and a beginner to advanced electronics, I have been working on computers for over 15 years, which includes playing with caps. I should have probably been a bit more descriptive and noted that I'm not a complete beginner to electronic repair, I have a deep understanding of how things work, but I have no experience using an osciliscope, laboratory diagnosis, electrical engineering, and making my own circuits. However, I have plenty of experience figuring out a general issue and repairing it. 

Thanks however for the warning.

[Mediarocker]

Replaced the BD237s and The power light still doesn't come on..

So I'm willing to think it's the PSU. Going to take more advanced diagnosis of it...

[91dedios]

hello,
I just recieved a phillips pm3212 and was wandering how can i make sure it is working properly?
thank you!,
Luis

[c4757p]

Luis, I don't mean to be rude, but you really should just look around for an oscilloscope tutorial. This is a nearly one month old thread on repairing a known broken oscilloscope; testing one is just knowing how to use it and trying all the features. For a basic test you can connect each channel to the probe adjust output and try to get a trace displayed on screen, but I'm not going to hijack the thread to explain how to do that.

[alejandro.sela]

Hi Mediaroker and all!
I know that this is quite an old post, but I just received  a  PM3212 and started troubleshooting it. Did you have any progress on it? 

I found some information regarding replacement (and built!!) of the main transformer, and other guy that claim that the issue could come from the "buzzer-shaped" piece in between the two BD237.

Mine happened to work for a couple of minutes, but after some noise died again...

I am begginer on oscilloscopes reparation, but will work to have this one working! First thing I am doing is documenting myself, and trying to find similar faults. I have the service manual and some equipment, but very little knwoledge about troubleshooting/ testing, so I will learn about this and come back with information if I ever manage to go one step further!

thanks!

Alejandro

[jaycee]

The BD232's appear to be part of a blocking oscillator type circuit which drives the primary side of the transformer T202

One immediate thought is that dried up electrolytic capacitors on this board might be stopping the circuit from oscillating. The two transistors might have turned on simultaneously as a result, effectively shorting the supply across them.

Manual with schematic is here - http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Philips/pm3212.pdf
Page 80 shows the power supply board schematic in the bottom right hand corner. Page 76 shows the power supply board layout.

[Mediarocker]

Thank you Alejandro and Jaycee!

It has been sitting shelved for a while as current work projects have had me a bit busy. I'll be digging it out to check this!

Thanks for the suggestions!

[mikes112]

I just got a Phillips PM3214 oscilloscope and the back is exactly like yours, but my plug is the European or rather English big three pronged type, so I can 't even plug mine in yet. I read the manual and it doesn't indicate which post to solder the white and black wires coming in from the wall plug which is the wire colors we use here in the US. Also mine doesn't even have a grounding wire from the plug. Just wondering is it possible to tell me which wire goes where? Right now one brown wire coming from the chord is soldered to a post which has a black wire going directly into the transformer, and the other wire  which is blue is soldered to a post which goes through a capacitor.  Any help would be appreciated. Also I read on another website that the rubber boot which supports the video tube became conductive over time causing some sparking sounds, so he had to clean the outside surface of this rubber boot and hasn't had a problem since. Thanks.

[mikes112]

So I found that the plug is marked with L N E  L I assumed will be live and N will be neutral or rather white wire, but the ground wire is not attached, and I see that others for sale of my and your model dont have the ground attaced, there only 2 prong. Also the plug should be polarized so you get the L and N plugs inserted properly, wondering if it is suppose to be grounded? Or is it suppose to be like an isolation transformer? So when you test your isolated from ground?