So thanks to a generous fellow here (Thanks
![Grin ;D](https://www.eevblog.com/forum/Smileys/default/grin.gif)
) I now am the proud owner of a Phillips PM3212 25MHz Analogue Osciliscope.
![](http://i11.photobucket.com/albums/a163/MediaRocker543/IMG_20121010_141244.jpg)
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.
![](http://i11.photobucket.com/albums/a163/MediaRocker543/IMG_20121010_165818.jpg)
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...)
![](http://i11.photobucket.com/albums/a163/MediaRocker543/IMG_20121010_172440.jpg)
And upon further inspection... lookie what I found...
![](http://i11.photobucket.com/albums/a163/MediaRocker543/IMG_20121010_172456.jpg)
![](http://i11.photobucket.com/albums/a163/MediaRocker543/IMG_20121010_172510.jpg)
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.
![](http://i11.photobucket.com/albums/a163/MediaRocker543/IMG_20121010_203441.jpg)
![](http://i11.photobucket.com/albums/a163/MediaRocker543/IMG_20121010_203507.jpg)
![](http://i11.photobucket.com/albums/a163/MediaRocker543/IMG_20121010_203618.jpg)
They were located here:
![](http://i11.photobucket.com/albums/a163/MediaRocker543/IMG_20121010_172440a_zpscd447509.png)
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.