Philips Oscilloscope

Hello!

Glad I've found this forum!

I've nearly completed a 2 year degree in Electronics, which is to say, I don't know much! Actually, I do have some hands-on experience, but I'm still a young buck when it comes to troubleshooting. That being the case, may I bend your collective ear on a piece of test equipment I'm troubleshooting???

I recently picked up a very nice, albeit old, Philips PM 3320 Oscilloscope to round out my bench equipment. The scope was free, but tragically, it has no raster on its CRT

I started off by listening for a chirp, and crackle from the flyback, and yes, a chirp is definitely present, but no crackle of high voltage. Running my Fluke Volt-Alert voltage detector near the HV anode wire that goes to the CRT confirmed this. There is no high-voltage on the anode wire. The filament can be seen glowing at the neck of the CRT, so that's a good sign.

I downloaded the service manual for $15 from "manuals-in-pdf.com", but I feel they kind of got the better of me. The manual doesn't really present much direct information on troubleshooting. Their most insightful blurb on my symptom reads as follows..."If there is no signal, or text visible on the C.R.T. screen, then check: Intensity Control, Power Supply To CRT..."
No kidding!
So, anyway, the manual does contain some schematics, but mercy, there is some pretty darned complex stuff in there. I think it's for people with a bit more experience than I. If you need a good laugh, I've uploaded the manual, and you can peruse it for yourself at:
https://www.electro-tech-online.com/custompdfs/2006/08/PM3320.pdf
Please be advised though, it's about 24 MB in size.

My suspicion, is that the flyback transformer, or the circuitry that drives it, has failed. Would anybody care to hazard a guess on where one would find such a critter? I'm guessing, for a late 80s O-scope, I'm not going to have much luck. That being the case, I'm wondering about substitution?

In any event, any thoughts you fine people have on the whole matter are greatly appreciated. I desparately need an O-scope, but my cheap nature simply will not allow me to spend 'big dollars' on one.

Thanks in advance,
Jason

My first thought would be that the power supply is shutting down due to an overload on its output or a failure in the power supply itself. Bad capacitors, shorted power transistor, prime suspects for PSU failures. Overloads outside the power supply may be much harder to find.

Maybe post just the schematics and more people might jump in and look. I saw 500+ pages. I was too lazy to go to them. Just a thought..

You can download screenhunter from download.com and grab the schematics off the screen. Save them in jpg or png format and upload them. Also, not sure if you are supposed to be putting the manual/pdf on the internet, I would think it is copyrighted.

Good point on the copyrighting...I doubt Philips would care, considering this is a nearly 20 year old, obsolete unit, for which they do not offer support. Still, one cannot be too careful about things like this. Don't want to wind up in court or something. So, I have removed the manual from my server.

Here is a screenshot of the CRT section. Now, I noticed something interesting last night. When I turned all the lights in the house off, the face of the CRT glowed. Would this be happening if HV was not present? Perhaps it's a focus issue? Perhaps HV is making it to the anode cup, but if the focus was wonky???? Have a look at those four transistors in series in the "FOCUS" section of the diagram. Perhaps I'll have a look at those first. What do you think?
**broken link removed**

What color was the glow. If the phosphorus (sp) was glowing then I would think you had HV. Problem is, you will probably need a working scope to trouble shoot it.. I will look at what you put on your site. It has been a long time since I have done TV/scope stuff, I was just pointing out, if the schematics were here, other would make comments.

OK, what I recall after looking.

You have the screens, they should have constant power on them. You said you see the filiment glowing in the back of the tube right? If you have an input to the scope I would think (on another scope) you would see X and Y doing something. Also, I recall the filiment sometime would have to be super heated to fix a problem with old tubes. Rejuvenated or something..

Only other thing is a clever way to check the power at the anode/flyback. That is why I stopped doing TVs a long time ago.

Hang loose, someone will be along to help you better that I can. Sounds like you have a clue, so I figured I would share the above. But be careful, there is 15K plus probably on that anode.

If the timebase or vertical amplifier is completely farked, the beam current will be shut off, or blanked to prevent screen burn.

just a thought...

Check the horizontal drive signals feeding the h-out stages and make sure, same goes for the sawtooth on the vertical output stages.
Check the B+ on the lopt as well...just in case...
Before you do anything, check over all the power supplies and verify that they are indeed operational.
These old Philips scopes suffer timebase troubles as often as I have coffee, my guess is your trouble lies therein...

rgds

Philips oscilloscope update

Just an update folks. I found what I am hoping is the source of the problem. I found two disc shaped components, which in the schematic, are labled with "R" designations. These devices are on the main power supply board, "A19". I suspect they might be some sort of surge suppressor, rather than actual resistors. Varistors maybe? Have a look at this video, and you'll see how I found the problem...**broken link removed**

You'll also notice the capacitor in the lower portion of the screen is also arcing internally as well. That's a fairly standard component, so there's no problem finding a replacement for it, but the disc shaped components...any thoughts? How does one go about finding replacements, as there is no info printed on the components. I should also mention, there's a physical chunk missing from the one with all the arcing in the video!

Here's the schematic of the area in question...**broken link removed**
Hopefully, there's nothing downstream causing the failure. I checked the bridge rectifier, and it appears to be functional.

They are thermistors, used for surge limiting.

But I'm not very hopeful about the scope?, the capacitor and the thermistors aren't related in any way - if they have BOTH failed I would suggest it's had lightning up the mains!.

For a test you can simply remove the capacitor (it's only to help prevent mains borne inteference), and try replacing the thermistors with a resistor - 10 ohm 5W should be OK for a test. Make sure you test the chopper transistor in the PSU, and carefully look at any other components around for damage.

Stand well away when you switch in ON, and see if it goes BANG!!.

I learn something new everyday. I had never heard of "Power Thermistors"! Ha, ha! I had only used thermistors in comparitor circuits to act as crude temperature sensors.

So, I am assuming the theory is that their cold resistance starts off relatively high...like maybe 50-100 Ohms, and then, as they are placed under a load when the scope is turned on, they begin generating heat, which lowers their resistance...is that how they function in this scheme??? I'm thinking of surge suppression in the wrong terms then. This functions more like a "buffer" for the power supply to protect it from overloading, say, as large capacitors charge on initial power-up.

I love this stuff!

martinihenry said:

I learn something new everyday. I had never heard of "Power Thermistors"! Ha, ha! I had only used thermistors in comparitor circuits to act as crude temperature sensors.

So, I am assuming the theory is that their cold resistance starts off relatively high...like maybe 50-100 Ohms, and then, as they are placed under a load when the scope is turned on, they begin generating heat, which lowers their resistance...is that how they function in this scheme??? I'm thinking of surge suppression in the wrong terms then. This functions more like a "buffer" for the power supply to protect it from overloading, say, as large capacitors charge on initial power-up.

Click to expand...

Yes, they are to reduce the switch-on surge - using a thermistor rather than a fixed resistor (which is also often used) is more efficient, because it wastes less power once it's warmed up.