Service manual - Protek P3502 (Hung Chang) Oscilloscope

[shimniok]

PPS: when you run the suggested tests above ...

Now that we're really diving in, whenever you film the trace behaving a particular way, leave the position alone and film the trace for say 10-20 seconds. That will help me see exactly what the trace looks like without any additional effects from the position knob.

To set up the scope---

1. Make sure you have the time/div knob set to where you can clearly see the square wave -- it's period should be taking up about 2 of the major divisions on the screen.

2. Set the position and volts/div so that the trace is taking up 1/3 to 1/2 of the screen and showing a normal square wave.

3. Set it to trigger off of INT (internal, ie, trigger off the waveform being displayed) and using the AUTO trigger to make sure the square wave looks "still" on the screen.

Sorry if you know how to do all this already. I forgot to ask how much you know about using 'scopes.

When you film, see if you can reposition the trace and film 20 seconds EACH of:
- the trace in the top half where it looks like a normal square wave
- the trace on the lower part of the screen where it is all wonky
- a position on the screen where the top half of the square wave is ok and the bottom half is wonky (adjust volts/div to have the trace take up the whole screen if you want)

I'm hoping this will further verify that the problem is in the vertical amp Y- plate driver circuitry. That'll narrow us down from block to circuit (following the methodology of that troubleshooting manual)

After you do these tests I have another test I want us to do, to see if we can get a clearer picture of what the trace looks like and maybe get a sense of what components might be failing. Since the position of the beam on the screen is telling you precisely what the voltages are on the Y+/Y-, X+/X- plates we will take advantage of that to see if we can get any sense of what component(s) might be failing.

Michael

 

[pierreretief]

Ok, so I took the video, but it was kind of hard to do everything that you said, since the scope didnt always respond as expected. But have a look at the video, and then just ask me to take another one if you want more info... I kept it on one position for a few seconds at the end, but did not really have enough time on the clip to do it for each volt/div, but it doesnt make a real difference, since what you see, is where it stays for that 10 or 20 seconds.

But I had all the settings as you wanted...

Do you think I should open it up and start measuring some voltages on the vertical amp circuit? Or should I follow the book approach and try and figure it out before opening it up?

Thanks again for the help

 

[shimniok]

Fantastic video -- nice work. That actually is superbly informative. I am seeing something very similar to what I was expecting, actually.

The square wave displays in the upper half or 1/3rd of the screen and when you position it lower (no matter what volts/div) the trace goes haywire below a certain point. To me this further confirms a problem with the circuitry driving the Y- plate. I also realized another thing... the trace itself is actually scanning down to the bottom of the screen... so that proves there *is* voltage applied to Y- --- though it is erratic and not very related to the signal coming in from the pre-amps! The square wave shows that the top 'half' of the vertical amp is fine but whenever the preamp drives the negative half, things go bonkers.

When we get to measuring components in a little while, we'll play it safe and test components in circuit with power unplugged and machine off and see if we turn up anything.

Btw, even though the trace goes wonky at slightly above the halfway point on the screen I still believe it is the bottom 'half' of the circuit with the problem. The mid point for trace position (ie, the balance between the top and bottom halves of the vertical amp) is adjustable as far as I am aware. I'm assuming it is just out of adjustment somewhat. Not a big deal, no need to fix that (now or ever, probably).

Stay tuned...

 

[shimniok]

Before we dive into the component tests...

I'd like to do one more movie/test to see if we can get a better idea of what the trace is doing when it goes weird.

I want to see the trace sweeping slower in hopes that we will see an interesting pattern in the wonky part of the trace.

1. Hook the probe up to the cal pin again
2. Adjust volts/div to .2 (I verified against my scope this time)
3. Adjust position so that you can see the top half of the square wave and the bottom half has the multiple lines running across (make it look like this):

**broken link removed**

4. Now, set time/div to 2mS
5. Film the trace for a few seconds each for: 2mS, 5mS, 10mS, 20mS and 50mS
6. While we're at it, repeat 5 with the probe unhooked or with GND coupling selected

For giggles, you might also try using LINE coupling to see if the wonky trace becomes 'stationary' which may tell us something.

Here is the circuit diagram, btw:

**broken link removed**

 

[pierreretief]

Ok, I took the video.. When it is set to line, it does not become stationary, hope this video helps.. strange things again...

 

[shimniok]

Super! Once again, nice going. That is really interesting. Particularly the sort of repeating pattern for 2mS and 5mS. I will re-watch this a few times.

So, we've isolated down to the block level for sure... and at least a subset of circuits at least in my view --- I still think the trace is telling us that the bottom 'half' of the vertical amp is wonky.

Ideally we'd want to try and isolate the various circuits within but not sure if that is going to be fruitful. Life is a bit harder since the svc manual doesn't show proper waveforms at these points.

Btw, do you have access to another scope??

So anyway I think it'd be worth testing a couple of key resistors and diodes; and also the transistors in the bottom half of the vertical amp (I am assuming, btw, that Y- corresponds to the lower half of the screen...)

Please be safe!! Ideally, the power supply caps should be discharged and you should try to work with one hand and the other behind your back (at least that is what they taught us in EE lab 15 yrs ago )

So, the two resistors are the ones on the right of the ckt diagram that are inline with the Y plates and in parallel with a couple of inductors. They are 680Ω each. They are 3W -- fairly large and easy to find. IIRC you will find them and the two TO-220 power transistors on the left side of the board, midway between front and back panel. To help orient yourself -- the power supply caps are back left. The high voltage section encased in aluminum cage is back right. You can check the service manual for help on board layout, too. And the component IDs (e.g., Q14) are labeled on the board, too.

Beyond that I would check diodes next. It's easy so why not test them all. I'm assuming you have a diode tester on your DMM and that you know how to test diodes... if not lemme know.

D2,3,4,5 (the circuit diagram shows D14 where I'd expect to see D4 -- this is probably a typo).
D6,7,8,9.
D10,11,14.
Am I missing any?

After that I'd do a diode test on the transistors (do you know how to do that for NPN versus PNP?)

Q6,8,10,12,14,16,18,20

Not sure all of the diodes and transistors will test right in circuit, though. If not, make a note of them and we'll think about removing them and testing out of circuit.

Good luck!

Michael

 

[shimniok]

PS: also look for any signs of burned up resistors or capacitors in the vicinity...

 

[pierreretief]

Great

Great, I am going to do that..

Let me clarify plz

1. Can I just test resistors in circuit?

2. I know that a diode measures OC one way and Closed Circuit the other. I also know that my DMM has a diode setting, and just to check, am I correct in saying that on this setting, the value I measure will be say.. 0.6 which is also the volt drop over the diode when it is in operation? Plz let me know if that is correct. So do I then measure it in circuit or out,? on diode scale? or measure the volt drop?

3. Transistors... Know about NPN and PNP ... know how to identify them... to test, there is some uncertainty, do I use my multimeters transistor socket, which will mean I have to remove the transistor? Or do I measure the voltage between base and emitter, which I think should be 0.6V or do I use the diode setting again, using the concept of the transistor modeled as a couple of diodes?

Sjoe, thats a few questions for you to answer! Sorry bout that, but I love electronics, and I am sure if you explain these things, like you would explain to someone who really does not understand electronics.. I will really be able to follow, since people sometimes answer m 3 questions with answers that that just brings up 10 more, if you understand what i mean...

Thanks again for all the help. I think the things you wil teach me with in circuit measurements will be helpful in fixing other electronic equiptment to!

Thanks again

 

[pierreretief]

ps

Oh yeah, I dont have another scope, but I have 2 DMM's if that will help. One even has the cap measurement ability

 

[shimniok]

Great, I am going to do that..
1. Can I just test resistors in circuit?

Those two, sure. If it's like the horizontal section they'll measure 680Ω in circuit. Generally speaking I have been measuring in circuit and if it looks off, I'll make a note of it and desolder/test later if necessary.

2. I know that a diode measures OC one way and Closed Circuit the other. I also know that my DMM has a diode setting, and just to check, am I correct in saying that on this setting, the value I measure will be say.. 0.6 which is also the volt drop over the diode when it is in operation? Plz let me know if that is correct. So do I then measure it in circuit or out,? on diode scale? or measure the volt drop?

Correct--measure in circuit. I've been using diode scale to measure 0.6 one way and some high value or ∞ the other way. If it isn't clear whether the diode is ok, then I would desolder a leg and measure out of circuit.

3. Transistors... Know about NPN and PNP ... know how to identify them... to test, there is some uncertainty, do I use my multimeters transistor socket, which will mean I have to remove the transistor? Or do I measure the voltage between base and emitter, which I think should be 0.6V or do I use the diode setting again, using the concept of the transistor modeled as a couple of diodes?

Since Bipolar Junction Transistors (BJTs) act like they have diodes between B-C and B-E, you can use the diode test on them. What I have been doing is to measure in circuit and use the diode test to measure B-E, E-B; B-C, and C-B. I can't find the site I was using but here are some links that will help.

**broken link removed**
Testing semiconductors with analog and digital multimeters

Since you're measuring in circuit it may not be clear whether a transistor is ok. I'd make a note of those and we can come back to them later since it is a PITA to desolder and there is risk of making something worse. Want to keep our explorations non-invasive, I think.

Thanks again for all the help. I think the things you wil teach me with in circuit measurements will be helpful in fixing other electronic equiptment to!

Happy to help! I am learning too! I realized recently that I love fixing electronics (I already knew that I love fixing lots of other things). I actually like to buy old, broken oscilloscopes to fix That's how I got my Protek in the first place.

We'll see if the diode and transistor testing turns up anything obvious. If not, we'll try something else. Maybe testing with it powered up...

Good luck!!

Michael

 

[pierreretief]

Hey, just to let you know, I got my results back and I have to rewrite one of my exams on wednesday. :-( It sucks since all I want to do is open that bad boy up and find the fault. but if I dont put all my effort into this subject, I wont have a electronics carreer in the future, so best I spend my time studying, but wednesday night, that bad boy is oing to be opened and fixed!

 

[shimniok]

Hey, just to let you know, I got my results back and I have to rewrite one of my exams on wednesday. :-( It sucks since all I want to do is open that bad boy up and find the fault. but if I dont put all my effort into this subject, I wont have a electronics carreer in the future, so best I spend my time studying, but wednesday night, that bad boy is oing to be opened and fixed!

Sorry to hear you have to re-write the exam! That does suck. But kudos on having the discipline and the right priorities! For what it's worth, I was a comp engr major (basically EE and Comp Sci mix) and still remember the hard work and studying (the pain has faded slightly) and it paid off with a great career so far -- not EE or software related, but the engineering mindset is what has carried me far and served me well. I know the same will be true of you. Your initiative and gumption to fix a scope on the side speaks volumes -- I suspect few EE majors would touch a scope outside of class let alone try to fix it.

So best of luck and if I can help with anything (as long as it isn't thermodynamics or electromagnetics ), PM me or email me and I'll try to dust off the cobwebs and help and/or consult my old textbooks.

Michael

 

[pierreretief]

Thanks alot man, I appreciate it, in a few days that scope is going to show a perfect wave... Im sure of it

 

[pierreretief]

back

Hey guys. wow its been a long time

I got se busy with the studies that I had to put that oscilloscope away. But now, in my 4th and final year, Im ready to tackle it again. First thing Im going to do is check out that power supply. Just thought Id keep you posted (thanks for all the help you gave way back when

 

[shimniok]

cool, good to hear from you. I remember starting on this thread late at night after my newborn baby fell asleep. Now she's 2 years old!

 

[teor ino]

Links are dead

 

[RobDF]

Hello Simon, I would like to express my sincere appreciation on the way you have explained the troubleshooting on the Hung Chang 3502 that I also owe and suddenly started giving some "funny" results even when connected to my service monitor. I am not particularly advanced on the use of oscilloscopes but this has been a present from my late Mom and I need to make it work good again. And besides I still like it. Maybe the age has started giving problems. I didn't manage to download the second pdf on the troubleshooting just to have a bit more guidance. I guess if you have a bit more patience we can touch the 10 years with this post ........ hehehe