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CRT ocilloscope issue

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brandon3055

Sun Aug 02 2015, 10:39PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Been a while since I visited these forums but I find myself in need of some advice regarding my oscilloscope so here I am again.

The last time I used my oscilloscope was about 2 weeks ago and it was working flawlessly. But today when I went to use it I turned it on and instead of the usual trace which stretches rite across the screen I noticed it was only about half the width of the screen. So I assumed I just left it on the wrong setting when I put it away and started checking the settings. But within about a minute the trace slowly shrunk down to a width of about 5mm and I have not been able to figure out what is wrong.

I did a quick search online for a possible solution but did not find much and my knowledge of the inner workings of CRT oscilloscopes is very limited so I am hoping someone here can help or at least point me in the rite direction. When I took the cover off I noticed the faint but unmistakable smell of an overheated or fried transformer. But the transformer wasn't even warm.

The only thing to identify the scope is what's on the front panel "Griffin 15MHz single trace oscilloscope" I could not find a model number. Here are some images that better show the problem aswell as one of the interior of the scope (sorry for the horrible camera work)

Edit: Whatever the issue is it only seems to affect the horizontal width everything else seems to work fine. When attach the probe to the .2v square wave output the trace splits into two lines indicating the positive and negative voltages of the square wave.

Dr. Slack

Mon Aug 03 2015, 05:20AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The fact that the vertical deflection is still working to spec is very encouraging, as it tells you a lot is still working.

I notice from the internal shot you have labelled, leaded, components. The fact that it's also a very simple analogue controlled scope means it's very unlikely to be doing anything clever inside. These give you a fighting chance of tracing the circuit and figuring out what is/isn't happening, even if you can't find a circuit diagram, but if you can find a servicing diagram then it will be much simpler.

The timebase chain in an analogue scope (and you should be able to google and wiki for actual diagrams to see what I'm talking about, for instance

consists of a ramp generator, with a variable speed controlled by the timebase setting, with a way of starting it controlled by the trigger setting, which has an auto setting that will trigger the ramp if nothing's happened for a while, with an amplifier to the tube deflection plates, which will usually have a contrallable offset in it. That's pretty much it. On a slow timebase setting, you can see what these blocks are doing with a meter, not really needing a scope.

As with any debugging exercise, if you have a simple linear chain, which you have here, then 'divide and conquer' works well. Break the path, is it working before or after? Break the non-working one, rinse and repeat. Of course that assumes you know what good looks like!

You can check some stuff from the outside. I notice it has an external trigger input. Try using this, it might be the auto trigger or the trig detector.

The first thing to observe before heading in is that, Â£1 to 1p, it's the pots and the switches that fail open circuit due to dust build-up rather than actual component failure. Of course you still have to find the right one and clean or replace it. Take all your timebase controls and swing them quickly end-to-end many times. That may clear the fault. If not, blast any open switches/pots with an air-duster.

If that doesn't fix it, then try to track down a servicing diagram on the interwebs. It's so much easier to do the jig-saw if you have the picture on the box.

brandon3055

Mon Aug 03 2015, 07:20AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Thanks for the information. Unfortunately it looks like this isn't going to be as simple as a dusty switch. And so far I have not been able to find any documentation of any kind for this scope mainly because there is no model number or any sort of unique name anywhere on the scope. So I guess I am going to have to do this the heard way...

I think I understand what your talking about with the time base generator but assuming I have some understanding of how this works I don't think that is the issue. The reason being before the trace completely shrunk down to nothing but a little dot I messed around with the timing knob and it seemed to be working normally. So I think the issue is most likely outside all that and closer to the CRT in whatever component(s) actually take that dot and stretch it across the screen.

There may also be a clue in the way it died. it did not just stop it seemed as if the power getting through to the horizontal deflection plates started at about 50% and slowly died down to 0. Which makes me wonder what type of components die in this way. I'm thinking something passive like a capacitor.

But I am basically just thinking out loud at this point and given my limited knowledge of how these work I may be way off.

BTW you mentioned trying to use the trigger input. Is there a simple way to do that? because I assume you would use a function generator for that which I don't have.

Dr. Slack

Mon Aug 03 2015, 11:47AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

The timebase chain starts with the trigger, and ends with the deflection amplifier, and it's a question of tracking down where in that things break.

You can deliver a single trigger by connecting a 1.5v battery to the ext trig input.

If by looking at the board you can guess which connection carries the ramp generator output to the deflection amp input, then that would be a good diagnostic place to break it. Looking at the construction, I strongly suspect that the ramp generator and the amp will be in seperate boxes connected by a wire, you might even find it's labelled. The time-position knob will probably go to the amplifier. The seconds/div knob will go to the ramp generator.

Move the time-position knob, does the dot move sideways? That will tell you whether the fault is before or after the position knob.

If before, check the timebase output with a meter to see if there's a ramp there on auto trigger. If after, then the bad news is you have to dig into the amplifier, the good news is that it should all be very obvious and straightforward in there, it's just an amplifier. You have a working Y-amp to look at, with luck they are layed out similarly, though a Y-amp is likely to be tweaked for a higher bandwidth.

BTW, don't get bitten by the odd 3kV on the CRT!

Ash Small

Mon Aug 03 2015, 06:54PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Not sure if this is much use, but aren't a lot of these old 'scopes fitted with electrostatic deflection plates, or something?

I imagine there's a capacitor in that circuit, if it's there, also old insulation.

Do you know the year, or even decade, of manufacture? A lot of these old 'scopes were made for TV diagnostics, and are pretty similar in construction, maybe we can find a schematic for something of a similar age and specification?

Antonio

Mon Aug 03 2015, 10:10PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

I had an analog oscilloscope quite similar to this, that I had to repair several times. The horizontal amplifier may have some transistor (or IC) burned, or, as mentioned, the sweep generator may have some problem. Check if you can locate the source of the smell, or some visible burned or too hot component, My oscilloscope ended its career when the sealed power transformer burned. Too much work to replace it. If there is a dot on the screen and the vertical amplifier is working, the power supply is probably ok. If it works in XY mode the problem is in the sweep generator.

brandon3055

Tue Aug 04 2015, 03:21AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Ok so first off the external trigger. When I attach a AA to the trigger input the dot on the screen just flashes off briefly and it dose the same when I detach the AA battery but that's it.

The time-position knob dose not do anything but it did before the display completely shrank down to a dot. It moved the ends of the line left and right as you would expect but it would not move ether end past the center of the screen (which I suspect is normal) And I'm pretty sure that means the issue is after rite?

Unfortunately it dosnt look as modular as you describe and there are almost no labels on the board but here are the labels I have noticed I'm just not sure if any of them will help.

The .2VP-P goes to the calibration output on the front panel and as If we think its not the trigger system that doesn't help much ether but atleast I know where not to look.

This points to the bottom half of the same mess of components that the trigger sys points to. I'm guessing that's the sweep generator so maby a point of interest To the right are the 12 and 24 volt regulators which are working.

Not sure what this is supposed to mean. I tried checking for a 1MHz signal with my dmm and did not find one but then I remembered my dmm only goes up to 10KHz so that probably dosnt mean anything. The 4 devices down the bottom are bf459 high voltage transistors with their collectors attached to the back of the tube. I'm guessing they control the horizontal and vertical deflection plates if that's the case they may be a good place to start looking for problems.

Unfortunately I don't know anything about this scope except that I got it second hand from the electronics class in my old college. So far I have not been able to find any capacitors or other components that look fried.

And thanks for the warning regarding the HV but I was already hv risk. Although I was expecting it to be a flyback transformer similar to a TV but instead it has a 400VAC winding the transformer (was a little nervous about putting my DMM on that) which goes to what looks like a voltage double or Tripler. Haven't tested the output of that but that should be fine because I pretty sure that's used to power the electron gun without which there would be nothing on the screen.

Edit: Here are some more detailed images of the interior of the scope.

Voltage multiplier

What I believe to be the trigger and sweep generator on the pcb

Other side and the time scale adjustment

Time scale adjustment next to the voltage scale adjustment

Back of the pcb

Back of the tube and those 4 transistors connected to the tube

The smell may be coming from the transformer because it is humming as if under load. I know some transformers especially old ones do this even when not under load but I do not recall if this one did or not before this. It may also just be under its normal operational load. Because of this I have been trying not to keep it running for more then a few seconds at a time but I have been able to confirm that all of the transformers outputs are working.

Edit: can anyone decipher this capacitor code? "Line1:DEC/MFR (F may be an E), Line2:01/10, Line3:100V-, Line4:F7" That is the small cap next to the 4011 Q NAND gate in image 2. It reads as 11nf but I can figure out if that is what its supposed to be.

Edit: I just discovered that there is a sign on ether the base or emitter of one of those 4 BF549 transistors that ranges from <1Hz to >200Hz depending on the time scale setting but there are no detectable signals on any of the other 3 bit I'm not sure if that means anything.

Edit: seems my dmm wasn't getting a good reading. The two on the right are getting a signal and the other two arnt. So it stands to reason the two that arnt control the horizontal deflection. The signal to the two that are also changes when I adjust the V/Div and goes over 3KHz. So I think the next step will be to track down where the two that are not getting a signal are supposed to get a signal from.
Edit: Those transistors are connected directly to the V/Div control module via the green and blue wires that can be seen tied together on some of the pictures. But it just occurred to me that thay may be working just at a frequency to high for my DMM to detect.

Update: I think I may be getting closer to solving this. Messing with those 4 transistors I have found that the two that are not getting a signal control the vertical deflection which explains why they are not getting a signal. When I bridge the collector and emitter of those with a 50 ohm resistor (safely) and create some noise trough the resistor by rubbing the that is connected to the emitter on a free pad that is connected to the emitter I can see the dot deflect ether up or down. However when I try the same thing on the two that I suspect control the horizontal deflection nothing happens. This suggests the issue is in the power supply for the horizontal deflection especially because when I try bridging the collector of one of the horizontal transistors with the emitter of one of the vertical transistors the dot is deflected horizontally. The weird thing is there is actually 200VDC across both of the horizontal deflection transistors so I don't know why its not working... I will continue to investigate.

Update: Ok so I think I can rule out all that... At this point I'm thinking there is something wrong with the signal getting to the horizontal deflection transistors but unfortunately I cant confirm that without another scope... I tried connecting the signal from one of the vertical deflection transistors to one of the horizontal transistors and it turned the dot into a short horizontal line so that tells me there is nothing wrong with the horizontal deflection transistors. I think I might just try replacing some of the IC's and see what happens...

radiotech

Wed Aug 05 2015, 05:48PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Based on comments about transformer in your scope, a power measurement is
essential before going any further.

On my bench, dealing with mostly old equipment, I can measure AC wattage
and AC line current of anything plugged in immediately. As things are powered up,
a variable trip setting is possible.

You probably don't this equipment, so a simpler method is called 'Dim Bulb'
testing. It is widely used by radio restorers.

brandon3055

Thu Aug 06 2015, 12:36AM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

Assuming my DMM (which supports AC current) is correct the current draw is 60ma which is significantly less then the rated 160ma@240VAC. Also the transformer seems to have stopped humming.

brandon3055

Fri Aug 07 2015, 12:58PM

Registered Member #4548 Joined: Mon Apr 23 2012, 03:52AM
Location: tasmania
Posts: 271

haven't been able to get replacement ic's yet but from what I can tell with my dmm the existing ones seem to be working. I must have gone over every inch of this thing a doze times now and haven't found anything wrong with it yet... Starting to wonder if its worth spending any more time on it. The annoying thing is everything in this scope looks very simple and easy to fix. I just cant figure out what needs fixing. What are the chanced of the CRT itself failing in this way? I'm wondering it got knocked around when stored in my cupboard and is that could damage the tube internally.

Where are the best and cheapest places to look for a good second hand scope? I have been looking around but so far haven't found much under $300-$400. Except for one I found for auction on ebay which was previously owned by a now deceased electrician. Its a bit of a gamble because the seller dosnt know anything about it other then the screen shows a green dot when you turn it on but if the bids don't go to high it may be worth the risk. Unless ofcorse someone here knows of a better place to get a scope.

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