[SOLVED ]Trio Oscilloscope blanking & intensity problem

[gkmaia]

I am a bit stuck trying to find why the intensity potentiometer (VR3) has no effect. did remove and tested most transistors and diodes on the intensity block and could find nothing.

Can anyone give me a light? I included the circuit description and the schematics and some measurements.

[bob91343]

It seems to me that tracing the dc paths is the way to go.  If the intensity control's wiper is changing its voltage, then the subsequent circuitry needs to control the CRT grid accordingly.

If the CRT grid isn't responding, the problem has to be in between.

However, without perusing the diagram, I warn that often these circuits are floating at rather high voltages, not exactly safe for ordinary people and test gear.  So watch your step and take appropriate action to avoid trouble.

[gkmaia]

It seems to me that tracing the dc paths is the way to go.  If the intensity control's wiper is changing its voltage, then the subsequent circuitry needs to control the CRT grid accordingly.

If the CRT grid isn't responding, the problem has to be in between.

However, without perusing the diagram, I warn that often these circuits are floating at rather high voltages, not exactly safe for ordinary people and test gear.  So watch your step and take appropriate action to avoid trouble.

Thanks for your reply!

If you look at the schematic bellow. All the green and orange dots are tested components that are working.
Then only thing that does not match the manual is the node 2. It should be a squarewave. At the moment it is flat at 0V.
But when you look at P408 you see pin 3 comes from 5v rail, that is working. Feeds into VR3 and VR3 works. But I get no voltage on node 2.
If you look R438 & R439 are both working. Not sure why node 2 is low.

Do you think that is probably where my problem is?

Is it possible for a fully a CRT that display images perfectly to have just it is -V grid of the control grid to be damaged? Therefor it is stuck into high intensity?

Other thing I plan to test tonight if the blanking signal that comes from the horizontal amp board and feeds into VR3. I will do that and post the result here. Maybe that is one of the reasons why node 2 is low?

[bob91343]

That node 2 comes almost directly from the intensity control.  The difference voltage across the two series neon bulbs N401 and N402 is what actually controls brightness.  If node 2 is wrong, trace back to the intensity pot.  If it's shorted to ground or something that might be the trouble.

The missing square wave seems to make the entire intensity control move with it.  If you aren't getting the wave (off the diagram) then look there.  But I keep thinking the control itself may be the problem.  Perhaps its shaft should be insulated and isn't, for instance.

Does the Z axis input have any effect?

Best would be to model the circuit to see how it works.  I don't know how to do that.  But the dc values have to be such that the CRT is biased on.  What is that voltage?

[gkmaia]

But I keep thinking the control itself may be the problem.  Perhaps its shaft should be insulated and isn't, for instance.

Can you elaborate that a bit more? By "control itself" you mean the CRT internal control grid. Which mean the CRT itself is faulty?

Regarding the Z axis. I know it is a Z axis modulator intensity but never used and not sure how to test it. So not sure what kind of signal or voltage to supply there and not sure what the CRT reaction would be for such input.

Also, in regards to the NEON lamps. I imagine as the intensity circuit tries to reduce the CRT brightness the NEONs will suck current. By doing so as the CRT brightness reduce AND the NEONs light up? is that a correct assumption?

Sorry for some many questions....

[bob91343]

The control to which I refer is the Intensity control.

As for what signal to apply at the Z input, see the manual.  Generally a volt or three, and the polarity matters.

[bob91343]

The neon lamps are simply overvoltage limiters.  They aren't very good at it but in this kind of circuit they are okay.  They help avoid CRT damage from excessive drive.  So they won't light except when protection is needed.  They should be dark in all ordinary operation.

[mzacharias]

How about a shorted C426, 1uF at 250 volts? That would completely disable the intensity control.

I had that exact thing happen on a Tektronix 465.

[gkmaia]

I will have a look at it thanks for the tip. An odd thing i noticed was a custom jumper not in the schematics between the +10v rail and the -10v rail. Any idea why the factory or previous owner would do that?

[Andy Watson]

Have you checked that the high voltage (-1.5kv) is correct at the power supply and t on the terminals of the tube?

[gkmaia]

I am struggling with this one. Can't really find where the fault is... I put apart the whole intensity blanking block and checked each and every transistor/resistor/cap/diode & contacts. Absolutely no faulty component. Did also checked and re soldered all contacts of that board. Matched everything to the service manual.

I've made a video showing what is happening with blanking and intensity. When I reduce the intensity it seems to kill the blanking and also increase brightness.

Is it possible for a transistor to be faulty even if it passes the diode check? That must be it because this board connects directly to the CRT. There is nothing in between. Trigger & vertical / horizontal amplifiers seem to be working perfectly.

https://youtu.be/DjnekZekF8o

[pbarton]

In your first post the third attachment is your note book. The second waveform is 0V. If your scope was working this waveform would be switching between 5V and 0.6V in time with the sweep speed. The manufacturers schematic drawing shows the expected waveform at your node 2, the base of Q414.
An almost identical waveform (phase and magnitude) should be observed on the wiper of the INTENSITY pot. One end of the INTENSITY pot is fixed at 5V the other end should have an alternating TTL logic level signal, relating to the sweep speed (from IC303).
Can you confirm that of the INTENSITY pot has an alternating TTL logic level signal (the blanking pulse), relating to the sweep speed (from IC303) on it?

Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth. Arthur Conan Doyle.

[gkmaia]

please see the attached image for updated test points.

POT pin 1 = 5V steady
POT pin 3 = signal from IC303 switching between 5V and 0V.

At node F & 4 which are after the output of POT pin 2 you get at counter clockwise 5v and clockwise the pulse from IC303. In between the mix as per the video bellow.



Signal comes in fine and reaches nodes F & 4 but do not pass to node O. but also it's frequency does not seem entirely correct as per the image attached.

Nodes A, B, C are as per spec. Node H +8.97v
Rail 5v = +5.2v
Rail +10v = +9.96v
Rail -10V = -9.96v

Node E is not exactly perfect as you can see in the video bellow but it is somehow responsive.



The base of Q414 is stuck high. Not sure why. I do not understand really well what makes it's base oscillate.

[pbarton]

Your video “Trio probe E node” shows the correct ‘envelope’ waveform (it’s responding to you turning the INTENSITY control) but the ‘modulation’ from the DC/DC converter is missing!
Compare your video with the expected waveform for node E, which can be found by following the dotted line back, to above ‘P403’ on the schematics.
The expected waveform for the base of Q411 is shown just above the DC/DC converter and should alternate 3.2V above/below ground. Please verify this waveform for the base of Q411.

[gkmaia]

Your video “Trio probe E node” shows the correct ‘envelope’ waveform (it’s responding to you turning the INTENSITY control) but the ‘modulation’ from the DC/DC converter is missing!
Compare your video with the expected waveform for node E, which can be found by following the dotted line back, to above ‘P403’ on the schematics.
The expected waveform for the base of Q411 is shown just above the DC/DC converter and should alternate 3.2V above/below ground. Please verify this waveform for the base of Q411.

Thanks for your response. I did check the base of Q411 and it is fine as per image TP1. Also tested the emitter of Q411 at TP2 and image is attached.

Also toke a chance to test D404 and its fine.

That is so intriguing... any more ideas?

[pbarton]

The ‘envelope waveform’ comes from the collector of Q413 and is imposed on the collector of Q411. Your video “Trio probe E node” shows this is correct (it’s responding to you turning the INTENSITY control).
Q411 is supposed to modulate the ‘envelope waveform’ with the higher frequency signal from the PSU DC/DC converter, as shown by the signal diagram on the schematic (just above P403).
Apparently, Q411 is not conducting (not switching current, according to the waveform applied to its base). Why is this?
I believe that you have removed and tested Q411 and it appears good. However, is it possible that under (test meter) small signal conditions that Q411 appears to be OK, but when installed in circuit its operating under very different conditions and that it fails to conduct in circuit? It may sound improbable but I don’t have a more credible explanation.
What would I do at this point? I think that I would remove Q411 from circuit and remeasure your “Trio probe E node” waveform. If the ‘envelope waveform’ is still present and the scope display behaves identically, then clearly the absence of Q411 his having no effect. Which implies that Q411 is not working when installed in circuit. If this is true than you need a new Q411. In which case Q411 would look interesting on a curve tracer.

[gkmaia]

Thanks pbarton! I will try that today.

On case it failed I do not have many options in stock here. Quite hard to find a ECB with similar specs. Mostly obsolete. I did order original 2sc983 but it will take a couple weeks.

The closest I have in stock is the 2N5551. I will have to tweak it's leads but that is ok. What is your opinion about the other specs (see image attached)? Close enough?

[pbarton]

Yes, I think the 2N5551 is close enough. NPN Silicon, meets working voltage, frequency and gain. You just need to re-arrange the leads to ECB format. 

[gkmaia]

Did follow the plan.

The video starts with a 2N5551 running as Q411 you can see the 50v square-wave with modulation.
Then at 4sec you see the modulation goes away for a few seconds but the main square-wave remains. That is when I remove the transistor with the scope running.
Then at 6sec the modulation comes back when I insert the original 2sc983 back in.

With or without Q411 the scope CRT behaves the same. No intensity control and the blanking issue is present.

[pbarton]

The modulating signal is tiny with either the original or the new Q411. The signal diagram on the schematic (just above P403) shows a large modulating signal. Try replacing R453 220K?

[gkmaia]

R453 should be good as I remember testing it previously, but will test again just to be sure.

One thing I noticed now or I may have missed.

My 150v rail is lower at 135v and the collector of Q412 instead of getting 136v is getting just 130v. Could that explain the modulation being 10v under or the base of Q414 not oscillating?

What is causing the rail to be under is also intriguing...

I did check the power supply. The secondary of the transformer is outputting AC 300v. Then 150v after the bridge. The decoupling caps after the bridge are new Nichicon. I don't think I have an issue with my power supply.

Something may be bringing the rail down? Do you think a failed cap working as a resistor on another board? How would you approach finding what is pulling it down?

[pbarton]

130V for the collector of Q412 is fine. I'm not concerned by the high voltage rails, they are close to being correct. I'm trying to understand why you don't have the waveform shown on the schematic (just above P403).
"My 150v rail is lower at 135v and the collector of Q412 instead of getting 136v is getting just 130v. Could that explain the modulation being 10v under or the base of Q414 not oscillating? " I don't think so.
"Something may be bringing the rail down? Do you think a failed cap working as a resistor on another board? How would you approach finding what is pulling it down?" I think that if you had a failed cap then its failure would be more obvious (hot, discoloured, bulging or leaking). Is the voltage selector set correctly for your supply voltage?
What voltage is on the emitter of Q411?
Edit: The power supply appears to be a SMPS design without any feedback from one side to the other. There appears to be provision to adjust the "LOW VOLT ADJ" which will change the secondary rail voltages. I'm not aware of the procedure to set the "LOW VOLT ADJ". Apologies: You previously included the Q411 emitter waveforms.

[pbarton]

Today I realized that collector of Q411 is an analogue summing point. It is responsible for producing the complex waveform shown just above P403 on the schematics.
The ‘ceiling’ of the waveform (which is 95V in the diagram above P403) is formed by D406 and the components to the left of it (including VR405 and R455).
The ‘floor’ of the waveform (which is stepped between 30V and 75V in the diagram above P403) is formed by D404 and all the components to the right of it (including Q414, Q413 and Q412).
Between the ‘ceiling’ and the ‘floor’ lies the modulation (the frequency of which is determined by the PSU DC/DC converter) this is produced by Q411 itself.
The floor, ceiling and the modulation are summed together at the collector of Q411. The output of this circuit is via C423.
I am convinced that the floor of the waveform is correct (it’s responding to you turning the INTENSITY control).
However, the remaining problem is that Q411 is not producing the large amplitude modulating waveform.
In an attempt to restore the large amplitude modulating waveform on the collector of Q411 can you please try the following….
1. try disconnecting one end of C423 and re-measure the collector of Q411.
2. try disconnecting one end of D406 and re-measure the collector of Q411.
3. try disconnecting one end of D404 and re-measure the collector of Q411.

[gkmaia]

Let me see if I got your plan.

It is the similar troubleshooting technic we applied when removed the Q411.

By removing C423, D406, D404 one at a time you want to see if it makes a difference. If it makes a difference means the component is doing it's job. If it makes no difference means the component may be faulty or something near it is.

One thing I replaced on this board was a badly burned R459. It was still providing 22k but was quite carbonised.

[pbarton]

Yes, you can remove them one at a time.
I'm hoping that when one of the three is removed that the large amplitude signal on the collector of Q411 will appear.
This would indicate an abnormal load that we can investigate later.