[SOLVED ]Trio Oscilloscope blanking & intensity problem - Page 1
Forum section: electronics / repair

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 17, 2019, 06:20:53 am
content: 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.

author: bob91343
author_rank: Super Contributor
timestamp: April 17, 2019, 04:14:56 pm
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 18, 2019, 01:01:22 am
content: Quote from: bob91343 on April 17, 2019, 04:14:56 pmThanks 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?

author: bob91343
author_rank: Super Contributor
timestamp: April 18, 2019, 01:54:40 am
content: 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?

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 18, 2019, 02:12:43 am
content: Quote from: bob91343 on April 18, 2019, 01:54:40 amCan 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....

author: bob91343
author_rank: Super Contributor
timestamp: April 18, 2019, 05:01:11 am
content: 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.

author: bob91343
author_rank: Super Contributor
timestamp: April 18, 2019, 05:04:48 am
content: 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.

author: mzacharias
author_rank: Frequent Contributor
timestamp: April 19, 2019, 11:37:54 am
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: April 20, 2019, 09:04:04 am
content: 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?

author: Andy Watson
author_rank: Super Contributor
timestamp: April 20, 2019, 12:07:05 pm
content: Have you checked that the high voltage (-1.5kv) is correct at the power supply and t on the terminals of the tube?

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 04, 2019, 07:35:00 am
content: 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

author: pbarton
author_rank: Frequent Contributor
timestamp: May 04, 2019, 09:06:04 am
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 05, 2019, 04:35:16 am
content: please see the attached image for updated test points.POT pin 1 = 5V steadyPOT 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 +8.97vRail 5v = +5.2vRail +10v = +9.96vRail -10V = -9.96vNode 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.

author: pbarton
author_rank: Frequent Contributor
timestamp: May 05, 2019, 12:01:48 pm
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 06, 2019, 06:38:16 am
content: Quote from: pbarton on May 05, 2019, 12:01:48 pmThanks 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?

author: pbarton
author_rank: Frequent Contributor
timestamp: May 06, 2019, 08:56:27 am
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 06, 2019, 08:41:39 pm
content: 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?

author: pbarton
author_rank: Frequent Contributor
timestamp: May 06, 2019, 09:19:04 pm
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 07, 2019, 05:54:52 am
content: 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.

author: pbarton
author_rank: Frequent Contributor
timestamp: May 07, 2019, 09:05:02 am
content: 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?

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 07, 2019, 09:40:53 am
content: 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?

author: pbarton
author_rank: Frequent Contributor
timestamp: May 07, 2019, 10:35:57 am
content: 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.

author: pbarton
author_rank: Frequent Contributor
timestamp: May 07, 2019, 08:45:34 pm
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 07, 2019, 09:49:07 pm
content: 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.

author: pbarton
author_rank: Frequent Contributor
timestamp: May 07, 2019, 10:38:26 pm
content: 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.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 08, 2019, 06:31:51 am
content: There you go!The only one that made some difference was d404.Possibly because it is the bridge between Q412 emiter and Q411 collector?And as expected when D406 is out VR504 has no effect.c423 makes not difference at all...But I don't really know what to make out of it.Trio collector q411 no c423 spin intensityTrio colector q411 no d404 spin intensityTrio collector q411 no d406 spint intensityTrio colector q411 no c423 spin vr405Trio colector q411 no d404 spin vr405Trio colector q411 no d406 spin vr405

author: pbarton
author_rank: Frequent Contributor
timestamp: May 08, 2019, 07:24:50 am
content: Halleluiah, you have located the large amplitude signal on the collector of Q411 by disconnecting D404 and everything to the right of it (including Q414, Q413 and Q412).So, something to the right of D404 is causing the modulating waveform (or the waveform envelope) on the collector of Q411 to collapse.No problem with D406 and everything to the left. No problem with the output via C423. No problem at all with Q411 itself.However, we know that many functions of the circuit to the right of D404 appear to be working correctly, as ‘floor’ is responding to you adjusting the INTENSITY control.Now we need to identify the failing component/circuit to the right of D404 which is causing the large amplitude waveform on the collector of Q411 to collapse.Can you restore normal connections and try disconnect one end of C422? Then re-check the Q411 collector waveform?Edit: while you have C422 disconnected, can you check it with a meter?

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 08, 2019, 08:28:30 am
content: Just did this one.Trio q411 colector no c422 spin intensity

author: pbarton
author_rank: Frequent Contributor
timestamp: May 08, 2019, 08:37:51 am
content: The large amplitude waveform on the collector of Q411 has collapsed again, even with C422 removed. So that's not the problem. I need to think some more.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 08, 2019, 08:52:43 am
content: ok, so basically we don't want the large amplitude wave to colapse.Just the modulation should change by interacting with the intensity?What is the function of C422 in there?If you look at the image bellow. The wave we are getting is the blue one and the missing bit is the red bit? The modulation should not fall, right?

author: pbarton
author_rank: Frequent Contributor
timestamp: May 08, 2019, 10:01:04 am
content: "ok, so basically we don't want the large amplitude wave to collapse. " Yes, that's correct."Just the modulation should change by interacting with the intensity?" Varying/rotating the INTENSITY control alters the level of the envelope 'floor' and hence the spacing/distance between envelope 'ceiling' and 'floor'. As the ceiling is static (unless you change VR405)."What is the function of C422 in there? " Conveys the INTENSITY 'floor' waveform to the CRT control grid. It's OK."If you look at the image bellow. The wave we are getting is the blue one and the missing bit is the red bit? The modulation should not fall, right?" Well, when the Q411 collector large amplitude signal has collapsed its tiny, actually about 5V in magnitude. When you disconnected D404 the Q411 collector large amplitude signal was about 100V peak to peak (according to your video Trio collector q411 no d404 spin intensity).

author: pbarton
author_rank: Frequent Contributor
timestamp: May 08, 2019, 02:00:12 pm
content: On the schematic diagram, close to Q412, are three test points marked A, B and C. The corresponding waveforms are shown in the top right corner of the schematics. Can you scope these three points with everything reconnected? The note above the waveforms mentions that the INTENSITY control should be on max.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 08, 2019, 07:59:06 pm
content: There u goFirst wave A,then B at 0:06then C at 0:11They all seem V spot on. The duty is a bit different but there is no value for that on the schematics. Should it be of concern?

author: pbarton
author_rank: Frequent Contributor
timestamp: May 08, 2019, 08:48:56 pm
content: I have to agree, the three amplitudes (A, B and C) appear to be correct.The difference in the duty cycle could be explained by the sweep speed that you happened to use during the tests.The drawings within the schematics used a slower sweep speed than you (the long part of the duty cycle is the scan and the short part is the blanking bit during retrace).You happened to use a faster timebase sweep speed, where the scan is fast and the blanking bit during retrace takes as much time as the scan itself. Hence the duty cycle appears more symmetrical.It was the amplitudes that was important. I had hoped that it would have uncovered a fundamental problem. I will have to think of something else that you can test.EDIT: Are you sure that D404 is OK? I would ask you to replace it, but I fear that it may be an exotic part. Silicon Epitaxial Planar Diode for High Voltage Switching. Peak reverse voltage 300V.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 09, 2019, 05:17:43 am
content: I could try replacing it. There is an image with what I got in stock. The closest thing I got I think is a BAV21. Peak reverse 250 but I don't think we no were near 250v anyway... also current seem to match.https://www.vishay.com/docs/85543/bav17.pdf

author: pbarton
author_rank: Frequent Contributor
timestamp: May 09, 2019, 07:48:34 am
content: Thinking about it again and I’m even more convinced that D404 is breaking down in operation and is effectively conducting both ways. It obviously measures as a diode using a test meter (using low voltage signals). However, we have seen the tiny modulation signal when in operation (which was about 5V, my guess from your videos). So, I believe that its working as a diode up to about 5V, then it breaks down and conducts both ways. The reason that the Q411 collector modulation collapses is due to the diode D404 holding the Q411 collector within 5V of the envelope floor (which is set by the INTENSITY control). Yes please replace D404 with what you have in stock.

author: gkmaia
author_rank: Frequent Contributor
timestamp: May 09, 2019, 08:55:13 pm
content: Hey Phil... you may want to see this!You were right, D404 was leaking under higher voltages.I can't thank you enough for all the time you spent helping me fix my Trio.

author: pbarton
author_rank: Frequent Contributor
timestamp: May 09, 2019, 09:24:21 pm
content: Congratulations, its working. Don’t throw the old D404 out, its valuable. Many EEVbloggers would use that one component as an excuse to buy a curve tracer! It sure would make a nice tracer display. Perhaps somebody would volunteer to put it on a curve tracer and take a photo for you?If anybody has asked me a week ago to design a circuit to produce a modulated envelope waveform with adjustable floor and ceiling DC offsets, using one transistor two diodes and a few resistors, I could not have done it. After pouring over Q411 for a few days it all becomes clear. So simple, a very clever analogue circuit. I learnt a lot.

author: neil
author_rank: Contributor
timestamp: May 09, 2019, 10:25:41 pm
content: Kudos to both of you for working through this one and sharing the educational process with all of us lurkers so effectively! And congratulations on the successful outcome.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 03, 2019, 09:30:34 am
content: Quote from: pbarton on May 09, 2019, 09:24:21 pmHey pbarton,Thought you would like to see the faulty signal diode on a curve tracer.Here it is reverse biased. It is leaking like a zener.You were right on your diagnose.

author: pbarton
author_rank: Frequent Contributor
timestamp: July 05, 2019, 09:38:14 pm
content: +1 for the curve tracer.What make and model is the curve tracer?Is it the kit one, on flebay from Thailand for US $18.52 ?

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 05, 2019, 10:21:23 pm
content: Quote from: pbarton on July 05, 2019, 09:38:14 pmIt is a BK precision 501A. I did not want to buy those chinese as they are not good and have very low voltage rating for testing. This BK goes up to 100volts.I really wanted the Leader one but was twice the price for kind a similar result.https://www.torontosurplus.com/test-equipment/curve-tracers/bk-precision-501a-semiconductor-curve-tracer.html