Author Topic: [SOLVED ]Trio Oscilloscope blanking & intensity problem  (Read 2846 times)

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Offline gkmaia

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #25 on: May 08, 2019, 06:31:51 am »
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 intensity


Trio colector q411 no d404 spin intensity


Trio collector q411 no d406 spint intensity



Trio colector q411 no c423 spin vr405


Trio colector q411 no d404 spin vr405


Trio colector q411 no d406 spin vr405



« Last Edit: May 08, 2019, 06:37:07 am by gkmaia »
 

Offline pbarton

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #26 on: May 08, 2019, 07:24:50 am »
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?
« Last Edit: May 08, 2019, 07:51:47 am by pbarton »
 

Offline gkmaia

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #27 on: May 08, 2019, 08:28:30 am »
Just did this one.

Trio q411 colector no c422 spin intensity
 

Offline pbarton

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #28 on: May 08, 2019, 08:37:51 am »
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.
 

Offline gkmaia

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #29 on: May 08, 2019, 08:52:43 am »
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?
« Last Edit: May 08, 2019, 09:04:14 am by gkmaia »
 

Offline pbarton

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #30 on: May 08, 2019, 10:01:04 am »
"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).
 

Offline pbarton

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #31 on: May 08, 2019, 02:00:12 pm »
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.
 

Offline gkmaia

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #32 on: May 08, 2019, 07:59:06 pm »
There u go

First wave A,
then B at 0:06
then C at 0:11

They 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?

 

Offline pbarton

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #33 on: May 08, 2019, 08:48:56 pm »
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.
« Last Edit: May 08, 2019, 09:46:20 pm by pbarton »
 

Offline gkmaia

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #34 on: May 09, 2019, 05:17:43 am »
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

« Last Edit: May 09, 2019, 05:30:28 am by gkmaia »
 

Offline pbarton

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #35 on: May 09, 2019, 07:48:34 am »
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.
 

Offline gkmaia

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #36 on: May 09, 2019, 08:55:13 pm »
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.

 

Offline pbarton

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #37 on: May 09, 2019, 09:24:21 pm »
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.
 

Offline neil

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Re: Trio Oscilloscope blanking & intensity problem
« Reply #38 on: May 09, 2019, 10:25:41 pm »
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.
 

Offline gkmaia

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Re: [SOLVED ]Trio Oscilloscope blanking & intensity problem
« Reply #39 on: July 03, 2019, 09:30:34 am »
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.

Hey 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.  :-+

 

Offline pbarton

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Re: [SOLVED ]Trio Oscilloscope blanking & intensity problem
« Reply #40 on: July 05, 2019, 09:38:14 pm »
+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 ?
 

Offline gkmaia

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Re: [SOLVED ]Trio Oscilloscope blanking & intensity problem
« Reply #41 on: July 05, 2019, 10:21:23 pm »
+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 ?

It 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
 


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