Labtech 3502 (Hung Chang) Scope No intensity control

[cam245]

Hi Everyone, my first post.

Thanks to Dave for the great videos and all contributors on this board,
it is great reading, and clearly a great wealth of knowledge here.

I would like some help with a cheapie scope purchased on Ebay that had
an EHT fault, since repaired but now I have no intensity control.

Both A&B traces are there and intensity varies with the timebase control but
 the Intensity control does nothing.

So far, all the .01 2KV caps and the 2 22Meg resistors have been
replaced and a couple of burnt carbonised spots on the board have been
scratched out which got the EHT working and the scope traces back.

My issue now is the intensity control has no effect.

Focus works, Astig, Symetry works. Trace rotation works. The main Power
supply voltages are correct, however I don't have the gear to measure
the EHT.
I have 74.8V (Max clockwise) and 122.5V (Max anticlockwise) at the
junction of C424/D409 when I turn the Intensity control but the voltage
between the Cathode and Grid of the CRT only changes by .5V, and
depending on the timebase setting this voltage is anywhere between 3.6
and 21 volts.

I am measuring this across the 2 22Meg resistors R422 and R423. What is
weird to me is that the 3 neons are lit constantly at the same
brightness... always... 3.6 - 21 volts having the CRT plug connected or disconnected makes no difference.

The manual is here
https://elektrotanya.com/protek_p-3502c_user_sm.pdf/download.html

Thanks in advance, any help appreciated.

Grant

[Armadillo]

Hi Welcome;

If you varies preset VR405, does it do anything to the intensity?
 
Measure the DC voltage as shown in the attached. How many volts can you get?

[cam245]

Thank you. No. Varying the intensity range pot VR405 has no effect on the trace.

Also the Z axis input does not work... I'm assuming that is because there is no illumination control.

[Armadillo]

This is in fact puzzling ?

But it is worth to investigate further with the suggested measurement points to gather more clues to work on.

 

[cam245]

Thank you.

Here are the voltages.
I can not measure the G1 voltage at the cathode of D410 as I assume it is in the order of =~1.9Kv.

[Armadillo]

Thank you.

Here are the voltages.
I can not measure the G1 voltage at the cathode of D410 as I assume it is in the order of =~1.9Kv.

Capacitor C424 is only rated at 250V, so G1 cannot be any greater than that. 
Q408 is hard ON but only 4mA flowing through it, so I suggest you check R427, Q408 and surrounding circuit.

[cam245]

Thank you.

In my work on this scope so far I had neglected to realise I had the HORIZ MAG set to x5.
Now I have it set to NORM and this is my trace. It has a slight flicker to the right.

I have remeasured the indicated voltages. Maybe this new clue will help lead us to the problem.

[cam245]

Pic too big. I'll try again.

[Armadillo]

Did you check R427, Q408 ?
If in doubt or having low hfe, just change the transistor and the resistor. And check on traces or solder joints.

Edit: I am still on your intensity control.
It seems like you have horizontal problems as well.

Edit2: The measurement of 60.3v and the 50.1v is it a measurement Error? Different by 10V across the inductor don't seems right.   

[cam245]

Thank you.

I checked Q408, Q409, Q410, Q411, using base emitter method with diode tester on my multimeter in circuit and found no abnormality.

I have checked in circuit R428, R427, R429, R432, R430, R434 and have found no abnormality or any thing worth further investigation.

Today however, I turned on the scope and the traces were full width. So it seems I also have a intermittent horizontal issue as well.

You are correct, my previous voltage readings were wrong due to the other horizontal issue. The correct reading across the inductor is 0V. This inconsistency is due to the horizontal issue. The voltage changed with the length of the traces.  New pic attached.

What else have I done or found...

I had previously ordered some new neons, thinking maybe they had become ionised/faulty/damaged/ungassed due to the previous HT issues I bought the scope with and I replaced them. But the problem is exactly the same. They made no difference. They look just like the originals. Glowing lightly.

I also measured the voltage across D411. From my research this is a 5.2V zenner. But I measure 4.6V.

I also installed a 1K resistor in series with G1 on the circuit board at the back of the CRT and measured the voltage across it and with the traces at maximum brightness (.5Ms is where it's brightest) the G1 current is 0 (no voltage drop across the resistor) G1 current maximum I could get by altering the timebase is 5uA, that seems tiny.

I have also attached a screen shot of the signal at JP19. It does not look correct. I was expecting to see a saw tooth. But it looks more like sine wave.

Thank you

[cam245]

Can only attach one file at a time.

[Armadillo]

And now Q408 B-E is reverse biased ?
Then it would be wise to confirm that C405 voltage is solid [not varying due to intensity adjustment].
And change Q408 since measurement results are still again suspected.     

[cam245]

I don't have a replacement for Q408. Because Q412 circuit is functioning correctly I swapped Q408 for Q412 but there is no change. The problem exists.

The voltage at D401 (across C405) is constant as the intensity control is varied. But the voltage at the junction of D409 and D410 stays fixed.

While I had the transistors out I buzzed out the tracks between P403 and connected components and everything checks out. I also checkked between pins on P403, suspecting maybe there was burnt material under the on board conector. That too revealed no issues.

So a bit mysterious... I can't get my mind off why the 3 neons are glowing when there is only such a low DC voltage across them. PLUS 2 of them glow between the internal electrodes which seems to indicate some AC.

Thanks

[cam245]

Using the scope I have measured the oscillator frequency of the HV supply at the collector of Q404.

I'm surprised to find it is 15Khz. That seems really low. I was expecting 40Khz.

The cal signal measures correct. .5Vpp @ 1Khz so I'm assuming the scope is measuring correctly.

Can any one confirm this to be correct or a problem. The oscillation frequency I believe is determined by the impedance of the transformer so maybe the transformer is at fault.

[cam245]

Solved. The EHT transformer is faulty.

Thank you for your help.