CRT oscilloscope display problem !!

[SM-Piyes90]

Hi every one

To be honest i was really confused where to put this topic and since the scope i got contains a CRT screen on it i thought maybe its a good idea to put it in this section , but any ways if its not mods can move it to the right section .


Ok back to the heart of the matter .

Today , as usual i was planning to use my scope to check the output signal of a PWM ic ( ka7550 ) , so i turned the scope on , connected the ground clip to cold ground , and tried to measure the output with the probe , and by mistake i shorted some pins of the IC .

Now i get a constant trace leaning downward ( see picture bellow ), and i cant show any measured signals on the screen .

Here is a list of the inside of the scope content :

- CRT screen tube
- Main card containing power supply and channel 1 and 2 voltage/ vertical position controls circuitry .
- Side card containing HV CRT power transformer + time , sweep mode , horizontal position controls circuitry .

[stj]

Re: CRT oscilloscope display problem !!

same problem on both channels?

[SM-Piyes90]

Re: CRT oscilloscope display problem !!

same problem on both channels?

Yes , i get the same display on both channels , but the short was caused by probe on channel 1 .

[tom66]

Re: CRT oscilloscope display problem !!

Have you tried adjusting the trace-rotate control? Does the ground lead still have continuity to the mains protective earth/ground? Have you tried other probes?

[Ltank]

Re: CRT oscilloscope display problem !!

Most Scopes have Rotate trim pot on front or back of scope.

[tom66]

Re: CRT oscilloscope display problem !!

I meant trace-rotate, not sure why I wrote trace-width!

[SM-Piyes90]

Re: CRT oscilloscope display problem !!

I meant trace-rotate, not sure why I wrote trace-width!

Hi every one .

I can rotate the trace , but i can't get any signal to show up on the screen unfortunately ,

[stj]

Re: CRT oscilloscope display problem !!

you need a schematic to investigate the Y-amp but you still didnt list the make & model!

[SM-Piyes90]

Re: CRT oscilloscope display problem !!

you need a schematic to investigate the Y-amp but you still didnt list the make & model!

Yes i forgot that , sorry , its HUNG CHANG 5502 20 MHz made in korea .

And here is a partial schem , i hope we can get something out of it :

[budm]

Re: CRT oscilloscope display problem !!

"Today , as usual i was planning to use my scope to check the output signal of a PWM ic ( ka7550 ) , so i turned the scope on , connected the ground clip to cold ground , and tried to measure the output with the probe , and by mistake i shorted some pins of the IC " By any chance you are trying to make measurement in the Primary hot side of the circuit? Also if you put your probe on the scope 1KHZ output test port on the front of the scope you see nothing?

[SM-Piyes90]

Re: CRT oscilloscope display problem !!

"Today , as usual i was planning to use my scope to check the output signal of a PWM ic ( ka7550 ) , so i turned the scope on , connected the ground clip to cold ground , and tried to measure the output with the probe , and by mistake i shorted some pins of the IC " By any chance you are trying to make measurement in the Primary hot side of the circuit? Also if you put your probe on the scope 1KHZ output test port on the front of the scope you see nothing?

Nothing at all , by cold ground i meant IC's GND , the power supply i was testing is also out of service .

I still can't believe how a spark can cause this damage , i guess there should be some protection like a fuse or something , the IC i was testing has a max voltage of 12 Vdc

[budm]

Re: CRT oscilloscope display problem !!

Is the IC in the primary side of the circuit?
Sounds to me that you are working in the primary side of the circuit so what ever you are working on should be connected to the isolation transformer so the the Line and Neutral are floated.
So right now the scope will not show any wave from when connected to the scope built in 1KHZ generator?

[SM-Piyes90]

Re: CRT oscilloscope display problem !!

Is the IC in the primary side of the circuit?
Sounds to me that you are working in the primary side of the circuit so what ever you are working on should be connected to the isolation transformer so the the Line and Neutral are floated.
So right now the scope will not show any wave from when connected to the scope built in 1KHZ generator?

The IC is actually on the middle of the power supply ,taking power from the primary side and the output of the IC (ka7552) is connected to a MOSFET gate , i will post some pictures of the power supply later of course .

But the problem is i have already desassembled the scope , i didn't notice the 1 khz generator until now

[stj]

Re: CRT oscilloscope display problem !!

that schem is the gun-drive,
we need the other page(s)

[SM-Piyes90]

Re: CRT oscilloscope display problem !!

that schem is the gun-drive,
we need the other page(s)

Do you mean the card containing the HV transformer ? i only got this page btw .

[stj]

Re: CRT oscilloscope display problem !!

no, not the HV, there is a sheet with a connector caled p3something on it.
that handles the Y deflection.

if you look at the schem you posted at the tube, the Y coil wires are on the left and it just shows a connector to another board.

[stj]

Re: CRT oscilloscope display problem !!

o.k. i did some searching.

they trade under the name "Protek"
and i found a newer manual that may be close.

[R_J]

Re: CRT oscilloscope display problem !!

May be the same as a Protek, stj ,you beat me to the manual

[SM-Piyes90]

Re: CRT oscilloscope display problem !!

o.k. i did some searching.

they trade under the name "Protek"
and i found a newer manual that may be close.

Nice find stj , thanks a lot , i hope its similar to the 5502 .

So right now after having disassembled the scope a part , i see two main cards :

-one is related to the time control , horizontal control and contains a high voltage section, witch i guess is supposed to power the cathode ray tube .

- the other one is certainly the vertical and voltage controls , also contains a power supply section coming straight from 220 V transformer's secondary .

[budm]

Re: CRT oscilloscope display problem !!

What happen when you change the time base, and when change the the VOLT/cm knobs from min (mV) to max (V)?