Trouble with Iwatsu ss5711d oscilloscope

[andrejsstrumfs]

Hi everyone!

Couple of weeks ago i got a cheapish Iwatsu-5711d oscilloscope, which all in all works ok with one slight problem i would like to fix. The problem is - it won't trigger from channel 2, while triggering from ch1 and ch3 is ok. So i got a manual http://exodus.poly.edu/~kurt/manuals/manuals/Other/IWATSU%20SS-5711%20Instruction.pdf , hooked up ch2 to calibration source and started poking around with my other oscilloscope.
Trouble looks to be in the ch2 pre-amp circuit (page 111 in manual) - at first it looked like dead Q35 transistor - when i unclipped the P90 connector i got the same signal but with ~2x smaller amplitude when compared to ch1 trigger output. But with the connector on there is no signal at all. So i removed the transistor, checked for shorts and replaced the transistor with a new one. And got the same problem. Power sources are all ok. Resistors arund Q35 are all ok. Maybe someone can come up with idea?

Regards

Andrejs

[edavid]

But with the connector on there is no signal at all.

That's the first thing to fix.  Shorted cable or switch?

[andrejsstrumfs]

That's the first thing to fix.  Shorted cable or switch?

Yup, that was the very first thing i thought. But nope, the resistance for both ch1 and ch2 is ~47 ohms - looks ok according to schematics.

[edavid]

That's the first thing to fix.  Shorted cable or switch?

Yup, that was the very first thing i thought. But nope, the resistance for both ch1 and ch2 is ~47 ohms - looks ok according to schematics.

What happens if you disconnect the cable from P90 and connect a 47 ohm resistor there?

[andrejsstrumfs]

What happens if you disconnect the cable from P90 and connect a 47 ohm resistor there?

Same thing - no signal. But i noticed that with the resistor or connector in place i get -0.3V DC there. But even with no resistor or connector in place the amplitude is too small - about 50mV. On ch1 same signal was ~0.3V

[edavid]

What happens if you disconnect the cable from P90 and connect a 47 ohm resistor there?

Same thing - no signal. But i noticed that with the resistor or connector in place i get -0.3V DC there. But even with no resistor or connector in place the amplitude is too small - about 50mV. On ch1 same signal was ~0.3V

I guess you should connect the same signal to ch1 and ch2 and compare stage by stage.

[andrejsstrumfs]

I guess you should connect the same signal to ch1 and ch2 and compare stage by stage.

Thanks for help, turned out the problem was a tiny hair crack on pcb. Soldered and fixed

Andrejs