Electronics > Repair

Analog oscilloscope repair

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Jim95:
First off I'd like to say this is my first post. Long time reader and subscriber to eevblog. Just finished my first year at Uni of Sheffield doing EE. So I decided to buy a scope for various summer projects. After loosing many bidding wars a finally bagged myself a Hitachi v209 (Dave had mentioned that Hitachi are a reliable company to go with).

It finally arrived and after a bit of fiddling around, it seems the trigger doesn't work. I know this because; a) I cannot in anyway get the wave to stop, b) switching the trigger source has no effect on the display.

So it seems my first project is to repair the trigger.

Luckily the scope came with the service manual including schematics and the trigger circuit has its own board.
So this is the stage I'm at:
-I've worked out that the problem is on the trigger board as unplugging the input from the channel preamps has no effect to what's being displayed and if I unplug the output of the trigger board. absolutely nothing is displayed on the screen.
-I have measured the voltage rails to the board as you can see there are +5v +8v -8v. I measure ±0.01 +8 and -8 on those rails but on the 5v rail I am measuring 4.2v max.
-If i unplug the power cable from this board I measure exactly 5v, 8v and -8v on each point(basically voltages from the power supply). So this rules out the power supply being the problem.

After a visual inspection all the components seem fine and I have cleaned the pots and switches.
So my guess is that a transistor has burnt out and shorting causing this voltage drop?

Any other help would be much appreciated!

Jim95:
P.S Attached is the trigger schematic

RazSlack:
Have you checked it with an external trigger signal as well? Although I doubt that it makes a difference, but might worth asking.

Check the capacitance of the electrolytic caps. I doubt they cause the whole circuit to fail, but still a good idea to check them.

According to what you said, I'm thinking that the differential amp block in the middle might have some fault to it. There's a lot of components that can cause such a voltage drop, most probably a semiconductor's junctions are shorted. You should check the input FET (TR401) for good measure, and the differential pair transistors (TR404 and TR405) by desoldering and diode-checking their Base-Emitter and Base-Collector pins (Gate-Source for the FET respectively. If it's a JFET, don't be suprised if Drain-Source is shorted, that's how it should be).

If all transistors turn out to be good (i.e: have an actual diffusion voltage that you can measure, ~0.5-0.7 volts for silicon transistors), your next bet could be the logic circuits. Take one chip out, turn the scope on (high voltage, be careful! Don't even try to fiddle around the CRT while it's live), measure the voltage again. If it's still bad, solder in a socket, put the chip back, and try another one (it's good practice to socket your ICs, but not necessary).

Best advice I can give. Maybe some CRO gurus can give you better tips on this. Gotta tell, you're a lucky bastard to have such a simple schematic design in your scope there, I rather like it in fact.

KJDS:
If you want a summer project fixing Hitachi scopes, I've got a dozen in the repair pile.

Jim95:

--- Quote from: RazSlack on June 20, 2014, 06:23:33 pm ---Have you checked it with an external trigger signal as well? Although I doubt that it makes a difference, but might worth asking.

Check the capacitance of the electrolytic caps. I doubt they cause the whole circuit to fail, but still a good idea to check them.

According to what you said, I'm thinking that the differential amp block in the middle might have some fault to it. There's a lot of components that can cause such a voltage drop, most probably a semiconductor's junctions are shorted. You should check the input FET (TR401) for good measure, and the differential pair transistors (TR404 and TR405) by desoldering and diode-checking their Base-Emitter and Base-Collector pins (Gate-Source for the FET respectively. If it's a JFET, don't be suprised if Drain-Source is shorted, that's how it should be).

If all transistors turn out to be good (i.e: have an actual diffusion voltage that you can measure, ~0.5-0.7 volts for silicon transistors), your next bet could be the logic circuits. Take one chip out, turn the scope on (high voltage, be careful! Don't even try to fiddle around the CRT while it's live), measure the voltage again. If it's still bad, solder in a socket, put the chip back, and try another one (it's good practice to socket your ICs, but not necessary).

Best advice I can give. Maybe some CRO gurus can give you better tips on this. Gotta tell, you're a lucky bastard to have such a simple schematic design in your scope there, I rather like it in fact.

--- End quote ---

Thanks I checked all the suspicious transistors and sadly they were all fine.
Then I check the ICs taking each one out.
There are two NAND gate ICs that I swapped with each other and it turns out that IC402 is the bad one.
So I've got a new one on the way, I'll keep you posted.
Luckily I've taken your advice and socketed them so it should be a quick swap over!
Cheers

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