Hitachi v212 Scope Repair - Trace Rotation Off (Despite adjusting the pot)

[padagra]

Hello all,

First of all, thank you to everyone taking their time to help me. I've always dabbled in electronics but only recently starting diving deeper - which was triggered by the community here and a newly sparked interested from Dave's videos.

Anyways, I picked up this Hitachi v212 oscilloscope (first scope - analog as recommended by Dave) from an auction a few hours away. I got it with a function generator for $31.00 - I figured it was worth it to drive and get it at that price.

To cut to the chase, the trace rotation is off - it stays too far rotated left (trace heading to upper right edge of tube). At first I found the trace rotation pot had come un-soldered on one of the pins (probably from someone jamming too hard on the pot). I resoldered the joint, checked for shorts, etc. Now I can adjust the trace rotation but not to the place it should be. Its like I'm able to adjust it within a range but the range is off in the first place?

I started to take it apart, looking for any other obvious damage - so far the back of this board (as seen in picture below) seems awfully crusty.. problem is the whole board has that. Old age?  I've done some searching online but have yet to find a more obvious issue.. I did gently attempt to rotate the tube as one post stated some older scopes had this option but I couldn't seem to rotate mine. At least not with the force I was comfortable exerting on it.. I'm hoping it isn't a faulty amplifier as I read in one post..

I have checked for magnets around it and that doesn't seem to be the issue. I stopped digging deeper into it so I could hook it up enough to get pictures of this issue for this post. Please disregard how rough it looks at the moment.. :/ The picture with the solid line as the trace show the rotation pot turned all the way the right - this is as close as adjusting the potentiometer will get it.


Again, I appreciate all of your help. I also have the service manual but aside from the obvious (adjust trace rotation) I haven't found much more related to the trace rotation in the manual. Not to mention, I'm new to reading circuits and this scope goes a bit over my head.


I really hope I'm just missing something obvious.

-Adam

Note: I can submit links to higher quality pictures if needed

[smjcuk]

The pot may be knackered if it had that sort or force applied to it. I'd take a look at the service manual and see if you can work out the max and min voltages (these are usually voltage divider arrangements) from the wiper and poke a meter at it at both extents of travel (or desolder it and measure the resistance). Also worth checking the power supply voltages against the service manual. All sorts of odd things happen if the PSU voltages are off.

[tautech]

Also worth checking the power supply voltages against the service manual. All sorts of odd things happen if the PSU voltages are off.

+1
Thats always the first port of call with repairs.

Looking at the Cal waveform, the rising and falling edges ARE perpendicular to the what should be the horizontal trace. IMO this indicates a problem with the Trace rotation circuitry only. Check/test all componentry associated.

[Grapsus]

You'll find attached the part of the circuit responsible for trace rotation. It's quite simple, there is a coil around the CRT neck which is driven by a constant voltage. The voltage is chosen between -8 and 8 V by RV1348 and buffered by TR1348 and TR1349 forming a push-pull arrangement. From your photo, the image looks strongly rotated, so the coil voltage must be stuck at one the the extreme voltages, -8 or 8 V.

Here is an improvised troubleshooting procedure :

1. check -8 and 8 V power rails in this circuit
2. check that RV1348 works by measuring the voltage at the wiper terminal for different positions
3. check the output voltage of the push-pull buffer
4. check TR1348 and TR1349
5. if the fault is still not apparent, check the coil itself for continuity or bad contact with the rest of the circuit.

[smjcuk]

That's a beautifully simple circuit. Good show for Hitachi - should be relatively easy to work out the story there 

[padagra]

You guys are awesome.  So I just started testing the list that Grapsus posted. I unplugged the cable to the rotation coil and the trace is really close to being spot on in terms of rotation. In fact its actually higher on the left side of the trace now by just a little which seems to indicate something with the rotation circuitry as stated.. Will update in here in a sec, but here's a picture of the scope with the rotation coil unplugged.

I'd like to add that its pretty awesome to break something like this down because I'm beginning to understand how this part of it works.

[padagra]

Here are the current values I'm measuring. The rest of the testing I'm having to do some searching to learn the right way.

Power rails (as measured on the outer pins of the pot):
1st rail: -8.06v
2nd rail: negative voltage - changes with the pots wiper arm. Not identical amounts but close to the chart below.

Wiper pin:
Pot at "0" / left: -8.06v
Pot at "50%": -6.41v
"100%": -5.41v

Rotation Coil Resistance
219.1?

I tried something else that might be of use. I added a 1k? resistor in series with the rotation coil - this causes the trace to become straight. "Perfectly" straight as far as I can tell. Unfortunately it causes the trim pot to stop working.. I'm going to see if I have a 10? resistor as the rotation coil seems a bit low. Maybe that will even it out? This may not be helpful but just in case, there's a picture of it below.

I need to continue on this later so I'll have to update then.

[tautech]

The rails (+8, -8) should measure as such. At mid-range the voltage supplied to coil will be 0V.

If this does not happen 1 transistor is faulty.

[padagra]

Got it (with all your help of course)! 

It was a transistor - I believe the PNP one. I replaced both with new ones I had here in a parts container.

Can't say it enough - thank you.

The trim pot is pretty messed up so I'll probably replace it but for now its working.