Author Topic: Help me diagnose a scope problem? (Hitachi V1100A)  (Read 274 times)

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Offline zenrael

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Help me diagnose a scope problem? (Hitachi V1100A)
« on: December 04, 2017, 01:28:03 PM »
Hi!

Forgive me if this is a common problem - I have searched for images depicting the same issue on a single channel with no real success...

I have an Hitachi V1100A with a problem on the 1st channel. All other channels give a good clean trace, but channel 1 has this odd 'ghosting' issue, where the vertical image is sort of doubled. Could it be bad caps? This is my first scope and I've not yet had the case off to inspect. I was wondering if this could be some sort of calibration issue given its confines to the one channel.

Any help would be appreciated!

First pic shows channel 2 (no problems), second shows channel 1 (the issue occurs no matter the timebase or v/div), the third pic shows channels 4 through 1 (top to bottom), with channel 1 set to GND.

Cheers,

John
2E0KDK

EDIT: Ignore the '8' character on the images - I was dicking about with the labelling function and forgot to disable it.

« Last Edit: December 04, 2017, 01:30:20 PM by zenrael »
 

Offline w2aew

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Re: Help me diagnose a scope problem? (Hitachi V1100A)
« Reply #1 on: December 05, 2017, 01:24:11 AM »
Since this scope, like most analog scopes, only produces 1 trace at a time (single trace scope), then the "double trace" is a result of some small shift in vertical position or offset that is either concurring on alternate sweeps, or is bouncing back and forth very quickly.

Slow the sweep speed way down, to something like 100ms/div so that each sweep is 1 second long.  If you see a single trace that varies in vertical offset with each repetition, then you know you have some kind of sweep related offset issue.  If you still see a double trace, then *carefully* work your way up through the various sweep speeds, *including* slowly adjusting the vernier, to see if you can find the repetition rate that is bouncing the trace up and down.  You might then be able to relate that to something else going on in the scope.
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Offline Armadillo

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Re: Help me diagnose a scope problem? (Hitachi V1100A)
« Reply #2 on: December 05, 2017, 02:03:14 AM »
Can you show just channel 1;
1) AC coupled [see if voltage measurement is correct]
2) DC coupled [see if voltage measurement is correct]
3) Ground [again].

Then Change another probe;

One more time;

1) AC coupled [see if voltage measurement is correct]
2) DC coupled [see if voltage measurement is correct]
3) Ground [again].

Edit: Also check C36, C38 and C39 near the IC U10.
« Last Edit: December 05, 2017, 05:52:24 AM by Armadillo »
 

Offline zenrael

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Re: Help me diagnose a scope problem? (Hitachi V1100A)
« Reply #3 on: December 05, 2017, 02:41:22 PM »
Since this scope, like most analog scopes, only produces 1 trace at a time (single trace scope), then the "double trace" is a result of some small shift in vertical position or offset that is either concurring on alternate sweeps, or is bouncing back and forth very quickly.

Slow the sweep speed way down, to something like 100ms/div so that each sweep is 1 second long.  If you see a single trace that varies in vertical offset with each repetition, then you know you have some kind of sweep related offset issue.  If you still see a double trace, then *carefully* work your way up through the various sweep speeds, *including* slowly adjusting the vernier, to see if you can find the repetition rate that is bouncing the trace up and down.  You might then be able to relate that to something else going on in the scope.

Thanks for the reply, great videos by the way! The trace is actually doubled no matter what sweep speed. Even with a 1 second sweep there's two clear dots tracing across the screen. I guess its bouncing back and forth very quickly as you say?

Can you show just channel 1;
1) AC coupled [see if voltage measurement is correct]
2) DC coupled [see if voltage measurement is correct]
3) Ground [again].

...

Edit: Also check C36, C38 and C39 near the IC U10.


Thanks, I've attached the photos below - the two different probes were near identical so i combined the second probe pics into one image. From top to bottom they are - 1st probe with AC coupling, same with GND, then DC coupling, then the same again with probe 2. The final pic is a sweep on channel 2 with cursors for reference. As you can see the sweep seems rather accurate if you consider the lower of the 'double' vertical images.

The cal signal is 0.5v at a khz or so, which seems to be detected fine by the built-in meter. I haven't yet had the chance to take a look at the caps you mentioned, but i intend to do so tomorrow and will report back here. Again thanks for the help!


This scope is otherwise working fine, and there is no real urgency to this problem being solved - although it would be nice to have both main channels working right. I'm prepared to have a play inside, but if it involves changes that would drastically throw out the calibration then i'll probably just put up with it for now. Still though, would be nice to fix!

As i said above i'll take a look at the caps tomorrow and report back with pics.

Cheers again guys,

John
« Last Edit: December 05, 2017, 03:09:06 PM by zenrael »
 

Offline Armadillo

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Re: Help me diagnose a scope problem? (Hitachi V1100A)
« Reply #4 on: December 06, 2017, 01:28:37 AM »
If the voltage measurements are correct, then you are logically left only with the CH1 preamplifier section to focus at particularly those capacitors C36, C38 and C39 near the IC U10.  ;)
 

Offline zenrael

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Re: Help me diagnose a scope problem? (Hitachi V1100A)
« Reply #5 on: December 13, 2017, 04:19:55 AM »
Apologies for the late update on this guys, bit of a busy week...

Okay so i took the casing off and it turns out that the boards are rather tightly packed. I took whatever pics I could with the caps you mentioned, but i imagine they're not very useful.

None of the caps looked 'bad', per se - there are some coloured dots on the caps for the CH1 preamp section that may have been added by whoever last calibrated it? Maybe they indicate caps that need replacing but the job was never done? There are no such coloured dots on the caps surrounding the CH2 preamp section.

They *look* fine, but i don't think i'd be able to check further without pulling everything out of the chassis. I may try this in the new year but for now I guess i'll just put up with the working CH2 and the semi-working CH1.

Many thanks for the input guys, if i do work on it any further i'll update again.

John
 

Offline Armadillo

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Re: Help me diagnose a scope problem? (Hitachi V1100A)
« Reply #6 on: December 13, 2017, 04:44:34 AM »
Small value electrolytic cap fails without opening its top visually, so its kind of hard to tell except to take it out and measure its capacitances and ESRs.
Alternatively, if you have a second oscilloscope, you can always probe at the output of the preamp and compare it with CH2.

Always, don't feel compelled or rush to repair this kind of tedious electronics, particularly you don't have an urgent need of it. Wait until you feel the joy of opening it up, then you would enjoy the repair.

Treat it as a surprise present to be open up.  ;)

Cheers;   ;D
 


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