Triggering problem with Hitachi V212 Oscilloscope

[jhoweaa]

I have a Hitachi V-212 Oscilloscope that I purchased used a few years ago. For the most part the unit works fine, although I am currently having problems with the triggering system. If I switch the unit to 'Normal', I am generally unable to get a trace to appear at all. However, there are times when the trace *will* appear and then after a short amount of time (sometimes seconds, sometimes longer) the trace will again disappear. I've tried flipping switches and wiggling things to see if there is some sort of short with either the switches or the pots, but that doesn't seem to have any effect.

I'm fairly new to electronics but I'd be interesting in at least trying to debug what might be wrong with the triggering system. I do have the schematic for the unit, but I'm not sure how to proceed to figure out what might be going wrong. If anyone has some simple tips, or particular things to check out, I'd certainly appreciate hearing them.

Thanks!

Jim

 

[betwixt]

I'm not familiar with that model but on many scopes 'normal' means it needs a signal to trigger on and 'auto' means it free runs.

Are you giving it a signal to trigger from? Without a signal you may get occasional sweeps of the trace, especially if you turn the triger level control but it would be unlikely to show a line all the time.

Brian.

 

[KlausST]

Hi,

It sounds pretty normal to me.

Trigger "normal" means that the line on the scope only appears when trigger event took place.
So if the trigger event not happens, then you see no line.

The trigger event usually is set to "rising" or "falling" edge and the "trigger level".

Klaus

 

[D.A.(Tony)Stewart]

Auto trigger uses a sweep tracking clock (PLL) to trigger on edges so a repetitive waveform looks stable but sweeps after a brief delay if the edge does not occur when expected.

Normal is used in rdisplay ONLY when signal edge exceeds the set threshold for + trigger or goes under the threshold for -ve trigger. So you would use Auto for dc SOURCES unless you were looking for ripple. \

Often Scopes have a Trigger view so you can see what is being used to trigger. With null input (0V) in NORMAL + trigger mode you should be able to generate one sweep by reducing the trigger threshold from + to below 0V. There will always be some % of hysteresis , by design, to prevent false re-triggering. and your signals ought to be with short ground leads and low noise for a stable repeat trigger with an AC waveform.

With AC couple you set the threshold near zero for viewing start of waveform and higher for only detecting peaks.

 

[jhoweaa]

Thanks, I understand all of that. My situation is that I'm feeding in an AC signal (a wave form from a synthesizer) and on Auto I see the pattern but it isn't stable. I want to use Normal mode and the trigger control to get a stable waveform. Sometimes (but rarely) I can get it. Once I get it, it will generally disappear after a short period of time and I'm unable to get it back no matter how carefully I adjust the settings. It makes me think that there is some issue with the trigger detection circuitry, but I'm not sure how I can verify that.

Jim

 

[betwixt]

Those are not the same symptoms you originally described. It looks like you might have a problem in the comparator circuitry that finds the voltage in the waveform to trigger on. The trouble with that kind of problem is you usually need another oscillosciope to find it!

Check something else first: If the scope has 'line' trigger, feed it with a low voltage (< 6V) from a power transformer and select 'line' mode. I'm trying to establish whether the problem may be due to ripple on the power rail, in line mode it will still show a stable waveform but in normal and auto modes, the ripple will prevent a constant trigger point being detected. Ripple on the supply rails is common on older equipment as the PSU capacitors dry out.

Brian.

 

[jhoweaa]

Brian,

Sorry I wasn't clearer on my original post. My scope has three modes for triggering, Internal (using either channel 1 or 2), Line, which the documentation says to be used for triggering on signals running at line frequency, and External. The only one that I can feed a signal to is External so I tried feeding a 5 v signal from my power supply. The signal was DC since I don't have a variable power supply which can do AC and DC. When I fed the oscilloscope a 200 khz signal, it seemed to behave exactly the same as when I used the Internal trigger. Selected 'Auto' gave me an unstable sine wave, it basically was continually rolling across the screen. Switching to Normal make the screen go blank.

One thing did happen while playing around, however. I disconnected the external signal and went back to just using the the internal trigger. Some combination of events, switching from Auto to Normal, playing with the slope adjustment, etc. and suddenly when I switched to Normal I had a stable wave form. I could change the frequency and/or wave shape coming in and the wave remained stable on Normal. I was even able to play with the slope and get it to disappear and then reappear. I then pulled out the trigger adjustment too switch from positive to negative slope and the wave disappeared. From that point on, nothing I did allowed me to see the wave in Normal mode even though I had the slope setting in the same spot where it was working previously.

Are there things I can check in the trigger circuitry just using a multimeter instead of a second scope?

I appreciate the help.

Thanks.

Jim

- - - Updated - - -

The more I investigate this issue, the more I'm starting to think it is related to the push/pull slope adjustment switch which governs the trigger voltage. I fed a sine wave into channel one and made sure it showed up on auto. I then switched to Normal. Not surprisingly nothing showed up. I slowly adjusted the slope adjustment, turning the knob left and right. Still nothing. I pulled the switch out (which switches to negative slope) and did the same. Nothing. I then put the slope mostly in the middle of the range and just kept pulling on the switch and pushing it in, maybe about once a second or so. At some point when I did it I saw a trace flash on the screen and disappear. I kept doing this and when I saw the flash of the trace again, I started to adjust the slope slowly and I managed to get a trace to hold for quite a while. I could adjust the slope and I could change the starting point of the trace until it disappeared, and then I was able to bring it back again. If I pulled (or pushed, depending on m starting state) the knob, the trace would disappear again and I would have to do my push/pull trick until I saw it appear again. I was able to find the trace both on the positive slope as well as the negative slope, but I always had to do the push/pull trick to get something to appear.

Jim

 

[betwixt]

It is possible the switch is faulty but they are usually reliable unles the 'feel' like something inside them has broken. I would still guess a trigger fault has developed elsewhere.

Note that feeding DC from a PSU will not allow the scope to trigger in normal mode anyway, it needs a change in level to make it trigger and a good PSU will have a stable and hence unchanging output.

I'm away from my service manuals at the moment but if I remember correctly, that scope has a calibration output pin. Can you connect it to one of the channel inputs and adjust the amplitude range to give say 1 division and see how that triggers. Being a square wave, you should find it triggers well in all modes and in both polarities.

Brian.