oscilloscope problem

[dlt205]

When I put a sine wave to the scope it shows the wave perfectly for about 5 minutes then the signal on the scope will slowly sink to the bottom of the screen and disappear. I have a direct connection via bnc cable from the generator to the scope to test this. I check the signal generator and it's working properly. If I power off and let scope sit for a while, then power up the signal is back in the middle of the screen.
What could this be and how can I fix this?

I have a Goldstar dual trace 100mhz scope thats around 12yrs old. It has been used only a few times.

Thanks

 

[daddles]

Sounds like some kind of thermal drift. One of the first things to do is to use AC coupling on the scope. If it still drifts when using AC coupling, then there's probably something wrong with the vertical position part of the scope. It will likely be hard to troubleshoot without a schematic.

If the drift problem goes away on AC coupling but remains when you use DC coupling, then it's your signal generator that's somehow getting a DC offset.

 

[poor mystic]

Hi
Please tell us exactly what model scope it is, so we can see what we can see.

In the meantime... the first thing to check with all electronic equipment failures is the power supply. So measure that when it's good and measure again when the fault is up.

Another thing would be to set up 0V in the middle of the screen and measure a 0V referenced square wave - see how that changes. You want all the information you can get at first.

 

[dlt205]

Thanks for the responses.
It's a GoldStar / LP Precision Co model OS-9100P oscilloscope . It is a 100mhz, 2 channel, 2 traces,6"squared CRT and divided TV trigger signal . I have the manual but no schematics. I can not locate any schematics for this unit via internet search.

The problem is the same on AC coupling. Will look at P.S. next.

Thanks

 

[daddles]

Does it happen on both channels or one? That's a critically important clue. If it only happens on one channel, then you can compare the good channel to the bad.

 

[dlt205]

It happens on channel 2 also. It seems to take longer but eventually does the same thing.

 

[poor mystic]

It sounds like the kind of thing that a faulty electrolytic capacitor could do. How about have a very careful look at all the capacitors on the board and replace anything that looks at all damaged - especially swollen, even slightly.

Also~
It is my experience that the most common cause of electrical/electronics faults is connectors. I'd be looking very carefully at them all.

 

[daddles]

Since it happens on both channels, it's in circuitry that's common to both of the channels. I agree with Mark (poor mystic) that you should be looking at connectors and especially electrolytic capacitors in the power supply.

If you can get your hands on a function generator or filament transformer and you have a meter that can read AC voltages, you can make an approximate measurement of the ESR (equivalent series resistance) of the electrolytic capacitors in the device. Here's one way to do it. This can find bad capacitors even if they look OK.

However, you know from your problem description that it's something that happens slowly and is probably from something heating up. This might not be a bad capacitor. Another troubleshooting technique is to get a heat gun (e.g., hair dryer) and selectively heat up components you can reach to see if you can duplicate the problem. If you can, you've narrowed down the suspect parts. A complementary technique is to get some freeze-spray and cool down parts after the problem has occurred. This can also isolate the guilty component.

 

[pcarew]

Manuals for 9100P

Thanks for the responses.
It's a GoldStar / LP Precision Co model OS-9100P oscilloscope . It is a 100mhz, 2 channel, 2 traces,6"squared CRT and divided TV trigger signal . I have the manual but no schematics. I can not locate any schematics for this unit via internet search.

Hello,
I saw your post from last summer and while I can't answer your original question, I can offer info for the similar models.

I have just bought a OS-9100P off of ebay (Jan 2012) and started the hunt for a manual as well.

I have looked high and low, sent emails to LG and EZDigital but with no response.

I saw that you mentioned that you have a manual for the scope but no schematics.

Although I haven't found a manual or service guide for the 9100P, I have found both for 'close by' models.

I would be happy to send them to you.

I was also wondering if it would be possible to get a copy of the actual 9100p manual that you have.

You are the only person that I have come across that has mentioned that they have the actual manual and I would definitely appreciate and be grateful for a copy.

Thanks.

My contact info is:
Paul Carew
paul (at) CarewContact dot com
512-576-3030

 

[shrtrnd]

Yep, as already pointed-out, I'd be seriously looking at the power supply.
I have on occasion seen this happen with the vertical control circuit.
Bad cap usually, or a leaking semiconductor.
Can of freeze-spray on this job might help you find the fault faster.
Heat related, so freeze spray on each component until you see the trace rise back
to the center of the scope, will identify the offending component.
First though, I'd look at the large electrolytic caps that others told you to check.
good luck.

 

[(*steve*)]

It's worth pointing out that if the problem exists on both channels, but takes longer to happen on one as compared to the other (especially if both channels are simultaneously displayed) then the common factor you're looking for is a single type of component in a corresponding location in the (presumably) vertical amplifier of each channel.

If it happens simultaneously, and to the same degree on both channels, then the fault is more likely to be in a single component that is in part of the scope which is shared between both channels (think power supply).

As an example, I have had two pieces of equipment purchased at the same time, with serial numbers quite close to each other, fail with the same symptoms only weeks apart after about 5 years of operation. The fault turned out to be the same components in their power supplies. Clearly, in this case there was no common circuitry, but the components were nonetheless common.

 

[Sid723]

When the trace drops, does it happen right away, or does it slowly move down and eventually out of sight?

In either case, I believe it is heat related. If it moves quickly, then I would think it is a bad connection on the circuit board that opens or closes as it heats up. If it moves slowly down, then I believe it is a component that is changing it's properties when it heats up.

Freeze spray and a heat gun should help you to isolate the faulty component. You may also try tapping on the board with something insulated when it happens. This may flex the board and cause a loose connection to correct itself if only temporarily. If tapping on it changes the level, then you can always just shut the thing down and touch up solder connections.

Good luck.