Hi,

Recently, I got an analog oscilloscope OS3000 from my neighbor, free of charge. He says the device wasn't in use for about 30 years, was well kept in dry place and the last time it was used (30 years ago) - it worked fine.


When the scope is plugged in - the 'power on' led is ON but the screen doesn't show any sign of life.
On the bottom of the case, the scope has some fluid spread - the neighbor assumes it's a capacitor/s leakage, says it will be probably fixed after caps replacements.
When I opened the bottom case, the dielectric "blow" is clearly seen at least in 2 spots (I guess it's dielectric)
It's actually a transparent fluid...in the picture its somehow appears as some kind of dirt


During the investigation of the spot 1, a weird 'coupled' capacitors appear with a weird mark 'C228 AOT'...probably it's leaking

When I looked in the manual for this C228 - it has a blank row...hmmm


A several questions to you guys:
1. How to proceed with this C228 AOT ? why 2 caps are soldered together ?
2. The spread dielectric - how should I clean it ? alcohol ?
3. In some thread, here on the forum, an advice was made to replace all the caps...omg...there're a lot of them - should this be considered ?
4. Do you have an advice for me how to handle this '30-years-not-used' scope ? (I'm not that expert)

Thanks for help and a good advice.

 

First things first.

1) Get a copy of the service manual.

2) No beam can be the result of a few things:
- bad LV supply
- bad deflection plate voltages
- bad position settings
- bad triggering
- no HV
- bad intensity or focus
- bad blanking circuit

Check LV supplies first

3) Set Trigger to Free Run

4) Check voltages on deflection plates

 

AOT. Took me a while. Adjust On Test or Fiddle with it until it works.

What markings are on the cap?

 

You don't need to replace all of the capacitors, just the electrolytic ones. The electrolyte dries out with time and the value of the capacitors becomes drastically reduced.
The two capacitors wired in parallel must have been selected to give the best results in circuit. Don't mess with them.
I recommend that you replace all the electrolytics before you power it up again. Turning it on with the the regulated power supplies not working correctly could cause other components to fail.

 

perhaps the manual: www.kevinchant.com/uploads/7/1/0/8/7108231/gould.__os3000._service_and_operating..pdf

 

The manual will be a great help. When you have replaced the electrolytic capacitors, first measure the regulated power supplies. Nothing else will will make sense until they are working correctly.

 

Got the manual, like michael8 provided, but with text recognized ... easy to navigate and search for a specific text
(will post the link later, maybe some other OS3000 owner will need it)

According to the text search it has 9 electrolytic caps, so I guess it won't be that hard to replace them all. Will start from this.
I have some caps from AliExpress. Can I use them or it would be better avoid those ?

 

... The electrolytic capacitors have the + and - polarity marked on them, so be sure to orient the new capacitors in the same way as the originals.

 

As a rule I do not replace capacitors unless I know they are bad.
If you must, start with C84 and C85. Replace with 4700μF 40V-63V.

 

Got the manual, like michael8 provided, but with text recognized ... easy to navigate and search for a specific text
(will post the link later, maybe some other OS3000 owner will need it)

According to the text search it has 9 electrolytic caps, so I guess it won't be that hard to replace them all. Will start from this.
I have some caps from AliExpress. Can I use them or it would be better avoid those ?

I have never had a bad experience with AliExpress components yet so go ahead and use them. They will be at least as good, if not better than the original ones were 40 years ago. The technology for making them has improved quite a bit..

 

(will post the link later, maybe some other OS3000 owner will need it)

OS3000 Manual with text recognition

In the bottom there're download options (PDF with Text)

 

Good news - the beam appears on the screen.

There's no 'beam finder', but in the manual there's a procedure that describes some default knobs/switches positions to get the beam seen. I gave it a try, before diving into more serious troubleshooting - the beam was located !

2) No beam can be the result of a few things:
- bad LV supply
- bad deflection plate voltages
- bad position settings
- bad triggering
- no HV
- bad intensity or focus
- bad blanking circuit

Check LV supplies first

3) Set Trigger to Free Run

Fortunately, in my case it was Triggering knob (wasn't set to 'Free Run') and a Sweep switch (was set to 'SingleSweep' instead of 'Normal').
I'll receive the probes in a couple of days, then will see if some other shape, besides single straight line, can be shown.

My next step, as I see it, correct me if I'm wrong, to follow 'Fault Localization' procedure according to the manual, to make sure everything is fine (voltages measurements....I'll skip the HV [2.5KV and 9KV] ).
As usual, several questions:

1. 1. Weird ? There's no 'Turn On switch'...An electric cable must be plugged in/out every time. All scopes are like that ?

1. 2. Supply voltage Selector - What are the numbers on it ? Should I change it (the current setting was already set) ?

DMM shows 228VAC in my home socket (If I change the selector, I might have to do some recalibration to the scope? - I just have a regular DMM...no HV, no squarewave/sinewave/time mark generators)​

The manual just says - "Ensure it's set to suit the voltage of the supply to be used"​

​

​

1. 3. Tolerance - In the manual, for some electrolytic caps the Tolerance is specified (with +/- signs) for some it's not - just a blank.

For other caps the Absolute percentage is specified without +/- sign. What does each method of tolerance specification mean ?​

​

​

1. 4. Some caps , that I suspect of popping, are specified in the manual as "3300pF Erie Disc Style 801 250V" - they look like ceramic. Can they be replaced with ceramic (I think I realized the answer during writing this - "NO...Erie's voltage is higher than regular ceramics' " - Am I right ?

​

From manual: ​

In the www.mouser.com, for example, I had trouble locating those types of caps​

In the search I chose 'Safety Ceramic disc capacitors" - is my following filter correct - [My filter at mouser.com] ?​

250V --> Is it VAC or VDC ?​

Very excited to get my first oscilloscope...my wife, for some reason, doesn't share the same excitement on this matter

 

1. Weird ? There's no 'Turn On switch'...An electric cable must be plugged in/out every time. All scopes are like that ?

The top knob above the focus knob says supply off!
As you now have a trace, why are you persuing capacitor replacements?

 

I suspect the numbers on the voltage selector are the bottom and top voltage limits for that voltage setting.

 

1. 2. Supply voltage Selector - What are the numbers on it ? Should I change it (the current setting was already set) ?

DMM shows 228VAC in my home socket (If I change the selector, I might have to do some recalibration to the scope? - I just have a regular DMM...no HV, no squarewave/sinewave/time mark generators)The manual just

The numbers denote the mains voltage range for that selection.
No, you won't have to recalibrate the scope if you change the voltage selector setting. Just check it's within limits.
For a quick check to test the channels calibration, make sure to select DC input (CH1), turn the channel knob fully clockwise into the 'calibrated' position, set timebase to free run (time won't matter for this test) and put a known DC voltage to CH1 BNC input, eg: 1.5v battery. If the Chanel sensitivity knob is set to 0.5v, then the trace will deflect 3 graticule positions up or down depending on the polarity of your input.

 

The top knob above the focus knob says supply off!
As you now have a trace, why are you persuing capacitor replacements?

The power switch is on the "BRILL" control.

 

Very excited to get my first oscilloscope...my wife, for some reason, doesn't share the same excitement on this matter

Buy her a box of chocolates. That always works for me.

Power switch is on the top control labeled Supply Off.

There is Cal 1V output in the middle of the front control panel. Just connect a piece of hookup wire from Y1 to Cal 1V.

 

The top knob above the focus knob says supply off!
As you now have a trace, why are you persuing capacitor replacements?

You were all right about the power off. The knob was a bit stiff there, so I didn't want to put too much force without knowing for sure the PowerOff position is actual there.
I have at least 2 dielectric puddles. Some caps are out, it (bad caps) might lead to some problems or malfunction...I guess. So it should be addressed.

 

There is Cal 1V output in the middle of the front control panel. Just connect a piece of hookup wire from Y1 to Cal 1V.

The hookup wire advice worked...who needs probes ...haha
I played a bit (a lot) with Channel Y1 and some settings.
Tried to do the same with Channel Y2 (changed the Mode to Y2) - its' beam isn't there (tried also Y shifting) - seems nonfunctioning Channel - maybe because of popped caps ?

I also noticed that after getting back from "Y2 tweaks" to Channel Y1 - its' beam is vertically misplaced/shifted (not on zero y axis, as I set and left it before) - it occurred several times.

Both malfunctions (and the popped caps) encourage me to start the 'Fault Localization' according to the manual.

 

Keep the jumper wire between Y1 and Cal 1V.
Set the Mode switch to Alt or Chop.
You ought to see Y2 as a flat horizontal line.

Jumper both Y1 and Y2 at the same time to Cal 1V.
You should see square waves for both Y1 and Y2.