Oscilloscope (CRT type) repair

[Grumps]

Hi All A friend has acquired an old Goldstar OS 7020 dual channel CRT based 'scope, and I offered to check it out to see if it is all working. So, mostly it is fine except that the timebase wanders randomly, sometimes speeding up, sometimes slowing down. And it does this to the same degree in ALL timebase settings except the X-Y mode. I have the circuits, they come with the manual, and have scoped various points in the sweep generator circuit. When in X-Y mode the sweep gen is still active and is stable. There are at two main timing caps for the timebase depending on speed selected, but I have ruled these two out as the source of the problem as they're not used in all settings. There is also a little cap which must be part of the timing cct, but even if that was duff, why should it have a huge effect on the varying speed when in a slower setting as its capacitance would be dwarfed by the main timing cap.

So, I'm a little stuck. It is an easy unit to probe as it uses all conventional through-hole components and the sweep gen board is on the top when you remove the cover.

If anyone can give further clues that'd be great. Or if there are more relevant forums available to post to. Ta very much.

[Martin Brown]

There is often a potentiometer somewhere as well with the timebase only being strictly calibrated when it is at one end of its range. My first guess would be a dry joint causing trouble.

How much is the timebase shifting around by?

[Brian Gaff (Sofa]

I'm assuming there is no pot controlling the timebase speed that may be somewhat knackered in its ability to connect to bits of itself, ie the noisy volume control syndrome?

I also assume this is a semiconductor design, not valve. In that regard is there a way it can be synchronised, would not be much of a scope without such a feature, so maybe that part of the circuit is up the spout.

What about internal connectors between boards etc? Those can create some interesting faults, from memory, many are just crimped or idc ribbon types. Brian

[The Natural Philosopher]

Is there a pot involved? Most scopes have a fine tune pot for s3eep speed...if that is scratchy...

[gareth evans]

Perhaps look for electrolytic capacitors that are bulging or leaking?

(Not to be confused with the manufacturer Bulgin. I remember 50 years ago that we in the G3UOE radio club wound up the electronics dept store manager by enquiring as to whether he had 4 rubber feet and a Bulgin knob! :-) )

73 de Gareth G4SDW

[Dave Liquorice]

That would tend to produce step changes in the time base rather than, as how I read the OP, smooth variation in the rate. Also implied by looking at the timing capacitors, I've not known capacitors to step randomly step change. Of course if the variations are step changes then dirty/worn pot or switch or other connection is the likely source.

[The Natural Philosopher]

Another possibility is poor voltage regulation somewhere

[nothanks]

I don't know that model but the I had a Tektronix scope "back in the day" that developed dirty timebase switch contacts - might be worth checking if your scope has the usual stacked wafer switches. Alternatively (and I don't have the circuit diagram) might the output stages be wandering, or there be a problem with the power supply. If it's an old'ish scope, and without full test gear, some useful steps would be to clean all switches and connectors, look for pcb cracks, re-flow and dodgy-looking joints and replace any electrolytics.

[nothanks]

(Apologies for the typos; I hope it still makes sense)

[Spike]

I take it you get a stable Lissajous trace in X-Y mode?

[Fredxx]

In which case I would leave it running for an hour or so to see if the problem cleared up. It's more likely that same electrolytics will need reforming.

[Fredxx]

I would be searching for the manual. Apart from leaving the 'scope turned on for a while to see if settles down finding the fix is like shooting in the dark.

This might help:

You will also need DjVuLibre or similar app to view the file.

As others have said it could be a noisy pot, but then resistors can become noisy with age. Look at the wiper on VR601 on page 56 of the manual, and check how stable the voltage is.

Good luck.

[Harry Bloomfield, Esq.]

Check the timebase voltage regulation.

[Dave Plowman (News]

I'd first check for dirty/worn out switches.

[Custos Custodum]

The idea of syncing a free-running scope T/B went out decades ago. Modern scopes have "trigger level" control which determines the point on the i/p waveform at which the sweep begins. This also enables single-shot operation, although you would have needed a storage scope or a camera to make much use of this feature before digital scopes came along. (BTDTGTTS)

[Grumps]

That particular pot doesn't have the range that I remember when I last used a scope like this. It does vary the TB a bit though.

About +/- 25%.

[Grumps]

There is a pot like that. It has little effect on the TB (not as much as I recall this type of adjustment as having). I'll check it out.

[Paul]

OK, so how do you make a sweep ? What principle is involved ?

One way to do it, is take a capacitor and run a constant current source into it. The voltage potential across the cap, gives you a constantly-rising waveform. Then, when it hits a threshold voltage, you short out the cap (with a bit of resistance so the current flow level isn't ruinous) and that falling voltage interval (with beam gated off) is the retrace. In my sample picture below, the falling edge should be steep, but not exactly vertical. It takes some time for retrace to happen.

The result is a sawtooth waveform, with a slowly rising front half, and a quickly falling back half.

/| / | _____/ |_____ (Apply voltage to horizontal deflection)

__ | | _____| |_____ Beam gate (turn off beam during retrace)

A varying current flow, instead of a constant current flow, on the leading edge, makes the sweep rate non-linear. Speeding up or slowing down in proportion to the deviation from ideality.

If you have a schematic, look for the sweep circuit and see if you can fit some notion of that to the components present.

Unless there is conductive dirt near the capacitor, I'd be most interested in the health of the tube providing a constant current.

In the digital era, you could take a binary counter, and run it into a DAC, to make a ramp. But of course nobody does that, because the clock rate needed into the binary counter would be pretty high.

They also made waveform generator chips at one time, which is just the embodiment of various waveform generation tricks, into a single chip. The one I got came from TI, but I believe it was soon canceled.

Paul

[Grumps]

It's not a step change. It's smooth. It can be steady, then fall in frequency, and the next moment it can rise in frequency - all within the space of several seconds.

[Grumps]

I did quickly check it, and it looked OK at 12V.