I have picked up one of these, a 20MHz dual trace instrument made by the Good Will Instrument Co Ltd, Malaysia.

On a quick check all seems sweetness and light, except that traces that should be horizontal, aren't quite horizontal at the bottom of the screen. I guess this would need internal adjustment to fix, if that is possible for the user. I have found a users' manual online but no mention is made of remedies for such an error.

There may also be matters of calibration that need sorting out.

So I am after a workshop/maintenance manual for the unit if anyone can oblige.

This eBay seller says he has a number for sale. Might be worth contacting him about a service manual. If he has one he may make a copy for you for a small fee:

http://www.ebay.com/itm/GW-GOS-622G-20-MHz-Analog-Oscilloscope-Dual-Channel-20-MHz-/111326056928?_trksid=p2054897.l4275.

Thanks GTC, I did spot these but thought I would try the forum first.

Differentiating: Is it only at the bottom, or is it on a lean for the whole screen. You should be able to tell from the graticule.

Marc

After setting the traces horizontal at the centre of the screen, both are not quite horizontal when moved towards the bottom of the screen. OK at the top.

Since there are no external adjustments for such errors seems to me the fix would be inside the box.

Have yet to put a signal on the y axis to see if there is other distortion. Will set it up to make a circular trace. 30 years since I have driven one and getting back into the swing of it.

Could have dodgy filters, looks a bit like a bit of astigmatism for which there is adjustment. It may pay to degauss it.

I have recollections that these were marketed by someone who also made good ones here. I did get to have a look at one (earlier model) that had a flash over on a pot (there were two pots) with about 2KV on it and someone got zapped.

For reasons unknown the (metal body, metal shaft) pot was insulated from, but on the front panel? There was sufficient clearance between knob & pot nut for the 2KV to jump, as it had found a path (probably eroded carbon from the track) to the shaft.

I think in the bookshelves, or boxes (not unpacked since moving) either one of the books of the three CRO's here has the info, or its in one of the books.

Its vague but try this:

http://www.eserviceinfo.com/index.php?what=search2&searchstring=GOS+622%2F623.

If you can't get it, I did minus the add on Hate sites that try that on; better thing to do than remove their crap, or fix up the problems it caused.

I say the paper work is vague as I looked at it. Most modern stuff like it is.

Marc

Thanks Marcc - but thats where I found the operation (user) manual.

Re pots - no nips yet.

"It may pay to degauss it."

Marcc, if this the solution, would one of the old tape head demagnetisers be adequate? Or would I need to cannibalise an old TV or monitor for a degaussing coil?

When I first saw it, it was on an electrical fitters test bench; also welders in the shop - so plenty of opportunity for magnetisation.

Have had a careful look at the case and don't see any signs of abuse - so hopefully it hasn't been dropped. Also no tinkles when I turn it over so the tube seems intact.

Will open it up tonight and see what it is like inside. Came from an industrial environment so expecting lots of dust.

Opened the case (appears to be all aluminium) - not nearly as much dust as I expected. Webs indicate spiders have been in residence - could only have got in through 2.5mm ventilation holes. Cleared these out.

Only other indication of trouble was crazing of the board lacquer and apparent scorching around the connections of two flat pack transistors in the high voltage area. These flat packs had no heat sink. No smell though. Maybe ozone damage?

Year in date stamp on tube practically indecipherable but possibly 1988 or 1998.

The electrolytics didn't show any signs of distress though of course might be worthwhile replacing them.

Going back to my previous posts, my quick check was to hook up two alligator clip leads to produce very dirty sine waves through the air from the house AC, indicating both channels and time base working.

Since then have got proper probes and hooked up to AC through a soldering iron transformer. The traces are not symmetrical, with the upward slope less steep than the downward, confirming I guess distortion in the tube or output circuitry somewhere. In general the pattern seem to lean to the right evenly down the screen - trapezoidal distortion?

Have tried to set up a circular trace in order to better see distortion, but don't seem to be able to do this from the controls. Guess I need to produce 90deg out of phase signals to do this.

I actually have a tape head degaussing device which will work just as well as a wand.

One of the reasons that you get retrace on one side is the pot cannot centralise the trace. 1998 is a prime time for some very dodgy Taiwan capacitors that were made from a formula pinched(hacked) from I believe Murata (?) who had left out, probably deliberately an essential chemical.

Not a new idea. The Britts deliberately designed a magnificent battle ship, plans to be stolen, during the war. It was built without the thieves checking them and in accordance with design, it was built, was "top heavy" and rolled as soon as it was turned and sank.

So the nitty gritty is that it may have lost some voltage on the deflection plates and they are uneven. I am finding car radios losing volume from electrolytic's drying out, you may have ripple on the B+ circuits exacerbating the problem. I note the ancient Heathkit has quite a few filter electrolytics to kill ripple.

With new probes there is a procedure for and an adjustment to set the probes to the CRO with a square wave. That CRO may have a square wave on its reference voltage?

I would prefer to see the sine wave off a transformer say 12V. Do note the limit on input. Many modern solid state bits of apparatus will not handle tube radio voltages. That includes the mains. I have 2KV caps on the input of the Heathkit. It had to be rebuilt as all of the resistors & caps failed.

Re degaussing: I don't really know what I need to degauss. The case is all aluminium so it can only be sundry steel fittings, nuts & bolts etc.The mains tranny should degauss itself. There is a metal sheath around the gun; I didn't want to use a magnet to find out if that is steel!
I read in another forum that degaussing can be done with a coil of wire about 5ohm and pass a decent AC current through it; then take the item to be degaussed up to it from a distance, through the coil and away to a distance, all slowly.

"One of the reasons that you get retrace on one side is the pot cannot centralise the trace." Not sure what you mean by this.

Re Taiwanese caps: will have another look and see what the brands are.

"So the nitty gritty is that it may have lost some voltage on the deflection plates and they are uneven." The way I see it for everything to lean to the right, there needs to be a steadily changing error in the voltage applied to the X direction plates going down the screen. Or a magnetic field to do the same thing. Or both.

Re calibration voltage: a 2V pp square wave is available at the front panel for compensating leads. Three probes came with the cro, one dud, the other two adjust nicely as far as I can tell.

Re input voltage limits: These are marked on the front panel, so no excuses there. 400V peak X & Y, 100V peak trigger. probes are HP-9060, 10Mohm, 600V, 150V DC + peak AC.
I have used a 3.3V Scope soldering iron transformer and a 24V Christmas light transformer. Have managed to set up a wobbly ellipse using the cro line voltage as trigger (must be out of phase with mains AC). A very nice ripple is produced on this ellipse when the off-peak (20Kc?) control voltage come on.
Presumably I need a nice clean signal from a signal generator to properly see what is happening.

The glass can be magnetised. The "Position" pot centres the trace; on magnetic, or voltage controlled deflection plates, the two need to be even.

With no sweep you will get a spot: That spot must fall in the centre and not favour any of the quadrants when the pots for X & Y are centred.

These were the forerunner of TV: Their tubes were in TV's so early TV picture tube theory can apply.

Marc

"With no sweep you will get a spot: That spot must fall in the centre and not favour any of the quadrants when the pots for X & Y are centred."

If the spot for either channel is centred, and the spot then moved vertically, it moves off the vertical axis by about 10 degrees; when moved horizontally it pretty well stays on the horizontal axis, but at the bottom of the screen it is off by a few degrees.

That is very weird behaviour for a CRO. It is possible that it is an electrical problem but it would require horizontal deflection voltages feeding into the vertical deflection and vice versa. Not a very likely scenario.

It is more likely to be caused be some residual magnetic field near the base of the CRT. Another possibility is the CRO has been dropped and the deflection plates in the tube are now misaligned. If this is the case the tube is permanently damaged.

To to determine if the problem is electrical or mechanical/magnetic you need to measure the voltages on the four deflection plates with the spot centred and again with the spot shifted vertically down to where you see the fault. When shifting the spot vertically there will be no change in the voltage on the horizontal (X) deflection plates. If it does vary then that confirms some weird electrical fault.