Dead Goodwill GW GOS-522 Oscilloscope

[davenz]

Hi team. I posted previously asking for e circuit diagram for this scope. I have since acquired one from Goodwill themselves so if anyone else needs it, I have it available in pdf format.

I have succeeded in getting the beast going (which is no mean feat for this old tinkerer) only to find what appears to be a dimming CRT. The thing is, when operating with a slow trace, (the 'blip' taking about a second to traverse the 5-inch screen) the CRT is as bright as I would expect it to be. However, if I start running the trace faster, it soon dims and almost disappears. Is this a symptom of a dying CRT or perhaps something like the blanking circuit not doing its job properly?

I know this is an old slapper of a scope but I have invested a lot of time and energy into it only to have to admit defeat and chuck it in at this stage. If the tube is gone, so be it, at least I have tried all I can to make it work. It irks me to see it nice and bright with a slow trace but increasingly invisible and dim with faster traces.

Thanks and the best of 2010 to al of you.

Dave Thompson.

 

[Joan]

Gos 522

I have a GOS 522 but one channel is not working. Would you share the schematic?
Thanks a lot

 

[davenz]

Circuit diagram

Sure thing, send your email address to me at dave at davethompson dot co dot nz and I'll email it directly to you.

Cheers

Dave

 

[Blind Bruce]

Dimming scope trace

You are correct in your selecting the CRT as a possible cause of the dimming trace. There are several other factors to consider as well. I have worked with $10,000 Tecktronic scopes and they did the same thing even they were fresh out of the box so you may not have a problem after all. Check all the relevant voltages in the bias circuits from slow to fast trace speeds and see if any changes are noted. Failing that carefully examine the trigger circuits. If you find nothing, get back to me. I have a friend, now retired who spent most of his working career repairing and calibrating scopes.
Bruce in the Peg

 

[KeepItSimpleStupid]

Is this at full intensity?

 

[davenz]

Dim Trace

Hi all and thanks for the feedback and suggestions. Yes, the trace is at full intensity (as per the intensity knob). There is a secondary focus and intensity control (or primary, depending on which way you look at it) on the board in the form of trimpots - the focus alters things but the intensity seems to do nothing to the trace. It has been a long time since I had it apart now, even though it is still sitting cluttering up my workbench, so my memory is now dimming as to the exact symptoms and what I did and didn't try. I remember going through a bunch of stuff with Goodwill techs (they went above and beyond actually) and in the end they gave up and said it was likely a dead tube, but it is so bright in very slow traces that I just can't accept that diagnosis, and the fact it starts bright at any given setting and fades over about 30 seconds to a minute to nothing indicates to me something wrong in the sweep/drive circuitry.

As it stands, I have run out of talent and even though the scope is what I would call analogue - that is, it isn't all surface mount this and digital that, but solid old recognisable op-amps and transistors the likes of which I have in my parts bins. I have already located and replaced a few dead trannies and diodes which got it to light up in the first instance, so I was pretty pleased with myself in that regard, though now I have pulled and tested every component in that area or part of the system, including all resistors and caps in the vain hope I found another dead component and it was magically fixed. Alas, this didn't happen and the danger now is the board's traces are very fragile with age (and my ham-fisted soldering/desoldering) and as a consequence I have had to repair many lifting and broken tracks and pads.

I have learnt an incredible amount from doing this so nothing has been a waste of time. To be honest, I would have chucked it long ago except for the fact my uncle went to a lot of trouble to ship it over to me from Australia and I thought it at least worth having a go at fixing as I believe they are a reasonably good scope at the time. I don't have the motivation to buy some new digital whizz-bang thing that would be wasted on my analogue/audio-level hobby anyway so it would have been ideal but there we go. One day I'll dig back into it and have another shot at the title - who knows, I might just get lucky.

Cheers and have a great Xmas and New Year for those who celebrate such things.

Thanks again and I'll report back anything I discover.

 

[KeepItSimpleStupid]

Posting/sending some schematics and we could help more. Bad CRT's are usually an all or nothing or interactions where control elements are shorted not the kind of wierd stuff your getting. Power supply issues would always come to mind first. I hope Santa brings you a good ESR meter for Christmas.

 

[davenz]

ESR meter

I have been fortunate enough to have 'won' some Peak electronics tools, including two Atlas LCR analysers, an Atlas ESR analyser and an Atlas DCA semiconductor analyser, all of which I used extensively in testing the suspected components from the scope. As mentioned I found some dead caps, diodes and transistors this way and after replacing them the scope went; except for the fainting trace everything else works perfectly.

I even took a punt on a local auction and purchased a brand new, old stock 5 inch scope CRT with the idea of if not using it permanently, at least using it to test the theory that the tube is OK or not. Unfortunately the plug on the scope is the wrong size (too small) to fit the new tube and I'd need to use either an adapter (which I don't think exists anyway) or to temporarily jury-rig/wire it in to test it. As I said, it has been a while since I had it all out to work on so one day when I have time I will rig it up to use the new tube - spec. wise the tube is almost identical so there shouldn't be any danger of ruining the new tube or burning out the circuitry on the scope. The exercise will soon tell me if it is a tube problem or whether the fault lies in the driver circuitry.

Thanks

Dave.

 

[FreeThinker]

I replied to your original post here https://www.electro-tech-online.com...-schematic-circuit-diagram.100025/#post919573 with a link to a manual.My scope had the same problem as yours but the cal signal was also a little iffy.I checked out the patch board and tweeked the pot to get a nice equal space /mark and tested again,at this point I notice the trace seemed to be brighter and on further investigation this proved to be the case, I may of moved something or (I think) the intensity pot loops up to this board as a via may be worth a look around this area.

 

[sparkman001]

Sounds like it could be a problem with low eht to the tube. Also check all the high value resistors in the focus/brilliance and eht generator circuits. If its an old 'scope the chances are that these resistors will have gone high in value and cause the problem you describe. Hope this helps