Electronics > Repair

GW Instek GOS-6200 200MHz Oscilloscope - repair log

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sean0118:
The scope repair marathon continues.

I thought I'd crack open this GW GOS-6200 because I'm waiting on parts to fix my Tek TDS2024, Tek 7603 and GW GDS-2204.

I haven't powered it up but I was told the flyback had failed, but I don't think anyone has attempted to fault find etc. So I assume it just means the CRT doesn't power on and the flyback may or may not be okay.






Turns out it has a similar issue to the GDS-2204, the epoxy used on the power supply board has gone conductive and corrosive. I ended up pulling all of the caps to clean it up. I tested them with some smart tweezers, they seem pretty much okay, so I don't think they have leaked, all of the corrosion is from the epoxy. They are Jamicon SK series which aren't great, but not too bad. Strangely C1023 (1uF, 160V) is a TK series part even though there is a 1uF, 160V SK right next to it. The TK part seems to be double the ESR, 30R from memory but I might be wrong.

Some of the traces under the rectifiers looked a bit crispy, although the rectifiers test okay, I need to investigate this further.

The glue had got in under one of the rectifiers and caused corrosion to the pads, so I pulled it to clean it up properly.


Power Supply Board:













Unfortunately they also used epoxy to glue on the connections to the vertical and horizontal deflection plates. Looks like the corrosion isn't too bad, might have got to it just in time. The lug that grounds the CRT was also a bit loose, I'll add a lock washer to this.

Vert and horizontal deflection:









The main board looked okay, it also uses Jamicon caps, although I probably won;t replace these. They've used the glue here again to attach a coax cable, so I scraped off as much as I could and added DeOxit.

Main Board:







So turns out the CRT is a D14383GH-123, looks like it was also used in Fluke scopes.

The HV board looks okay, although would be difficult to tell if there's a fault inside the flyback module. The switching transistor is a UTC 2SD880 NPN, tested okay just using the multimeters diode mode.

There were only three Jamicon caps in the HV module so I replaced them with Panasonic and Rubycon parts, although the Jamicon parts did test okay.

CRT & HV Board:









The next step is to test some more components on the power supply board. The crispy rectifier traces might indicate an issue, hard to say. I've already pulled all of the electrolytics from the power supply board, so may as well replace them with better quality parts too.  ;)

sean0118:
Ok, time for an update.  ;)

I recapped the whole power supply and the vertical amp board (except for one non-polarised cap).

I powered it up and all of the rails on the power supply look good, very low ripple too (+110V, +55V, +12V, -12V, +5V). Except I'm not getting a proper trace, it appears to be scanning off the bottom of the screen (I can see the glow of it sweeping if the intensity is turned right up). When switched to X-Y mode and set to high intensity I get an out of focus vertical line in the centre of the screen.

Because it's sweeping I assumed the horizontal amplifier was okay, so I probed the outputs of the vertical amp (Y+ measured at R1159 and Y- at R1156). Y- (blue) isn't being driven at all and is sitting at 55V (the supply), but Y+ (yellow) is being driven.

Is it Y+ which has an issue? If it's set to X-Y mode with the inputs set to GND coupling should both Y+ and Y- both be at 55V and no signal? There shouldn't be an offset between them right? Or should Y- be driven the same as Y+?

I have the schematic (attached to previous post) so should be easy to troubleshoot, I just want to check I'm on the right track first.

Vertical amp outputs:


edit: I also tried all of the focus and astig pots, but with very little change

sean0118:
Okay, another update, turns out the transistors (2SC1252) which drive Y- are dead.

sean0118:
I put in some 2N4238's temporarily and it came to life!  :-+


The 2N4238's are too slow, but I'll have a look and try to find a proper replacement soon.




tautech:

--- Quote from: sean0118 on November 13, 2021, 07:46:28 am ---Okay, another update, turns out the transistors (2SC1252) which drive Y- are dead.

--- End quote ---
Found BF259 to be a good GP plate output amp. Check their specs against what you need.

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