Author Topic: GW Instek GOS-6200 200MHz Oscilloscope - repair log  (Read 1053 times)

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Offline sean0118

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GW Instek GOS-6200 200MHz Oscilloscope - repair log
« on: October 04, 2021, 03:07:11 am »
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.  ;)
« Last Edit: October 04, 2021, 03:14:34 am by sean0118 »
 
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Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #1 on: November 13, 2021, 06:53:40 am »
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
« Last Edit: November 13, 2021, 06:55:48 am by sean0118 »
 

Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #2 on: November 13, 2021, 07:46:28 am »
Okay, another update, turns out the transistors (2SC1252) which drive Y- are dead.
« Last Edit: November 13, 2021, 07:48:41 am by sean0118 »
 

Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #3 on: November 13, 2021, 08:31:13 am »
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.




 

Offline tautech

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #4 on: November 13, 2021, 09:16:52 am »
Okay, another update, turns out the transistors (2SC1252) which drive Y- are dead.
Found BF259 to be a good GP plate output amp. Check their specs against what you need.
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Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #5 on: November 28, 2021, 08:54:37 am »
Okay I found some 2N3866's in a junk box at work which seem to be good replacements for the vertical amp.


The next issue I'm looking at is the horizontal scaling at divisions around 1ms. The horizontal amplifier board only has a "5ns ADJ" trimming capacitor, I assume this is for some high frequency timing or compensation.

All the other horizontal trimpots are under the bottom cover on the main board. I thought this would be an easy fix, but I've reached the end stops on both the "H_GAIN" and "1ms/div_ADJ" trimpots and the scaling is still out. Is this a common issue? Have the trimpots or other resistors drifted? Or would there be some reference frequency I need to adjust somewhere?


1kHz from HP 226A Time Mark Generator:


1kHz ref from Rigol DS1074Z:


Trim pot locations:
« Last Edit: November 28, 2021, 08:58:17 am by sean0118 »
 

Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #6 on: November 29, 2021, 12:33:25 am »
Okay, I've been probing around the adjustment trimpots and I found the horizontal ramp waveform. If the GW is set to 1ms, shouldn't the rising part of the ramp be for 10ms because the GW has 10 horizontal divisions?

Although, I would have thought only having 9ms ramp would give me the opposite problem to the scaling issue I have. Maybe it's the amplitude I should be focusing on?

 

Offline TheBay

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #7 on: November 29, 2021, 12:16:13 pm »
Great work and looks like you caught that right in time. I've seen that glue eat the legs off components and eat away the tracks on the circuit board.

Looks like this is coming along well and looks to be a very useful scope.
 
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Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #8 on: December 04, 2021, 04:24:26 am »
Okay, I've had another look at this, but haven't made much progress.

I realised the trace width only overscans the division lines by a little bit, not enough to explain the horizontal scaling issue. I suspect this means the period of the ramp waveform is roughly correct, but I'm only guessing...

I probed the ramp signal coming in and out of the "1ms/div adj" trimpot. The trimpot (1k) and some series resistors (120R, 2.7K) reduce the amplitude of the ramp. I pulled one leg of the 2.7k which let me measure their resistance, all three seem okay.

The ramp then goes into some transistor based inverter circuit (is inverter the correct name? It's level shifted to +5V and the ramp slopes down). I guess it's possible the ramp on the output of this inverter isn't being pulled up enough, which is what I'll look at next.

I might re-cap around that area of the PCB too. But it is tempting to just replace the 1k timmer with 2k or similar, but that feels like cheating...   

Let me know if you have any ideas, I'm not really sure what these waveforms should be looking like...      ;)
 
Trace width:


Horizontal ramp going into trimpot (1ms/DIV):


Output of trimpot (1ms/DIV):


Output of inverter (1ms/DIV):
 

Offline tautech

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #9 on: December 04, 2021, 06:23:06 am »
Typically setting up the timebase on a CRO requires an adjustable mark space generator so to get pulses to line up with graticules. Today any reasonable AWG easily can do this.
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Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #10 on: December 04, 2021, 07:16:56 am »
So I have a time mark generator that I borrowed, it agrees with my newer Rigol so I assume it's correct. The adjustment trimmers in the GW seem to work, but they reach the end stops before it's calibrated correctly.


I thought about it a bit more and I realised both the positive and negative outputs going to the horizontal amp board are roughly the same amplitude. So I think if the amplitude of the ramp is wrong it must be before the inverter circuit I mentioned earlier.

One interesting thing I did notice is it looks like there's heaps of high frequency noise on the inputs to the horizontal amp board. At first I thought it was pickup from measuring incorrectly, but even when using short gnd probes it stays, so I think it is really there and not measurement error:

 

Offline tautech

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #11 on: December 04, 2021, 09:03:56 am »
So I have a time mark generator that I borrowed, it agrees with my newer Rigol so I assume it's correct.
Oh, so you do, sorry.

Yeah that looks gawd noisy however if you look at it faster does it align with any of the supply frequencies as it might be just the scope picking it up and not part of the sweep ramp at all ?

Looking back at previous screenshots and particularly this one:


Can you not adjust a whole period between the graticules as there should be like in the attached ?
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Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #12 on: December 05, 2021, 05:23:45 am »
Yeah that looks gawd noisy however if you look at it faster does it align with any of the supply frequencies as it might be just the scope picking it up and not part of the sweep ramp at all ?

Yeah I think I'll need to investigate it a little bit more, might be from the PSU as you say. The vertical amp has similar noise from memory.

Can you not adjust a whole period between the graticules as there should be like in the attached ?

Almost but not quite, that's as close as it gets before the trimmer hits the stops. Pretty tempting to increase the resistor in series with it.


I actually forgot I had the schematics for this (attached to first post). I went back and had a look and noticed something interesting, what do they mean by "0.65V/div" in this context? SWP_OUT is the horizontal ramp signal where I'm getting close to 10Vpp.


 

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #13 on: December 05, 2021, 06:26:39 am »
Can you not adjust a whole period between the graticules as there should be like in the attached ?

Almost but not quite, that's as close as it gets before the trimmer hits the stops. Pretty tempting to increase the resistor in series with it.


I actually forgot I had the schematics for this (attached to first post). I went back and had a look and noticed something interesting, what do they mean by "0.65V/div" in this context? SWP_OUT is the horizontal ramp signal where I'm getting close to 10Vpp.



There should be a SWP_OUT waveform somewhere in the documentation and 0.65V/div refers to the monitoring scopes vertical settings to replicate the same waveform.
Unusual yes that 0.65V/div is used however until you line up the example waveform with what you actually get it might not be clear why this is done.

How much additional resistance do you envisage needing in series with the trimmer ?
Could it just be things has drifted some and their totals marginally exceed spec ?
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Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #14 on: December 05, 2021, 06:38:56 am »
There should be a SWP_OUT waveform somewhere in the documentation and 0.65V/div refers to the monitoring scopes vertical settings to replicate the same waveform.
Unusual yes that 0.65V/div is used however until you line up the example waveform with what you actually get it might not be clear why this is done.

Thanks, that makes sense, I don't have the service manual unfortunately so I'm not sure what to set the monitoring scope too then.

How much additional resistance do you envisage needing in series with the trimmer ?
Could it just be things has drifted some and their totals marginally exceed spec ?

I guess it probably isn't that much as the trimmer is only 1k, so maybe a few more 100Ohms? I might give it a go and report back.

I've also been recapping that area of the board, but surprisingly difficult. The pins of the 47uF are bent flat against the board, I've damaged three pads already even after trying to straighten the leads out, they just pull the pads with them. I have a repair kit though.  ;)
 

Offline tautech

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #15 on: December 05, 2021, 06:54:02 am »
How much additional resistance do you envisage needing in series with the trimmer ?
Could it just be things has drifted some and their totals marginally exceed spec ?

I guess it probably isn't that much as the trimmer is only 1k, so maybe a few more 100Ohms? I might give it a go and report back.
It could be the trimmers need some exercise too and some also give them a drink of IPA or Deoxit.

Quote
I've also been recapping that area of the board, but surprisingly difficult. The pins of the 47uF are bent flat against the board, I've damaged three pads already even after trying to straighten the leads out, they just pull the pads with them. I have a repair kit though.  ;)
Yeah that's a pain when legs are hard bent over generally for wave soldering and probably with that Pb free muck in which case you'd be well advised to flood the joint with some leaded solder first to lower the melt point to help save ripping pads up.
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Offline sean0118

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #16 on: December 05, 2021, 07:59:44 am »
Yeah that's a pain when legs are hard bent over generally for wave soldering and probably with that Pb free muck in which case you'd be well advised to flood the joint with some leaded solder first to lower the melt point to help save ripping pads up.

The 47uF seem particularly bad for some reason, not sure why, all the others came out okay.

I replaced the 2.7k in series with the 1ms trimmer with a 3.3k. I can now trim it at 1ms, but the problem is the trace stops at 9 divisions and doesn't reach the edge of the screen. Then if I switch the time make generator and scope to 1us it's out again.

I'll need to think about this a bit more, I thought it would work but didn't really have the desired effect.   ::)






 

Offline tautech

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Re: GW Instek GOS-6200 200MHz Oscilloscope - repair log
« Reply #17 on: December 05, 2021, 11:34:36 am »
H Gain would seem like it needs adjusting to at least have a blank sweep extend past the graticules.
It's between Q905 and 906 in the Hori Preamp schematic on P26.
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