Electronics > Repair

GW Instek GDS-2204 oscilloscope - repair log [fixed]

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sean0118:
Time to fix another scope.  ;)

This one is a GW Instek GDS-2204, made in 2007 judging by the IC date codes.

It seems to have some intermittent noise issues, things like spurious spikes, that don't appear on other scopes. I tried to reproduce it today but couldn't, might be temperature dependent?


I opened it up and straight away saw a lot of corrosion on the power supply PCB. At first I thought the capacitors were leaking, but turns out the epoxy used to secure the components is the type that turns corrosive, and it's on everything!  :'(

I'm not sure if the capacitors are actually compromised, I put a probe on its output and couldn't see much ripple. There was high frequency noise ~0.1V but that's probably due to how I was measuring it.


I need to investigate further, I suspect the power supply might not be causing the other issues. But here's some photos anyway:
















Grandchuck:
Wow; that is a lot of corrosion.  Was the scope in a high humidity environment?  Good luck with it.

sean0118:

--- Quote from: Grandchuck on September 18, 2021, 12:26:55 pm ---Wow; that is a lot of corrosion.  Was the scope in a high humidity environment?  Good luck with it.

--- End quote ---

It's even worse than I thought initially. I'm really surprised it put out the correct voltages still.  :D

I haven't had it for long, but it was stored in a normal lab environment before I had it (but not humidity controlled). I think the epoxy must just be really corrosive. The weird thing is there's a few places where the epoxy is still a light colour and hasn't caused corrosion yet, maybe it was a different batch?


I'm starting to suspect the power supply might be the cause of the odd behavior. Some pads on the bottom of the PCB are even corroded, one pin was completely cracked and loose.

I was hoping to not do a complete re-cap, but even some that looked okay initially are lumpy with corrosion under the plastic sleeve. I couldn;t even get a capacitance reading of one of the bad ones, so I suspect they are damaged internally too.









 :-BROKE

Grandchuck:
I am wondering if there was some kind of fumes/corrosive atmosphere in the storage environment?  You could be right about the epoxy.  I wonder if other items from this manufacturer have exhibited this?  Anyone know?

Good chance you will wind up with a decent scope if you follow through.

sean0118:

--- Quote from: Grandchuck on September 18, 2021, 02:27:06 pm ---I am wondering if there was some kind of fumes/corrosive atmosphere in the storage environment?  You could be right about the epoxy.  I wonder if other items from this manufacturer have exhibited this? 

--- End quote ---

The brown glue is known to go corrosive, it's more common on 1980s audio amps etc though. But I think you are right, seems high humidity might be required as well, and I guess here in Sydney it is high at times.


Also, the PSU is a LYTEC LP5422, turns out there's already repair threads about it (and a partial schematic):

https://www.eevblog.com/forum/repair/help-with-a-smps-(lytec-lp5422/
https://www.eevblog.com/forum/repair/gw-instek-gds-2104-power-supply-failure/
https://www.eevblog.com/forum/beginners/lytec-lp5422/

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