Electronics > Repair

Tektronix TDS2024 Oscilloscope - Repair Log

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sean0118:
Sydney is still in Covid lockdown so the scope repair marathon is continuing. I'm not sure if this one will be fixable though, might be parts only.  ;)

Someone has already had a go at repairing this, symptoms were the LCD backlight and a few buttons were lighting up. I checked the 1kHz cal with another scope an measured it as being close to 3kHz which is probably a bad sign.

It gets worse though, I had it turned on while measuring power supply voltages, everything seemed okay but after a few minutes the +35V rail suddenly jumped to +70V. I turned it off quickly, so didn't measure the other voltage rails again, but I assume they doubled in voltage as well. But I did see a few wisps of smoke rise from around the backlight transformer circuit. The main board was attached at the time, and because someone else has already had a go at repairing this I wonder if it's seen sustained over voltage for a long time.

I'll worry about the mainboard more later, for the moment I'll focus on the power supply. The first thing I noticed is they are using Lelon electrolytic capacitors, in my opinion these are trash C or D grade capacitors, very surprised to see them used by Tektronix. Also, if you are trouble shooting one of these take note that the MOSFET heatsink is floating at -160V.

It looks like the feedback to the controller (KA7552) is from the 3.3V rail. There's a little Lelon 2.2uF capacitor that's on the 3.3V side of the optocoupler, it's hard to see from the photo but it looks like the bottom bung might be bulging out slightly, I wonder if this might be overheating and going close to short, making the controller bump the voltages up.


I've had a bit of a look around, can't see if this is a common issue, but with the Lelon caps I assume it might be. I also need to work out how to power up the supply without the main board attached, there's an LT pin, might need pulling up or down?

 :-BROKE










Capacitor in feedback path:



edit: nvm LT is Line Trigger of course

tautech:
No issue with Lelon here over some 8 years but your TDS is probably some years older.
3KHz on the probe compensation output was the first sign there was something wrong with my TDS2102B which turned out to be a corroded pin on a 100 pin IC. These non fan cooled top vented scopes can allow dust to settle that can be corrosive when atmospheric moisture is added and the only sign you might see is a black spot on a IC pin that when carefully poked the corrosion falls off to show there is no connection.
Good luck with the repair.

BTW do you have the SM ?
Can email the 2ch version if you need it.

sean0118:

--- Quote from: tautech on August 01, 2021, 09:29:15 pm ---No issue with Lelon here over some 8 years but your TDS is probably some years older.
3KHz on the probe compensation output was the first sign there was something wrong with my TDS2102B which turned out to be a corroded pin on a 100 pin IC. These non fan cooled top vented scopes can allow dust to settle that can be corrosive when atmospheric moisture is added and the only sign you might see is a black spot on a IC pin that when carefully poked the corrosion falls off to show there is no connection.
Good luck with the repair.

BTW do you have the SM ?
Can email the 2ch version if you need it.

--- End quote ---

Okay I may have been a bit too harsh on the Lelons, the main output capacitors are RXK, which maybe aren't too bad. I've pulled them all though, their capacitance and ESR look okay, but I'll replace them anyway.

I'm starting to think maybe the mainboard has a fault, maybe it's overheating and pulling the 3.3V rail low? I had a look over it but couldn't see anything obvious, I'm yet to look at the board for display / user interface. It's interesting your cal was also 3kHz, maybe that's just the default when the board is dead. Was your faulty IC on the main board?

I don't have a schematic, only a service manual with block diagrams. The 2ch schematic would be very useful.

tautech:
No schematics are available for these AFAIK. Only the SM that I got after registering on the Tek website back before TekWiki was launched.
Here's the page for yours:
https://w140.com/tekwiki/wiki/TDS2024

My TDS2102B boot faulted with a USB fail from the IC with the corroded through pin which then triggers the 3 KHz probe comp output which IIRC is a replace mainboard flag in the fault finding flowchart.

For yours indeed the PSU instability is worrying as it may have done some irreversible damage to the mainboard.  :scared:

sean0118:

--- Quote from: tautech on August 03, 2021, 01:14:41 am ---No schematics are available for these AFAIK. Only the SM that I got after registering on the Tek website back before TekWiki was launched.

--- End quote ---

Oh, that makes more sense that SM stands for service manual. Wishful thinking on my part that a schematic was available.

Still it's interesting to know how yours failed, I'll need to take a closer look at mine, will update once the new capacitors arrive.  ;)

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