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Electronics => Repair => Topic started by: k1mgy on December 05, 2019, 02:28:48 pm

Title: TDS744A Color Display Flicker
Post by: k1mgy on December 05, 2019, 02:28:48 pm
My TDS744A, which has already been re-capped (to include all caps in the display driver) is exhibiting a symptom a few minutes after power on (heating) where the colour display flickers.  Best shown by video:  https://drive.google.com/open?id=1pX_4fu-fTLoiURP0z4U-7bewY4jmLD4u

Service manual mentions shutter board, but have no idea where that board is.

Any ideas?



Title: Re: TDS744A Color Display Flicker
Post by: Jwalling on December 05, 2019, 02:59:28 pm
Shutter board is on the HV board which drives the CRT.
I have lots of them - if you're in the USA I'll send one to you for cost of postage.
Title: Re: TDS744A Color Display Flicker
Post by: k1mgy on December 05, 2019, 03:20:47 pm
Thank you Jay.

I just sent you a message (pm).

/mark
Title: Re: TDS744A Color Display Flicker
Post by: k1mgy on December 08, 2019, 09:15:12 pm
Found and resolved the problem with the TDS744A.

First, many thanks to Jay, who generously provided a replacement shutter board.  Turns out, that was not the issue.

In, "TDS Color CRT Driver Repair (700 series): all by sniffing" (https://wonghoi.humgar.com/blog/2018/11/03/tds-color-crt-driver-repair-700-series/ (https://wonghoi.humgar.com/blog/2018/11/03/tds-color-crt-driver-repair-700-series/)) I found this:

Quote
"The next problem is that the shutter color changes out of sync back and forth a slow then fast rate till it gets stable after warming up for a while. I did a lot of troubleshooting, changed a bunch of capacitors and transistors and shutter board, but no avail. In the process, I smelled electrolyte evaporating with the flux and I decided to give the board a full wash with dishwasher detergent and waterpik (then dry the big part with a leaf blower, spray with 99% rubbing alcohol to the water out and left it dry). Bingo!"

So I removed the entire CRT assembly.  There was a layer of dust on the component side.  A few bits of contamination on the underside.  Flushed it with warm water, then cleaned the bottom with flux cleaner, then used anhydrous isopropyl to wash everything off, and then thoroughly dried.

Voila!  No more shutter issue.


Title: Re: TDS744A Color Display Flicker
Post by: george.b on December 08, 2019, 09:50:46 pm
So there was a leaked electrolytic capacitor?
Title: Re: TDS744A Color Display Flicker
Post by: k1mgy on December 09, 2019, 12:31:35 am
Didn't see evidence of that.  And last year when I tackled this same unit, I re-capped the entire scope.

However, there was a coating of dusty crap on top, and some spots on the bottom side where flux and other residue were present.
Title: Re: TDS744A Color Display Flicker
Post by: george.b on December 09, 2019, 07:00:57 am
Ah, right. It's just that the excerpt you quoted there mentioned a smell of electrolyte, that threw me off for a bit there.

So, congratulations on the repair! Washing stuff is the ticket sometimes, I know I've repaired a couple of wonky boards by washing before. :-+
Title: Re: TDS744A Color Display Flicker
Post by: Jwalling on December 09, 2019, 10:59:42 am
Didn't see evidence of that.  And last year when I tackled this same unit, I re-capped the entire scope.

However, there was a coating of dusty crap on top, and some spots on the bottom side where flux and other residue were present.

Glad to hear you've sorted things out! But I wonder if the real issue is that there was some cap goo somewhere that your cleaning removed...
Sometimes it collects just under the cap itself and conducts between the two pads. This is fairly common on the power supply. After some time it starts burning a hole in the PCB.
I've seen this on the HV board as well. It can get pretty bad if left unchecked!