|
|
Post by gjaky on May 18, 2016 10:38:51 GMT -5
I just bought a Yokogawa DL1740 oscilloscope off E-bay with an issue. The scope is not here yet, but I found a topic on an other forum regarding to the exact problem (what was pronounced). It points out that the input preamplifier IC has to be replaced, which is a custom Yokogawa L6294LG IC. In the other thread the OP could buy a few ICs from www.jotrin.com. I also asked them as well, but unfortunately they don't have it anymore. So anyone can help me to track one IC down? |
|
|
|
Post by gjaky on May 25, 2016 10:51:18 GMT -5
The oscilloscope showed up yesterday. I got false information on the IC part number, it is A6294LG, and indeed it could be found on the above mentioned web page. so I guess the case is solved.
|
|
|
|
Post by gjaky on Jun 10, 2016 15:07:36 GMT -5
I got the IC yesterday and could repair the oscilloscope well for a day... The picture below shows the state how the input IC for CH1 looked, you can imagine removing the IC even with a hot air station was more than problematic, a lot of pads were destroyed or damaged, fortunately despite the QFP64 package only about 10 legs are actually connected, still it was a bit of work to install the new IC properly.  After repairing this channel showed the similar behavior than the other (no distortions etc.) in fact I could measure a fairly nice rise time of my pulse generator:  670ps rise time, not bad! I've played with the scope yesterday quite extensively, with some on/off switching as well, it worked wonderfully. Today I switched on to check a few things again, and I barely got through the welcome screen, all lights started blinking and the machine cycled power, then came the usual smell of burnt electronics... I had a good guess where to look for the problem, but I was wrong, at least partly. Now CH 3's input IC gave up, and died by a very similar way, it just not that burned.  Those burnt legs are +/-5V supply lines and it looks like as there would be a short inside the chip since the package is bulging. I have bought 4 pieces of those ICs so I can change this no problem but certainly there is a pattern in this failure. |
|
|
|
Post by mastertech on Jun 10, 2016 16:42:11 GMT -5
When I have a multi leg ic that I know is bad or am not concerned with saving it, I cut all the ic's legs off at the ic body with a utility knife (not cutting the board naturally), remove the ic body and then remove all the remaining legs. No board damage then.
|
|
|
|
Post by gjaky on Jun 11, 2016 9:26:35 GMT -5
Ha funny, look at this:
I've spent a lot of time with checking the PSU behavior, but could not find anything noteworthy. I did not feel like removing the bad IC yet, instead I cut only the burnt power supply legs to eliminate the short. During power supply testing I put back the analog assemblies to see how the lines behave with normal load, and one thing that was interesting the scope booted up without an error message, then I tested and in fact even Channel 3 is working despite the burn out! How is this possible? The preamp IC has power supply legs on two sides of the package and as it seems they are identical.
So now it is more confusional what happened with it? There is some grease-sweat of the silicon pad that is transferring heat to the case, I saw this earlier too, but the DMM showed no condustion. Now this was the first thing that came into my mind again, but then the IC package would not bulge from INSIDE. Perhaps the bond wires within the package can't stand the inrush current of the chip, and acting as a fuse, and accidentally cause an internal short? If this is the case then CH3's preamp ic has its days counted since it is already running on half supply legs count... To support this theory it is interesting that there are no series resistors throughout the supply line (not one I've seen at least), so basically the +/-5V from the PSU reaches this preamp IC with "zero" resistance (not counting the losses of the PCB tracks and connectors) this I find rather strange.
|
|
