Electronics > Repair

TDS3014 adventures

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james_s:
Well if it makes you feel any better, I never win anything and am never the first to encounter and jump on a deal like this, so this is probably a one-off experience  :D 

As far as the oscillator, once I'd checked all the voltages, it kind of seemed like the next logical place to look since there was no sign of any activity anywhere. Once I get sucked into a project like this I tend to obsess over it and power through it until I'm finished.

giosif:
Ok, no grudges anymore.  :) :)

That is good, to get focused on a project until it's done.
Others... ok... Me, I tend to start a few projects in parallel and I have the feeling that leads to more of them ending up in limbo state.

james_s:
Oh I absolutely have that problem as well, but then occasionally something like this gets me excited and I really focus on it and don't get stuck to the point of burning out.

I ended up taking this apart again last night after I noticed the RTC was stopping when it was powered off. Obviously the battery in the dreaded DS1742 was failing although it still was keeping the data in the SRAM. At one point in time I thought those Dallas chips with the integrated battery were a neat idea but after dealing with discontinued older ones I now in a handful of devices think they are absolutely stupid, I mean how hard would it have been to build a coin cell holder into the top? Lithium coin cells almost never leak but I digress. Anyway I desoldered the chip and installed a socket, then popped it in my TL866 and read it as an EEPROM saving the contents to a file.

Then on to the surgical procedure, first cut the plastic shell with a razor knife and carefully peel off a section to expose the epoxy. With the help of a little hot air I softened the epoxy and carefully dug out the old battery which I then measured out of curiosity and found it was only 0.6V, pretty remarkable that it held the NVRAM contents. In this scope there was sufficient space that I was able to graft a CR2032 holder right onto the top of the chip, it's not pretty but it does the job.

Pop it back in the TL866 and load the contents back on it, then install it back in the scope and power that up for a test. All looks good, reference waveforms still intact, date and time are waaaaay off so I set those and it appears to be working fine. Then I check the error log and wait a sec, ok that's not right, I'm reasonably confident that this scope has not been powered on continuously since the year 213 BC unless I've stumbled into something *really* strange. I suppose it would explain the brittle plastic though.

Ok so back to the drawing board. Pop the newly modified Dallas chip out and back into the TL866 it goes. A couple hours of messing around and dozens of back and forths between the scope and programmer and eventually I figured out that reading these Dallas NVRAMs in the TL866 is not reliable, it seems the first read is almost always corrupted to some degree but read it a couple more times and it gets a clean read. Unfortunately I had only that one dump that I did initially so now I'm *really* glad that the calibration values are not stored in this as on some older scopes, incidentally neither are the power-on cycles. I worked out experimentally that the power on hours are stored in a series of bytes starting at x7E0. I'm not absolutely positive of the format but I suspect the time is stored as seconds, before realizing this I experimentally found that x0210 resulted in a display of 2252 hours which is pretty close to what it had prior to this little adventure. At this point I decided to quit while I was ahead and call it good before I break a pin off the DS1742 or plug it in wrong and fry something while trying to get the hours exactly right.

Sooo back together, put it through its paces and now I'm fairly confident that it's 100% working now. I bit the bullet and bought a GPIB/RS232/VGA comm module for $240 which is more than I had invested in the whole project up until now but I figure I'm still ahead as this should allow me to turn it into what is effectively a TDS3054 which I can take advantage of when needed by borrowing probes from my TDS784C boat anchor.



Once the dust settles and I'm satisfied that this is going to stay working I think I might try to repay the generosity that got me this thing and offer the TDS320 I've been using as my "portable" scope until now to someone who needs a scope but I need to find a suitable box. It has displayed a calibration error ever since I tried to back up the stupid Dallas chip it uses and getting it calibrated would cost more than it's worth. I'll post that elsewhere when I decide what to do.

TheSteve:
Will you trace out the serial portion of the plugin module, it does look like a DIY version of that should be possible.

james_s:

--- Quote from: TheSteve on September 29, 2019, 06:37:18 pm ---Will you trace out the serial portion of the plugin module, it does look like a DIY version of that should be possible.

--- End quote ---

Yeah that's the plan, it's probably not worth replicating the whole thing but the serial part looks like nothing more than a level shifter and possibly a buffer.

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