Electronics > Repair
TDS3014 adventures
techman-001:
--- Quote from: ArcticGeek on October 13, 2019, 12:59:13 am ---I don't know why Tek continues to use these Dallas/Maxim RTC chips that aren't replaceable. They started to use them in the 2465B scopes and have continued since then. I haven't opened any of their brand new scopes, but I know that even the higher end TDS5104 and TDS7104 used these things.
--- End quote ---
This thread had me worried, then I remembered the scope I use everyday is made by HP (54601A HP DSO circa 1994) , and has no Dallas/Maxim RTC chip in it :)
I'm pretty sure my only other scope, a Tek 7000 series mainframe doesn't have one either ;-)
james_s:
I remember thinking years ago that the Dallas chips seemed like a neat idea, this was after spending a lot of time cleaning up huge messes caused by leaking NiCd memory backup batteries on 80s arcade boards and vintage computers. I've also seen a number of computers completely destroyed by leaking lithium thionyl chloride batteries however it's very rare that I've ever seen a lithium coin cell leak and when it has happened the damage was localized to the battery holder. Anyway in retrospect it's an idiotic idea to pot a battery inside a module, just use a battery type that is not prone to leaking!
In case anyone else is interested, I reverse engineered the serial portion of the TDS3GV comm module, there was already another thread on that which I'll link here rather than fragmenting information all over.
https://www.eevblog.com/forum/testgear/reverse-engineering-tektronix-tds3gv-module-for-tds3000-series-oscilloscopes/new/#new
ArcticGeek:
There are some benefits of using these encapsulated RTC modules. Obviously, one benefit is that design is relatively simple compared to the added complexity of having a battery, holder, RTC, and SRAM.
But another benefit is guaranteed battery life. You would be surprised how poor the battery life of a NVRAM/RTC can be if the PCB is not completely clean of any type of flux residue/contamination.
I worked on a project some 20 years ago where I designed a custom Intel 386/486 computer platform, and that design used a Dallas RTC chip with an external lithium battery. As I recall the normal battery current was in the 200nA range; this would yield at least a 12 year life for the product. However, many boards failed early due to the battery being exhausted, and analysis found that the battery current on those failing cards was in the 20-50uA range - nearly 200 times the current! It was all caused be boards that were not perfectly clean, but yet even under the microscope they looked good and clean. The circuit itself was fairly simple, but yet just a little bit on uncleanliness caused dramatic reduction. it took quite a bit of effort to get the card supplier to keep the process in control. The lesson I learned from this is you have to be very careful on these sub-uA circuits to make sure you keep stuff very clean.
Using an encapsulated module gets rid of any of these issues, and I can kinda understand how Tek sees the advantage of using them.
texaspyro:
--- Quote from: ArcticGeek on October 13, 2019, 02:21:40 am ---The lesson I learned from this is you have to be very careful on these sub-uA circuits to make sure you keep stuff very clean.
--- End quote ---
One thing that is often overlooked is handling of watch batteries. A finger print across the edge of the cell can cause the battery to drain rather rapidly. Never handle the batteries with bare hands! Also, it can help to first clean the cells with alcohol.
james_s:
I think the PowerCap arrangement is a much better design though. One type of PowerCap can work with any number of RTC/NVRAM types, it's a lot more economical than keeping a whole selection of old products in production that all have a use-by date due to the encapsulated battery.
Also I'd much rather have a battery that doesn't last as long as it should than an instrument that needs to be sent in for an expensive repair. I mean Tek didn't even socket the part, clearly they did not intend it to be field replaced and 10 years is not all that long of a life for such an expensive piece of equipment.
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