Upgrading the MS-15 battery
A few weeks ago, working on another project, I needed to do a bit of signal testing on a circuit, and fired up my Non-Linear Systems (NLS) MS-15 Miniscope. When I turned on the machine, it did not light up and soon after started generating a quite acrid smell, and the case started to get warm! *Pull Power Now*!! I set it aside for another day.
As best as I can tell, the MS-15 was manufactured from the late 70s and apparently as late as 2005. This one was originally my father’s, from the late 80s and 90s, when he maintained a number of mountaintop radio systems. The gold plated Tektronix stayed in the shop, and this mini scope suffered in the back of his truck.
One of the scope’s significant features, was an integrated battery pack, that allowed the device to be used anywhere. No need to find a open power socket. My uncle used the same device in his auto repair business to troubleshoot engines.
Enough blathering, time to dig into the problem… Today, when I opened the machine, to no particular surprise, I found that the lead acid batteries were in very bad shape. The terminals were corroded, and electrolyte had started to leak onto the control board.
After a bit of digging on the web, I was able to find an appropriate schematic for the device (hiding down at the bottom of this post!), and was pleasantly surprised to find the power/charge circuit is completely in parallel to the batteries, and isolated from the rest of the system. The battery power leads could be used for an alternate power source. Hmmm.
My initial plan was to use a standard USB battery bank module. Charging a single Lithium cell via USB, and running the scope from the 5.1v boosted output. Using a bench power supply, I quickly found the machine won’t work at all below 5.4 volts, too high for a regular USB power bank. The other surprising finding was that power draw was less than 500 mA, so runtime on a small 1800 mAH lithium cell could be 2-4 hours.
The nominal voltage for the lead cells, and thus the power supply was 6v, going as high as 7.5v with the charger attached. In my toys box I found a USB lithium cell charger with an adjustable boost controller, allowing the output to be set to voltages significantly higher than either the cell’s voltage or the USB voltage.
I found that the best results were achieved when the power output is set to 6.5v. The boost controller maintains a constant voltage on the output at all times. However I found the boost controller would occasionally reset when load was applied. With the output voltage at 6.5v, this issue went away. Under load, the output voltage settled to 6v.
The charge module I have is capped at a 1A charge rate, and this is sufficient to cover the scope’s ~500 mA operating current and top off the batter. Charging from USB power eliminates the need for a dedicated power source for the scope – any available USB charger and mini USB cable can charge the scope.
Overall I was quite surprised how easy it was to upgrade the scope with a modern power source. An external USB port is now installed and the whole setup runs surprisingly well.
As long as I had the machine open, I took the opportunity to calibrate the display. Then reassembled it and returned it to its proper place on the workbench. I have left the original power supply in place – even the funky power jack. The new USB charge port will go over the old jack and protect me from making the silly mistake of actually using the old port!
If you made it this far into the article, most likely what you were REALLY looking for was the MS-15 schematic. Found one just for you! Enjoy!
Elmars Tinkering Again 