Rebuild Tektronix 326 Scope NiCd Battery Pack

[nevetslab]

Back in 2011, my cute little battery powered Sony/Tektronix 326 2-Ch 10MHz scope died. That is, the 9-cell C-size NiCd battery pack died. I pulled the battery pack out of the instrument for inspection and got the size dimensions, I did some digging. The cells in this battery pack were a bit shorter in length from what I was finding on-line...these measure 25.5mm dia x 48mm L, while what I was finding (at that time), were 25.5mm dia x 50mm L. And, of course, the tabs are Welded to the batteries, with my not having the facilities to spot-weld. I used to see Nicd Lady at the TRW Radio Amateur Technical Swap Meets all the time, and called them to see what it would cost to have a new battery pack built. Around $150. The caveat to that rebuild is using the shorter length C-cells, otherwise, the rebuilt battery pack wouldn't fit into the scope.



So, while on line at Amazon, looking for a quad set of NiCd D-Cells for a Simpson 460D (whose batteries had died), I came upon a listing for a Sub-C NiCd battery (actually a 15-pack). Smaller in diameter and shorter. The Tek battery pack's side pieces are molded to fit the standard C-size battery pack they used, so I'd need to increase the diameter so these would fit without flopping about in the nylon housing. I ended up using gaffer's tape wound around their bodies to reach the 25mm dia size. I wasn't sure if the metal spot-welded tabs would take solder, but after removing the insulation that was applied to them, I found they did, so I cut them down in size, and was able to flow solder onto the tabs, and cut 5-mil brass straps to length to solder the tabs together in series to build up the 9-battery assembly.



When I had first reassembled the battery pack, having first soldered the brass straps into place cleanly, then applied thin rubber straps cut to length across the tops of the two rows, taping them down in place, to both insulate the straps and to build up the height of the new battery pack, I lost track of where the 0.3 ohm sense resistor was to be located. I didn't immediately find that particular schematic showing it's location, so I placed it in series with the Hot output terminal which mates with the scope's contacts inside the chassis housing, with the charger end plugging into the panel. On the surfaces of the two nylon shaped panels that hold the battery pack in place, Tek fashioned insulated flat conductor straps that carried the battery connections form one end to the other, so it placed the battery pack and the sense resistor in series from one end to the other. So, I got the connections made, which was tedious having to reach in with a long tip iron working in cramped space. Plugged it into the instrument, and the scope powered up for the first time in 10 years. So, I put the charger assembly back into place, and plugged it into my variac/power analyzer, and let it charge up. I checked on it a little later, and the charger was QUITE HOT to the touch. The instrument was only drawing around 30W, but powered it down immediately, removed the charger assembly and the battery pack, and found the batteries were also QUITE HOT. Damn.....did I have a shorted cell. The battery pack measured 12.2VDC, so I didn't think that was the issue.

I dug thru the service manual further, and finally found the schematic diagram I had missed, which showed the charger and battery pack details. Tek had placed the 0.3 ohm sense resistor on the output side of the battery pack in the GROUND LINE. The charger's current sense circuit measures that potential between ground and the bottom of the battery pack. I didn't have the current adjust regulation for the charger, so not sure just how much current was being dumped into the battery pack. I carefully removed the sense resistor from the positive side and got it fitted to mate with the negative side.

When I was initially checking the charging current with the charger assembly, I had the battery pack installed into the scope, and cabled the charger's Ground Line thru my DMM's Current Connections and back to the Ground to the battery pack, and cabled the mating connections between the charger assembly to the back of the scope, unwittingly placing the DMM's Current Sense resistors in series with the battery pack. Using the 2A range, it placed a 0.1 ohm resistor in that path. With the wiring removed, and the battery pack in place as I had incorrectly wired it, there wasn't a sense resistor to regulate with!



I then found their proper procedure setting the charging current with the battery pack plugged into their charger, cover panel removed to access the current adjust trim pot, which I could now set for 45mV across the 0.3 ohm resistor to yield 150mA at Full Charge. You had to add a ground wire from the output of the battery pack to the Ground connection on the side of the charger to compete the circuit. After I had let this run a good while opened up like this, no excess heat or abnormalities were found, and I was able to put the cover back onto the charger, install the battery pack into the scope, with the charger then mounted onto it, and let it charge up overnight.



After I came in yesterday, now with a fully charged battery pack, I opened up the bottom cover and tweaked the Trace Rotation trim pot to set that. There's still some other minor adjustments to make in the vertical channels. Nice, crisp sharp CRT beam. Some of the refinements found in the Tek 7000 series are present in this instrument...the Horizontal Position pot has a course and fine adjust on the same control, and the push button switches, while not illuminated, are the same type used in the 7000 series. It being battery powered, it comes in handy where you need to be fully isolated from the AC mains, such as working on the live side of switching supplies.

[tedmich]

wow back to NiCd! way to stay "original"! Many would have built a small linear supply and be done with it!

I too am rebuilding a NiCd pack:


in an old Roland Promars MRS-2 synth, after looking into alternatives, thank god they still make these!

[g1]

in an old Roland Promars MRS-2 synth, after looking into alternatives

Depending on the voltage and current requirements, I seem to recall people subbing in Lithium backup batts and disconnecting the charging circuits.

[nevetslab]

I've gone to N-size Lithium cells to keep my HP 41CV running. I haven't yet modified or built a suitable charging circuit to handle those N-cells. I do have a 4-cell AA to N-size adapter, two of which have an LED to indicate charging. All one unit, plugs into a wall socket, but set up for NiCd's. As I recall, you can't use that circuit for Lithium. So, I have that chore to tackle, now that the three packs of rechargeable Lithium cells are low.

For now, I'll stick with this rebuilt NiCd battery pack for the Tek 326, as it worked fine for years.

Anyone have a handle on cobbling a spot-welder to handle welding straps onto NiCd cells (or Lithium Cells, for that matter) when you need an array to retrofit into the mechanical housing that fitted the original NiCd batteries?

[nevetslab]

wow back to NiCd! way to stay "original"! Many would have built a small linear supply and be done with it!

I too am rebuilding a NiCd pack:


in an old Roland Promars MRS-2 synth, after looking into alternatives, thank god they still make these!

Only problem with the small linear supply is....you're tied to the AC Mains. Having a battery-powered scope was one of the reasons I bought this decades ago.

[nosaj]

Anyone have a handle on cobbling a spot-welder to handle welding straps onto NiCd cells (or Lithium Cells, for that matter) when you need an array to retrofit into the mechanical housing that fitted the original NiCd batteries?

Ho about this?

[Enzo]

It has been a while since I last looked, but DIGIKEY used to have NiCad batteries with the welded tabs, AND they would configure them into groups.

[nevetslab]

Last week, checking raw materials on the McMaster-Carr website, I was looking for some hard rubber edging material that could be used to replace the aged & broken rear bumper feet that must have been another one of Tektronixs' custom parts, fitted on each end of the heat sink assembly of the battery charger (which bolts onto the back of the scope). When I first bought the instrument in April 2001 on ebay, the foot on the right rear was well in process of falling apart, while the one on the left rear was still mostly intact, but brittle, chipping away bit by bit. The right foot finally fell apart, and I removed what was left.

I looked thru McMaster-Carr's rubber edging material, setting the gap to 1/8", overall depth front to rear to 1", and then looked at their drawing to see the details. The depth from the end of the edging to the inside of the material was 3/4", yielding 1/4" thickness out beyond the where the heat sink fins would potentially sit. I'd have to cut one of the sides down to be the same size as the depth of the heat sink fins. Both sides of the extrusion has drilled and tapped #4-40 holes, and, inside the first fins, near the upper and lower corners are the mounting holes to attach the heat sink to the battery charger body. So, I'd both have to cut one side short to match the heat sink depth, and then cut a relief at the ends of those shortened sides to accommodate the screw heads.



The material I found was neoprene rubber, short hardness rating around 70, I think. It was around $3.25/foot, but had to buy the minimum length of 10 ft, so the material got expensive quickly.....plus tax & shpg. Still, it seemed like the most suitable material I had found,so, splurged, since I didn't have to spend as much money as I expected to on the NiCd battery pack rebuild. The material came in yesterday. Once I got it out of the box, and sliced open an area of the plastic bag to unravel the first turn of the 'wound' batch of edging, I saw this would definitely do the job. Cutting it to size was the next challenge. I fitted a fresh X-Acto blade into my medium-size knife, got my adjustable machinist square & calipers out, and measured the height of the heat sink extrusion and placed the square in from the end of the square-cut material, and scribed a cut line with the knife blade, then transferred that line across the top and other side, so I had a pattern to follow. Cutting thru that thick material took some time, but finally got the first piece cut off the 10ft roll, only nicking the tip of my middle finger once, so there was blood extracted in the procedure. I guess that was to be expected.

I then set the machinist square onto the fins, and set the blade to the depth of the fins, and transferred that to one of the sides of the edging. Then, resetting the blade for the length of the cut, I used the square as a cutting guide, and proceeded to hack away carefully until I had cut the one side to the depth of the fins. I still had the heat sink mounting screws to deal with, so got that depth and set-back, and transferred the cut lines to the now-shortened side, and hacked away at that until I had the two reliefs cut into the shortened side. Now I checked the fit, and it looked as I had expected. New rubber bumper foot in progress. All I had to do now was to transfer the location of the two #4-40 threaded holes to the uncut side of the edging. A few moves with the calipers and machinist square, and I had transferred those scribe marks onto the material. I first pushed my machinist scribe thru the material, checked the alignment with the edging in place on the end of the extrusion, and verified I had located the holes correctly. I fitted an drill bit into my small T-handle tap wrench, and manually drove the drill thru the hard rubber. That produced a suitable hole without a lot of effort, and then fetched some #4-40 x 5/16" PHMS, with flat and ITL washers, and mounted it. While not the precision fit that the original Tektronix part made, it was good enough to do the task.

I then repeated the steps, after first hacking away at the remaining original Tektronix part. That came apart in chunks, while leaving behind hardened adhesive on the extrusion, so it took a while to finally get to the aluminum surface on both sides of the extrusion. Then, repeated the same steps I went thru in cutting the first piece of edging. I was a bit more accurate with the overall cut than on the first piece. Got that mounted, and finally had nice cushioned feet to set the scope down onto for travel. I did have to cut a relief into the four corners to allow for the battery pack mounting screws to back out during unthreading during removal.