Redesign Of Tek Battery Pack

Got this old Tektronix 422 here which still works. It has the battery optio n. Of course the batteries are no good at this stage of the game.

I am considering replacing them with more modern batteries. As such it woul d require at least modification to the existing charging circuit, though si nce things have changed so much since this wasn't close to boat anchor mate rial, we might just be talking building a charger from scratch.

I am fairly sure it is 12 volts. I will make sure before actually setting d own any money for anything, but I am wondering about the options. Cost vs. capacity and all that, and the complexity of the charging circuit.

Could go Li-ion, Nimh, whatever. But what would be right for this ? It is a cathode ray oscilloscope so the current drain is not trivial.

Ideas ? I think it would be nice to have a battery powered scope. I do find occasion to want to use one on a car for example. Actually fixed one once with another 422, found the crank sensor was cutting out when hot resultin g in a stall and no hot restart. The car was a 1990 so it did not throw a c ode for that. Well 12, which means no pulses but then with the engine not r unning it always reads 12. But there are other times here and there, and ev en if I do a road call and think I might need a scope, it would be nice to have one less cord, like when I would take my laptop sometimes on the job. One less cord is a good thing in alot of places.

In fact maybe a cordless soldering iron would be nice as well. I know they already have them. Lights ? Need I say more ?

At any rate, who knows, I might be able to find some cellphones that have m et with some not-so-pleasant fate and stack up a few of their batteries. I' m sure the Tek will be tolerant of a little different voltage if necessary. Also, I figure to just tap right Off the AC mains to feed the new charger circuit. The system they got in there is too cumbersome. It's a selector th at works kinda like the voltage selector on some PX type equipment. Like th e kind you pull the fuse and then part of the holder is a plug that rewires it for 240, 120, whatevr, just no fuse in the middle. (I could be mistaken , maybe it just turns) I don't plan to use that, and if I get my way it wil l simply work and charge whenever plugged in.

I think it also has a 12 volt input. I know you can't charge 12 volts with

12 volts, and I am just not sure I want to mess with trying to put a boost convertor in there and then deal with all that.

I know these modern batteries have temperature sensors in them and all what ever else. Looks like some of them have enough pins to charge the cells ind ividually, but not sure if they do., They probably should. If the circuitry is cheap enough I might be inclined to do that.

Throw some ideas if you're so inclined, even wacko ideas. I like wacko idea s.

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Hand crank?

Modern digital oscilloscope?

John Larkin         Highland Technology, Inc 
picosecond timing   laser drivers and controllers 
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John Larkin

First wacko idea:

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All will be revealed.

First examine your objective. What are you trying to accomplish? If you wanna restore old equipment, get the right batteries and be done with it.

If you want a battery powered scope to use occasionally, you've got problems. The batteries will always be dead when you want it. The constant current charger will kill the batteries if you leave it on charge. Can you plan 16 hours ahead to charge it? Much better to get a 12VDC to 120VAC converter and run it from that. Make sure the input circuit can run off a modified sine wave, aka square wave AC input. Try that on a TEK 211 and you'll blow the fuse instantly. An extension cord is a cheap alternative for many applications.

If you want a scope to use frequently, start from a more modern base.

I find procrastination is the solution to most problems. The more you think about the cost/benefit ratio of the project, the less likely you'll complete it.

I went thru the same process with a TEK 1501 TDR. The design of the internal charger wouldn't be friendly to any battery technology. And the damn thing won't run without a battery installed.

I put a socket in the back and piped a wall wart into the battery connection. If I find that I ever need the instrument, I've got multiple ways to run it.

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If you do a search on yahoo tekscopes group, you may find a modification where a 2200 uF cap and a 1.5 k resistor is installed where the battery goes. That will allow the TDR to run from normal AC power. Also, search on

1502 and 1503 TDR as they all use the same type power supply.

Yahoo has really screwed up the groups with neo so if you don't find the details, I may be able to provide a link.

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Tom Miller

Turns out that it will, actually.

The reason it won't normally work without a charged battery installed, is that it has an undervoltage detection circuit intended to keep you from draining the battery all the way down to zero and killing it. No battery means no volts on the battery terminals means "severe undervoltage" from the circuit's point of view.

All you need to do, if you wish, is disable the undervoltage detection circuit... and you can do this by unplugging one transistor from its socket (sorry I don't recall which one it is, but the information's out there on the Intertubes).

Seems like a fine solution.

Our repeater group was gifted a couple of these older TDRs, including a 1501 in working condition. It's been quite useful on a couple of occasions... it spotted an odd impedance discontinuity in our 440 repeater feedline which turned out to be due to water infiltration into a heliax connector.

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Dave Platt

thanks, It's more like 200 ohms. I am concerned about the voltage at the battery node without the battery installed. Seems like a lot of hoping going on. But it's hard to argue with success.

I'm more interested in being able to run it from 12VDC. One thing I hadn't considered is that I could use the the disconnect on the barrel connector to switch the resistor out and have it work on AC or DC.

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Start on page 188 of

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for the power supply / battery charger manual. Quick version: it will run on external DC from 11.5 to 35 V, at 23 W typical. The internal NiCd pack is 20 cells, 24 V, 3.5 Ah, which provides "approximately four hours of running time". 24 V * 3.5 Ah = 84 Wh; 84 Wh / 4 h = 21 W.

You can probably get the original pack rebuilt at a Batteries Plus or similar store that rebuilds power tool batteries. They may even be able to install cells with a slightly increased capacity. It cost me about $35 to get a DeWalt 12 V, 1.5 Ah NiCd battery pack rebuilt at Batteries Plus.

If I didn't care about it being 100% original, I'd be kind of tempted to make a stack of NiMH cells, install them where the NiCds were, and then either run the NiMH pack into the "external DC" input, or disable the charger and connect them up like the old NiCd pack. Then use a separate NiMH charger to charge the pack.

For absolutely low cost, but more weight, use a couple of gel-cell/AGM

12 V batteries to replace the NiCd pack.

Matt Roberds

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It's a 200 ohm resistor in parallel with a 2200 uF cap.

Quoting the post you refer to: (#8139 in the tekscopes archive)


It can be "convinced" to turn on without a battery pack, or you can build an external battery eliminator (power supply). My 1503 powers up with a 200 ohm 10 watt resistor and parallel 2200 uf

25 volt cap across the battery terminals. I plug in the AC cord, wait a couple of seconds for the cap to charge, then power it on.

When using the AC line cord. the 1503 tries to charge the battery. It needs to "see" the battery drawing current (charging) - thus the need for a resistor. It attempts to charge the battery even when it is turned off. Initially (at turn on), it also needs to "see" a voltage available from the battery. The cap provides the voltage, and the resistor draws current, so teh 1503 "thinks" there is a battery installed.

I take no credit for this idea - someone else pointed it out. I would credit him if I remembered who it was. That same person also came up with the design for a battery and planned possibly to make them available for purchase. I hope he'll chime in here as he may be able to sell you a battery.

If you decide to "convince" yours - prior to adding the resistor & cap: If you put a meter across the battery terminals, you'll see a pulse when you turn the unit on. That lest you identify the positive and negative terminals. Do it several times to make sure you get the polarity right. My cap and resistor "live" in the empty battery compartment - they are connected to the 1503 by bananna plugs. The plugs are installed on a small piece of plexiglass, which in turn is mounted on a threaded rod, long enough to allow plugging the bananna plugs in when the unit is fully assembled. I also made an aluminum plate that exactly fits the recess on the back panel such that it is flush with the back panel when installed. The threaded rod sticks through a hole in that plate, and the plate is sandwiched between two nuts on the threaded rod. I also have a power jack mounted on the plate and a diode in the circuit, When I insert the plug from the wallwart, the resistor is removed from the circuit, the AC is rectified by the diode and filtered by the cap and powers the 1503 as if it had a battery in it. With the plug from the wallwart removed, the resistor is in the circuit

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If y'all like this batteryy shit, my buddy has a Fluke 8060 that neds batteries.

eI am game for trying to get thatt stuff runniung. Bsatteries or not, but WITH batteries it is so coool.

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You can get most batteries from the past still.. even NiCd. About the only kinds that are not available are primary cells such as mercury and portable tube radio plate and filament batteries.

Why not just rebuild it to spec and be done with it?

Best regards,  
Spehro Pefhany 
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Spehro Pefhany

I druther have it run 20 hours and recharge in ten minutes.

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