1. TekScopes@groups.io

Re: 1502 TDR Battery problems

Chuck Harris
 

Tektronix tried to be good engineers, and keep the battery
safe from over charge, or under charge.

So, they put a threshold circuit that disables the power supply
if the battery is "dead" (>9.5V), and disables the power supply
if the battery is open circuit, or missing (14V?).

The battery fooler loads down the AC charger built into the 1502
to a voltage that is within the operating range of the battery,
and it adds a capacitor to clean the ripple that the missing
Nicad battery isn't filtering.

In my not so humble opinion, the 200 ohm resistor is a bad idea,
and should be replaced with a nice big heat sunk 12V 10W zener
diode. Or a synthetic zener diode made from a 2N3055 and a smaller
zener diode.

-Chuck Harris

Jeff Kruth via groups.io wrote:

Hello Folks!

Went to use my 1502 TDR, wouldnt light up. I had built a battery cable for a 12 Volt lead acid external battery, so I charged the 12 V battery up and got it working. I remembered the Battery Fooler mod and looked it up on the Tek site here: 2200 uF cap paralleled with 200 ohm 10 watt resistor. I built one of these in a kaput battery back, plugged it in, and it doesnt work! The little meter is pegged in the green, looks kinda normal.  Yes, I got the capacitor polarity right.  SO I plugged a power supply into the unit: 12V @ 275 mA or so and the unit operated fine. At 13.5 VDC about 200 mA or so.Am I missing something? The Battery Fooler seemed like a good thing.  Should I thinker the cap value? Why the 200 ohm resistor- is this needed to keep the SCR thingy working right? Should the value be increased?Any insight would be appreciated. TIANew info: Tried several caps and resistor values around ther original stated ones: No joy!Regards,Jeff Kruth
BTW Brooke Clarke's 1502 webpage is quite good.
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