Batboard

I have a bunch of UHF SABERS, In particular I have a SYSTEM SABER i regularly use. I have had problems with an aftermarket W&W NIMH battery dying while the radio sits idle and turned off. So I got curious and decided to measure the leakage resistance of the radio when turned off. I used an analog Radio Shack VOM with the leads arranged so that the ohmmeter polarity was correct tot the battery terminals.

I was surprised to find the following variation among a sample of radios I have:

SS III #1 200K ohms (NTN5835A DES-XL module installed) (edited corrected 02/03/2014 10:20PM edt)
SS III #2 29K ohms (NTN5832A DVP encryption module installed)
SS I infinity ohms (No encryption module installed)
and
Astro SABER III 15K Ohms (Don't know if encryption module is installed)
Astro SABER II 20K Ohms (Don't know if encryption module is installed)

Calculating for 7.5Volts, these are numbers in the range 0.04 to 0.5 ma. Not much, but it is consumed while the radio is off.

Can anyone share why such variation, especially among the SYSTEM SABERS? I would expect some leakage to keep the encryption key alive. But how much?

If you have an analog VOM. could you measure your radio and tell me what you see? Thanks Joe

Sounds like some bad PA modules to me

My first thought, but the service manual implies that type of failure is measure above approx 105 ma in standby. This is powered off, and much smaller drain 0.5ma. I will peek at the schematic to see what parts hang directly on the battery + line. Hoping to interest some others to measure their radios. Surprised that two System Sabers and two Astro Sabers have this sort of drain.

Bad PA modules were a common drain in that line. The other thing is that any secure-capable radio equipped with a crypto module will draw power to keep the key alive, even if no key is loaded. This is because the zeroize function requires it; if there is no power, the key is lost, which eliminates the problem.

(Emergency zeroize: turn radio on, take battery off.)

I guess I will have to open these up and check the PA modules and the secure modules. Now I have to pull batteries of not in the charger.

What I am saying is that the power draw by the secure module is normal.

Hi Slim Bob;
Are you saying the 200K Ohms for a DES-XL and 29K for DVP modules are normal? Did Motorola intentionally make the DVP draw down faster? I always wondered if you could defeat the zeroize function by installing a large capacitor on the B+ line. Now maybe it seems a bleeder resistor inside the module is the "key" to zeroize the key (pun intended)?

Yes. The DVP is probably older, less power-efficient silicon or germanium.

There is no bleeder resistor in there to remove.