Problem with Amiga 3000T , Please help !

[Damion]

Check the chips inside your Amiga 3000 computer. Most of them did NOT have the military-rated versions as supplied by Commodore only to the government. If this is the case, you are wasting your time. Throw that POS in the dumpster where it belongs, and get yourself an Amiga 2000 computer. Connect it to your stereo, load up a sh!tty Amiga game of your choice, and enjoy. Just don't forget to TURN UP THE VOLUME!

[Damion]

Hi Anthony,

To look at the maths behind it...
From the RP5C01 datasheet we can see the minimum input voltage is 2.2V at a current of 15µA.  The RTC has a 1.2k Ohm resistor in series with the battery (see schematic).  So it'll develop (1.2k x 15µA) = 18mV, or round to 20mV across it.  So the actual minimum voltage at the RTC becomes (2.2V + 20mV) = 2.22V

Question - is there any harm in removing and bridging that resistor (R179), when replacing the original battery with a diode and 2032 coin? I've done this on my 2000's, by simply installing the diode (1N60 in my case) in place of the resistor (R803). Based on what you've written, it would seem the RTC battery should last quite a bit longer. Any potential issues with this?

Edit: Just pulled my 2000 from the closet, it's been sitting off for well over a year, and the battery is nearing 3 years old. I was surprised to find that it had only lost 20 minutes. Looks like the RTC on the A2000 is good all the way down to 2.0V, which helps.

[Damion]

The purpose of the resistor in series with the battery is to limit charging current to a safe value when a rechargeable nickel based battery is fitted.  With the lithium cell + diode, the resistor serves no purpose so can be bypassed (short circuit) if you want to, although the gain in battery life in doing so will be miniscule; hardly worth the effort.

As I recall, the 1N60 is a germanium diode so should have forward voltage characteristics comparable to a Schottky diode.  Can't remember the exact specs of them, but put your voltmeter across the diode.  If you see any more than about 0.3V with the computer off, then you can make a worthwhile gain by replacing the 1N60 with a BAT85 or whatever.  




The A2000, A500+, A501, etc, use a different RTC device, a Seiko/Epson MSM6242 which does have a lower data retention voltage of 2V as opposed to 2.2V on the Ricoh RP5C01.

Of course it depends on the device and temperature.  While the minimum data retention voltage of 2.0V is the specification, in practice some devices may operate below that.


A little off topic but perhaps worth adding is the fact that the crystal oscillator on the RP5C01 and probably the MSM6242 varies in frequency slightly according to the applied voltage.  I generally calibrate them for f0=32.768kHz at 3.6V (for nickel batteries).  If the voltage drops much below that, the oscillator runs slightly slow.  When the computer is running and the supply voltage is 5V, it runs slightly fast.

And of course as quartz crystals age, they usually drift low in frequency, so that and the lower operating voltage explains your time drift.  Not to mention that those crystals aren't brilliantly accurate over wide temperature ranges.

While you can trim the crystal frequency using the trimming capacitor, you'll need a high impedance input (10M Ohm or better) scope or frequency counter to do it correctly.  Any load on the oscillator circuit from the measuring equipment will change the frequency.

Many thanks for the thoughtful reply. Measuring a 0.3V drop here, and 3.05V total on the anode side - looks like it'll go a few more years yet, with the potential to do a bit better with a Schottky.

This is probably obvious, but it's worth mentioning I've had great luck with the Renata (or anything made in Japan) 2032's. The Chinese ones die quickly.

[Damion]

WOOT! Nice job!

:pint: