48g upgrade to 768k but only getting 640k?

[caribet]

I have done two 768k upgrades on 48g's. One I successfully got 768k but the other for some reason only has 640k though I have done the same thing to them (apart from the difference in the 47k and 100k resistor positions since one is the older PCB and the other is the newer version). So on one (newer) I get ports 0-5 and the other 0-4. Any ideas why this might be the case? Thanks.

[Han]

Well, firstly that's not supposed to be a 47k and 100k resistor on the hex-flop chip. It's supposed to be a 47k surface mount resister and 22uF capacitor. These two make an RC circuit that acts as a lowpass filter, so that the bank-switching chip does not reset itself during power fluctuations.

The fact that Port 1 shows up is an indication that your problem is likely with chip used for card slot 2. I would check that you have properly soldered the hex-flop chip on correctly as well as the pins to the 512K chip.

[caribet]

Is that even for the newer PCB's (ie a 22uF capacitor and not a 100k resistor). Seems like that some upgrade authors have different info in that regards....

[Han]

That's for ALL HP48G/G+/GX PCBs as far as I know. Many of the current online guides or ones from the old newsgroups postings are wrong about the two resistors. If you read up on the hex-flop chip and understand how it works, then you can easily see why an RC (resistor + capacitor) circuit is needed there as opposed to two resistors. In short, it's to prevent the reset line of the hex-flop chip from resetting the bank-switching mechanism during a voltage fluctuation.

But as with the zener diode, many of these extra circuits were for "just-in-case" scenarios.

[caribet]

Well I have check all the wiring and it all seems good. I even removed the hex-flop and put another on to make sure that it was contacting properly. All the wires on the 512K are also connected properly. Any chance that it is the ROM verion (M) that is giving me this problem? Has anybody been able to put 768k on an M version? Thanks