Texas TI-58C: (in)famous constant memory issues

[jebem]

I have this Texas TI-58C that is not able to store in any of the registers or program codes.
Otherwise, the calculator operates normally and all the math and scientific functions gives the expected results.

The previous owner left the batteries inside and it leaked badly.
I have dismantled the machine piece by piece, put everything in the kitchen sink in warm water and dish soap for 15 minutes, then used a soft brush to remove the battery remains from the PCB and plastics.
Two days later I powered it on and it worked normally, except that it doesn't store in the registers or in the program steps. Just get zeros when reading.

I am not able to find the service guide or the schematics, so I'm looking into the PCB traces and try to figure out its architecture.
There are some information in the Internet concerning the TI-58 and TI-59, including the service guide, but these share little with the TI-58C at the hardware implementation, despite being very similar at the architecture level.

Does anyone has this documentation on the TI-58C and willing to share or sell?

1) For this TEXAS TI-58C from 1981 (ATA3381), the main PCB components are:

• TOSHIBA TC5047AP-1 (SRAM 4-bit 4K 1024X4 550nS)
• TI TMC0591NL (SRAM I/O, DoM y1980 w18)
• TI SN27882N (Display Driver, DoM y1981 w22)
• TI SN27882N (Display Driver, DoM y1981 w22)
• TI TMC0573NL (ROM1, DoM y1980 w18)
• TI TMC0501-NL (CPU, DoM y1981 w27)
• TI TMC0580NL/CD2400NL (ROM2, DoM y1981 w22)
• TI TMC0580NL/CD2401NL (ROM3, DoM y1981 w31)
• NICHICON 47uF/10V (NiCd battery bypass, DoM y1981 w15)
• NICHICON 470uF/10V (SRAM battery backup, DoM y1981 w16)
  

It seems that the registers and program area are sharing a common storage sub-system based on two IC's (TC5047AP-1 and TMC0591NL).


2) A few things I have checked:


2.1) I have recorded the current consumption to be about 220mA, which seems on the high side for me, but I do not have another machine to compare.

2.2) The voltages from the DC-DC converter power supply for the Texas IC's are fine:

• Vdd = -10VDC (Pin 2 at CROM socket);
• Vgg = -16VDC (Pin 1 at CROM socket);
• Common point = Vss at +Vbatt. (Pin 8 at CROM socket).
  

2.3) The power supply for the SRAM TC5047AP-1 is fine (reading taken while powered ON):

• Vdd2 = -4.9VDC (pin 8);
• Common point (pin 20);
  

2.4) The SRAM backup power supply (470uF cap.) looks fine as well (reading taken while powered OFF):

• Vdd2 = -3.6VDC (Pin 8) --> the voltage will drop with the passing time;
• Common point (pin 20);
  

2.4.1) This large 470uF capacitor near the DC-DC power supply is a energy storage for this 5VDC SRAM IC supply, allowing the battery to be removed without losing the SRAM contents.
Basically, the Capacitor positive terminal is connected to the +Vbatt (this is the common return), while its negative terminal is fed by two power supply sources:

• When in power OFF state, the -Vbatt (-3.6V) feeds the SRAM thru a Germanium diode;
• When in power ON mode, the -Vdd (-10V) is feed thru a transistor/regulator (I had no time to investigate what this component is and how it is driven).
  

3) So, I believe that the SRAM chip may be faulty. Why?
Well, I do not have the required equipment to do the circuit analysis to be sure, but:

• The SRAM IC is basically in parallel with the battery;
• The battery is fed thru a full wave rectifier bridge from the AC power adapter;
• The battery was dead and in short circuit, so probably it was removed at some point;
• This means that if someone powered On the calculator without the battery pack in place just to test it, the SRAM chip did receive an unregulated high voltage at their power supply terminals, killing the chip.
  

I'm ordering a replacement SRAM chip from China. Wait and see from now on.


4) Some pictures from the internals: