HP-21 repaired

[PANAMATIK]

I think many HP-21 calculators were repaired in the past, sometimes there is only a battery contact problem, sometimes there could be a chip replaced from another calculator, sometimes a new ACT chip could have been helpful. But some of them refused to be repaired, because the display drivers were defective.

I already build a replacement for the display anode driver 1818-0153, which is described in the thread ACT available. But this time, I achieved a new kind of repair.

One of my HP-21s had a bad display cathode driver 1820-1382, this is the 20-pin chip at the right side below the display. Until today there was no replacement for this chip. Because I urgently need new cathode drivers also for my HP-19C and HP-97 repair projects, I decided to build one. After some measurements with the oscilloscope it turned out, that the logic was very easy. And I used one of my rejected second quality ACT boards for programming it as a cathode driver.



CH1 STR signal, generated by the display anode driver
CH2 K1 Signal generated by 1820-1382 cathode driver
EXT unvisible RCD signal at "T", generated by the ACT chip, Reset Counter Display

CH2 shows the timing for all display segments for "-1. E00". You can see at the left two exponent zeros, followed by a single minus segment, then three segments number 1 with dot.



The HP-21 board with missing cathode driver.



Same board bottom with replacement driver, for space reasons it had to be placed away from the original chip.



The result is a reborn HP-21 calculator.

Code:

// Woodstock Calculator Cathode Driver Replacement with PIC16F1516
// simulate a 12 digit shift register with one bit low, all others high

// 12 digit outputs RA0-RA6, RC0-RC4, connected to K1 - K12
// RCD Input RB4
// STR Input RB0
//
// Suitable for HP-21/22/25/27/29 Cathode driver 1820-1382 and also for HP-19C HP-67/97
// digit 0  = leftmost digit, mantissa minus sign
// digit 11 = rightmost digit, exponent ones
// digit 0-11 from left to right

void DoDisplayScan()
{
uint8_t digit,mask;

  TRISA=0x00; // RA0-RA7 output
  TRISB=0xD9; // RCD input
  TRISC=0xA0; // RC0-4 output, RC5 input, TX output RX input not used

  while(TRUE)
  {
    while(STR==0); // wait for STR high, will be high for 5,5 us

// from here to while(STR); only 22 machine instructions are allowed, 5500 ns/250 ns = 22

    if(RCD==0)   // if RCD is low during STR high, then reset the shift register
      digit=11;  // reset digit count to 11, rightmost digit will be shown after RCD low

    PORTA=0xFF;  // switch all digits off during STR high
    PORTC=0xFF;
    if(digit<7)  // first seven digits are connected to RA0-RA6 Port bits
    {
        mask=~(1<<digit); // precalculation to save time
        while(STR);       // wait for STR negative edge
        PORTA=mask;       // set digit low immediately after STR goes low
    } else if(digit<12)   // next 5 digits are connected to RC0-RC4 ports bits
    {
        mask=~(1<<(digit-7)); // precalculation to save time
        while(STR);       // wait for STR negative edge
        PORTC=mask;       // set digit low immediately after STR goes low
    }
    else
    {
        while(STR);       // wait for STR negative edge
    }

    if(++digit>=12) // increment shift register counter
      digit=0;
  }
}

I need to mention: Now every woodstock calculator electronics can be repaired, regardless of the cause of its malfunction, because all components have found a modern replacement.

Bernhard

[rprosperi]

Another great repair story Bernhard, thanks for sharing it. The HP-21 has a special place for me, as this was my first HP Calculator. I really wish I had kept that one, but I sold it to buy an HP-25C, and it has only spiraled out of control since then...

[PANAMATIK]

(06-20-2017 08:35 PM)rprosperi Wrote:  Another great repair story Bernhard, thanks for sharing it. The HP-21 has a special place for me, as this was my first HP Calculator. I really wish I had kept that one, but I sold it to buy an HP-25C, and it has only spiraled out of control since then...

My first HP- calculator was the HP-25. I didn't know about the existence of the HP-21 calculator until some years ago. When I saw it the first time I thought: very peculiar how naked it appeares without any printing on the keyboard bezel and only one blue prefix key! Now I really like this calculator.

My next restoration will be another HP-21, with both display drivers and ACT chip defective and need to be replaced. But this is a child's play now.

Bernhard

[rprosperi]

(06-20-2017 09:21 PM)PANAMATIK Wrote:  I didn't know about the existence of the HP-21 calculator until some years ago. When I saw it the first time I thought: very peculiar how naked it appeares without any printing on the keyboard bezel and only one blue prefix key! Now I really like this calculator.

It was not until many years later that I saw an old HP ad noting that the HP-21 was the updated replacement for the original HP-35. All these years later, my fingers still have 'muscle memory' for some functions on the HP-21. My strongest are the HP-41 and HP-71 as I used those for many more years each, but it's interesting that some things still feel 'familiar' on this machine more than 40 years later.