HP-19c almost works

[teenix]

(11-30-2021 06:37 PM)Jonas Sandstedt Wrote:  I think that the calculator makes a quick restart when the printer is running, probably due to a voltage drop such that more complicated calculations are not completed, and always only one line is printed.

I also have a question, can electrolytic capacitors be used instead of the tantalum capacitors?

It is hard to say why only 1 line is being printed. From a software point of view, the Microcode will format and output data to the PIK chip which will then start printing a line. If more than 1 line is to be printed for a particular operation, eg Print Stack, then the after the first line starts printing, the microcode will just sit there waiting for the PIK chip to tell the ACT that the print carriage has homed and then the microcode will immediately send the next line of print data to the PIK.

The home signal is meshed in with the transistor logic which is responsible for sharing a few hardware states to one of the ACT flag inputs. The logic is required because the ACT only has two flag inputs available. One of these, Flag 2, is connected to the transistor logic and inside the ACT is connected to the Status[3] bit. The microcode has to process this single bit for all those sources from the transistor logic depending on what is happening at any one time.

This sort of leads back to the print problem being around this logic if there was false triggering on this flag line. You might like to check the connections for ACT EN pin (pin 22) which controls the state of the transistors as does the connection from the PRGM switch.

The only way to test for correct print logic operation is to monitor the flag input to the ACT with an oscilloscope and capture the moment that the print head homes. You will see a short logic low pulse on the flag line. (Pin 16 of PIK, Pin 4 of ACT)

The LLD output from the cathode driver (pin 14) is connected to the other ACT Flag 1 input (Pin 3) and controls the Status[5] bit. The LLD pin is monitoring the power supply voltage and when it drops too low, normally the low battery LED will light in the display like in the HP-67. The 19C and 29C don't have the extra LED and when Status[5] in the ACT goes logic low and the Microcode will jump to code that toggles the decimal point LED on and off.

If the voltage drops too low, if the printer starts for example, there is a chance the processor will reset but if that is the case, the printing would most likely stop, and the display should show what was there before, and the print head should home. For a full reset, the load would have to discharge the RAM supply capacitor, in which case you will see "Error" on the display when things get going again. However, the power spike could cause anything too happen, hard to say. If you suspect this, then a check of all the battery connections would be needed. Check the wiring where it is soldered to the boards. Sometimes, the wires have moved during servicing and the small strands can break. This may limit battery current in high load situations like the printer use.

Yes, electrolytic capacitors can be used, but are generally more bulky that tantalum types. However, they may have been used on purpose by the HP engineers.

cheers

Tony