HP-19c almost works

[Jonas Sandstedt]

A couple of months ago I acquired an HP-19C that needed some overhaul. It now functions normally when the printer is in “MAN”. The settings “TRACE” and “NORM” both function as the setting “NORM”, and when any one of these setting is chosen, the calculator will not calculate more “advanced” calculations such as sinus, logarithms and y^x, while "ordinary" calculations such as addition, multiplication, squares etc. work. When pressing for example 15 SIN, the printer prints “15 SIN” but with the result 15 on the display. My theory is that the printer motor somehow creates a disturbance that affects calculations that take longer time, such that they are not completed. Has anyone encountered such a problem, and/or have any tips how to approach it?
The calculator is powered by 4 AA NIMH cells.

Best regards
Jonas

[teenix]

(11-17-2021 08:26 AM)Jonas Sandstedt Wrote:  A couple of months ago I acquired an HP-19C that needed some overhaul. It now functions normally when the printer is in “MAN”. The settings “TRACE” and “NORM” both function as the setting “NORM”, and when any one of these setting is chosen, the calculator will not calculate more “advanced” calculations such as sinus, logarithms and y^x, while "ordinary" calculations such as addition, multiplication, squares etc. work. When pressing for example 15 SIN, the printer prints “15 SIN” but with the result 15 on the display. My theory is that the printer motor somehow creates a disturbance that affects calculations that take longer time, such that they are not completed. Has anyone encountered such a problem, and/or have any tips how to approach it?
The calculator is powered by 4 AA NIMH cells.

Best regards
Jonas

The HP-91C has some transistor logic to expand the functions of the flag input to the ACT. Part of those functions are for the printer switch. Maybe one of the transistors is faulty.

cheers

Tony

[Jonas Sandstedt]

Thanks Tony,

Interesting, I will take a look at the HP-19C schematics. Perhaps this transistor logic is present for the HP-97 as well, where the complete service manual is available?

All the best
Jonas

[teenix]

The 97 is different to the 19C in that respect. The 19C ACT chip vs the 97 CRC chip.

cheers

Tony

[Jonas Sandstedt]

Thanks Tony,

I'll stick to the Hp-19C schematics, and will come back to tell the result.

All the best
Jonas

[Jonas Sandstedt]

Hello,
It has taken some time, but now I have checked the transistors in question, and they seem fine.
I have also analyzed the calculator's behaviour more in detail, and to summarize:

In print mode “MAN” it works normally, except print commands that require printing more than one line. PRT STK results in that on empty line is printed, but is not followed by the four registers. The same for PRT REG and so on. PRX works, only one line being required.
In the print modes NORM and TRACE only one line is printed no matter what print action that required, it seems as if the printer only prints one line even if more lines are requested no matter what print mode that is selected. The printed line is printed correctly.
Furthermore, in the print modes NORM and TRACE, one line is printed when a calculation such as +, SIN, LN and so on is chosen, but only the “simple” calculations such as +, - and so on works, for SIN, LN etc. the X register is unchanged.

I thought that the motor might create some disturbance, and tried to change the motor capacitor, but that did not help.

I am grateful for any suggestions regarding how to handle this.

best regards
Jonas

[teenix]

It could be ACT problems.

There has been some recent chatter about failing old capacitors, so maybe replacing the ones in the power supply might help.

cheers

Tony

[Jonas Sandstedt]

Good idea, I will try replacing these capacitors.
Until there is a replacement ACT available (as I hope there will be one day), this is a doable way forward.
Best regards Jonas

[Jonas Sandstedt]

In the meantime I can mention that I have given some thought regarding how to accomplish a versatile battery solution that enables “normal” charging of the calculator, as well as external charging in an easily handled manner. My solution is found in the pictures, a factory-mounted AA 2400 mAh NiMH battery-pack with a miniature connector, intended for RC cars. A corresponding connector is soldered to the power terminal of the calculator, and yes, the back cover can be fit and stays in place. The battery pack can be easily removed and charged externally with the USB charger that was delivered with the battery pack, the only extra item needed is the connector that is soldered to the power terminal of the calculator.
best regards
Jonas
       

[Dreato]

This is a great idea for the battery pack. I'm going to try this on my 19C.

Regards,
Dave

[Jonas Sandstedt]

Hello,
I hope it works fine for you as well. The one I bought is labelled "Hootracker 4.8V 2400mAh Ni-MH AA Rechargeable Battery Pack". Separate connectors with short cables can be bought in bundels of 5, 10 etc. These separate connectors have wires that are black and red, but watch out - they may not match the polarity of the battery pack (black may be positive and red may be negative). Just check to be sure before you connect.
best regards
Jonas

[Dreato]

This battery idea worked perfectly for me. In addition, I changed the big 15 ohm current limiting resistor to 68 ohms, to allow safer charging for the NiMH batteries. Now I can charge from the AC adapter or the external USB charger.

Any luck with your problem? I routinely change out the tanatlum capacitors in any Classic or Woodstock series. This usually corrects power supply problems, and often the removed capacitors test good. Even if there's no visible corrosion, I still clean the holes that connect to the gold pins, and remove and clean the wire grid that connects to the keyboard.

BTW - where did you find the schematic for the 19C?

Again, thanks for this great battery idea!

Regards,
Dave

[Jonas Sandstedt]

Good to hear that it works, and thanks for the tip regarding changing charge resistor.

I have not had the time to sit down and remove the tantalum capacitors, I think they can be a cause of the problem. I have now arrived at a theory regarding what happens when the printer is running by observing the calculator’s behavior. 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?

I have found the schematics at http://www.teenix.org, “Reprinted schematics for Classics, Woodstock, HP-10, and HP-67. Zipped PDF files” at the bottom of the page (thanks Tony for this source of valuable information).

I will come back when I know more.

Best regards Jonas

[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

[Jonas Sandstedt]

Thanks for the exhaustive explanation, Tony.
This is really a small mystery, but I will continue to work with it.
best regards
Jonas

[Dreato]

Jonas:
Thanks for the reminder about Teenix's extremely helpful archive of schematics!

Regards,

Dave