Hardware design
HP-35 Power unit
Clock driver
Power-on circuit
Display & Keyboard
HP-35 Schematics
HP 35 hardware in depth analysis.

In the following pages, I present an exhaustive analysis of the HP35 hardware. As of Sept 2009, I have completed the hardware analysis.

This study was started during the months of April 2006 on a bunch of fully working HP35, (starting with SN 1212313213 type 2, ) with a few multimeters, LCR bridge (Capacitance, Resistance & Inductance tester), Transistor tester, and my old oscilloscope HITACHI (model 422 40 Mhz, double trace).

It has been constantly updated and augmented since.

Many calculators HP-45, 55, 65, 21, 25 and 67 were analyzed at the same level of accuracy to understand differences in the hardware.

I hope I'll have the courage to publish all this material.

The reader can recognize on this photo the components of the dismantled HP 35 calculator, split into two PCB, connected by 25 pins and powered by a genuine HP power pack.

That was the very start of the story.
I reconstructed on 'breadboard machines' -with modern components- many basic hardware parts (power supply unit, Led display etc).

I swapped parts between different models (HP-35, HP-45, HP-55).

I used Spice to simulate dynamically bipolar sections (Power Supply Unit, Clock Driver). 

I constructed a ROM reader to validate object code (buggy HP-35).

Finally I traced every signal on the 2 PCB (clocks, addresses and instruction data ...), so even if it may remains errors,  I believe honestly it is a fairly good description of the calculator electronics - statically and dynamically.

1) The small PCB 1 can be divided in 6 parts:

a- left, the white chip is the Control and Timing circuit(1820-0849),
b- center, the 3 ROMS in their 10 pin TO99 metal cans,
c- right, the 2 chips are - with a golden top- the Arithmetic and Register circuit (1820-0848),
d- and above it the clock driver,










e- Top right, on the PCB 1, 2 transistors and 7 passive components form the "one shot" Schmidt trigger used to deliver a PWO (power on) signal to the rest of the machine,

f- Top left, 2 transistors, a pulse transformer and 15 passive components form the DC to DC converter (PSU - Power supply unit), that creates from a single 3.75 V DC the three supply voltages of the machine:
Vcc = +7.5 V,
Vss = 6 V,
Vgg = -12 V.

2) The bigger PCB 2 -that is linked to the first by a 25 pin connector - can be divided only in 4 parts:
- the keyboard (golden "cricket" principle 35 keys),
- the 15 LEDs (in red) and the 2 chips, respectively Anod
e driver (below) and Cathode driver (above) and a few passive components resistors, LC circuit etc...). 

The unit is powered by a battery pack (3 rechargeable NiCad units of 1.25 V each - total V+ = 3.75 V) or by a power unit delivering a regulated and limited current to power the calculator and charge the NiCad unit. 






I will structure this my study as follows:
The chapter "Hardware basic design" will put the pieces together, showing the function of each module and bus line, and  will also discuss the system timing and the synchronization issue,

Then (as sub chapters), I will cover in detail, the main 5 MOS/LSI chips:
- "The Control and Timing circuit",
- "Arithmetic and Register circuit",
- "The 3 ROMs"
- the Clock Driver,
- the Power on unit,
- the Anode driver,
- the Cathode driver,
- the Keyboard,
- the 3 inductor driven 5 LED clusters.                                         
- the Power unit.

To establish a comparison, the same review as been conducted at the same time on a HP25 (Woodstock) unit.

J. Laporte
May 05 2006, May 28 2007, revisited Sept 2009.


(Photos Daniel WEED, J. LAPORTE)