HP-35 hardware testing help sought

[afernan10]

Hi, greetings!

I am new to the forum. I am a HP collector, calculators and also instrumentation. My interest is on items produced during the golden age of the company (i.e. in the BF -before Fiorina- era).

I recently acquired an “untested” HP-35 with wall plug-in adapter in the auction site and -as assumed before purchase- it could be better described as a “not working, for parts” unit

The supplied wall plug-in adapter (an original HP item) is not appropriate for the HP-35. It is a two-pole unit supplying just 8V AC, like those of later models.

I powered-up the unit with a lab power supply at 3.6V and limited the current to 150 mA to no success. Something was clearly wrong as supply voltage dropped to 2 V and limiter entered in action. Then I open the calculator and on first inspection I realized that transistor Q3 (the NPN in the switching power supply) was a 2N3565, strange! As this type is not in the original specs. Q4 was OK and within specs (it was the rare Motorola 3020). Tested all PS components (inc capacitors ESR) and everything was OK. Diodes D4 and D8 had 0.270 V forward and 1.7V reverse when in the board, but tested ok when removed from the board. Same for D2 (0.6V both fwd and rev when on-board but OK when removed) Both transistors, Q3 (2N3565) and Q4 (3020), tested OK when removed.

To make a long story short: after a number of tests, I replaced Q3 with a 2N3904 and the rest remained original. Powered the unit and the PS unit started switching! (good) but no lights on the LED display, PS current abt 50 mA (Consistent with no LED display on). Checked voltages: Vbat 3.6V; Vgg -11V; Vcc 7V and Vss 5.3V. lower than nominal but not that far, imo. Switching frequency measured on the scope was about 250 kHz, much lower than the 800 kHz reported by the late Laporte (http://home.citycable.ch/pierrefleur/Jac...ceDeck.htm) . The Q3 base swing in my case is exactly as reported by Laporte (same shape in the scope) but much lower frequency. Is this relevant? What could be the cause?

As the display didn’t power up next thing I checked was the INIT circuit sequence. Again, all components OK, a nice pulse (4.5V peak) on the scope when powering-up. Phi signals are noisy but evident, although I do not know what to expect.

And this is where I am stuck ☹ in summary: switching PS at lower frequency, voltage values slightly below nominal, INIT ok and LED display does not light. Any help or suggestion on what could I test next?

I also have a concern, given the fact that I received the wrong power supply I wonder what kind of damage could have occurred if the previous owner used this wall unit that supplies 8 VAC instead of abt 4 VDC as the proper wall unit.

Thanks very much in advance for any comments or suggestions!

Toni