My HP-35 has a 1 second counter mode!!!



I don't know if this has ever been seen before but the other day I was trying to find out why an old HP-35 with a serial No. 1302S xxxxx wasn't operating(I bought it inoperable for spare parts).

What I found was so cool that at first I thought it was a glitch. The HP-35, when raising the input voltage to approximately +6.1VDC enters a special "1 second counter" mode...This is how I found it:

I connected my benchtop variable DC supply to the battery input on the HP-35 and I initially set the voltage to +3.6VDC(which is the specified battery voltage). I found out that a wire from the power jack to the PCB was broken so I fixed it but by then I had increased the voltage to 6.5VDC and I unwittingly connected the supply to the HP-35 at around that higher voltage. As expected, the LED display was erratic then I noticed that I still had the supply at 6.5V so I began turning the voltage down slowly and then IT HAPPENED right at about 6.1VDC the LED display started counting from 0 up at a frequency of approximately 1 sec per count. This is how it looked :

000000000000.** then 0000000000001.** >> 0000000000002.** >> and so on. I counted 10 minutes and it was still going. The counter at 10 minutes(my modern watch) was off on the HP-35 by 14 seconds so I'm assuming the crystal on the HP-35 is not that accurate by modern standards but then again back in 1973 who cared.

It's repeatable and I was amazed that the circuitry wasn't blown at this higher voltage (6.1VDC). By the way, for you techies and engineers, at this voltage the HP-35 was drawing 215mA...which seems excessive but then again I'm way out of spec on the voltage so anything can happen.

I'm thinking HP engineers added a factory counter mode to test the display and other circuits in the HP-35. This is so cool. My HP-35 still doesn't work but I know that at the very least I can enter my special "counter" mode and leave it on as an eye catching display.

Has anyone seen this before? I know it's not in the user manual for the sole reason that you're not suppossed to be cranking up the voltage past 4V.

If anyone has any other questions go ahead and email me.




The HP-35 hardware and microcode has been studied thoroughly, and there is no factory test mode that does what you describe. What is happening is that your HP-35 malfunctions when operated out of spec. Some kind of glitch is causing it to enter a loop that happens to increment the B register, and because the loop was entered abnormally, the usual termination condition for the loop is not met.

I'm not sure what you're seeing in the display that you've represented as "*". You might not see the low order digits of the count in the B register because the A register is used as a display mask, for instance to prevent the exponent from being shown when the display is in fixed format (vs. scientific notation).

If there had been a factory test mode, it most certainly would not have been invoked by raising the voltage beyond spec.

The HP-35 doesn't have a crystal. Like most HP calculators up until the mid-1980s, it uses an LC oscillator, which has nowhere near the frequency stability of a crystal. The HP-55, HP-01, HP 82182A Time Module, and the time module portion of the HP-41CX do use a crystal.


If it is a glitch, it is a very repeatable glitch. The '*' represents a fast changing digit. The right-most digit is basically a static blur, the 2nd digit is blurry but you can see that the frequency at which it changes is about 10Hz approximately. The third digit(I correct my earlier description) is a flashing "-" at a rate of 2 to 3 Hz. The fourth digit is where the count started at "0".

You are correct Eric, there is no crystal, I see that an LC circuit was utilized for that operation.

I haven't come around to checking the output voltages on the three internal supplies (should be +8, +6 and -12) but I have a feeling that they're likely higher than expected.

The fact still remains that this is a counter that is entered by a rising battery voltage at around 6.03VDC. As soon as I drop the voltage it exits this counter mode and if I raise it to 6.03VDC it enters the counter sequence. You're probably right, it's not a factory test mode but it's certainly repeatable.

Does anyone want to try this? I have a second HP-35 1143-0xxxx model of the first 10k units produced. It's a vintage and there's no way in heck I'm jacking up the voltage past 4.7VDC on that one. So if anyone has a spare HP-35 that's not operational maybe you can try this experiment to find out if it's repeatable in other units.



I saw this happen a few months ago. I had an AC Adapter/Charger I had just bought off eBay. Plugged it in to my 35 and saw a variety of strange sequences. Started with all zeros flashing, then some counting, then some randomness, more flashing.

I didn't leave it plugged in long because I thought it was going to self destruct. I put a multimeter to the charger and it showed right around 6.1V.

I agree, while interesting, it isn't a likely test mode of any sort, just a behavior at a certain voltage that causes certain signals to get crossed in a repeatable way.

The Classics do interesting things when operated out of spec. Have you ever heard of the HP-67 PPC Black Box? Or NNN characters (Non-Normalized Numbers)?


I expect you mean that the next-to-the-last digit changes at approximately 100 Hz. The rate of the "-" flashing is consistent with digit 2 of the A register cycling at 10 Hz. (My previous description in this thread had the A and B registers reversed; B is the mask.)

An approximate 1000 Hz count as you are seeing is thus what one would expect from an increment loop using three or four microinstructions.

I can't identify any specific code segment in the ROM that would easily be expected to show this behavior, but if the machine is malfunctioning due to elevated voltage, anything can happen.

The fact that it is reproducible just means that you are raising the voltage to the point where the "weakest link" starts malfunctioning. This may or may not be the same malfunction that would be see in any other specific HP-35.

I don't recommend doing much experimentation with this, at least on any HP-35 that you would not want to damage.


Please don't put higher voltages into an HP calc!

I accidentally put 12 volts into my HP-22 and now
it is a paperweight.

IIRC, decades ago an electronics magazine had an
article about converting an LED calculator to
read capacitance.

I think it involved changing the capacitor for the
clock circuit with the DUT (Device Under Test),
and some other modifications so that as the DUT
charged a proportional number was displayed on the LED.

I don't think any HP's were harmed by the article, as
it specified using CHEAP calculators!


dona nobis pacem

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