|My HP-35 has a 1 second counter mode!!!|
Message #1 Posted by oscarHPfanatic on 20 June 2007, 1:33 p.m.
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.
REPLACE THIS TEXT WITH YOUR LISTING