Early (Buggy) HP-35 - Insides



#2

I have just got an early HP-35 into my collection - one with "The Bug "- exhibits all the bugs documented here on MoHPC. It's a Type/Model 2 (dot on 5 key, "Hewlett Packard" without the 35, S/N is 1230A).

It has intermittent power switch and battery "switch" contacts, and since the label had already been damaged, I disassembled and cleaned it up a little.

Very interested to see some "tinplate" EMI shielding in the case bottom and around the display - I haven't seen this in other Classics (HP-45, HP-55) - was this the norm for this generation? Do other 35s have this (I have a Type/Model 4 but have not opened it yet)?

Looks just like tricks I've had to play on my own products to get through FCC, but this wasn't as much an issue in the 70s, or was it??

Cheers


#3

Quote:
Very interested to see some "tinplate" EMI shielding in the case bottom and around the display - I haven't seen this in other Classics (HP-45, HP-55) - was this the norm for this generation?
My 45 does have this shielding around the LED, too. Opened it not too long ago.

#4

Folks,

At the time the Classics were launched, the FCC limits for radiated EMI were not as low as they are now. However, the "tinplate" shields may have been included in these calculators to improve their immunity to spikes and static coming from the outside. HP imposed a high level of immunity to static discharge, even though this was not a regulatory requirement. It was an HP quality requirement, and one of the reasons why HP products were among the most reliable on the market. Of course, that was then.

My interpretation is that these shields were included in the earliest units which needed them for protection against electrostatic discharge spikes, then removed at a later time when the noise immunity of the MOS ICs improved. At that time, MOS-LSI chips were a new and delicate technology!

FWIW...

Joel Setton


#5

Ah, yes, perhaps it was for external source protection, also ESD...

Interesting...


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