Early HP35 again: Let's play "spot the difference"

[Harald]

This is a display PCB as I know it:


And this one is an early one that surprises me:



Can you spot the difference?
Ok, yes, the left one is missing a anode drive, I know .
But it is also missing an inductor for the clock generator!
It has 380pF from pin 9 of the anode driver to ground.
Ususally there is a 150pF in series with 330µH from pin 9 to ground.
However the clock is working fine.

Does anyone know if the anode driver initially had a different design?

Cheers,
Harald

[jebem]

Interesting.

I checked my archived HP-35 photos and what i have from pin 9 is a 330uH inductor in series with a 150pF to ground.

It seems the machine on your second picture (the one missing the inductor) have the 150pF cap disconnected. Was the machine modified on the PCB bottom side to wire the cap to ground?

[teenix]

(11-18-2016 04:13 PM)Harald Wrote:  This is a display PCB as I know it:

Can you spot the difference?

The PCB design is a bit different below pin 1 of the chip so might have some bearing on the change.

cheers

Tony

[PANAMATIK]

(11-19-2016 05:50 AM)teenix Wrote:  The PCB design is a bit different below pin 1 of the chip ....

I can see only assembly differences. There were small assembly modifications also in the woodstock boards. Perhaps they learned they could remove some parts after having units in the market.

Bernhard

[teenix]

(11-19-2016 08:31 AM)PANAMATIK Wrote:  I can see only assembly differences. There were small assembly modifications also in the woodstock boards. Perhaps they learned they could remove some parts after having units in the market.

Bernhard

Bit tricky to spot, there are also other track differences above the led display.

cheers

Tony

[Harald]

There is a slight modification in the PCB. The point where inductor and capacitor can be connected has moved a little and the trace going around it has moved as well. That allows a slightly longer inductor to be installed, but electrically it is the same.
On the first board the 380pF goes from pin 9 to ground. The ground plane on the bottom side is a bit larger than on the top. But the capacitor is connected to it.
The Anode driver has a fault, but the clock was still working. So I am sure this anode driver (1820-0853) does not require an inductor for the oscillator.
I have now inatalled 330uH and 150pF in order to install a new anode driver. I don't have a 0853 available as a replacement

[PANAMATIK]

(11-19-2016 11:46 AM)Harald Wrote:  There is a slight modification in the PCB...

Very tricky to spot indeed.

The electrically identical modifications can be understood, when you know, that the layout was "painted" manually by slim black tape strips on a transparent film. During mass production the film had to be reused several times and some lines could have moved and had to be repaired or readjusted. Just an assumption.

Bernhard