Early HP35 PCB with 2.02 bug question

[Harald]

I got lucky and received this board with a broken HP35 I bought on ebay:



It is an early version with the 2.02 bug. What I am wondering is, why does it have an extra 3 capacitors compared to this board I found in the HP35 with 2.02 bug which I have recently acquired?

[compsystems]

Old art on PCB +1

[jebem]

From what I can read in the pictures, the two orange axial ones have a capacitance of 0.0018 +/-10%, 500DC, so this translates to 0.0018uF or 1800pF, 500VDC. Can't identify the dielectric type.
The radial one is a 33pF +/- 5%, 300VDC and it looks like a silver mica type.

The other ceramic radial capacitor located at the power supply is a very high quality X5R category unit, 470K, 1KV (47pF +/-10%, 1KVDC).

While those three additional caps are of good quality, they looks to be of inferior quality when compared to the X5R cap used in the power supply.

All the 3 caps seems to be well soldered, like it was a factory modification or testing.
Can you tell from what you see on the bottom side?

I don't know this machine in particular, but I can think of a couple of reasons to have those 3 caps added.

Hypothesis 1: Power supply IC rails bypass. As i see it, they have too small capacitance value to be useful as power supply rail bypass and they are at the wrong place for that (too far away from the IC's).

Hypothesis 2: They were installed during troubleshooting to eliminate suspected noise and then forgot to remove them at the end of the work.

Hypothesis 3: Over-clocking/under-clocking. Can you tell if the speed is the same as the original PCA?

It seems that at least the radial cap goes to one of the pins of the clock driver (8-pin IC).

Hypothesis 4: Machine fails to reset on power on. So they modified the clock driver to bypass the situation.

It would require to have a detailed inspection to the PCA and draw the schematics to have more information.

[teenix]

(11-18-2016 12:28 PM)Harald Wrote:  I got lucky and received this board with a broken HP35 I bought on ebay:



It is an early version with the 2.02 bug. What I am wondering is, why does it have an extra 3 capacitors compared to this board I found in the HP35 with 2.02 bug which I have recently acquired?

There are a few more similar PCB images on this old thread

http://www.hpmuseum.org/cgi-sys/cgiwrap/...ead=212650

[Harald]

(11-18-2016 11:25 PM)jebem Wrote:  Hypothesis 3: Over-clocking/under-clocking. Can you tell if the speed is the same as the original PCA?

There is a clock driver in this PCB, but the two phase clock is generated om the display PCB by the anode driver. So the frequency is set there.