The 540B C5 Mystery

[Jeff]

Hi All,

While building the 540B, I have come across an unexplainable discrepancy between the schematic and reality.
According to the schematic (both Sams and the hand drawn one), C5 does not connect anywhere. However, in the layout included with the Sams schematic, it exists on the PCB. Sams even lists it in the parts list as a 150pf capacitor.

C5 in Sams.png (1.02 MiB) Viewed 6198 times

I checked my stock 540B from OSI and it is indeed there. I then checked several images found on the internet and indeed, it exists on those boards too.

C5 on 540B PCB.png (864.81 KiB) Viewed 6198 times

Does anyone know if this is simply an omission from the schematics?

/Jeff

[dave]

Interesting. I have 2 540B cards, and it's installed in both. The 540A doesn't have it, but the colorburst clock does have a small cap in the same position, which is documented in the schematics.

The cap provides a bit of phase lead for the first 180 degree phase shift stage. I wonder if it is there to help provide a more reliable oscillator startup by providing a bit of asymmetry from the initial powerup condition.

[Jeff]

I installed it on the 540B I am building and it would not startup until I removed it.

Strange.
/Jeff

[dave]

What frequency crystal are you using? Are you using a serial crystal type, or parallel? This should be a serial crystal, and most modern microcontroller oscillators use a parallel crystal.

Dave

[Jeff]

Hi Dave,

I honestly don't know what kind of crystal I am using.
The frequency is 12.000 Mhz.

The part reads:
321 0163
12.000MHZ
VM 6
9250 KOREA

/Jeff