Scoping out a UK101

The first thing to measure when repairing a retrocomputer is the main oscillator. In a UK101, this is called CLK and found on U58 pin 3.

CLK (8MHz)

 This is divided by U29 and sent to the processor from pin 11 as Phi0 and reaches the counting chains as C0.

Phi0 (1Mhz)

The processor uses Phi0 to produce the processor clock Phi2. This is sent from U8 pin 39 to the hardware devices which populate the address and data buses.

Phi2 (1Mhz)

The counting chain is made up of U59, U60, U61 and U30. These divide C0 into C1-C15. About halfway along the chain are C8-C10, which are fed into the character generator's address lines A0-A2.

C8 (7.8kHz)

Ultimately this produces a Composite Video signal on J2 pin 12.

Video (15.5kHz)

So it's fairly certain that when this is connected to a monitor, there will be some sort of picture!

(This work also turned up an error in the serial clock generation. C3 (125kHz) is used to produce the transmit  clock TxCLK. However while the former is OK, the latter is missing: further investigation is required.)