Thread by @TubeTimeUS

this is the computer you wanted to have in your electronics lab in 1983. it's like a cross between a scientific calculator and a computer. it runs BASIC, has a built-in printer, CRT, and tape drive. an HP-IB add-on lets it talk to GPIB instruments.

...and mine is broken!

hmm.

here's the main board. there are test points for voltages and the four clock signals.

so the voltages and clocks seem ok.

there is bus activity. but only read operations. maybe it's stuck in a loop, waiting for something that never happens.

time for my new toy, a EBTKS from @PhilipFreidin! I have it on good authority that it could be useful in this circumstance.

oh yeah it has a logic analyzer mode. let's see what the CPU is doing.

caught in a loop around 3BFB. hmm, time to dig up the source code.

looks like it is waiting for the screen retrace.
(3BFB is 35773 in octal which this machine uses quite a bit.)

here's the CRT controller chip (ceramic and gold). think I'll probe some signals on it.

there's a clock, RCL2, going to the CRT controller chip, and it is *absent* on the chip, but it is present at the output of the gate U7D. maybe the resistor went bad.

resistor's fine. the chip is pulling the signal up to 12v somehow. it's an input pin! c'mon...

I reseated the CRT controller in the socket, and voila

ok so it was working BUT new symptom: totally dead, not even a power light 😅

this thing seems to be shorting out something. when I unplug it, the computer works fine.

took it apart and nothing was wrong with it. the caps were ok, no shorts, no blown chips. weird.

so I put it back together again and it works.