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Data Precision 2020 Function Generator repair

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ioprev:
Started debugging the main logic board today, however it's a bit difficult to take measurements on the board while it is inside the board drawer.
Then noticed that there is a 64-pin header on the back side of the main logic board so I decided to follow the traces from this pins; most of them are directly connected to the 68000 processor socket.
For anyone that may visit this in the future, here is the pinout of this debug header



Now I quickly noticed that on mine the clock is running but ~RESET and ~HALT lines are kept at zero volt. Will not continue searching on the reset circuit.

Here is the clock signal:

fcb:
Not familar with that MCU, but would expect _RESET to be high when running.  If you trace that line back, you might find a watchdog/brownout circuit.  This might well give you a clue as to what voltages to expect in a correctly functioning unit.

Neomys Sapiens:
I have a 2020 which should be ok. As I also have the 2045, that is the one that gets used. Another user has aquired at least a 2045, or more. But none of us has a service manual.
I could provide measurements on the 2020, but not instantly.

duak:
I've attached a simple description of the RESET and HALT lines.  I would trace out the circuitry attached to these lines to see if there is some extra logic that asserts these signals when starting.

I remember the AVX DIP caps from the early 80s.  AVX claimed lower impedance and were easier to insert by automatic assembly equipment.

srb1954:
The ~RESET and ~HALT lines should both be high for normal running of the MC68000 processor.  I think you need to hook up an oscilloscope to look at the sequence of signals on these two lines from a power up.

The ~RESET line should start out low on power up and transition high when the power supplies and clock oscillator stabilizes. The fact that it is stuck low may mean that the reset circuit has not recognized the main power rails as having settled. A quick look at the PCB shows there is a LM393 comparator (Z22)and a LM385 voltage reference (CR1); these could be part of a voltage detector for the main +5V. I think you need to trace out the reset circuit and verify it is working correctly.

Another reason the ~RESET line is stuck low could be a watchdog time-out. If this is the case the ~RESET will transition high after power up and then transition low again after a time delay, probably in the range 100us to 10ms. I don't see a dedicated watchdog timer IC on the PCB but the equivalent function could be built into one of the PAL chips. Normally a 68000 design would have a watchdog timer or bus time-out circuit to recover from a processor hang if a device on the bus doesn't respond.

The ~HALT line being stuck low probably indicates that the processor has detected an bus error condition and is unable to continue running. This could occur if some device on the bus is not responding or if there is a failure in the handling of an interrupt signal e.g. from the PF circuit. Trace out the circuit around the interrupt encoder chip (Z51 74LS348) and see if there is an active low IRQ signal on any of the input lines to this chip or if there is an active low signal on any of output lines to the the processor interrupt inputs, ~IPL0, ~IPL1 and ~IPL2.

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