my mc68000 cpu short or not? - Page 1
Forum section: electronics / repair

author: galaga1982
author_rank: Contributor
timestamp: July 30, 2023, 08:09:14 am
content: Hi,The resistance value between the vcc and the gnd point of the mc68000 cpu not mounted on the pcb is 80 ohm or less. Is this a short one? A multimeter open circuit test produces a connected beep sound.Is there an easy way to read whether CPU is a fail or not?Thank you.

author: srb1954
author_rank: Super Contributor
timestamp: July 30, 2023, 01:34:42 pm
content: Quote from: galaga1982 on July 30, 2023, 08:09:14 amIgnore the beep from the multimeter and just rely on the resistance measurement. 80is probably quite OK for an MC68000 as this is an older generation chip that draws quite a bit of power so its resistance will be lower than modern CMOS chips.QuoteNot really. You need to plug it into a PCB that provides at least the minimum ancillary circuitry require to operate a MC68000. Only then can you check whether the chip is functioning or not.

author: galaga1982
author_rank: Contributor
timestamp: July 31, 2023, 06:58:03 am
content: @srb1954,Thank you. The cpu wasn't broken. My hobby is repairing arcade video game pcb.I was suspicious because there was a beep that VCC and GND were connected only when the CPU was installed on the PCB, but there seems to be a problem somewhere else.Is there an effective way to find the short part of the PCB?

author: srb1954
author_rank: Super Contributor
timestamp: July 31, 2023, 09:54:02 am
content: What is the resistance across the power rails of the fully populated PCB? Is it a hard short < 1?The easiest way to find a short is to supply the PCB from a current-limited (set to no more than 1A for safety) bench supply and scan the board with a sensitive thermal camera to find the hot components.If you don't have a thermal camera you can use a DMM to measure the voltage drop along the supply tracks. Most of the current will flow through the short and the track to that short will develop the greatest voltage gradient along its length. Other tracks not connecting to the short will have minimal voltage drop along their length. This method can also be used with boards that have internal power and ground planes but you may need a more sensitive micro-voltmeter to measure the smaller voltage drops across the lower resistance power and ground planes. As the current through a power plane doesn't follow a narrow path it will be more difficult to determine the path to the short. It may be necessary to plot the voltage drops at multiple points across the PCB to determine where the voltage gradient is greatest.

author: galaga1982
author_rank: Contributor
timestamp: August 02, 2023, 01:00:11 pm
content: The resistance values of vcc and gnd of fully populated pcb are about 72.3 ohm. Does that mean it may not be a short failure?If the pcb is in a short state, it would not be an easy way to find a voltage drop without a thermal imaging camera or a sensitive microvoltmeter, as you say.

author: srb1954
author_rank: Super Contributor
timestamp: August 02, 2023, 09:28:26 pm
content: Quote from: galaga1982 on August 02, 2023, 01:00:11 pm72.3is not a short and just represents the current draw of all the chips on the PCB. You can confirm that there isn't a short by measuring the Vcc - it should be between 4.75V and 5.25V for normal operation.The basic sequence for initially debugging a basic MC68000 based circuit is to:Measure all the supply voltages and check that they are within specification, usually +/-5%. Checking the supply rails with a scope is also advisable to ensure that there isn't excessive ripple.Check the supply current on each of the rails if there is a convenient access point e.g. a removable fuse or link.Check that the clock source to the 68000 is operating and has good logic levels conforming to the requirements in the datasheet.Check that the RESET* and HALT* signals to the processor are correct with good logic levels and timing.Check for activity on the address lines and address (AS*) and data strobes (UDS* and LDS*). Note: some address lines may be not be active depending where the program is executing.Check the address decoding logic is working and that a chip select signal is being applied to the boot EPROMs.Check for activity on the data bus from EPROM reads. Check logic levels are good on all pins and timing is correct in relation to the data strobe signals and DTACK* signal.Check for accesses to the RAM showing good logic levels and timing for both read and write cycles.

author: Bud
author_rank: Super Contributor
timestamp: August 06, 2023, 01:53:35 pm
content: Quote from: galaga1982 on August 02, 2023, 01:00:11 pmIf a PCB has a hard short you can power it with a current limiting power supply (set to 0.5...1A current limit) and spray with alcohol. Watch where on the PCB alcohol evaporates quicker. That will be the warmer location.