To short out the uP to remove any remaining charge that may be locking the logic in an unwanted state requires that you first ensure that no significant voltage remains on the board. That means first ensuring all rails are nominally at 0.5 volt or less. Also make sure that the battery is not fitted.
If these conditions are met then get a piece of aluminium foil (cooking foil/chocolate wrapper kind of stuff) and press that firmly onto the pins of the chip so that they are all shorted out. Leave it like that for a couple of hours. Then retest.
Its a long shot, but one that has definitely paid off numerous times in a busy workshop environment.
If these conditions are met then get a piece of aluminium foil (cooking foil/chocolate wrapper kind of stuff) and press that firmly onto the pins of the chip so that they are all shorted out. Leave it like that for a couple of hours. Then retest.
Its a long shot, but one that has definitely paid off numerous times in a busy workshop environment.
I've got the foil pushed on. I'll leave for a few hours as you say, and then put it back together and switch on.
In the meantime, study the schematic in the service manual and look out for the reset circuitry in particular, as well as any power sources involved. Stuff is not going to work if the µP is stuck in reset (usually because some electrolytic cap is leaky/shorted) or some chip does not receive power because the voltage regulator is dead.
I've put it back to gether again, powered it on and its the same as when I started the thread.
Unfortunately I dont know enough about electronics to make much sense of the schematics to know where any reset circuitry would be.
Would it be worth removing some key capacitors to check their capacitance?
Unfortunately I dont know enough about electronics to make much sense of the schematics to know where any reset circuitry would be.
Would it be worth removing some key capacitors to check their capacitance?
Never replace parts in hope as that will just lead to more problems...
OK, so I've finally looked at the service manual.
1/ Check the 4.5v supply on pin 42 of IC501.
2/ The reset generator that is on pin 18 of IC01. Check for 0 volts here.
3/ Check the 5 volt regulator Q011 is giving the correct voltage.
4/ Check the 4Mhz oscillator on pins 19 and 21 of IC01 is running (scope).
5/ Check the 7,142 Mhz oscillator on pins 40 and 41 of IC501 is running.
6/ Check all outputs of voltage regulator IC701.
7/ Check pin 30 of IC501 (remote in) to make sure the pin isn't being held low by the remote receiver.
I still think it is worthwhile checking the multiplexed keyscan to the keyboard in case there is some issue here that is inhibiting the microprocessor. You can isolate this by lifting R531 through to R534 and D507 through to D511.
Do all the voltage checks first.
OK, so I've finally looked at the service manual.
1/ Check the 4.5v supply on pin 42 of IC501.
2/ The reset generator that is on pin 18 of IC01. Check for 0 volts here.
3/ Check the 5 volt regulator Q011 is giving the correct voltage.
4/ Check the 4Mhz oscillator on pins 19 and 21 of IC01 is running (scope).
5/ Check the 7,142 Mhz oscillator on pins 40 and 41 of IC501 is running.
6/ Check all outputs of voltage regulator IC701.
7/ Check pin 30 of IC501 (remote in) to make sure the pin isn't being held low by the remote receiver.
I still think it is worthwhile checking the multiplexed keyscan to the keyboard in case there is some issue here that is inhibiting the microprocessor. You can isolate this by lifting R531 through to R534 and D507 through to D511.
Do all the voltage checks first.
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