The first step in all troubleshooting is to determine what is or is not working. The following information should be used when the set is totally dead or nothing happens when the power button is pushed.
- Once the main fuse has been eliminated as a potential problem, measure the +1.8Vs supply at IP551 on the SSB board: The +1.8Vs voltage is the main supply for the main microprocessor, IR001. Without it, nothing will fire up. If missing, troubleshoot the Standby Power Supply.
- Once the +1.8Vs supply has been confirmed as operational, check the clock (pin 6) and data (pin 5) lines to the main NVM, IR005.
In the first 100mS after AC power is applied the CLOCK and
DATA lines must rise to very near +5V and have at least few cycles of data.
This indicates communication between the main microprocessor and NVM (EEPROM).
Note only a limited amount of Data is transferred when AC is applied.
Communications
will cease until the power ON button is pressed to start the
chassis. However, the short amount of data transfer between the microprocessor
and NVM is a good indication that communications between the main microprocessor,
ROM and RAM were successful and the main microprocessor is alive. From observing clock and data activity,
there are two failure indications. First, if there were no signs of data or
clock suspect the main microprocessor is defective. However if there is
constant communications it indicates the microprocessor is trying to
communicate with the NVM and cannot. Suspect a defective NVM (EEPROM). In
either case the SSB should be replaced.
NOTE: In some cases it may be required to check the communications
again. To do so requires the main micro be in a zero voltage state. Be aware
the mains doublers can hold a significant voltage for a long period of time. AC
must be removed for at least several minutes to give the +1.8Vs supply time to
completely bleed off. To save time the +1.8Vs supply may be bled by shorting
CP555 on the SSB module. Check the
+1.8Vs supply to make certain it is less than +0.2V before attempting to
reapply AC power.
RESET CAP: LOCATION
INF_PWR_FAIL
Power Fail (INF_POW_FAIL)
Another useful preliminary check is the INF_POW_FAIL
voltage located on the PSD board. The voltage monitors an unregulated rectified
output from the +7Vs winding. This can be located on one end of RP231 as shown.
During normal operation this voltage will be less than -1V and normally stays
around -2V. INF_POW_FAIL can indicate severe loading problems on the standby
supplies. Absence of a negative INF_POW_FAIL signal will not allow the chassis
to start.
ITC222 Power Board & Small Signal Board Cross-Reference.
POWER BOARD-BAR CODE LABEL LOCATION
SMALL SIGNAL BOARD [SSB]: BAR CODE LABEL LOCATION
