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Service Addendum18
Output Held Low Troubleshooting (sheet 2)
Figure 3-8 Output Held Low Troubleshooting (Sheet 2 of 2)
Check IMon voltage at A1U403-7
(TP28) to common (TP9).
A1D413 Conducting
?
Yes
A
No
Yes
From Sheet 1
A1D413 Conducting
?
CC Comparator A1U403 defective.
IMon Amp A1U403 defective.
No
Yes
Check voltage at A1U408-3 and
R686 (TP36) to common (TP9).
Gross Current Limit circuit A12U409
defective.
Check A1U408, D420, loading of
voltage by Tunnel Circuits.
A1U408-3 Neg
?
A1R638 > 12mV
?
Yes
No
No
Check if supply operates OK with
no load but cannot get full scale
output with load connected.
Yes
Disconnect cables from tunnel boards
A4A1-A4A4 one at a time to see if
output rises.
Output Rises
?
Troubleshoot or replace defective
tunnel board.
Yes
Check A1U408 and D420
No
Partial output attained
?
Disconnect cables from Tunnel
boards one at a time to see if one has
little or no effect. Troubleshoot or
replace the defective board.
Check Gated +9V and Shutdown
circuit. See Test Points 18-21, Table
6-3 and Figure 6-5 Sheet 2.
Yes
No