M850-00
T858/859 Fault Finding
D4.3
Copyright TEL 31/09/98
4.1 Visual Checks
Remove the side cover from the T858/859 and inspect the PCB for damaged or
broken components, paying particular attention to the surface mounted devices
(SMDs). Also check for defective solder joints.
Refer to Section 4.7, Section 4.8 and Section 3 of Part A for more details on repair
and replacement of components.
4.2 Component Checks
If you suspect a transistor is faulty, you can assess its performance by measuring the for-
ward and reverse resistance of the junctions. Unless the device is completely desol-
dered, first make sure that the transistor is not shunted by some circuit resistance. Use a
good quality EVM (e.g. Fluke 75) for taking the measurements (or a 20k ohm/V or bet-
ter multimeter, using only the medium or low resistance ranges).
The collector current drawn by multi-junction transistors is a further guide to their per-
formance.
If an IC is suspect, the most reliable check is to measure the DC operating voltages. Due
to the catastrophic nature of most IC failures, the pin voltages will usually be markedly
different from the recommended values in the presence of a fault. The recommended
values can be obtained from either the circuit diagram or the component data catalogue.
4.3 DC Checks
Note:
No RF power is to be applied during these checks.
Check that +13.8V is present on the collectors of Q3, Q4, Q5, Q6 and Q7. Make
this measurement when the transmitter is not keyed.
Check that approximately 12-13V is present on the collector of Q1 (in the T859 the
level is dependent on RV69 being set to maximum).
T858 Only Check that +13.8V is present at pin 4 of IC1 and pin 8 of IC3.
T859 Only Check that +13.8V is present at pin 4 of IC3.
Check that approximately +12V is present at pin 4 of IC1 (the
level is dependent on RV69 being set to maximum).
Check that +7.0V is present at the output of regulator IC2.
To Next Page
Loading...
Need help?
General
