Potomac Instruments FIM-21 - Troubleshooting and Repair

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SECTION 4. MAINTENANCE (Cont.)
4.8 Preventive Maintenance (Cont.)
Thoroughly remove dust and dirt from the wells around the antenna
contact plungers on top of the case, and from the fixed contact insulators
on the cover. The insulators for both sets of contacts wi ll appear white
when clean. The we l ls are best cleaned with a cotton swab (
11
Q-Tip
11
) or
similar article. If the contacts should get wet, dry them as soon as possible.
Wipe any dust from the antenna contacts with a soft cloth. Do not
use an abrasive (such as a pencil eraser) on them, as this will remove the
gold plating.
When cleaning, inspect the unit for loose hardware and physical
damage, and correct as necessary.
4.9 Troubl es hoot ing and Repair
If the FIM-21 or FIM-41 appears to be functioning improperly, or
not at all, s yst ematic checks may be made to try to find the problem, at first
wit hout openin g the unit. If the unit must be opened, internal components and
connections can be iden tifi ed by using the Component Locat i on Diagrams, Figures
4-2 and 4- 3, and the Schematic Diagram, Sec. 5.1.
Make checks as fo l lows:
1. Check the setting of all switches, especially the RF INPUT and
METER switches.
2. Check the battery voltage, with AUDIO and LIGHT switches on if
they are normally used.
3. Determine whether the unit can be calibrated using the normal
procedure. If no FI reading is ob tain ed with the calibrating oscillator on
(FULL SCALE switch at CAL), either t he receiver or oscillator may be faulty .
If an FI reading between 3 and 5 is obtained but no null can be obtained, the
null circuit is probably faulty. The FI reading due to t he oscillator should
not vary more than 30 percent across the full frequency range; a large varia-
tion indicat es an RF tu ned circuit problem, which n ormally requires return to
the factory. If the reading cannot be peaked within the range of the CAL osc
control, the calibrating oscillator tuned circuit needs adjustment.
4. Determi ne whether the FIM gives normal FI readings on several
signals of known strength. This indicates whether the receiver is working
properly.
5, Determine whether switching the AUDIO and LIGHT switch in and
out affects field strength readings. If so, these circuits may be faulty,
drawing excessive current, and the FIM may read correctly if they are left off.
4-5