Electron Tube Replacement Techniques

TM 11-6625-542-14-1

6-7. Electron Tube Replacement Techniques

NOTE

To prevent the discarding of good electron tubes, follow

the procedures given in a through j below when

If a replacement for a bad tube soon becomes

troubleshooting the AN/USM-108A. Refer to figures 6-3

defective, check the adjustment and condition of

and 6-4 for tube locations.

parts in the tube circuit. Otherwise, continued

Inspect all cabling, connectors, and the

tube replacement will result in only temporary

repair, and more serious troubles may result. f.

general condition of the AN/USM-108A before removing

the electron tubes.

If the tube substitution does not correct the

Isolate the trouble, if possible, to a particular

trouble, reinsert the original tubes in the original

circuit. (Use troubleshooting table 6-2 as a guide.)

sockets before forwarding the defective for

If a tube tester is available, remove and test

higher category repair.

Discard tubes when-

one tube at a time. Substitute new tubes only for those

(1) A test by a tube tester or other instrument

that are defective.

shows that they are defective.

If a tube tester is not available, troubleshoot

(2) The tube defect is obvious. For example,

by the substitution method.

the glass envelope is broken or a connecting prong or

(1) Replace one of the suspected tubes with

lead is broken.

a known good tube.

Do not discard tubes merely because the

(2) Check to see whether the circuit is

tubes have been used for a specified length of time.

operative. If the circuit is operative, discard the original

Satisfactory operation in a circuit is the final proof of tube

tube. If the circuit is inoperative, remove the new tube

quality. The tube in use may work better than a new

and reinsert the original tube.

one.

(3) Continue to check the suspected tubes,

Do not discard tubes merely because they fall

using the procedures given in (1) and (2) above, until the

on, or slightly above, the minimum acceptable value

circuits become operative or until all the suspected tubes

when checked in a tube tester. Some new tubes fall

have been checked.

If the circuit is inoperative after the procedure

near the low end of the acceptable range; yet these

outlined in a through d above is completed, continue as

tubes may provide satisfactory performance throughout a

long period of operational life at this near limit value.

follows:

Be careful when withdrawing a miniature tube

(1) Replace the suspected tubes, one at a

from its socket. Do not rock or rotate the top when

time, with known good tubes until the circuit becomes

removing it from its socket; pull it straight out. The

operative or until all the suspected tubes are replaced by

external pin and the wire lead sealed in the glass base

new ones. Note the sockets from which the original

are of two different metals that are butt-welded together

tubes were removed. If the circuit begins to operate,

where the pin appears to enter the glass. Rocking or

discard the last tube removed.

rotating the tube causes bending, which tends to break

(2) Reinsert the remaining original tubes,

this weld or causes a resistance or intermittent joint to

one at a time, in the original sockets. If the circuit fails to

develop.

operate at any time during this step, discard the original

tube last reinserted. Do not leave a new tube in a socket

if the equipment operates satisfactorily with the original

tube.

6-8