Service manual

Shure LX2 Hand-Held Transmitter

Bench Checks

25D1006 (CG)

RF Problems: Deviation

Domestic (FCC/IC)

n There must be a good carrier to get any deviation.

n If R217 cannot be adjusted to obtain 15 kHz deviation, try to iso-

late the problem to the audio or rf section. Check for –2.2 dBv at

I118; if not, refer to the Audio section of the circuit description.

n If the level is correct at I118, check R217, C220, C227, R208,

R216, D201, L209, and C214. The value of C214 is critical to

the deviation sensitivity.

n Last of all, try replacing D201 and Y201.

Euro (ETSI)

n There must be a good carrier to get any deviation.

n If R201 cannot be adjusted to obtain 15 kHz deviation, try to iso-

late the problem to the audio or rf section. Check for –2.2 dBv at

I118; if not, refer to the Audio section of the circuit description.

n If the level is correct at I118, check R201, C201, C202, R202,

R203, D201, L201, and C203. The cathode of D201 should be

getting 5.0 Vdc bias from the 5.0 Vdc line through R202 and

R203. The value of C203 is critical to the deviation sensitivity.

n Last of all, try replacing D201 and Y201.

Low Output Power

Domestic (FCC/IC)

n The output of the transmitter should be terminated in a 50 Ω load

from I210 to ground during testing. If the unit is being tested as

a two board assembly with the rf and audio boards soldered

together, temporarily remove C240 to disconnect the battery

antenna. The rf levels shown on the schematic were measured

with a FET probe without an rf ground and are only approximate.

n Probe the output after the oscillator stage (I230). If there is

none, refer back to the “Distortion” section.

n Make sure that when C217 is rotated 360 degrees, two separate

peaks in the carrier output amplitude are produced. If there is

only one peak, make sure the C217 is the correct color (value).

Check values of L205, C225, C234, L202, C233; and check bias

on Q201. Last of all, replace Q201.

n Repeat the procedure for the second rf stage. C216 should also

be turned one complete rotation to check for two peaks in the

output power. If only one peak is observed, make sure that

C216 is the correct color (value). Check values of L204, C238,

C236, L210, C235, C222, C226; and check bias on Q203. Last

of all, replace Q203.

n Repeat the procedure for the last rf power amp stage. Check the

dc bias on Q204 and values of all parts from the base of Q204 to

I210. Last of all, replace Q204.