TTI TCB-1100 won’t power off
Problem
TTI TCB-1100 CB radio wouldn’t power off when the volume/power control was in the off position. The switch functioned correctly but the internal power remained on.
The power output was higher than expected and varied a lot with a small change in power supply voltage.
Diagnosis
The volume control includes a power switch, which we found was working. The switch was signalling to a switching voltage regulator enable/disable pin.
The voltage regulator was not providing any regulation, a short in the output transistor meant the power supply passed through unregulated.
Solution
Replacing the damaged regulator IC restored the correct function of the power switch. The regulator formed part of the adjustable supply voltage for the RF power stages, so the output power could once again be set properly.
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