I'm trying to diagnose a problem with the higher bands where the VFO display will change to random frequencies while transmitting.??
Specifications for the PB-2153 are 300mV output on Pin 3, where I am seeing about 20-25 mv peak to peak.? The output is at the correct frequency for each local oscillator.? Supply voltage switches properly to each LO pin on the board, but seems low at 5V each band.? Since the collector voltages are listed (for the earlier board, very similar circuits) at 6.6-6.7 volts, 5V supply seems too low.
The board supply comes from a regulated 78L08 chip mounted on the chassis as a heat sink.? It's fed by a 12V output from the Rectifier A board.? It's regulated 8V output goes to the bandswitch segment S1A through a 22 ohm dropping resistor, then from the switch directly to the board pins for each band.? The 22 ohm resistor would be carrying about 136mA to drop the voltage 3 volts, with about .4 watts dissipated, which seems rather a lot.? Also, 5V seems too low for the circuit as described in the service manual.??
I'm about to start tracing the power from the 78L08 to see if it's actually putting out 8V, and where it drops to 5V.??
Any hints, suggestions, lessons learned etc. welcome.
Chuck, WS1L