Repairing a BT 40-550b modulator

[trinescope]

I just got one of these modulators off the 'bay as-is, plugged it in and got the dreaded flashing power light. Checking the power supply voltages found the 5 and 12V lines OK, but the 30V line was low and had ripple. Replacing the electrolytic capacitor for that supply got the 30V back to normal but still with the flashing light and noisy output. A scope on the RF output showed a signal varying wildly in frequency. I found a noisy signal on one side of a surface mount electrolytic which turns out to be the VCO voltage for the second up converter. The first upconverter seems to work OK at this time. Right now the second upconverter will not tune and is out of lock and the unit outputs a low (10MHz) signal. I can tune the upconverter oscillator with an external voltage source, so it is probably working normally. There is a buffer opamp between the PLL chip and the upconverter chip which seems to have failed since both input pins are at about a volt and the output is more like 6 volts when it looks to be used as a unity gain buffer. I believe the microcontroller needs to see that both PLLs are in lock to not give a fault indication with the power LED.

For now I have placed an order at Mouser for the opamp and the PLL chips. I should have these parts Monday since I ordered today and it's a weekend (I'm within driving distance of the warehouse and normally get orders the next day). The opamp is a TI TLE2141 and the PLL is ADF4112BRUZ from Analog Devices. Hopefully this will fix the problem and help others who run across these units with similar problems.