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.

[trinescope]

Further work I did tonight makes me almost certain that the PLL chip failed in the second upconverter. Pin 14 is the output to the microcontroller that goes high when the PLL is locked. It was high in the first upconverter but not in the second. Forcing that output high made the power light stay on solid, so it does in fact need to see both PLLs locked.

I was incorrect about the OP amp; it is configured with a gain of about 3 instead of unity like I said above. This is shown in the ADF4112 datasheet for use when a wide range of frequencies is required. This is also the reason for the 30V supply. It powers the op amp in both upconverters, so they can supply a relatively high voltage to the varactor diode. This also may be why I have read of others who had the flashing power light and fixed it by replacing capacitors in the power supply. The ripple was affecting the operation of the op amps and causing the PLL to go out of lock.

[trinescope]

I was successful in repairing this modulator. It had two bad electrolytic capacitors and the PLL chip. Later on I need to do a short guide on troubleshooting the two mixer/converters. There really isn't much to them. I found two causes for a flashing power light: One or both PLLs is not locked or there is an "illegal" DIP switch setting such as the ones digit being set greater than nine.