|
I was able to connect a good TV to the video out as well as the standard TV channel RF output (tried separately) and the pattern generator seems to work fine.
Lastly I moved on to the programmable 30-50MHz IF output which was off frequency and drifting. By the way the VA62 frequency display is the intended output not the actual measured output. The board that contains the microcontroller for the RF and IF outputs is called the PLL (Phase Locked Loop) board. The board in my unit has a small piggyback (daughter) board that subs for a 8 pin op-amp IC. The small daughter board contains Surface Mounted Devices, one of which is an op-amp. This op-amp was running very hot, I suspected it was shot. I ordered a new one and installed it but it did not solve the frequency problem. PLL circuits are tricky to troubleshoot because, when they are working right, much of the circuit is in a self correcting feedback loop.
One good way is to "open" the loop. I cut the wire that fed the voltage output of the op-amp I replaced to the VCO (voltage controlled oscillator). I used a set of molex pins and some heat shrink tubing to make a connector to re-connect the wire later. By opening the loop I found that the VCO was not responding to the voltage. The VCO is in a small module on the RF board. You have to remove two screws and un-solder a wire that comes up in the middle of the board in order to remove the VCO. Everything I could test on the VCO tested OK. I cleaned the five position connector and re-installed the VCO, re-connecting the ends of the wire I has cut. That fixed the problem! The connector had been dirty or oxidized.
|