All of these problems are par for the course. One tackles them as they come, normally to a successful conclusion. But that doesn’t always happen.

Mrs Shirley brought in her 1990 Panasonic NV-F70A VCR which had been involved in a lightning strike, along with her TV set, which was dead. This VCR had the unusual symptom of not having any sound in the E-E (tuner) mode. It could play and record sound via its AV inputs but there was nothing coming from the tuner – neither was there any stereo LED display.

Being a stereo machine, the sound IF is taken from IC701 (M51366SP) on the TV demodulator pack and fed into the audio decoder where it is split into 5.5MHz and 5.74MHz signals before going to separate sound IF amplifiers and FM detectors. The outputs of these are then fed into IC7301 (TDA3803A), which is the Zweiton stereo decoder.

Knowing how something works is one thing; fixing it, another. Unfortunately, I have no equipment that can accurately indicate whether the sound IF is working correctly although I do have an RF probe that just gives a lot of noise and there is the CRO, which shows the same.

Fig.1: part of the sound IF circuitry in the Panasonic NV-F70A VCR. All the symptoms suggested that IC701 was the culprit but in the end, there was no way to be sure.

The decoder module is almost totally inaccessible when soldered into the main CBA (Circuit Board Assembly VEP1353B). All I could do was measure the inputs and outputs of the module but that indicated only that the +12V was present and that the mute Audio Off was in the right mode. And before anyone asks "what about the simulcast mode switch", yes, I checked that first and it was OK.

At this stage, I felt that the decoder IC (IC7301) could be faulty and decided to change it. Doing this meant removing the entire module before unsoldering and replacing it with a new one. This all took time and patience and as is my wont, I scrubbed the module with methylated spirits to remove any unwanted flux and check for faulty joints. Everything looked fine and eventually I was able to solder it back into the main CBA. I also checked for faulty joints on the demodulator module, especially around the metal screening cans where they are soldered onto the board.

I switched it on and was disappointed to find it still didn’t work. There were two possible courses of action now. One was to check the +12V rail and any other rails for ripple that might be affecting associated circuits; the other was to check the sound IF. I had already checked the +12V and +11V rails associated with the voltage stabiliser circuit, involving Q7305, on the decoder module and they were spot on.