[Tascam 424] Need Repair Advices

[ToxicBoi]

Hi everyone ! I recently bought a Tascam 424, and I need some help to make it work perfectly again !

Overall it works fine, but here are the problems I'm having with it :
1) The "Line Output" doesn't work (haven't tested it cause I don't have the gear to now, but the previous owner warned me about it)
2) The "Monitor Output" doesn't work either
3) The L and R channels of the volume (levels ?) meter on the screen don't work. Like, they are not climbing up when I plug an instrument and follow the instructions of the manual. The 1 / 2 / 3 / 4 channels are working tho.

I suppose all the problems have the same cause, since the L and R channels of the VU meter are supposed to show the same signal that is supposed to come off of the "Monitor Output".

Here are some pictures if it can help :

Feel free to ask me any picture of a specific element ! I can also scan the diagram that's in the manual if it can help any of you

 

[SkywaveTDR]

Put all channels in safe and apply a 1KHz signal at -10dBV to each input one at a time then apply it to the two busses and test pan. You want to test each input module as they used to call them to see if someone blew out any op amps for that input. Then when all 4 inputs work then you move on to the master and other outputs tests. A AC millivolt meter and a scope would help in troubleshooting if there are missing signals. Connectors that people play with in these units can be intermittent and the soldering on the boards is not aerospace grade. many joints on connectors of reel decks I see broken by the bulls that work in China shops technicians.