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Old 13th January 2020
Gear Head

Fostex A8 Faulty Tracks

Hi all,

Been bringing a Fostex A8 that had been sat in a basement for a few too many years back to life. Almost everything is functioning now, but I'm having a problem with one of the channel boards.

To clarify, the Fostex A8 only has four inputs despite being an 8 track machine, you have to switch between recording to tracks 1-4 and 5-8.

The Problem:
The problem is that the slot for the second channel board (originally where the channel board with tracks 2 and 6 sat) isn't working. By that I mean I've swapped the actual second channel board itself into another slot and it works perfectly, so the problem lies somewhere on the connector board or with the inputs.

Track 2: normal meter movement when recording, but on playback the level is very quiet and heavily distorted.
Track 6: Completely dead, no meter movement and no audio signal.

I have reheated almost all of the solder joints on the connector board, so I can't see it being that. I've also replaced the majority of the caps on the connector board. I've checked the voltage on all the logic chips, and they're fine.

I can't believe it's a problem with the phono input jacks because in that case the outcome would be the same on both tracks, but it isn't.

Does anyone have any thoughts on how I should go about trouble shooting this? I presume just following the audio signal from the phono connector until I hit a problem, I'm just finding it very hard to trace the route on the PCB...