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Fostex A8 Faulty Tracks
Old 1 week ago
  #1
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...
Old 1 week ago
  #2
Gear Head
 

Having a problem with the pitch control/capstan motor now.

The varispeed knob had never worked, so assumed it was a dud but upon testing it was fine. Problem must lie somewhere on the logic board.

Currently when the machine is on the capstan will not spin at all unless I unplug the pitch control from the logic board.. it must be completing a broken circuit when connected.

Attached a screenshot of pitch control/capstan schematics above. There aren't too many components in this subsection so must be fixable. Capstan works, pitch control potentiometer is OK.. there must be a problem in between.

If anyone has ever worked on one of these and might be able to provide guidance or point me in the right direction that would be massively appreciated! I do have an oscilloscope.
Attached Thumbnails
Fostex A8 Faulty Tracks-screen-shot-2020-01-18-23.42.34.jpg  
Old 5 days ago
  #3
Gear Head
 

Sorry for the bump, anyone got any thoughts?
Old 2 days ago
  #4
Gear Head
 

Quote:
Originally Posted by kinjack View Post
Having a problem with the pitch control/capstan motor now.

The varispeed knob had never worked, so assumed it was a dud but upon testing it was fine. Problem must lie somewhere on the logic board.

Currently when the machine is on the capstan will not spin at all unless I unplug the pitch control from the logic board.. it must be completing a broken circuit when connected.

Attached a screenshot of pitch control/capstan schematics above. There aren't too many components in this subsection so must be fixable. Capstan works, pitch control potentiometer is OK.. there must be a problem in between.

If anyone has ever worked on one of these and might be able to provide guidance or point me in the right direction that would be massively appreciated! I do have an oscilloscope.
Pretty please, any advice would be massively appreciated!
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