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Fostex A8 Faulty Tracks
Old 13th January 2020
  #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 4 weeks 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 3 weeks ago
  #3
Lives for gear
 
ruffrecords's Avatar
The IC following Q16 is a comparator. One input is the amplified and rectified signal from the capstan motor tacho. it is compared to a voltage provided by the vari-speed board. Every time the tacho signal exceeds the vari-speed dc input the comparator triggers and sends a pulse to charge capacitor C27. it is this voltage that controls the actual tacho speed. So check the vari-speed board actually gives a variable dc output and check the comparator operated and C27 charges.

Cheers

ian
Old 2 weeks ago
  #4
Gear Head
 

Quote:
Originally Posted by ruffrecords View Post
The IC following Q16 is a comparator. One input is the amplified and rectified signal from the capstan motor tacho. it is compared to a voltage provided by the vari-speed board. Every time the tacho signal exceeds the vari-speed dc input the comparator triggers and sends a pulse to charge capacitor C27. it is this voltage that controls the actual tacho speed. So check the vari-speed board actually gives a variable dc output and check the comparator operated and C27 charges.

Cheers

ian
Hi Ian,

Thanks for the reply.

I recapped all of the ceramics within the capstan drive/varispeed circuit and that did the job. Varispeed and capstan work perfectly now.

The Fostex is pretty much there, minus a couple of tracks that don't seem to record/pick up signal properly.

Thanks again,
Will
Old 2 weeks ago
  #5
Lives for gear
 
ruffrecords's Avatar
I assume you have given the heads a thorough cleaning? I had a Recox A77 once that had one low level track. There was a hard to see speck of ground in grime that took a lot of cleaning to remove. After that it was fine.

Cheers

Ian
Old 2 weeks ago
  #6
Lives for gear
 
ruffrecords's Avatar
Just another thought, the first Fostex R8 I had worked fine for a while then part way through a session Track 5 died. Took me a long time to find the fault but it turned out to be a broken track on the main PCB the channel board plug into.

Cheers

Ian
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