Fostex 280 firing on 3 cylinders

[falderguitars]

Hi all,

Been a while since I posted here - hope this is the correct forum...

A long shot, I've been fixing up a handful of old Fostex Model 280's. Got most of them up and running (excluding the worst which I've stripped for parts).

The problem unit only works on 3 tracks - track 1 is dead. No audio comes through, the meters don't even flutter. However, I can record onto track 1 from any of the other 3 tracks and listen back to the audio through aux 1, so I know it records.

On top of that, if I play back track as normal, I can crank the input fader and trim to max and head a faint sound. It's loud enough that I can test the pan pot/fader/trim/EQ which appear to be working. I've also swapped the mixer board out into functioning units and the problem remains, so I'm confident it's the mixer board which is at fault.

All other track function, as do the aux sends/returns/master fader etc. According to the block diagram, the signal splits via the track/mix switch post EQ. It should pass through the track 1 pan pot, on through the master fader, through the relevant circuitry (which is all located on another board) to the tape heads. But it doesn't...

I've tested the continuity of the switches and faders, all are fine. I've followed the signal path for continuity as well as I can with no available schematic, can't find any breaks or dodgy compnents. Input jack, fader and trim pot are also fine.

Here's a link to the block diagram:



Any suggestions? Have I missed something obvious?

Thanks.

[kslight]

On Tascam 388s I have solved this issue multiple times by replacing the fader. Don’t know about fostex.

[jacksaturn]

I run a business repairing cassette-based multitrack recorders, and I've worked on a good number of Fostex 280's. My guess would be either a bad fader (as the other commenter suggested) or a failed op amp near the fader. It's a shame that schematic isn't more detailed! (I just checked the service manual and that simple block diagram is unfortunately all they provide.)

Jack

[falderguitars]

I run a business repairing cassette-based multitrack recorders, and I've worked on a good number of Fostex 280's. My guess would be either a bad fader (as the other commenter suggested) or a failed op amp near the fader. It's a shame that schematic isn't more detailed! (I just checked the service manual and that simple block diagram is unfortunately all they provide.)

Jack

Hi Jack,

Thanks for the input. The fader and trimpot both tested fine on my meter, but I went belt and braces and replaced them with spares I know are working, but the channel is still dead. Ditto the input jack.

A faulty op amp seems a good bet, I'll try swapping the gain op amp for one I know works.

However - as I mentioned - when I crank everything up on playback, I can very faintly hear the signal on track 1 (having bussed it there from another track during recording). All the controls appear to work as expected (fader/trim increase/decrease signal levels, pan pot pans, EQ EQ's, etc). So it would seem that the problem lies elsewhere... I think the signal is passing through the mixer as intended, but not leaving the mixer board. Unfortunately the board is so complicated my efforts to trace where exactly the signal leaves the board have come to naught. It's got me stumped!

The block diagram from the manual is useful, but only to a point. There is a service manual for sale online, but the description says the component values on the schematic are illegible. I'll probably bite the bullet and buy one in the end, but I'd rather not spend £15 on a useless PDF.

Thanks again,

Matt.

[jacksaturn]

Matt,

I acquired a service manual for the Fostex 280 directly from Fostex, and the block diagram was all that was there - no schematic. I don't believe Fostex provided any more detail than that, circuit-wise.

Without having a 280 open in front of me, I can't be of much specific help, but do keep in mind that there are a lot of op amps in that machine at various stages. So while you might find that the pre-fader op amp is fine, there might be a post-fader one that is the cause of the problem. I've definitely experienced op amp failures where I could still hear faint or crackly/distorted signal like what you're describing.

Jack