Fostex G16 (with Dolby C) issue

[petereugenefoley]

Hi all,

I'm new here and am in dire need of help. I have a Fostex G16 (with Dolby C, not the Dolby S model) and I messed it up yesterday like the imbecile that I am. I tried to measure the master bias frequency on the J type connectors on the R/P cards ("measure the bias frequency at points J 2-7 per the manual), but I must have accidentally crossed the multimeter with the wrong connecting pins because I saw a little spark and when I put it back together, it no longer recorded or played any audio. The transport works great and the input monitoring functions are flawless, but it is silent when it comes to recording and/or playing anything back. I have a feeling it has to do with the mute line on the Connector PCB, but I am otherwise very lost. Can anyone lend a hand?

Here's a link to the service manual: http://www.miroslavmusic.com/oddsnends/Fostex_G16C_ServiceManual.pdf

 

[gyraf]

Having sparked, check and verify all power supply voltages...

Jakob E.

 

[petereugenefoley]

Jakob -

At this point, all channels play back audio, but only 9-16 record audio. I believe that the power supply is not the issue since this is the case and because input monitoring works on all channels. The headstack is either erasing but not recording, not getting bias or not erasing. Thoughts?

 

[JohnRoberts]

If you have some channels that record and some that don't I would be suspicious of the bias oscillators for those channels. Probably uses a master oscillator and individual buffers.  Compare the channels that work to the one what don't. with a scope if you have one (bias frequency is probably >80kHz  ) or even a multimeter for outlier node voltages.

The Dolby C is probably handled by a large IC... If the the playback side audio works, the record side audio is probably OK.

JR

 

[moamps]

At this point, all channels play back audio, but only 9-16 record audio.

Check out osc/bias amp for 1-8 channels. Page 88, transistors Q17 and 19.