DBX problem on Tascam MS16

J

James K

Member
Hi,

I was mixing a song yesterday when I found that track 16 on my MS16 wasn't putting out any audio, despite the VU meter moving. I found that playback was fine with DBX disengaged but it cuts out when I switch the DBX on. This is a problem since the track was recorded with DBX and therefore sounds hissy and strange being played back without.

Any ideas?

Thanks,
James
 
I'm not sure how the noise reduction switching is done on the MS16 but on other machines I've serviced NR switching is done using FETs or electronic switches like the 4066 CMOS chip. Hopefully one of those has gone wrong because they are fairly easy to replace. Presumably the NR is done using a dBX chip - if this has gone wrong it could be harder to find.

It is possible to decode dBX in software but I guess you want to stay in the analogue domain. If so, you may need to start looking for an external dBX box if the problem lies with the chip.
 
I can't find the schematics for the DS-8DX, but it seems to use cards for pairs of channels. So you can try swapping the card to a neighbour and see if the problem moves too.
There is one such card on ebay but it does only seem to have a 5218p op-amp and a couple of single inline components for ICs so it's possible that the switching is done elsewhere.
It is entirely possible the DBX is switched in and out on the channel cards in the MS16 itself (I can't find schematics for that either) so it is also worth swapping audio cards to see whether the fault lies in the DBX card, the channel card or somewhere else entirely.
 
I had a maybe similar sitch with an MSR-16S (Dolby S not DBX and different machine) I bought off eBay years ago and after pulling the cards and cleaning it, the tech place (TASCAM support praised them but forget the name) they either couldn’t or didn’t know how to fix the problem. Mine was acaross several tracks though that Dolby wasn’t working
 
Seeker of Rock said:
I had a maybe similar sitch with an MSR-16S (Dolby S not DBX and different machine) I bought off eBay years ago and after pulling the cards and cleaning it, the tech place (TASCAM support praised them but forget the name) they either couldn’t or didn’t know how to fix the problem. Mine was acaross several tracks though that Dolby wasn’t working
Click to expand...

Some of the Dolby S chips are known to be unreliable and spares just aren't available. Later examples of the Fostex G series machines suffer from the same problem when they started using the single Dolby chip rather than the 6 chip module.
 
If all else fails, you can buy a used dbx I unit and patch it in. The stand alone 150 would work just fine.
 
TalismanRich said:
There is a site that has free copies the DX8 and 16 schematics. Two different ones, depending on serial #.

Free TASCAM Semi-Pro Tape recorder Schematics and Manuals - from AnalogRules.com and AudioSchematics.com

Free TASCAM Semi-Pro tape Recorder manuals and schematics
www.analogrules.com

The rar file is password protected, but if you read the page, the password is AnalogRules.COM. It is a PDF file.
Click to expand...
Thanks, that MS16 schematic is the most useful one as it includes the DBX schematics.

From that, I can see that the switching is done inside the DBX unit, and that the two SIP ICs as seen on the ebay board are in fact the VCAs used for the DBX process. I had hoped those were resistor packs or something. If those chips are the reason for the failure it may be difficult to replace them. The OP would be advised to snatch that DBX card on ebay if it's still available.
However it would still be worth swapping cards over to make sure it is the card and not some fluff in the DBX switch or something (this did happen on my TSR-8).

Given suitable tooling it may be possible to trace the fault on the card, e.g. by feeding a test waveform into several channels and using a 'scope to compare the good and bad channels to find the point at which the signal goes away. But replacing the card is a quick option (and and external DBX module for that channel is, as has been mentioned, an option of last resort)
 
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