Otari MKII: crackle on channel 24?

[Tragabigzanda]

Hey all. Bought my Otari MKII 2" machine about 9 months back. We're winding up our 4th session on the machine right now up in VT, and we're experiencing quite a bit of crackle on the 24th output of the machine. This is actually the first time we've used track 24, so we're a little stymied. We've done all the obligatory testing, and have isolated the problem to only track 24 output. Any ideas what this might be? Perhaps because #24 is frequently used for SMPTE, could there have been some damage to it? Should I look to the card, the output, the head stack? Any experience with this before? It sounds not unlike a dirty pot on an old console. Any input is appreciated.

[klangtone]

I have an Otari 8 track and I was experiencing that same sort of crackle thing on track 5 and 6. I swapped out the Rec EQ Amp PCB on those channels and it seemed to fix the problem. I haven't debugged the bad cards yet, but I bet it's a failing capacitor or something.

Roy

[Tragabigzanda]

Right on, Roy. We actually left VT without getting to swap in another card to test, but I'll be sure to check it out next time I'm there. How do you debug an entire pcb board?

[klangtone]

I would start off by looking for any obvious problems usually in the form of burnt components or capcitors that have oozed out their dielectric. If you find anything like that, replace the part(s).

If everything looks ok you have to get a hold of the schematic for the card. Figure out which edge pins are power and ground and get a bench supply that you can use to provide power to the right pin(s). Figure out which pin is the input and provide a 1kHz sine wave (via a tone generator or your computer's soundcard). Using an oscilloscope, start probing the signal from the input through the entire circuit. Hopefully you'll find the place where the signal starts getting some kind of noise on it and your nice sine wave will have sporadic glitches. Stop there and replace the last component you passed in the circuit. Retest and see if you can get the sine wave clean all the way through.

That's debug 101. Hope you have success!

Roy

[Tragabigzanda]

Figure out which pin is the input and provide a 1kHz sine wave (via a tone generator or your computer's soundcard).
Roy

How would I apply a 1kHz test tone from a CD to a loose card?

[The Scum]

The thing you really need is an extender card or cable.

You remove the card, plug in the extender, then plug the card into the extender, where it's accessible for troubleshooting.

The tough part is that if you don't already have an extender, it'll be hard to track down. I could design a PCB for you, but fabricating one-off PCBs isn't exactly cheap.

[klangtone]

The more brute force method would be splice together a special cable that goes from whatever connector you have on the source audio to alligator clips. With the alligator clips, you can clamp on to the edge connector pads that correspond to the input of the circuit. It's not going to be the best connection in the world, but it should work.

A more robust connection could be made by temporarily soldering some wires to the right spots on the edge connector. When you're done debugging, you can desolder and clean up the pads with solderwick. There might still be some solder remaining, but if it's a thin coat I doubt it will be a problem.

The other thing to check on that Otari machine is the connection between the machine's electronics and the headstack. There was one or two channels that still were giving me some static even after my card swap and I tracked it down to that connection. I did a DeOxit job on it and it seemed to improve things.

Roy