My school's "new" 100A went down this week. The instructor/engineer said the
levels dipped (all of them) a couple of times before they went "out"
altogether. He checked the machine room and found a burned resistor smell
coming from the deck. I checked it out and found that the levels were down
about 20dB on all channels, but they are there. The levels were equally low
(and highly distorted) on Sel-Rep and Repro, if that tells you anything.

Chasing the signal flow (and the smell), I found a connector (CN3) on the
power supply at the rear of the unit, the Dolby SR supply, was charred at
one end (three wires burned off at the connector). The folks at Otari
identified the power supply as the Dolby unit and recommended pulling the
Dolby circuits from the signal path. That seemed to recover the repro
levels, but the +20V audio supply was about 6V low and the unit continued to
record and playback with significant levels of distortion (4% at all
frequencies). I pulled the power supply and, as usual, the +20V "magically"
came back. However, that hasn't resolved my distortion problem. The
auto-cal feature seems to be backing up my reaadings, in an odd way, since
the unit continually fails to be able to set record EQ to its own
satisfaction. The unit reports the HiEQ compensations setting fails spec,
for random (but most) channels.

Since schematics for the Dolby components seem to be unavilable, I'm stuck
for a solution to this problem. I suspect there is more to disabling the
Dolby than I've so far found in the Otari manual. I also suspect lightning
or a power surge caused the original problem, though the storms we had were
the night before and the unit played on for a few hours before taking a
dive.

Any advice would be appreciated.

Thomas Day

Wirebender Audio Systems



http://www.wirebnder.com

"T. Day" wrote:

My school's "new" 100A went down this week. The instructor/engineer said the
levels dipped (all of them) a couple of times before they went "out"
altogether. He checked the machine room and found a burned resistor smell
coming from the deck. I checked it out and found that the levels were down
about 20dB on all channels, but they are there. The levels were equally low
(and highly distorted) on Sel-Rep and Repro, if that tells you anything.

Chasing the signal flow (and the smell), I found a connector (CN3) on the
power supply at the rear of the unit, the Dolby SR supply, was charred at
one end (three wires burned off at the connector). The folks at Otari
identified the power supply as the Dolby unit and recommended pulling the
Dolby circuits from the signal path. That seemed to recover the repro
levels, but the +20V audio supply was about 6V low and the unit continued to
record and playback with significant levels of distortion (4% at all
frequencies). I pulled the power supply and, as usual, the +20V "magically"
came back. However, that hasn't resolved my distortion problem. The
auto-cal feature seems to be backing up my reaadings, in an odd way, since
the unit continually fails to be able to set record EQ to its own
satisfaction. The unit reports the HiEQ compensations setting fails spec,
for random (but most) channels.

I don't know anything about the MTR100 but if I was in your situation I
would remove all the audio cards, check the supply voltage and then
replace the cards one by one, checking the voltage as I added each card.
I would assume that you have a dodgy regulator on one of the audio
cards. I wouldn't try running any of the automatic calibration stuff at
this stage because you could well cause problems.

Hopefully Chris Notton will chime in soon with a properly informed
answer though.

Cheers.

James.

T. Day wrote:
"James Perrett" wrote in message
...

I don't know anything about the MTR100 but if I was in your situation I
would remove all the audio cards, check the supply voltage and then
replace the cards one by one, checking the voltage as I added each card.
I would assume that you have a dodgy regulator on one of the audio
cards. I wouldn't try running any of the automatic calibration stuff at
this stage because you could well cause problems.

Yep, I've done that and found nothing "interesting." All cards, now, are
providing the proper audio voltages and the +/-20V supplies are holding
steady. I have no idea what drug it down originally.

Start inspecting decoupling capacitors, even little tantalum ones.
Check for even small cracks around the leads or goo coming out.

If the Dolby supply went and the Dolby cards threw a large DC offset
onto the record or repro amps, it could have damaged something there and
started pulling the supply down.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."