Hi Brent,
Responses in BLUE interspersed below. I am also going to send you a
report on work done on the console in 2002, off list.
best,
Ike
Brent Punshon wrote:
Hi Ike, Matt and Monte,
Thanks so much for the input. Lots of ideas there. Believe me, right
about now I can use them so it's very much appreciated. No days off
here until the week after next so I'll be doing a bit each night post
session. I'm preparing myself for a slow and frustrating hunt on this
one. Sounds like I was vaguely on the right track at least.
To respond to Ike's questions:
-Do you only hear the noise when a solo is engaged?
Yes.
OK, then this implies that the fault is on the stereo solo buss, if
Matt and Monte are on the right track. This signal is on the lowest
ribbon cable and is derived post Mix Pan thru a TL0-74, U-18. It goes
through the pan solo switch, through 20k to the buss...doesn't go
through any FET's at this point. The same pair of outputs from U-18
are run up to the top part of the module through a pair of sheilded
cables (W-101, W-102). The Mute FET's for the 2-Mix signal are located
on the upper board of the module, with the buffered signal from U-18
going through the FET's Source -->Drain and the un-buffered signal of
the pan pot going to the Gate. If there was a problem with these
FET's, you couldmight hear it on the solo buss. Another thing to try
is to go through the console and engage every solo switch, one at a
time and see if you hear any extraneaous noise, pops, clicks, bangs
etc. when the switch is engaged. BEWARE THE SWITCH YOU NEVER USE! I
have had many occasions when a fault on a console turned out to be a
switch for some function that was never used. Switches can fail in the
"Out" position, too. One more note about the muting to the 2-mix buss:
Twice in the last month, I have had Elite clients call with either
intermittent, distorted, or muted (but with no Mute Led indication)
channels. In each case, this turned out to be the nut and bolt that
joins the lower edges of the upper and lower circuit boards together
being loose. This nut and bolt carries the ground connection from the
lower to the upper board, and if it is loose, the channel can exhibit
the symptoms described. BUT, this depends on how many wires there are
in the soldered ribbon cable that joins the two modules. Some consoles
have a 14-way ribbon, which omits the two ground connections at the
lower end, and some have a 16-way that includes them. I /think/ your
desk has the 16-way, but it's still something worth checking.
-Does the noise print to the 2-mix?
No, but the noise floor of the master left bus does read a little
higher on our DAW meters, but this noise is not audible in the way
the solo circuit noise is (eg with the the master fader up the noise
floor for the left master bus reads 5-10db higher than the right down
at -90 odd.. The DAW meters also show extraneous noise on the left
master bus when the impedance of the master fader changes (up or
down), and the settling time is a lot slower than the right side. Two
things here: 1) That master fader is long overdue for replacement.
It's an ALPS "K" fader, but a Panasonic EVB-JYBP08Y14 will work nicely
if you can find one. Or a P&G 3000 or 8000 series, of course. 2) The
last time I worked on this desk, which was a year or two before you
bought it, was for an intermittent noise on the Left mix bus. I ended
up replacing and / or upgrading a lot of components in the mix amps,
and it got better, but was still doing what you describe. Maybe it's
the fader. One test is to look at the noise floor at the 2-mix insert
send, which is pre fader (I can't remember what I saw there, and my
notes are at the shop...somewhere). If it's doing the same thing, then
it is in the Mix amp, or possibly on the buss, per Monte and Matt. I
sure like that -90db figure on the Right side!
-Are the monitors clean with another source selected?
Yes. This is only a problem when solo is engaged except for the
convoluted routing problem mentioned below.
You might also try removing U-4 on the Monitor module altogether.
If the noise is still present, but at a reduced level, than as Matt
suggested, it may be coming from a channel module. If the noise is
the same level, that would point to Q-103, the Left Solo switching FET.
With U-4 removed there is silence on the circuit.
Q103 has been changed out, as have Q101, Q102, 104, Q115, Q118 &
Q119. I would have a look at the passive components around U-4 (which
is the solo buss summing amp, called out as a 5532 on one of my
drawings, but I think I have seen TL-072's in there, too). It's
possible that one of those resistors has gone bad, or was never
soldered in properly to begin with...found many of those over the
years. Likewise the 10uf BP cap on the input to the chip, as it is
included in the feedback loop. Bear in mind, too, that if U-4 is
having problems, you will only hear it when a solo is engaged. If you
can get a scope on U-4, you will be able to see if it is noisy all of
the time.
Matt's last email has triggered my memory with regard to mentioning
hum. We recently attempted some mixing to stems out the first 16
busses, summing in the box and bringing the DAW 2-mix back into the
control room extra input. This created ground loop issues but for the
first hour or so there was also a god awful hum when monitoring the
return to the control room extra input. (sounded like a ground was
missing and a lot like transformer hum). It slowly dissappeared.
Tightening all the screws on the power supplies, the ground bus,
individual module screws and cleaning the bucket frame and the
underside of the modules had no effect. What happens when you plug
something else into CR Extra? Was the signal going INTO the DAW clean?
Have you checked the ground strap to the motherboard, and the
grounding on the powe inlet panel? That's all for me for now...
Best regards,
Ike
I'm starting to see a pattern. Could this hum be a product of the same
short and also be an indicator that the short isn't necessarily close
to the solo amps?
Thanks again all, I learnt a lot from those few emails. I'll come back
after I've done a bit of prodding.
Brent Punshon
Head Gap
Melbourne, Australia
Ph:+613 9480 6280
www.headgap.com.au
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Ike Zimbel
Zimbel Audio Productions Ltd.
Toronto
416-720-0887
"Studio House Calls" - Repairs and upgrades to analog audio equipment including:
Amek, Allen & Heath, Neve, Neotek, MCI, Soundcraft and more.
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