ROSS PC-8400 powered mixer

[McBarry]

Hi, these seem rarer than rocking horse poo...

can anyone assist by chance ??

1989 vintage, 8channel desk, 200W +200W p/amp..
I've a hum in ch 6 and it's eluding me.. really don't wanna trace out the circuit if avoidable..

Thx heaps, David

[Enzo]

I don't know the unit, so these ideas are generic.

Are the channels on separate boards< or one larger board?

Can I assume the circuits are based on op amp ICs rather than discrete transistors? If so, you can probably identify visually which ICs associate with that channel. Check the power pins on each IC to make sure the power supply voltage is clean and present, compare to the next channel over. Then check the output pins on the ICs for unwanted DC offset.

Assuming it has XLR inputs on it, are there two or four small transistors up near the XLR for each channel - even if the rest of it is ICs?

The channel hums, does it matter if there is something connected to the input or not?

With no input, do ANY of the channel controls affect the hum in ANY way? i.e. do the gain or level control alter the volume of it? Do the tone controls affect the tone of it?

It appears to be stereo, so can I assume the hum comes out left AND right if panned?

Any AUX or FX sends - is the hum on them too from this channel, assuming the sends are turned up on the channel?

While I too like the schematic, we may not need one.

[McBarry]

Thx for lightning reply Enzo,

U assumes 100% - 2 x 720D opamps and 2 x A970 and 2 x C2240 trannies. !!
It's a +DC hum, worsened by volume slider but small residual if slider at zero - I'm not home so can't recall actual readings but they were small mV kinda range.
It's upstream from the op-amps (pulled last night, DC still at their input pad), clean + & - 14.96v supply. AC ripple between zero and 0.001mV on DMM - not CRO checked.
This is part of the circuit - seems to be accurate.. From a Russian forum..
R 314 and 310 will isolate both tranny pairs.. will advise..


Thx again, DB

[oc disorder]

Hi McBarry , Sorry I only found the smaller rocking horse *** by chance in reply to your inquiry here

http://music-electronics-forum.com/t30877/

The mixer was fairly simple , I wouldn't be surprised if it was the same.

I'll post the full manual for reference (1.4megs).

[McBarry]

Thx OC..
yr particular rocking horse is in a slightly different paddock - mine is the desk style rather than the block style of the 6400..
Where in Oz are U - I'm in "almost-snowing" Canberra..
Dave

[Enzo]

Yours is shaped different, but the circuits could be very similar. Oops, maybe not...

If the ICs have good power right at their power pins, good.

let me understand, if you pull the first op amp IC301, you still have DC on pins 2,3,5, or 6? If so , it LOOKS like C312 and C313 ought to block DC from the transistor input section. Remember you have a good working circuit in the channel right next to this one, so use that as voltage reading guides. One of those two caps - or both I suppose - could have dried up and gone leaky. I'd lift them to see if the rest of the circuit, the op amps , is OK.

It looks like IC301-7 is DC coupled to IC302-6, so DC in the first op amp can propagate to later ICs.

[McBarry]

Apols for tardy delay after great ideas/insight/help from U guys..
Quick update, as I haven't had much time to return this to the bench..
A re-check of supply to IC's shows a strange ripple on -ve. "Strange" as in slowly rising AC starting at 0.1vac, rising approx 0.01vac every 3-5 secs. Found it courtesy of discovering my CRO lead dead, so haven't visualised it yet.. And discovered that when I realised I checked ripple on wrong pin... duh.... Bifocals are pissing me off bigtime..!!
DC is rock steady -14.92v. Filter cap visually OK but not pulled to check or replace yet.
However, that ripple doesn't cause any hum on remainder of channels or on summing board.
Will be back next week and still leaning towards the 4 transistors as culprits. Def only an instinct tho, no evidence as yet..
Cheers, Dave

[oc disorder]

One minor point perhaps we should have covered is the 48v phantom power to each channel.
I presume you can turn that off .. the Ross PC 6400 has a switch . Here's one quick explanation of phantom power.
What Is Phantom Power? - A Mini Guide To This Common Feature
It's basically a clever trick to feed dc power to a microphone some distance away via the same wires that return the signal
however the DC is "invisible" to the audio signal hence the term "phantom" !!!
R301 & 302 two 68k resistors connect the phantom to your mic input and there's a 50uF smoothing cap there too !
The phantom power is between those 2 signal carrying balanced input wires and ground with C202 and 303 (100uF's)
blocking the DC from the balanced input so it only goes to the mic's. Phew !

If the switch is off there is no chance the phantom is faulty .. I just thought should expand on the phantom mystery !

And YES .. those transistors are the most likely to get damaged if Ted Nugent plugs into channel 6 !

Edit: Just had a thought try disconnecting the wire that carries that 48v to that channel .. can do that by lifting one end of
both R301 and R302 or at the power supply lifting the wire that carries the 48v to all channels.

The reasoning behind it is the phantom switch disconnects one end of the AC to the phantom power circuit still leaving
a pathway to ground.

If one of the DC blocking capacitors was damaged (C302,C303) it could unbalance the balanced circuit and perhaps
create an undesirable earth loop hum.

Just a random thought.

[McBarry]

Brilliant thought OC..!! but alas no phantom power on the 8400..
C301, R301, 302 are not populated on the board.... ;(...

Not sure wot u were getting at re Ted Nugent..?? except maybe an ultra hot signal into the channel..??
FWIW, Ted used a Gibson Byrdland, orig pickups, low output by todays standards... Rarely effects.
His monstrous sound came courtesy of Fender super twins- each with SIX 6L6's.. master dimed and preamp vol to taste for venue.
Rest was controlled from gtr vol knob.

Getting into shed prully tomorow to wrestle the " Ch6 beast..."

[oc disorder]

"Not sure wot u were getting at re Ted Nugent..?? except maybe an ultra hot signal into the channel..??"
My apologies for the obscure comment and to Ted Nugent fans it's my euphemism ?? "for an excessive distorted overload signal"
well there's THD (total harmonic distortion) and TED (Totally Excessive Distortion).
Anyhow you managed to decipher it = "an ultra hot signal into the channel" .
I suspect we are shortly to hear some encouraging news !