M520 aquisition... Crazy??

Yes, you did. :D

I suppose I could have said "Regarding the AUX busses you're only maybe partially right..."
 
proof of life!!

Well picked up the first board this weekend and hooked it up. a few scratchy pots and faders that got better with just moving them a bit.

Pretty clean overall. I am surprised at how quiet this board is considering the age and the rep that they have for being noisy. But it really IS quiet, by most anyone's standards.:thumbs up:

Sounds great as well.:D

The only bugs are there are some missing screws and there is a biggie that I need to figure out.

The stereo A master hums like crazy!!!! :mad:

You get signal and the meters register, but it hums. It does not hum through the headphones, which I expected seeing how they are linked (I think)

Right now I am using the stereo B master which works fine, but sucks because I cannot solo with that fader to feed the monitors. In the control room I need master A.

I got some digging to do and some cleaning, But overall I am very pleased. Even all the lights work without flickering.

PURDY:D
 
Last edited:
Just a thought, would the M520 have pin 3 hot or pin 2 hot on the XLR's, some early gear had pin 3 hot (pin 2 is now standard) and I think the M520 was pin 3 hot. This could be the hum problem? Most of the time it does not matter it just reverses the phase 180deg.

Alan.
 
Alan, thanks for chiming in.

The stereo "A" master is for the control room and the stereo "B" master is for the recording (live room) It is set up so you can solo in place on "A" and engage talkback on "B". In the control room, you won't hear the talkback mic thus avoiding feedback issues, and in the recording room no one will hear a track being soloed. This could be very distracting to the talent. :D

So with master "A" not working properly, I'm forced to use only "B" and as a result have no solo function.

As far as the setup, both are on RCA jacks not XLR Each master has a pair of L/R outputs.
 
Last edited:
Look underneath. Any damage to the underbelly?

Hum through the stereo out and not the headphones will be a good clue as to the culprit. Will narrow it down at least. That means the cause is downstream of the fader and after it branches to the headphone amp. The hum is consistent regardless of the fader position, correct?
 
sweetbeats said:
The hum is consistent regardless of the fader position, correct?
Click to expand...

Yup. even with the fader pulled all the way down. You plug a jack in and instant hum on all 4 RCAs

I'll slide it off the table to see if there is any damage on the bottom. There is a possibility of something shaken around maybe? It did a 9 hour stretch on I-5 and some of the pavement was not the smoothest in the world.
Playing around I found one more tick. but just one. the talkback mic don't work. hmmmm. Not that I need it right now in a one room studio, but it would be preferable to have everything function.

One unrelated question..........what the hell does the slate pot do?????????? test tones work.


EDIT:

Just checked the underbelly. No visible sign of damage. However, the whole ass end (jack panel) including meter bridge wiggles.Not crazy wiggle like it is falling apart, but a bit loose. Looks like someone had this apart and was not all too conscientious about re-assembly. I will investigate further.
 
Last edited:
RFR said:
Yup. even with the fader pulled all the way down. You plug a jack in and instant hum on all 4 RCAs

I'll slide it off the table to see if there is any damage on the bottom. There is a possibility of something shaken around maybe? It did a 9 hour stretch on I-5 and some of the pavement was not the smoothest in the world.
Playing around I found one more tick. but just one. the talkback mic don't work. hmmmm. Not that I need it right now in a one room studio, but it would be preferable to have everything function.

One unrelated question..........what the hell does the slate pot do?????????? test tones work.


EDIT:

Just checked the underbelly. No visible sign of damage. However, the whole ass end (jack panel) including meter bridge wiggles.Not crazy wiggle like it is falling apart, but a bit loose. Looks like someone had this apart and was not all too conscientious about re-assembly. I will investigate further.
Click to expand...

I suspect somebody had it apart trying to resolve the same problem you are having with hum. There are 6 screws that fasten the backplane and meter bridge to the rest of the chassis...right there above the channel and master modules. Are they there? If not that would explain the loose meter bridge and such. With those 6 screws out the meter bridge and backplane tip back/open up like the hood of a corvette.

Question: When you have signal routed to the STEREO A buss, and you plug something into the output jacks and you get the nasty hum, what are the meters doing? Are they still happy and bouncing around to your source material you can still hear all nicey like in your headphones? Or do they go nasty steady to match the hum? I suspect they are still happily bouncing around to the source, yes? (make sure you have the meters sourced to the STEREO A buss and not the AUX 1/2 buss)

Regarding the slate pot...do you have the manual? Look on page 34. Study that. The SLATE pot controls the level of the talkback mic to the slate (busses 1~8) and AUX 1/2 and AUX 3/4 assign switches in the SLATE/TEST TONE switchrack. When the SLATE/OFF/TEST TONE switch is set to SLATE, then you can route the talkback mic to the SLATE, AUX 1/2 or AUX 3/4 busses using the momentary assign switches in the SLATE/TEST TONE switchrack. In that state the SLATE level pot controls the level of the talkback mic to those busses. The TALKBACK level pot controls the level of the talkback mic exclusively to the STEREO B buss when you press the T/B switch in the SLATE/TEST TONE switchrack regardless of the SLATE/OFF/TEST TONE switch position. My hunch is the talkback mic works, you just need to get cozy with HOW it works, but check it out and report back.
 
sweetbeats said:
I suspect somebody had it apart trying to resolve the same problem you are having with hum. There are 6 screws that fasten the backplane and meter bridge to the rest of the chassis...right there above the channel and master modules. Are they there? If not that would explain the loose meter bridge and such. With those 6 screws out the meter bridge and backplane tip back/open up like the hood of a corvette.
Click to expand...

Haha! I discovered that. All screws were there except for one. I read somewhere that the assembly hinges, but had never experienced it for myself. I took out the screws and sure enough, just like a Vette, it hinged back. It was loose just because the screws weren't tightened fully. Someone had worked on the first couple of channels, because there are wires that look stock but have tape with writing on them.


sweetbeats said:
Question: When you have signal routed to the STEREO A buss, and you plug something into the output jacks and you get the nasty hum, what are the meters doing? Are they still happy and bouncing around to your source material you can still hear all nicey like in your headphones? Or do they go nasty steady to match the hum? I suspect they are still happily bouncing around to the source, yes? (make sure you have the meters sourced to the STEREO A buss and not the AUX 1/2 buss)
Click to expand...
Meters are fine and are sourced correctly. And yes, they are happily bouncing around to the source.

Last night I hooked up my equator d5 monitors (which use a balanced line only) to the stereo A buss. I did this via a cheap ART unbalanced to balanced box. (ART cleanbox pro)
To the stereo B buss, I have my Ns10s with a hafler power amp hooked up.

The ART box has level control and I had it set @ 9 oclock and the D5 sensitivity at about halfway.

With this setup I was able to bring the a and b faders up to the same level with a good volume balance between the equators and the ns 10s.

The hum was very mild and unnoticeable once music was run though the monitors.

However, it should be as hum free as the B master stereo buss.

Here is something very odd.

With just one set of monitors (Ns10) and the power amp I tried switching back and forth on the busses.

Output B is plugged in. I pull out one plug from B and plug into output A (Left channel)

NO hum. I bring up both faders and have stereo coming out of the monitors.

As soon as I pull the right channel from stereo B, instant hum. I plug it into the right side of stereo master A and we still have hum...........Very odd

So somehow it would seem that A and B are funked up on the right side. However buss B is fine on it's own.

I sure hope all that made sense.

EDIT: I just tried flipping the connects. Now I have left on stereo B and right in stereo A No hum. But on either side, as soon as I unplug one from the b buss it hums, I plug into the A buss and it still hums. But on the A buss as long as I only have one plug inserted there is no hum. weird Huh!
sweetbeats said:
Regarding the slate pot...do you have the manual? Look on page 34. Study that. The SLATE pot controls the level of the talkback mic to the slate (busses 1~8) and AUX 1/2 and AUX 3/4 assign switches in the SLATE/TEST TONE switchrack. When the SLATE/OFF/TEST TONE switch is set to SLATE, then you can route the talkback mic to the SLATE, AUX 1/2 or AUX 3/4 busses using the momentary assign switches in the SLATE/TEST TONE switchrack. In that state the SLATE level pot controls the level of the talkback mic to those busses. The TALKBACK level pot controls the level of the talkback mic exclusively to the STEREO B buss when you press the T/B switch in the SLATE/TEST TONE switchrack regardless of the SLATE/OFF/TEST TONE switch position. My hunch is the talkback mic works, you just need to get cozy with HOW it works, but check it out and report back.
Click to expand...

I don't have the manual except for a PDF, which is better than nothing. I'll play around with it a bit more, but I think the TB is partially inop. When I have Stereo master B on, and engage the TB with the vol all the way up, it disrupts the source signal, but I do not hear my voice when I talk into the mic....I think I should be getting something.

Well, these old beasts certainly are an adventure aren't they.

Thanks for the help and suggestions.
 
Last edited:
I figured out what was up with the talkback. The internal mic was disconnected and someone soldered in an xlr jack. There was a cable inserted but had been cut off.
It started off with me being curious about the headphone jack being loose.
As close as i can figure, someone had undone it and routed the mic cable through the hole.........But why??????
Then they just cut the cable and left it to dangle inside.
I haven't hooked up the talkback mic. I just taped up the xlr for now. I'd have to pull some things that I'm not ready for yet.
How does the headphone jack tighten up?
 
Headphone jack...you need to remove the master section dress panels.

  1. remove all the knob and switch caps
  2. remove the faders (two scres each...they'll drop down inside the mixer)
  3. remove a couple more screws in the middle and top of the dress panels
  4. lift the panels away
  5. you'll now see the nut that fastens the headphone jack to the module frame
 
Thanks Cory.

I can do that, and can give the dress panel a good cleaning while I'm at it. Might as well get the toothbrush out for the knobs too.
once the faders are dropped, i might be able to hook up the boards talkback mic. I have no idea why they disconnected it in the first place.

Yes, I do have a multimeter.

Btw, this has the large frame faders, is that good?
 
Any number of reasons they modified the talkback mic. Best to not waste energy on that though and just focus on making it right.

With the console switched off, set your multimeter to ohms and measure the resistance between each of the 4 STEREO A/B outputs jack's hot pin and shield (i.e. STEREO A left, STEREO A right, STEREO B left, STEREO B right). Report back

"Large frame"? Can you post a pic so I can see what you mean? You mean the fader packs that are about 2" tall vs the ones that are about 3/4" tall?
 
I'll check ohms and report back.
the faders are the 2 inch ones.
no pics for now, my server is snail speed and they wont upload. But they will come.
 
no ground

UPDATE:
Stereo master b reads 45k, while master a reads open.
upon further testing. I discovered that on master B if I do a continuity check. I'll get full continuity when touching any of the grounds of the rca jacks. (Anywhere on the board)
Master A gives me nothing. So it would apeappear that whole block of 4 jacks is not grounded.
 
Last edited:
Both A jacks are open circuit and both B jacks are 45kohms?

---------- Update ----------

The 2" faders are the better ones for sure.
 
RFR said:
UPDATE:
Stereo master b reads 45k, while master a reads open.
upon further testing. I discovered that on master B if I do a continuity check. I'll get full continuity when touching any of the grounds of the rca jacks. (Anywhere on the board)
Master A gives me nothing. So it would apeappear that whole block of 4 jacks is not grounded.
Click to expand...

Yep! And that would explain why you get some resolve when your stereo monitoring device is straddled between A and B.
 
I think we're onto something there, Cory.
thanks for guiding me along.

Well, I took out that pcb. Gave it a thorough inspection and cleaning.
found out something interesting. Pulled out of the circuit, only left and right jacks share a common ground.

So the whole block gets its grounding from the harness, or rather where the harness goes to.
time to pull the master module again.
Hey, at least I have really clean jacks and know where the problem isn't.
 
You must log in or register to reply here.
Back
Top