M520 aquisition... Crazy??

I don't quite understand you're statement "only left and right jacks share a common ground". Can you clarify? Only left and right jacks of what? Usually in these era of mixers an entire jack PCB get's its ground from a single point. You need to use your multimeter set to ohms and check for continuity between that common ground point and the shield of each jack. My hunch is there is a broken solder joint or trace where the STEREO A output jacks tie to common. You can't rely on a visual inspection. You need to test it with the multimeter.
 
So you're saying that on the 8 input module, all inputs have a common ground?
I'll check it out again.
 
Ok, i checked again using an ohmeter.
there are 4 pairs of jacks. Why i am saying pairs instead of Individual jacks is because each pair (left and right) share a ground. This then goes to four of the 8 pins on the connector.
On the pcb they are grounded left and right but not grounded vertically.
So for all of the jacks to have a common ground, it would have to occur earlier in the circuit.
Looking at the traces they stay isolated from each other.
I wish i could do a pic but doing this post from a phone.
 
Okay I get what you are saying...that jogged my memory. I can see it in my mind now.

So can you just start tracing it out with the ohm meter and isolated where the circuit is broken now?
 
Yeah ill have to pull the master section.
i would imagine it is around the area they monkeyed with when the talkback mic was disabled. Its possible they just pulled off a ground.

I must say it is kind of fun learning the board from the inside.
I've had lots of experience working with a 520 on the top side, now I'm learning the guts too.

For a temp fix, I took an rca jumper and pulled out the center hot pins. I plugged one end in the bottom left of A and the other into the upper left of B. Just the sleeves. This gave it a temp ground and the buzz is gone.
 
Seems like tascam consoles are back in the forum thread stream.


I finally got to pick up my second m520 which had been sitting at my friend's house for about a year. Now I officially have in my possession two consoles.

I went through the whole board to test it and this one is in damn fine shape. Everything works, no unwanted noises, no scratchy pots. All good. :thumbs up:

One thing that that I'll have to remedy is a meter.

Meter 2 lights up, the peak indicator works, but the needle don't move.

Any ideas?
 
Check for continuity between the output of the driver IC and the meter coil. I don't recall from memory what chip that is or which of the two outputs drives that specific meter but it will be pin 1 or pin 7. If the continuity is good then run tone to the meter and start tracing back from the meter with your DMM and find where you pick up tone is just upstream of the failure. Could be a cold solder joint, failed ground path etc. can't remember if the peak LED and meter coil are driven by the same amp. I think they might be which would indicate it's not a bad driver (because your peak LED works). I'll look at the schematic when I can to refresh my memory...ask questions and otherwise check out what you can based on the suggestions above. Should be pretty easy to find the cause.
 
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Looked at the schematic.

So when you say "meter 2" you're talking about the buss 2 meter, yes?

The peak LED uses half of an LM339 quad opamp. The meter coil uses half of a 4558. So different amps, but they share the same signal path to the meter amp PCB. So...do what I said before for starters. Probe for continuity between pin 7 of U202 and the meter coil (making sure we have a path from the output of the meter amp to the meter coil), then probe the other side of the coil to pin 1 of connector P1 (making sure we have a good path from the meter coil to ground). If you have good continuity in those places, then feed tone to the meter and start looking for tone with your DMM starting at the meter and working upstream...look at the meter side of D203, then the amp side of D203...then the meter side of R211, then the amp side...then the meter side of C202, then the amp side. Now you should be to pin 7 of U202. No tone at pin 7 (the output)? Look at the input on pin 5. Report back.

If anything I'm saying doesn't make sense just ask. I'll clarify.
 
I'm on the road back from Namm so it will be a bit before I have it home and on a bench. But I will carefully look at it and follow your instructions.

Thanks!
 
The Ghost of FM said:
It connects together by taking the buss and aux/effect outs on the first board and plugging those into the cascade/SUB inputs on the second board. Once accomplished, it behaves as one giant board.

About the wood end caps, a member here on this forum made some up for his M312B and had some "B-stock" extras that he was kind enough to sell to me. I believe it was EVM-1024...memory's a bit foggy at the moment. And I made the top board myself.

Cheers! :)
Click to expand...

Yes, That was me - I had forgotten....

I have spare parts for a 512/20 if anyone needs something.
 
Evm, good to know.

Sweetbeats, I opened her up and i have no idea where and what im looking for.

Just popped open the corvette style hood.

I did however repair the bent up power supply connector on the board. I noticed it was loose and when I tightened it up it was severely crooked. No good.

Pulled it all apart and the metal base plate was all bent up. So it must have been banged at some point. I didnt want to unsolder the wires to be able to straighten the plate, so with a chunk of maple and a clamp managed to straighten it out. It took about 20 minutes but was well worth the effort, and without de-soldering and re-soldering. Now it's straight and solid

So anyway, first item fixed.

It sounds like you're saying there are 2 amps, as in transistors, running the meter?
 
The meter amp PCBs are right behind the meters...one PCB has two meters mounted to it.

Remove the metal panel on the back of the meter bridge. You'll then be staring at the foil side of all the meter amp PCBs. You'll then need to unmount the PCB with your dead meter on it so you can see the component side to start testing/probing/tracing out the problem.
 
sweetbeats said:
The meter amp PCBs are right behind the meters...one PCB has two meters mounted to it.

Remove the metal panel on the back of the meter bridge. You'll then be staring at the foil side of all the meter amp PCBs. You'll then need to unmount the PCB with your dead meter on it so you can see the component side to start testing/probing/tracing out the problem.
Click to expand...
Well I popped off the back. I can see how a pair of meters share the same pcb. However it seems like I'll have to take the whole meter bridge off. There is a metal plate that the boards are attached to, that would impede the removal of the pcb. It is held on by screws that are UNDERNEATH the meter bridge. No way can I get those screws off the bottom without bridge removal.

I think I'm gonna re-read the epic 520 thread.
:D
 
I use a #2 screwdriver bit and a 1/4" combo wrench to get into that tight space. Or you can take the meter bridge off. Remove side panels and there are screws revealed that hold the meter bridge uprights to the chassis. I'm pretty sure that's right. This is from memory. There should be enough slack in the meter bridge harness you can raise up and tip the bridge back enough to get a screwdriver in there. I'd be using the bit and wrench solution.
 
Yeah. The screwdriver bit didnt do it. All the bits I have are just a smidgen too long. I'll have to take the bridge partially off. Good thing I have another mixer, and I can at least compare a working pcb to this one.

In the meantime, while I had the side panels off, being a wood worker, I did the logical non productive thing.
I made some side panels and a top panel out of flamey black walnut. :D

If it don't work, well, at least she'll be purdy. : thumbs up:.

Some point I'll have to throw up some pics.

I don't like the looks of the optional factory wood panels. The stock ones look cool and have that Frank Loyd Wright vibe, but are particle board and vinyl veneer.

My design kind of follows the original, but I made the bridge panels bigger to accommodate a 7" top shelf, and they don't "float". They go all the way down to the long side panels with almost 7 inches touching the other wood. I did this for support reasons, so the weight of the shelf and whatever I have on the shelf is distributed more than just being supported by 4 screws.
This way everything is mechanically tied into the whole frame. When I want to tilt back the meter bridge, it's just a matter of undoing 4 screws and lifting the side panels and the shelf off as one unit. :D

I'm not going to put anything heavy on the shelf, but I wanted to ensure that it would be solid. I saw Cory's post about a bent up meter bridge bracket and that is what I'm avoiding.
:D
 
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Ok so I took both meter bridges off. Thats the only way I could access the screws underneath.
I swapped assemblys.
Took the known to be good buss 1 and 2 meter assy out and installed it into the non working desk. Lo and behold I got metering! :D

So at least I know the meter not functioning lies in the meter assy rather than the mixer.

Now how to figure the other one out.

Some progress.

Funny, the bulbs from one to the other are different. Board 1 ( with the good meters) is a lower serial number impliying older.

Board 2 (newer) has frosted glass bulbs that are a bit brighter, so ill either swap out bulbs or hopefully get the pcb problem solved and return the vu assys to their original mixers.

Is the plastic cover glue on or snap on?
Edit: never mind. It's just a piece of tape that holds the vu lens cover on.:D
 
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Yup...like "invisible" Scotch tape. Let us know how we can help. Isn't it nice having a spare unit???

:D
 
Yup! Like with vintage sportscars, gotta have at least one parts car. :D

The new board is a litte better than the older one, both are functioning units, but the older board has some small issues.

So right now i want to get the new one 100% then tackle the older one. Kind of like a reverse triage. :

I'm still trying to wrap my head around what you wrote about diagnosing the meter pcb. All I have is an older radio shack analog mutimeter.

I'll read your instructions over again and report back with at least some intelligent questions.
 
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