Tascam M-320 noisy only after it warms up

livingtree

New member
I purchased a mid-1980s M-320 mixer a few months ago for cheap and really like the sound. However, when it warms up after an hour it lets in noise which is most likely of the 60 hertz variety. It hums louder when the AC switches on or I futz with my light dimmer. But like I said, I have zero issues until about an hour after it's been on.

So I began my quest of figuring out what the problem could be. I recapped the power section, the output section, and the 4 channel buss section. The only things not recapped are the 20 input channels and the VU meters. I did, however, change out the lights in the meters and they all work now, however one is dimmer than the others.

The weird thing is if I go direct out from any of the input channels' own output (essentially bypassing the main outs and busses) I get no hum , even hours after being on.

I've tried multiple outlets in multiple rooms and unplugged virtually all that I can that I thought could be causing the outside noise.

Next up is recapping the VU meters and hope that that miraculously solves the issue, but I'm skeptical.

My other worry is that it could be the power transformer. I wouldn't mind swapping that out, but I have no idea how to match it. My electronic knowledge basically boils down to I can replace components fairly well. In fact, I've built my own clone pedals and even a JTM-45 amp clone, but if you ask me how things work, i'd be at a lost most times. I'm a "paint by the numbers guy", I guess.

So my question is: Could it be the power transformer? And if so, how do I find the right replacement? The only Identifying marks are: Teac QL76-504N C55. The Tascam Manual for the M300 series identifies it as: 5320033100

Any help would be great! Thanks, guys
 
Yep, Already did that. The power section, the complete output section (including the L R stereo and mono buss), talkback, and the 4 channel buss all recapped. I didn't think the meter caps were that important that's why they are left for last. All that's left to recap are the 20 inputs and the meters.

Could a bad cap on one of the inputs be the culprit? I read that the delayed noise could be a static build up due to a bad cap or cold solder joint. Let me be clear that the mixer was super clean on the inside when i bought it and no cap looked bad (ie nothing leaking or bulging). I didn't even think I would need to recap it until I heard the hum.

I pulled an audio clip of the hum into Logic and ran it through an EQ analyzer. It looks as if the hum occurs at 60hertz and every addition of 60. For example: 60 120 180, 240, etc....
 
You mentioned that if you use the direct outs from the channel strips, that there is no hum after an hour, so recapping them would be pointless unless you want a make-work project! :)

As to where else it could be coming from, I'm not sure. Hopefully someone with more experience will chime in when they get a chance.

Cheers! :)
 
What are you hearing the hum through? what kind of power amp, speakers, headphones, etc. Do you hear the hum through the on board headphone output and through the program buss outputs and stereo out? Is it the same whether something is plugged into the inputs or nothing is plugged into any of the inputs? Can you hear it through the headphone output when nothing else is plugged into any input or output? Is it the M-320 or an M-320B
 
So what outputs hum? Be specific.

It's not the transformer. The transformer just converts AC to various AC power rails of different voltages. You hear the hum because either you have a horrendous grounding issue, or a dying rectifier. Rectifiers convert the AC power to DC. The filter caps in the power supply clean up the rectified power, but if the rectified power is really dirty the filter caps aren't gonna be able to kill the noise.

If I recall correctly the M-300 series has +/-15VDC rails and +/-18VDC rails for audio. I'm wondering if the hum is heard only through audio paths powered by the 18V rails, and if that is the case id scope the output of the 18V rails and look for AC components. There shouldn't be any to speak of. If there are, I'd replace the rectifiers, whether it's a discrete array of 4 diodes, or an integrated bridge. I'd put in a new one and see if that did the trick. The direct outs are powered by the 15V rails which is why I'm picking on the 18V rails. You said the direct outs don't hum. The reason it is taking time for the problem to manifest could simply be heat. The failing part or parts might have higher resistance and as the system warms up those parts get hot effecting the circuit and maybe impacting its effectiveness.

Anyway, first step: list what outputs hum and which ones don't and we'll go from there.
 
I have the M-320. Not the B. Phantom power would be nice but oh well...

I currently use it as a summing mixer going out and back into a Focusrite Saffire Pro and I hear playback through a presonus monitor station via SPDIF on the Saffire. But that's moot, because I've tried listening to the board with everything else unplugged and off and I can still hear the hum though the onboard headphone jack. I did get a "different" higher hum when I tried plugging the 4 channel buss into an ADAT. I haven't tested that since I've recapped though. I suspect that to be a ground loop issue though.

I hear hum through all of the following: Headphone Output, Main Stereo and Mono Outs, L R & Mono Monitor out. Aux 1, Aux 2, Aux 3 and Aux4 and Effects; both 1/4 outs and XLR... Haven't tested the RCAs.

No noise: All 20 direct outputs.

Thanks, guys. The 15v versus 18v theory sounds like something worth exploring... Although, as an electronics amateur, it may get over my head.
 
BTW, here are the diodes in the power supply: KBPC-602, W03C(x2), U05B(x2), W02, 1S133T77(x8).

I suspect either the KBPC-602 or W02 are the culprits?

I have the service manual for the mixer in PDF form if that would help.
 
I currently use it as a summing mixer going out and back into a Focusrite Saffire Pro and I hear playback through a presonus monitor station via SPDIF on the Saffire. But that's moot, because I've tried listening to the board with everything else unplugged and off and I can still hear the hum though the onboard headphone jack.

That's we we have to ask for more info before proceeding. A guy might end up recapping the whole board when the problem is not the board... we never know. ;)
 
I have no skin in this game but gotta say you's guys are great with your tips and online diagnoses.
I gleen much in general just by reading,
 
Hahaha... I know, I know, but I was probably going to recap the whole board anyway. I've just always been a lurker on forums. Reading, but never participating. Plus, I'm always way too confident I can figure it out by myself. It truly is an awesome thing to have these forums as a resource. I've ordered the new rectifiers and even the metal poly caps next to them (on someone else's suggestion outside this forum). If that doesn't fix the hum, then I'm at a loss I guess. The jack is a two pronged unbalanced outlet, so I'm sure that not having a ground wire is letting in unwanted noise as well. But like I said, it sounds so good for about an hour. I've read that changing out the opamps improves the sound too, but I doubt I'll do that.
 
It won't hurt to change the rectifier diodes. Keep in mind some power in certain buildings/houses can be so dirty you have to condition it or run a new line. When I set up a new home/project studio I always run a new AC line from the breaker box to the studio room. You don't want air conditioner or fridge compressor causing interference. A light dimmer will wreak havoc. I mean, that stuff should not be showing up in audio. The op-amps, NE5532, are good IMO. A lot of people use those as an upgrade for other op-amps.

Hahaha... I know, I know, but I was probably going to recap the whole board anyway. I've just always been a lurker on forums. Reading, but never participating. Plus, I'm always way too confident I can figure it out by myself. It truly is an awesome thing to have these forums as a resource. I've ordered the new rectifiers and even the metal poly caps next to them (on someone else's suggestion outside this forum). If that doesn't fix the hum, then I'm at a loss I guess. The jack is a two pronged unbalanced outlet, so I'm sure that not having a ground wire is letting in unwanted noise as well. But like I said, it sounds so good for about an hour. I've read that changing out the opamps improves the sound too, but I doubt I'll do that.
 
By the way, I have the M-320B and it's as quiet as can be. No hum and good noise specs.
 
I live in a newly built home (like in the last 5 years built). Not to say they didn't screw up the electrical when it was built, but it should all be to code. Right now the mixer is in my little home office. I do plan on building a studio in the tandem portion of our garage, and I plan on even running a separate ground spike into the ground just for the studio. I was hoping to use the board as an analog summing mixer in addition to a separate monitor station for each band mender to get their own headphone mix.

I was lucky to find the board locally on Craigslist for $200. For that price I couldn't pass it up. I grew up recording on 4 and 8 track Tascam machines using cassettes. Reel to reel was a little before my time. My goal when building my studio is to keep a hybrid system going. Recording 100% in the box just isn't that much fun. I like twisting knobs and pushing faders. I would have greatly preferred the M320B so I could have phantom power, but I found it interesting that the input cards on the m320 have a unused hole on the PCB for +48V. Anyway, mostly I want to play around and educate myself and maybe move up to a m3500 or something similar. I'm just glad I found this forum.
 
Yes, the M-320 and M-320B as very similar. You got that for a good price for sure. Just wanted you to have a reference of how quiet subjectively speaking the board should be. Some gear has hum and noise problems from poor design, but the M-320 is a great board. I'm not looking to upgrade any time soon. In fact I've had many boards over the years in 30+ years of doing this stuff. I've always liked the M-300 series, so I finally got this one a few years ago. I wish they were 8-buss, but I can live with it. I like the sound and the layout... but the sound most of all.
 
Sounds like component/contact degrading as it warms up.Stand it on it's side and use a heat gun to isolate the problem.If you have a o-scope,you can find out the exact freq of the "hum". Once it is isolated to a particular section,you can use a combination of a soldering iron to heat up individual components and/or use freeze spray to further isolate the problem.You also might try cleaning the pins on the PCBs connecting all the cards together and re-seating them.
 
I bought an IR thermometer. Should a bridge rectifier read upwards of 120 degree F? Cause that's what I'm reading. It gets hot quick. Way faster than the transformer or power regulators. Seems to me if it gets that hot it needs a heat sink.
 
Update time. I changed out the 2 bridge rectifiers and the 2 chicklet gum-looking caps right next to them... Been running the board now for an hour. No hum. Zero. Zilch. Nothing when the AC kicks on, nothing when I turn on my ceiling fan light on a dimmer. Nothing. Just sweet sounding clean audio. I'm going to let it run for another hour or so. I did have to put a dual transformer isolator on the Master Outs because I was getting a ground loop from my computer & monitor when I hook up my interface outs to the mixer ins. Other than that, we are golden. Thanks for all the help, guys. I'm sure I'll be back.
 
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