M3700 Headphones Problem - can hear all incoming signals all the time

wolf728

New member
Hello! This is my first post. I bought an ATR80 and a M3700 a few months ago and I'm just in the process of soldering patchbays and setting everything up.

I powered the desk up for the first time today and apart from one light bulb in the master VUs, everything looks fine. The first problem I encountered is that I can always hear all 24 tape inputs on my headphones (including the SMPTE track on 24) even when the headphones volume is turned all the way down. When I push up a fader, I can hear the signal on the channel and the rest gets a bit quieter, but it always stays on.

Is this a grounding problem? I didn't try the XLR outs on the back side yet.

I'd be very happy if somebody with experience regarding this desk could help me. Thanks!
 
I have worked on one in the past but I am no expert. The likely issue is that the prefader listen buttons are all down. These are normally kept up or out.
look at the block diagram of the mixer to determine the possible cause. I get phone calls from past clients that are young and just entering into this type of
equipment. Part of the task of buying the equipment is also understanding how to use it. The larger a mixer is the more features it has on it and these can be confusing if you started with a 6 in 4 buss mixer like the Model 2. In all my years of working on all the Tascam mixers we never had a malfunction like this.
Mostly it is users error.
 
Thanks for your answer. I probably should have stated that I am by no means a beginner. I have worked on plenty of different desks for 20 years.

This is clearly a malfunction. There are no solo buttons pressed, no Aux channels open etc. Everything off. As stated above, the problem even occurs with the headphone volume all the way down. I have read a lot about this mixer in this forum and problems like this seem pretty common, especially with respect to the headphones out.

I really appreciate every comment.
 
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OK, It is not common for a part to do this by itself so I guess the only way to find out how it is happening is to get in with a scope and find out why. I looked on my computer and I see that I have no M3700 service manual on it. Are you sure someone didn't modify the unit to do this? Sometimes I see deck that were at universities that have all kinds of crazy mods in them.
 
Could it be that I'm simply feeding way too much signal into the -10db tape returns? The ATR80 delivers +4 or +10 ... that's my only guess so far. But it doesn't really explain the behaviour mentioned.
 
I'm just trying to remember if the monitor selection goes to the headphones. There is a row of buttons that selects what you hear in the monitors, aux 1, aux 2, etc etc. The phones hear the same as whats in the monitor out. Maybe you have one of these selected or there is a stuck button and the signal is coming via an aux?

I have never had this headphone problem.
Alan
 
Signal level will not cause this situation but it might be heard as a sputtering distorted effect. Best not to overload the inputs as then you might damage circuits. Some Porta Studio jobs get line in blown out by putting a AMP speaker output with plus or minus 30V designed for 8 or 4 ohm loads and the input spec can not take more that plus 12V so the chips get blown out. This means user error.
The examination of the block diagram in a complicated mixer would give possible routes that signal could take to the headphone outputs. The headphone output volume control should control the volume regardless unless this pot is damaged or broken. Most volume controls use the wiper arm to select from a ground terminal anywhere up to a hot feed terminal then feed it into some high power op amps to provide the current required to drive 8 ohm phones. If this pot is cracked or the wiper arm broken off, then the control has no control of the signal. The same thing can be true if the pot is fine but the solder joints on the board are cracked/broken and in most Japanese electronics through the years I have see thousands of low grade solder joints which I have to correct with Kester 44 solder- this is why I ordered a case of 1 pound rolls as most decks do need all these correction and mixers are not too different.

On your mixer specs it will say its range of limitations and so on inputs they usually have a attenuator that can be activated- some of them have a button for -20dB as well as a trim control. So most Tascam units should be able to take in +4dBu signals which are only like 1.58V plus or minus from that.
 
including the SMPTE track on 24

I did mean to ask in my last reply, why have you got the smpte track running to a tape input? It should be connect to the smpte in connection. Likewise the smpte out connection on the mixer should be connected to the tape machine 24 track in. Never run smpte through audio ins and outs as smpte is a good source of noise bleed due to the high pitch of the tone and the level it is recorded at.

Cheers
Alan
 
Thanks very much for your last reply! Of course the SMPTE has no place in an audio input. That's only because some tapes I have do have SMPTE on track 24 and others have audio. Sadly, there's no way to attenuate the signal coming from the tape machine, BUT the tape machine's outputs can be switched to unbalanced (the inputs on the desk are unbal anyway). I will try what this does first and then have a look at the phones pot.
 
A Technician will put a 1KHz signal into some inputs and see where the problem lies. If he can not get the problem to surface then sometimes the problem can be traced down to how a product is hooked up. I have seen people paralleling output through Y cords which is not acceptable. I would disconnect the source of the audio and start troubleshooting the mixer by itself and then if you find an anomaly then go after that in the mixer. This kind of headphone complaint I have never heard of before but often times one side is out or no output due to broken jacks or other strange reasons.
Make sure you are not hooking something into Sub Monitor inputs as this is used to hook in another mixer sort of like a cascade to add more sources to the Headphone system. The Teac Model 1 was made for this.
 
Oh, there is always a way to attenuate signals if you understand how to do it. There are attenuators of particular dB amounts that you could buy that were in XLR connection- just put then in line. The other way is to make your own. When I worked at ESPN we did have some remote kits that needed to interface +4 dBu signals to -10dB type products. An Attenuator is nothing but a voltage divider resistor network with the output taken off the smaller resistor in the series string.
Cutting a signal in 1/2 is 6dB so when you take a single output of a XLR balanced connector with reference to ground you are already getting a -6dB signal so then to get to -10 you are starting at -2dBu and just need 8dB more reduction. Then since it is known that 316mV is the -10dB signal it can be calculated as to what resistors you will need and putting them into a XLR connector is certainly possible using 1/4 watt resistors as they are small. The resistors will be selected to also present the right resistance (Impedance) to the output device as well. I think 600 ohms was the needed amount.

Later on MCM and now Newark has Stereo two way converters that can do the job. They are adjustable and I remember that there were these small blocks devices that would do the job but they were more expensive than MCM box.
Yes, They were called Stick on units and they required a 24Vdc supply and one such converter is the STA-1. I use to find these attached to the wall or furniture when I took studios apart- the 24Vdc wall wart usually gave away their presence somewhere. I am sure I installed some at Radio stations studios I put together for High Schools that has small 150W stations.
 
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