424 MK III Playback issues

[billeh]

Hey guys! I'm new here, so let me start by saying it's good to meet you guys. Anyways, here's what's going on: It seems like channel one is working properly and the volume goes down progressively to almost unhearable as you go to the subsequent channels. This seems to be an internal issue, but just to make sure I'm not missing anything, here's everything I've checked: I'm using an SM57, it should have enough gain. I use XLR but quarter inch has the same problem. Input switch is on mic/line. Masters and faders between 7 and 8. Output level is fine. Line out is on L-R, not mono and definitely not off. Headphone output works fine. Trim knob is as high as it goes. Anybody seen this issue before? If so, is a home repair possible?

 

[Levelanything]

So, channel 1 is fine and channel 2,3, and 4 are worse as you go?

 

[SkywaveTDR]

That is a strange one but can mean that the phase of the other channels are opposite of the #1.
As you mix the volume cancel out. Most problems with unit are dirty switches and fader or pots.
The other problem I see are people that plug guitar amp speaker output into the 1/4" jack and blow out the op amps- they are not made to take in plus or minus 40V from a speaker drive circuit.
I really can not give more info unless I was testing it- it could be a setting that is wrong still and maybe a miswired XLR connector. Some wires were wired pin 3 hot an other pin 2. When you mix these two kinds of cables phase issue can result.

 

[billeh]

Thanks for the reply. I did, while being vaguely aware that it would cause a problem but thought it would be fine, give my guitar amp a direct spot in the signal chain, running the headphone out directly into the 1/4 input on the tascam. This is a 250 watt head and cab. I think the opamps are fried. Any way I can fix that myself?

 

[SkywaveTDR]

Well if you have to ask the question then no. If you had the ability you would have just done the job. I am not sure unless testing that the op amps are bad and the headphone outs do not have the same power level as speaker output so that should have been OK except for excessive noise.
Some of these machines are kind of complicated to work on due to the size and construction. They just do not open up and everything is there- some of them are a clam shell design with layers of boards. It is best check on a bench before jumping to conclusions- the use of a scope and tone generator would be the way to see what is not working.

 

[SkywaveTDR]

I have had people bring in their units that they did hook the speaker out of the amp into the line input. The plus or minus 40-50V level will quickly overload and destroy the op amps that are made to take a 15 V max signal in. Just because something is 1/4" jack does not mean that you can plug just anything into it. Things like this are possible but the speaker outs should go into a 8 or 4 ohm load and then an attenuator should be used to limit the input signal. In this case the signal from the amp can be used but most people out there might not understand what I am talking about either. It is tapping off the speaker a very small fraction of the signal sent to the speaker and then every thing would actually still work.