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Ok Enzo, I will get the main board back in the chassis then I can proceed with the checks that you have already mentioned. I will just start all over again with the checks as I feel that I might have missed something. I think I have confused myself....Thanks for all your help....
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Ok, with the main board re-installed, I started the testing once again. Here are the results so far. MAIN OUT JACKS: I now have audio on the Left and Right but with the Right audio being a little weaker than the Left. With the To Main pot in the Cd/Tape control section and the main fader set wide open, the overall volume is ok but with my external power amp set for high output, the volume should be blowing me away but it's not. When pushing the Standby Button in the Cd/Tape control section (Standby Button Isolates the mono input section from the main mix), I get an increase in volume in the main mix and shortly after pressing the standby button, I get an increasing or rising static that sounds like it's out of control as to be running away. Also when pressing the Standby button in or out I get a loud popping. Other buttons are quiet. No leds are being displayed while working the Main Fader although the Main Fader is controlling the volume properly. The EQ is working normally. Now on to the MONO OUT JACK: I have Audio but it's weak even with the To Main pot in the Cd/Tape control section set wide open. The Low Pass Filter, Mono Fader and the EQ worked normally. The Low Pass volume is very low when the Low Pass button is engaged. No Leds are being displayed while working the Main Fader. Again, with my external power amp set for high output I should be blown away with volume but I'm not. That's the testing so far, any thoughts ? Thanks for your help....Comment
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Popping switch often is a symptom of DC on the signal path.
There are a few inout channels on that master board are there not? What happens when you input your signal through them? I can;t say as I am ever happy using the RCA tape jacks in or out on most mixers.Education is what you're left with after you have forgotten what you have learned.Comment
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Ah, Enzo to my rescue. I haven't tried the other inputs yet. I have been looking over the schematics trying to figure out where to go next. My input signal that I used for the above testing was through the Tape input RCA jacks. I have the mono side all pulled down but when I pushed the Standby button to completely isolate the mono side of the board from the main side, and got the popping, I thought that I should try to figure out that circuit and that maybe finding that fault might cure other symptoms like the low volume levels that I mentioned above. I intend to use this mixer in a band, so I will be asking alot from this board. Any suggestions of what I should check next ? Thanks Enzo....Comment
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Ran more checks, and here are the results. CHANNEL INSERTS: 1 thru 16 All working....AUX SEND JACKS: 1--2 Working....FX SEND JACKS: 1--2 Working....SUB OUT JACKS: 1--2--3--4 Working....Signal input was thru the channel line in jacks. Signal was assigned to the various jacks as mentioned above but when sending the signal to the mains, the signal was very low with the main fader pushed all the way up and there was no signal indicated on the Led display. The Led display worked when running solo on each channel and subs etc....Looks like the I'm down to the main fader section. Are the OP AMP outputs AC or DC. No oscope just a DVM. Thanks....Comment
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SO find the two ICs I mentioned in post #12.Education is what you're left with after you have forgotten what you have learned.Comment
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As a rule, the signal path op amps will have no DC offsets on their output pins.
No scope...
Signal is an AC voltage. Put a fairly strong signal in place and measure it with AC voltmeter. Try at the sub outs or someplace to get an idea of level. And if you are not sure if it is signal or not, then turn the signal up and down at the source, or at least earlier in the system, and see if your reading follows.Education is what you're left with after you have forgotten what you have learned.Comment
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More Checks Done. Signal Input was to the CD/TAPE inputs with the TO MAIN POT and MAIN FADER set to max audio. The MAIN OUT Jacks were used to send the signal to the inputs of my external amplifier. A TS Plug was used in the MAIN OUT Jacks and was pushed all the way in. Readings on IC's were as listed.
IC53
Pin 1 = 0 vac
Pin 7 = 0 vac
Pin 8 = +14.73 vdc
Pin 4 = -14.66 vdc
IC44
Pin 1 = .012 vac
Pin 7 = .005 vac
Pin 8 = +14.73 vdc
Pin 4 = -14.66 vdc
IC's 45, 51, 34, 39 all had very close readings as the above IC's....
The results I got during this testing were as follows.
The Main Fader controlled the volume of the the MAIN OUT Jacks.
The LED DISPLAY had no indications while using the MAIN FADER.
The Audio level was weaker than it should have been and with my external amp set for high output, I should have been blown away with a very loud volume level.
The EQ worked IN or OUT while using the MAIN/AUX pushbutton in the EQ control section.
The STANDBY pushbutton in the CD/TAPE section, when pressed in the standby mode, GREATLY INCREASES the volume level and then shortly after being pressed, a constant loud distortion or static is heard.
Also, I couldn't send my FX's to the mains even when the MAIN/SUB pushbutton is assigned to the Mains....
Where is my problem ? Thanks....Last edited by Twistedpair; 10-24-2010, 12:44 AM.Comment
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You happen to check the output pins of those two op amps for DC?
Can I again make a plea for using one of the regular channels as the test signal input rather than the RCA jacks. Modern mixers with RCA jacks have a terrible habbit of not wiring them up as plain old input channels.
The basic signal path is from the subs or main bus through IC53, through the GEQ - or not - through IC44, through the insert jacks to the main fader.
APparently we need a stronger signal for the AC voltmeter to work... or a signal tracer.
If you do not have a signal tracer, make one. Very simple. Imagine you plugged a scope probe into the input of your external amp and speaker. Then whatever you probed in a circuit would be amplified. Same thing you been doing, except now you are not limited to output jacks. Although in this mixer it probably doesn;t matter, you SHOULD add a series cap to block any DC from hitting the external amp. In tube amps that is especially important as the signal might be sitting on 300vDC.
A very simple probe can be made for this. Google signal tracer. This will be a useful tool in general. When I was young and learning, most of us built our own test gear from kits. Popular brands were the Heathkits and Eico kits. They all made signal tracers, a small amp in a metal box with a speaker in it. Every shop had one.
With a tracer you can plug a test signal into a channel, and then see where it gets to... well hear where it gets to. Then you can probe and listen to the outputs of IC53 and 44. If it is not on both sides of IC44, it won;t be on the faders or main outs either. If it is not on 53, then it won't get to 44. Etc.Education is what you're left with after you have forgotten what you have learned.Comment
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Other than making it easier to track down problems, why use the signal tracer ? Is it because of the low output from the IC's ? My guess is that something on the main board is dragging the signal down and I feel like it's going to relate to the standby pushbutton circuit. What are your thoughts so far ? I will go back and put my signal input into one of the regular channel to see what happens. Should I have an output on IC44 and 53, etc ? Regarding the DC Offset, I thought that I had done that test when I check the output of the IC's. The DVM was set to AC when checking output on pins 1 and 7. Should have I set the DVM to DC and checked the output pins again as well ? Thank you for your help....Last edited by Twistedpair; 10-24-2010, 02:21 PM.Comment
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I went back and checked for DC Offset on the IC's mentioned in post #23, and this is the result of those re-checks.
IC 44
Pin 1 = 0.012 vac ( -13.48 vdc )
Pin 7 = 0.005 vac ( -14.44 vdc )
Pin 8 = +14.73 vdc
Pin 4 = -14.66 vdc
IC 39
Pin 1 = 0.032 vac ( -.003 vdc )
Pin 7 = 0.008 vac ( +14.29 vdc )
Pin 8 = +14.74 vdc
Pin 4 = -14.66 vdc
Looks like these two are no good....Comment
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Which NJM4580 Op Amp did you order from Mouser, the E or the D package ? Thanks....Comment
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Beats me, look at the data sheet for the appropriate SOIC package. I can;t keep all the alphabet soup together in my head.
Um, that's not reason enough?Other than making it easier to track down problems, why use the signal tracer ?
DC on an op amp output is usually a sign of a bad IC, but just to be sure, remove the IC and make sure there is not some DC on the empty pads for the input pins.
I mentioned it because you have no scope. For strong signals you can usually follow signal with an AC meter, but what if the half volt of "signal" you are finding is just hum? A signal tracer is a siple thing to make, you already have other amps, so it all boils down to making some sort of probe. A signal tracer is good becaue you use your eyes AND ears. You probe a point int he circuit and you don;t have to move your eyes, just listen to the result.Education is what you're left with after you have forgotten what you have learned.Comment
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Ok, thanks for the signal tracer tutorial Enzo. My 25mhz scope stopped working years ago. It was a old bell and howell and if I remember correctly, I couldn't locate any parts for it so it got put in the corner. I guess that you saw the two bad 4580's I had. IC 39 and 44.... I checked all the op-amps on the main board and found these two had the dc offset that you had mentioned. Those checks were done with no signal input. Thanks....Comment
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I have 4580's coming. Haven't removed the bad ones yet. After removal, I will check the empty pads for voltage etc. Enzo, have you by chance remembered what 4580's you have used in the past ? Thanks....Comment
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