What are calling the input and where are you finding 96 Volts? What type of test equipment are you using? Let's start over, first check all of your DC voltages, + and - 15VDC, +5 VDC and the voltages for the output IC. The voltages for output IC are not spec'd on the schematic but I would imagine 40VDC or so, + and -. If those are OK you need to start tracing the signal path with an O'scope. If you do not have an o'scope you hunting in the dark without a flashlight!!!!
Craig
Craig
Craig, I do not have an Oscope. I am using a digital multimeter. The smallest setting for voltage is 200V and when I hook up to the non inverted input of the TDA (according to the datasheet) it is saying 096.9. I also pulled the power module plug out and measured at the pin all the way on the right (as you are looking at the front of the amp), and got the same reading. I can take pictures if you need me to. MAX input voltage according to the TDA datasheet says 90V.
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Something is wrong the only way to get close to 96 Volts is across the + and - of the output supply. Are you sure your meter is working correctly? With the -probe on the chassis(gnd) the most you should get is under 50VDC. Disconnect the output IC plug and measure each pin according to the schematic, page 3, upper right corner. Measure the PCB side NOT the IC side. Do you know what or where the 15VDC regulators are? 5VDC regulator?
Craig
Craig
Also there should be NO DC on the inputs (invert or noninvert) of the output IC. There should only be AC (audio signal) there and you may not be able to use your meter to measure that unless you have some kind of steady test signal. That's why I wanted you to measure it in the beginning, if the signal is good there the output IC and/or related circuitry(stby) is what to troubleshoot. If the signal is not there we need to go in the opposite direction, preamp secton. Have you checked the solder joints on the input jack?
Craig
Craig
Shoot man, I am an idiot. That reading I was getting was the power rail side. I was looking at the schematic backwards. That is coming in from the power rail side.
Okay, I am NOT getting a DC reading from the inputs. When I run the + probe to +power rail (13,14,15) and the - probe to - power rail (10,11,12) I get 96.9 V DC. When I run the - probe to output (7,8,9) I get 197.0V!! Hopefully this clears some things up. (BTW thanks a lot man, I am learning more than you would think from this experience) Is there any way to test the signal input without an Oscope??
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My tester was going wacky. I changed the battery and now from + power rail to - power rail I am getting 89V. From + power rail to output I am getting 45V.
So now from each rail to gnd you get 45VDC and from rail to rail is double that, so far so good. Now with a "working" meter check for proper + and - 15VDC and +5VDC. If those are OK we'll move on.
Craig
Craig
Okay Mr. Craig, Ill have to do that when I get home. Sorry for all the hassle earlier.Do you suggest I get a better multimeter that does a better job reading AC before I go home to night? Also, where do I measure the 15VDC and 5VDC currents from?
So I bought a better meter today. AC coming to the input jack of the fluctuates between near 0 and .3 VAC (when I hit the guitar strings when it is plugged into the input. Is this normal, or a very low signal?
That's at the input jack of the amplifier on the front panel? The max. you saw of .3VAC is plenty, I usually use a -20db signal which is less than .1VAC. Have you tried plugging into the CD input? If not try it and see what you get, it will probably be very low but if the output IC is getting signal you should be able to hear it. The CD input has a very low imp. and will load the guitar down quite a bit but we just looking to where the signal ends at the moment. As far as the DC voltages you're supposed to be looking for, I guess the easiest way for you is to pick one of the Opamps (4558 will do) and measure pins 4 and 8. The black lead on the chassis and the red lead on the Opamp, be careful as the pins are close together and test probes have a habit of jumping. And before you ask pin 1 will have a dot next it on the top of the case then count around CCW. 4 and 8 will be diagonally opposite of each other. You should have 15VDC on each pin, one + and one -.
Craig
Craig
I was measuring the input of the 1-15 pins leading to the output IC. I am not getting a signal when I plug into the CD jack. That is very useful info. Whats the deal with this though? I was checking some things with continuity and I realized that my main ground (chassis) is flowing continuity with the main input jack. This is not correct, eh?
Hopefully pins 4 of the opamps are -15v.
The tip contact of your input jack will be grounded when nothing is plugged into it. SO if that was the case, your cocntinuity to ground would be normal.
The tip contact of your input jack will be grounded when nothing is plugged into it. SO if that was the case, your cocntinuity to ground would be normal.
Thanks Enzo. So, I think the next thing that I need to check is the standby circuit? Do either of you guys have any clues for failure of this circuit or it's whereabouts?
So really the only places left to check is the standby circuit and the traces past the output IC?
The standby circuit is on page 1 of your schematic, middle bottom. It consists of D14, C98, TR2, R47, D17, C76, R45, and R46. Check the data sheet for the TDA again and see what the stby voltage is supposed to be. Then measure that pin on the IC or connector. It will be 0VDC or a DC voltage no higher than the 6.8V zener (D14). Check all of those parts while you are at it. I'd look it up but this your project. Also do you see the 5 volt regulator to the right of the stby circuit? Check your 5VDC also. Whilst on the data sheet see what the audio input voltage should be, hopefully the .3VAC you saw is enough. You also stated you replaced the TDA7293, just the IC or the whole module? If just the IC get a magnifying glass and check your solder joints, it's easy to screw that up.
Craig
Craig
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