Need Help with this Circuit Please

[rievax_60]

Re: Need Help with this Circuit Please

There are many possible causes plus extra damage now. Check all transistors and resistors to the right of and including Q224.
Pay particular attention to Q224 because it is usually mounted to the heatsink and can easily be mechanically stressed causing connections to break.
Q224 and surrounding components are for controlling output transistor idle current. A fault in this area usually results in more dead transistors.
Test run it with the bases of the output transistors disconnected or removed completely if they are suspect of being faulty.

[rievax_60]

Re: Need Help with this Circuit Please

Also check the 1N4148 diodes and the zenner diodes.

[tfs2015]

Re: Need Help with this Circuit Please

Test run it with the bases of the output transistors disconnected or removed completely if they are suspect of being faulty.

Is there any harm in doing this? I have put a lot of money into this thing already. I wish I could find a replacement circuit that I can use for this, but none of them run on 52vdc. They all to seem to run at about 40 vdc dual polarity power supplies. I have till May of 2016 to get this thing fully operational for the summer parade season. The other drawback that I have right now is not a lot of cash flow to spend on this. I bought it for $20 last year at a garage sale and I have put over $50 into it already.

I do have a question about some of the components that are listed on the schematics and layout because they don't match up to what is printed on the circuit board itself.

[rievax_60]

Re: Need Help with this Circuit Please

The reason for test running without the output transistors is to minimise possible damage if a problem is missed. Voltage readings are taken to be certain that it is safe to fit the output transistors.
You need to do your own searching to learn how typical audio amplifiers work so that the voltage tests make some sense to you.

[tfs2015]

Re: Need Help with this Circuit Please

Ok I have read how the transistors are supposed to work in the amplifier, I just am not sure what I am supposed to be getting for the + and - rail voltage as the manual just says 36 volts min rail voltage at which time the protect circuit cuts in and the amp will be shut down to protect the output devices.

[keeney123]

Re: Need Help with this Circuit Please

Have to tested the speakers to make sure there is not a direct short? you can use a ohm meter to check out the output circuity and speakers to make sure there not a short on the output.I would do this before you even power it up.

[tfs2015]

Re: Need Help with this Circuit Please

Found a problem in the mixer circuit:

one of the op amps, U18B pin 7 is has a reading of -15Vdc while U18 pin 1 is reading approx 90 mVdc is this normal the OP AMP is a TL072? Refer to page 24 of the schematic and let me know. I think that this was caused by one of the connections being connected up wrong. Please let me know.

Thanks
Tom S.

[keeney123]

Re: Need Help with this Circuit Please

One needs to look at the inputs to determine if the Op Am is being told to have the output. First make sure the -15 and +15 volts DC are on the power pins of the op amp. Then the two input pins will determine the output. It looks like you have a unity gain. The input cap to the + input blocks the DC while allowing AC signal. I don't have the equations remembered anymore, but you can find them on line easy enough. I will also look. Seems to be a voltage Follower with a differential input. I believe the negative input is used in the differential circuit to eliminate noise on the + input to the circuit. So with no input signal if you have both both -15 and +15 going to the power pins of the OP amp the input on the + pin should be gnd. through the 150K ohm resistor. That gnd on a follower circuit should be gnd on the output. I would say something is wrong with the output that has -15 Volts on it. One other note this would happen if considered in the design. If engineering does not consider a floating input on the negative pin when the circuit is power up then one could get a floating 2 volt input which could make the chip go to the negative rail and not act as a voltage follower.

[keeney123]

Re: Need Help with this Circuit Please

So here is the spec sheet.
https://cdn.badcaps-static.com/pdfs/...cbf37a8056.pdf

It looks like they are running this op amp at its maximum voltages where the recommended voltages are +/- 5 Volts

This is for those who what to know all about OP Applications.

[tfs2015]

Re: Need Help with this Circuit Please

ok got the bad op amp replaced and mixer is working good. However now when I put all of the output transistors in the one channel I am reading a DC offset of 46vdc which is is the rail voltage so what did I do wrong. I believe I have check all the connections and fixed all of the broken traces. I am pretty sure that I don't want to send 46vdc to my speaker.

Please help

Thanks
Tom S.

[keeney123]

Re: Need Help with this Circuit Please

I would make sure my GNDs are connected correctly. Switch Mode power Supply GND. and Power Amp Gnd. I would also check the 110 volt and 220 switch 2 to make sure it is set for the correct voltage. I would check the -/+ 52 volt are there then check if you have a short from the the +/-52 Volts to the output speakers and dragging it down to 46 Volts. Or you could check for the short first and then check the other things second.

[rievax_60]

Re: Need Help with this Circuit Please

The voltages across R214 and R218 might give us a clue. Have you checked the zenner voltages? What is the voltage between the output and the + rail?

[tfs2015]

Re: Need Help with this Circuit Please

I am starting to wonder if there is an easier circuit that I can replace this one with? Something like a drop in 2 channel amp module? Ok about the voltage from the + rail to output where do I need to check?

[keeney123]

Re: Need Help with this Circuit Please

Where are you exactly measuring the 46 volts From where to where and what probe is going to which. That is the negative probe and the positive probe. Is this a -46 volts or a +46 volts. Is this on one output to left or right speaker or both? Do you have both negative and positive 46 volts? Remember that the input dictates what the output is doing in a functioning circuit. There is nothing easy about any circuit until you understand how it is working.

[tfs2015]

Re: Need Help with this Circuit Please

ok with no input and the black probe on ground and red probe on the speaker output I was reading +46vdc on both of the speaker outputs.

[keeney123]

Re: Need Help with this Circuit Please

Is this the left or right speaker or both?

[tfs2015]

Re: Need Help with this Circuit Please

Both. I did find a bunch of open diodes, and shorted and open transistors on both of the channels. Due to the way this circuit board was put together and the cheap nature of the materials used, I have decided to shelf this project for now. The circuit board has far to much damage with broken traces and pads to be repaired. I am looking into a simple cheap way to replace the amp circuit that is in this with a couple of either drop in boards or a complete new circuit. It would have to run off of +/- 52 vdc from the SMPS. So any help or suggestions that you might have would be greatly appreciated.

Thanks
Tom

[keeney123]

Re: Need Help with this Circuit Please

You may or may not be able to replace that board. The Designers made that board to work with that circuit. It has feed back circuits. So do you have -46volts DC going to the Power amp board.

The things I think you need to study is single ended amplification and differential amplification. Then study the difference in parallel, bridge and parallel bridge amplifiers. You can find this information on the internet and it is relatively easy to understand. It will enhance your understanding on what you are doing when you flip switches and plug in cables.

[keeney123]

Re: Need Help with this Circuit Please

It seems that R213 and R242 on the power amp sets the offsets. One for one channel and the other one for the other channel. You may want to try adjusting these and see if the output voltage to the speakers changes. Just remember where you started at so you can return it to the previous location. Right now it would be good if a Stereo Tech would step in as they would be more familiar with the circuity than I am. They would be able to tell you what equipment you would need to set up this circuity. I am sure you will have to get a signal generator to induce a 1KHZ signal in to test out the different sections to see if they are within spec and if not you will have to replace those part so the circuits work to their best performance.

[keeney123]

Re: Need Help with this Circuit Please

With the Speakers disconnected. I believe if you disconnect the mixer board connections that go to the power amp and and leave the rest of the connectors in place, connected, you then can go power amp connector cn6 1 pin, left and connect it to pin 2 SGnd then go out to the left speaker and its output should be 0 Volts. If this is not the case then you should be able to go through the stages of the power amp to find out what is wrong. If this works then I would disconnect cn6 pin 1 to pin 2 and then connect pin 3, right to pin 2 SGnd. and look at the right speaker output voltage for 0 volts. If you read 0 Volts I would now check the left speaker and check that you are not reading O volts.