ElectroVoice 7300A trouble

[MarianB]

Greetings to all.

My name is Marian, i'm new here and I'm sorry to bother you with this but i'm kind of desperate, i have a problem with an EV 7300A power amplifier and i'm in great need of help.

The problem is with channel 1 with is silent, the protection LED on the front pannel stays on permanently, so the power amp inside is closed, this would seam to be cuse of some DC ofset voltage at the output that i have found but i cannot find the reason why it is, i have removed all the power transistors and checked them out and they all are ok, as are the other transistors on the board, then i have checked the other active components ( diodes, zenners ) and they are ok as well, i did found R31 to be foulty ( interupted ) and replaced it, and the power on LED on the board CR21 shorted out, i've replaced it either, now as i was nosing around i found what it first seam to be the the problem with OP Amp NE5532, i found a big difference of voltage between the rails at it's pins, as the negative one was ok at -15Vdc, the possitive one oscilating around +5Vdc, since all other components seam to be ok i have removed the OP and with it removed the voltage rails returned to good working size, +/-15Vdc, then i replaced the OP with a new one but the problem did not go away, i have put the old OP on the working module and it is ok there, but here every time i put the IC in the socket the possitive rail goes down allot, throug thoroughly verification i have found that the main positive rail goes down in fact ( that is measured against the ground ) so i have changed the power line with the one from the working module but still the same, with the NE5532 in it's socket the main positive voltage rail goes way down at a slow but steady rate, that i cannot understand why, i have also found an strange ( to me that is ) voltage difference between the power lines of the modules, i mean that as the working one has a +78/-53 Vdc the foulty one has +59/-72Vdc, i also verified most of the capacitors, searched for blown or defective resistors and came up empty..

Now i came here to you askin for help of any kind, i realy need to solve this amp as it's one of my best friend's amp, so any help is greatly appreciated.

Here is the link to the manual http://archives.telex.com/archives/E...s%20Manual.pdf, and thank you all for your time.
Marian.

[MarianB]

Does anyone has an answear please?

[kevinkr]

Hi Marian,
Welcome to the forum.. You'll need to give the right people a little time to find this thread and come up with potential answers to and/or suggestions addressing your question.

[MarianB]

Ok, thank you and i appologise for beying persistant.

[indianajo]

Output transistors "okay" is a touchy subject if you have DC on the speaker output line. I found doing the double diode test on BJT O.T's of a blown amp, that some were open, some shorted, some fine, and some a little weird. Fine is, on my meter, about 600 ohm on the 2000 ohm scale forward and 1 megohm or better backwards. My meter doesn't see any resistance forward on the 200 ohm scale, depends on the battery voltage what they do. The weird O.T.'s were 400 ohm forwards. I replaced them.
Get a good transistor of any BJT kind, or a diode, and see what your meter reads forward on a junction. Then the the "good" transistors should read that forward, not 2/3 of that.
Another way to test which transistors are putting out DC is to remove all but one upper rail and one lower rail output transistor (if you have multiple pairs). Then power on. No DC- those two are okay. If you can't find any pair of O.T.'s that don't produce DC, go buy some.
Pulling the wire from the drivers to the base of the O.T's may be necessary. O'T's should sit there with nothing connected to the base, except a disc capacitor to emmiter, holding off DC, as long as you aren't injecting RF with a cell phone or something.
Also check if you have a protection circuit in parallel with the output transistors, that is also blown up. Like, my PV1.3k had a triac on the output, that was also shorted. One of the two diac's (bidirectional switch) also.
A link to the schematic on the internet, or an upload of the one you have, would allow more detailed analysis of symptoms like "r31 shorted". People that run shops don't waste a lot of money-making time here, although some shop owners do drop in occasionally when they get tired of talking to customers.

[Enzo]

SInce fuses are not blowing, I am willing to believe the outputs are not shorted.

Indiana, he did link to the schematic in post #1.

This amp is reminiscent of QSC circuits. If the +/-15v is OK without the IC, but the one side falls when it is in, then look at the main power rails. Are they seriously out of balance? In other words, are the main power rails instead of something like 80 and 80, more like 40 and 120? The 15v rails derive from the high voltage rails.

[indianajo]

Enzo is a shop owner, you have had one of the forum expert's opinion.
I read the explanation on here of the tricky way QSC makes their speaker ground, and decided not to buy one, especially not a dead one . I'd have to buy Bob Cordell's book to understand it, I think. If the speaker ground is not in the middle of the supply rails, no wonder.

[MarianB]

Greetings to all.

Originally Posted by indianajo

Output transistors "okay" is a touchy subject if you have DC on the speaker output line. I found doing the double diode test on BJT O.T's of a blown amp, that some were open, some shorted, some fine, and some a little weird. Fine is, on my meter, about 600 ohm on the 2000 ohm scale forward and 1 megohm or better backwards. My meter doesn't see any resistance forward on the 200 ohm scale, depends on the battery voltage what they do. The weird O.T.'s were 400 ohm forwards. I replaced them.
Get a good transistor of any BJT kind, or a diode, and see what your meter reads forward on a junction. Then the the "good" transistors should read that forward, not 2/3 of that.
Another way to test which transistors are putting out DC is to remove all but one upper rail and one lower rail output transistor (if you have multiple pairs). Then power on. No DC- those two are okay. If you can't find any pair of O.T.'s that don't produce DC, go buy some.
Pulling the wire from the drivers to the base of the O.T's may be necessary. O'T's should sit there with nothing connected to the base, except a disc capacitor to emmiter, holding off DC, as long as you aren't injecting RF with a cell phone or something.
Also check if you have a protection circuit in parallel with the output transistors, that is also blown up. Like, my PV1.3k had a triac on the output, that was also shorted. One of the two diac's (bidirectional switch) also.
A link to the schematic on the internet, or an upload of the one you have, would allow more detailed analysis of symptoms like "r31 shorted". People that run shops don't waste a lot of money-making time here, although some shop owners do drop in occasionally when they get tired of talking to customers.

As i sayd before, the power transistors are ok, i have removed and checked each and every one of them, and trust me i know a good transistor when i see one.

Nothing is blown up, i did checked carefully for that, and the component seams to be ok and that is my dilema.

I did posted a link to the owners manual in PDF format, sorry if you missed it, check it and you will understand what i am talking about.

Originally Posted by Enzo

SInce fuses are not blowing, I am willing to believe the outputs are not shorted.

Indiana, he did link to the schematic in post #1.

This amp is reminiscent of QSC circuits. If the +/-15v is OK without the IC, but the one side falls when it is in, then look at the main power rails. Are they seriously out of balance? In other words, are the main power rails instead of something like 80 and 80, more like 40 and 120? The 15v rails derive from the high voltage rails.

The main power lines are like this:

->The 2 outputs of the power transformer are both the same at 100Vac.
->As for the DC power lines on the board, well each module has a separate power line so the good module has a +78/-53Vdc power, the foulty one has +59/-72, so there is an important difference between the rails, ( i will not comment it cus i did not worked with this cind of ground before and i do not understand it best ), but strange ( for me ) is that on the working module the negative rail is the smaller one, but in the other one it is the oposite, the positive one is the smaller one, if i swithc between them, if i put the line from the good module on the foulty one nothing changes, and if i conect the one from the defective module on to the good one it works just fine, so the main power lines must be ok both of them but still... they do not work the same and that is one other thing i cannot understand why..?.

It would seam to be a problem with the ground circuit, but this is where i am stumbling cus i cannot understand this cind of ground, i did not worked with it before now and despite of my intensive search on the net i did not find anything to clear my mind about it, the ground circuit does not seam to come from anywhere in particular but still it has great power, so can you help me understand it at least a bit?..

[anatech]

Hi MarianB,
It would be nice if they included the information on the protection / bias PCB.

So, you're really only misguided due to using the same name for more than two circuit nodes. When is "ground", not a ground? When it's the common between the filter caps as this point connects to the common point through the loudspeaker. It works the same as most other amps with this wrinkle and the two high current floating power supplies.

Let's clear a couple things up first. I'll assume your meter common point is the input common (ground) at some point. You are measuring floating supplies from that perspective. If you were to measure from the speaker positive terminal to each supply capacitor terminal, you would see close to the same voltages on each. We're talking about the absolute value with polarity ignored of course.

As i sayd before, the power transistors are ok, i have removed and checked each and every one of them, and trust me i know a good transistor when i see one.

Sorry, no. I don't buy that statement without knowing you.

You tested the power transistors (plus maybe others) and declared them to be good. So how exactly did you test them? Were you able to measure beta and leakage (in uA) for collector - base and collector - emitter? If not, you have not tested your output transistors properly. You may still have a defect. Swapping parts from one channel to the other should never, ever be done. That's a great way to end up with two dead channels instead of one and still have no clue what parts may be defective. Please do not do that ever again. It's a really bad habit.

Nothing is blown up, i did checked carefully for that, and the component seams to be ok and that is my dilema.

Again, no. We know something is bad. A damaged component does not have to be totally open or shorted to be defective. At this point we do know that either one or more parts are bad, or you have a problem on a circuit board or maybe even a short or leakage on the PCB itself. Did anything get spilled inside?

As for the DC power lines on the board, well each module has a separate power line so the good module has a +78/-53Vdc power, the foulty one has +59/-72, so there is an important difference between the rails,

Nope.
Neither channel readings are good. Your reference point is in the wrong place for these measurements. What you should be noticing is that both supplies have similar, but opposite offsets (they are pretty close for how this circuit is designed and where the reference for the meter is). Simply deciding that one is correct is an error in logic since they both show an offset from having the same reading for positive and negative supplies.

Enzo made an excellent suggestion on how to check the op amps. Let's add to that. On each op amp, measure pins 2 and 3, then pin 1. Measure pins 5 and 6, then pin 7 and record the readings. Op amps "want" to make the two inputs (2,3 and 5,6) equal in voltage. So there should only be low mV between each of the two inputs (allowing for meter errors). I'm talking about each section, not comparing between both sections. Look at the output DC voltages on the output pins (1 and 7) to see if they are in the linear range of operation, or pegged to one rail or the other. From there, you can follow the schematic to determine where the fault probably is. Please let us know what you come up with.

One thing will probably really help you do some troubleshooting. On a piece of paper, write down only your observations and measurements. Do not write down any conclusions at all. Once you have made the measurements you wanted, and inspected the unit visually, sit down and have a look at your observations and the schematic. Then, write down any ideas you may have on another piece of paper. Always keep your observations separated from any conclusions or ideas you may have. I can see that this is tripping you up already.

-Chris

[MarianB]

Originally Posted by anatech

So, you're really only misguided due to using the same name for more than two circuit nodes. When is "ground", not a ground? When it's the common between the filter caps as this point connects to the common point through the loudspeaker. It works the same as most other amps with this wrinkle and the two high current floating power supplies.

This is how i understand the concept of ground on a symmetrical power supply, it is the middle point on the power transformer's secondary winding, and at the same time the common point between the filter capacitors, that i understand to be the most tipes of ground used, the concept of floating ground to me would be something like that on the ATX power supplyes, but here it does not seam to be the case as that common point leads in fact to te positive output of the speaker, i appologise for my ignorance but as i sayd i did not worked with this configuration before, and i still do not understand from where does the ground on the board get's it's power, i did saw at the star ground screw on the case 2 ceramic capacitors that lead to the main power lines ( 220 ) but i do not think that is it's source of power.

Originally Posted by anatech

Let's clear a couple things up first. I'll assume your meter common point is the input common (ground) at some point. You are measuring floating supplies from that perspective. If you were to measure from the speaker positive terminal to each supply capacitor terminal, you would see close to the same voltages on each. We're talking about the absolute value with polarity ignored of course.

I used as my meter common point the big ground circuit, the one that is connected to the negative terminal of the loudspeaker and at the same time to the case of the power transistor, that was my reference point as with all the other amps i have worked with before, but i'l do as you say, i'll check the voltages on the caps terminals.

Originally Posted by anatech

Sorry, no. I don't buy that statement without knowing you.

You tested the power transistors (plus maybe others) and declared them to be good. So how exactly did you test them? Were you able to measure beta and leakage (in uA) for collector - base and collector - emitter? If not, you have not tested your output transistors properly. You may still have a defect. Swapping parts from one channel to the other should never, ever be done. That's a great way to end up with two dead channels instead of one and still have no clue what parts may be defective. Please do not do that ever again. It's a really bad habit.

I will not state that i'm some cind of expert on measuring the transistors, but still i am not a begginer either, i do have a digital multitester and i do measure it's beta factor as well as it's diode conection, and that is why i sayd that they are good, ok you do not trust me, i can accept that it's true we do not know each other, but still i would not have come here on this most respectable forum if i did not know at least that, but still i will check them again as you say and compare the results with theyr PDF's.

As for Swapping parts, well i do know it is not a good thing to do, what i did change i did it after checking to make sure i would not harm the other one as best i could, but i will keep in mind youre advice.

Originally Posted by anatech

Again, no. We know something is bad. A damaged component does not have to be totally open or shorted to be defective. At this point we do know that either one or more parts are bad, or you have a problem on a circuit board or maybe even a short or leakage on the PCB itself. Did anything get spilled inside?

Ok, maybe i overestimated my observations and i appologise, it is obvious something is bad since it's not working, i did check the board for circuit foult or short somewhere but i did not found any, and the owner did not say much about the reason for the amp malfunction or how it happened, he just sayd something like yesterday was working fine and today it did not.. he did say that he used the amp on bridge config for some time, now as i took a first lok inside i did not found any evidence that something was spilled on it, it loked clean but the thin is i am not the first one that tried to troubleshoot the amp, the other guy did not say much to the owner, just that he can't fixe it so i cannot be sure of anything, but i did found problems, as i sayd R31 was open, ( the one that reads the output of the speaker for the protection circuit ), and the power on LED on the board shorted, and another thing, i found some cold solderings on a coupple of power transistors, now i do not know if that could have caused some cind of chain reaction, i checked then all the soldering and fixed all that seamed not to be ok.

Originally Posted by anatech

Neither channel readings are good. Your reference point is in the wrong place for these measurements. What you should be noticing is that both supplies have similar, but opposite offsets (they are pretty close for how this circuit is designed and where the reference for the meter is). Simply deciding that one is correct is an error in logic since they both show an offset from having the same reading for positive and negative supplies.

Ok i was wrong it seams, maybe the reference point it is not well chosen, and i will check as i sayd on the caps terminal and see, but i was wright that the offset is opposite from one channel to the other and that i cannot understand why.

Originally Posted by anatech

Enzo made an excellent suggestion on how to check the op amps. Let's add to that. On each op amp, measure pins 2 and 3, then pin 1. Measure pins 5 and 6, then pin 7 and record the readings. Op amps "want" to make the two inputs (2,3 and 5,6) equal in voltage. So there should only be low mV between each of the two inputs (allowing for meter errors). I'm talking about each section, not comparing between both sections. Look at the output DC voltages on the output pins (1 and 7) to see if they are in the linear range of operation, or pegged to one rail or the other. From there, you can follow the schematic to determine where the fault probably is. Please let us know what you come up with.

The Op amps themselfs must be ok since both of them work fine on the good channel, but i will check them as you sugested and see what goes around there, sadly i do not have acces to an scope as i am sure the problem would be much easyer to find.

Now, i apologise for such long message, but i had to give as much detailes as i could, and i must thank you sincerely for your time and youre much apreciated imput, it helps me allot, so many thanks to you all for youre patience, and i wish you all the best.

Marian.