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PA mixer / amp repair help

newjeeper369

Aug 30, 2015
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some of this is from a thread i started earlier but all information should be here.

As far as the amp goes, when i received the amp it was taken apart in two parts, the mixer part which is the top piece and the amp and power supply, the bottom piece. it also was missing its main 5 amp fuse holder . (I will post pictures today when i get the chance) all i was told when i received it is that " it doesn't turn on right"..... I started by disconnecting the amp board and mixer boards from the power supply, i replaced the fuse holder and fuse and plugged in the power supply. output voltage was normal at ~+47 vdc ~-47 vdc, power supply work find. next i plugged in the power amp board, i found the power amp transistors (mj15002 & mj15001 two of each) were shorted causing the fuse to blow immediately. this is probably why someone removed the fuse and fuse holder.


I replaced the power amp transistors and connected the amp board to the power supply and all was fine, no fuse blow. i then connected the mixer boards and still all was fine. i connected a speaker and was getting a lot of noise. here is when things went wrong, the power supply and board are connected to the base of the mixer which is a metal chassis. the mixer boards are connected to the top of the mixer which is also made of metal so i thought maybe the need to be connected to ground correctly and the noise might go away. i connected a wire from the bottom chassis to the top portion of the mixer and as soon as i did the fuse blew and 2 of my power transistors got fried again and sorted. im not sure if me connecting the bottom half to the top half has something to do with it or it was just coincidence that it blew at the same time.


So where i am now is i installed new power transistors for the amp again and i am replacing all the preamp transistors on the amp board. keep in mind of my first post about the drawings i have and that they are not identical to my mixer. i still have not heard from musicparts.com about new drawings.


DSC_1299.JPG DSC_1300.JPG DSC_1301.JPG DSC_1302.JPG DSC_1303.JPG DSC_1304.JPG DSC_1305.JPG DSC_1306.JPG DSC_1307.JPG
 

dorke

Jun 20, 2015
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"drawings i have and that they are not identical to my mixer"
Are the differences small or major?

The grounding scheme seems to be separated: chassis and signal ground.
with very specific places they are joint together.


About the noise issue,
I think you should first determine if it comes from the Mixer part or the PA part:

1.Did you get the noise without any signal connected to the inputs(no connector physically plugged in)
2.Did you get the noise with "master volume"set to zero?
3.Does the master volume have any effect on the noise?
4.What goes on with signal connected?any output signal?output signal +noise?
5.Note that you can feed the "PA" directly from the "Line In",
In that case do you get noise?
6. You can feed the mixer and monitor the signal at "Line Out",
noise?clean?

About the heat-sink:
There are 6 visible Tr's on it :
4 TO-3 and 2 TO-220.
Can not see the TO-92HS case on the heat-sink,can you point it out on the photo?
 

newjeeper369

Aug 30, 2015
25
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"drawings i have and that they are not identical to my mixer"
Are the differences small or major?

The grounding scheme seems to be separated: chassis and signal ground.
with very specific places they are joint together.


About the noise issue,
I think you should first determine if it comes from the Mixer part or the PA part:

1.Did you get the noise without any signal connected to the inputs(no connector physically plugged in)
2.Did you get the noise with "master volume"set to zero?
3.Does the master volume have any effect on the noise?
4.What goes on with signal connected?any output signal?output signal +noise?
5.Note that you can feed the "PA" directly from the "Line In",
In that case do you get noise?
6. You can feed the mixer and monitor the signal at "Line Out",
noise?clean?

About the heat-sink:
There are 6 visible Tr's on it :
4 TO-3 and 2 TO-220.
Can not see the TO-92HS case on the heat-sink,can you point it out on the photo?


thank you, you raised some good questions.

first off the transistor with the heat sink i found to be a 2n3404.
i will answer in the order asked.
  1. yes
  2. yes
  3. didnt seem like it, i didnt have much time till the fuse blew
  4. didnt try, i will though when i get it that fare next time.
  5. i will try when i repair the damage amp again ( before plugging in the mixer...thank you, good point)
  6. i will try when amp is repaired
I will repair the amp section when the rest of the parts come in and the determin if it is the amp part or the mixer part causing the noise issue, also i will use your questions for trouble shooting.
 

newjeeper369

Aug 30, 2015
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Questions about the noise.

can anybody tell by looking at the schematic if noise would be caused by not connecting the mixer board to the amp if that would generate noise just because the nature of the circuit? i dont want to be chasing my tail troubleshooting the noise issue at the amp board if it is cause by not having the mixer signal plugged into the amp board.
 

newjeeper369

Aug 30, 2015
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going over what i wrote i may have left out some important details.

first i replaced the 4 large white 10watt resistors with the .47 ohm one you seen in the picture, the schematics calls for .47 ohm but it has .33ohm resistors installed. i ordered the .47 one before i realized that the schematic i have isn't for the assembly i have although it is very close. all the testing i have done as described above what done with the original .33 ohm FYI. i replaced them cause the sizzled when the fuse was blowing cause the power amp transistors were shorted. will the .47 ohm resistors cause any issues????

second i replaced the MJ15001 and MJ15002 (originally designed with) with MJ15004 and MJ15003, im pretty they modern replacements data sheet are identical. any concerns???

and last but not least, after replacing the power amp transistors as described above i turned the mixer on with the amp and mixer boards connected together for the first time. BUT!! i accidentally plugged the cable from the amp aboard to the mixer board ( J1-J4) one pin off making the connection look like the picture i attached and causing F1 to blow on the mixer board :(. after replacing the fuse and fixing the (J1-J4) connection i have ran into the noise issue and the power transistors shorting again issue as described above. could of damaged been done before the fuse blew?? i did a quick check on some of the components on the mixer board and everything seems okay.

sorry for leaving this stuff out from the earlier post....
 

dorke

Jun 20, 2015
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First thing to ask is what kind of noise?
Hum(line-50/60Hz buzz),White(shshshshsh type),Crackling,other?

In general,you can get noise if J1 is disconnected ,because the input to the PA is floating.
Most probably ,It would be the Hum type.
If you disconnect the cable J1-J4 at the mixer side you will get a lot more noise because of more pick-up noise on the cable(the longer it is the more noise you get).

You can connect a "shorted plug" to "Line-in",to be sure there is no signal and no noise coming from the mixer.
In that case the the output should be dead quiet ,otherwise the noise comes from the PA itself.

The resistors and TRs are important(but at low volumes shouldn't cause a problem).
Preferably they should be the original types and values.

Don't connect speakers at first!
Try running the AMP without speakers connected to it.
If you have a scope you can monitor the output signal for noise.

If you don't have a scope , headphones can help:
Measure their resistance and use a resistor in series such that the series resistance of both be about 1kohm (e.g. for 600ohms phones use a 470ohm resistor) and monitor the output for noise.
 

newjeeper369

Aug 30, 2015
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you suggest i put the original resistors back in? they ohm out fine im sure they are still okay. also do you agree the power transistors i installed are suitable for substitution of the original ones?
 

dorke

Jun 20, 2015
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In theory the Tr's should be fine as a "beefier replacement".
The question is where did you buy them from?
if on Ebay(and alike) they could be fakes!
If that is the case(Ebay),
I suggest you cut the burned ones open and compare to the photos in the following link:
http://sound.westhost.com/fake/counterfeit-p1.htm

About the resistors:
What is the actual value of R34, is it 75ohms?
What are the actual values of Q8-9 ,Q14-15? Do they match the schematics?
In case all the above match ,I think it would be better to go with the 0.47ohm.

BTW,have you check the resistance from out to ground without the final Trs in?
should be high
 

newjeeper369

Aug 30, 2015
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the TRs are from newark so im sure they are legit.
ill have to check R34 when i get the chance,
Q8-9 (i just replaced) and Q14 -15 (i have new ones im going to install) are what the schematic say they are.

i will check the resistance from out to ground, thats a good point it.

i have off work today so i should have to to work on it today ill let you know my findings.
 

dorke

Jun 20, 2015
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Well,
R34 value is another deviation from the schematics.
I would go with the 0.47 anyways.

Forgot to ask if you checked D15,D16.

In general,
would you say that the deviation from the schematics is just in values of some parts or that the board is actually different from the schematics?

One of the problems with fixing circuits someone worked on before is that they may have changed values,inserted "replacements" etc.
If that is indeed the case,the closer you get to the schematics the better.
That said,there are cases the schematics have errors in them as well...

Another thing is Q7 the bias Tr. ,Is it the 2N3404 ?

After reassembling the Tr's
You should check for the +/- 1.1V at the bases of Q8,Q9
and if needed adjust it with pot. AP-1 (left of Q7 in the schematics).

If all goes well ,the noise issue would be next.
Have you got a scope?
 

newjeeper369

Aug 30, 2015
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D15,D16 are one of the first things i replaced.

the schematic id a little different for example the schematics shows is should have 3 2n5210s but is only one and 4 2n5087s but only 3. the traces and the layout of the board aren't very organized so it is really hard to tell without taking all the parts off and tediously mapping the traces.

Q7, that seems to be another deviation from the schematic is not even on the parts list.

by "all goes well" you mean if it doesn't blow up when i turn it on? lol
 

newjeeper369

Aug 30, 2015
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i have scopes at work, i guess if i had to i could work on it during lunch or after work......i need a scope for home, what do you think about the computer adapters you can get for like 100 bucks?
 

dorke

Jun 20, 2015
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Yep, I meant no smoke....

I don't think the USB scopes are worth the price(they have poor resolution),
For about 100$ you can get a Tektronics 465B( I use one) or 475 etc. working analog scope.

For audio,
I have used the "Visual Analyzer" a free "software scope".
It is great for audio,utilizes the sound card of the PC.
You should take care not to blowup the sound card,
build a probe for it:
a voltage divider to get your signal to the 200mv level or about.
Then you should calibrate the scope and probe with a known signal source.

Did you get the AMP with D15,D16 failed?
 

newjeeper369

Aug 30, 2015
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the voltage drop over one of them was a little low, i had new ones laying around so i replaced them.
 

newjeeper369

Aug 30, 2015
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i use an old HP analog spectrum analyzer at work for offsetting sine and cosine inductosyn errors.
 

newjeeper369

Aug 30, 2015
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Got the last part i was waiting for today for the amp board. i installed and tested, i didn't have long so i didn't get very far but this is what i found.

  1. nothing blew up!
  2. still seems to be noise, i plugged headphones in the line out sounds like a hum. i used a 5k pot to adjust my headphone impedance to 1k. i then plugged my phones into the output and the noise is the same just a little louder.
  3. at the bases of Q8 is -47volts and Q9 +47 volts????
  4. with the phones in i shut the power off and the noise is there till the caps drain and the it make a low to high frequency noise. the best way i can describe the noise is like a old video game sounds of a motorcycle.....it made me jump when it started lol
 

dorke

Jun 20, 2015
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1.You probably measured at the collectors of Q8-Q9,
look at the pinout of the TIP31-32 attached.
2.Try the same "noise test" with Line-In (J3-in amp schematics):
insert a "shorted plug" into J3 (i.e center pin connected to outer one,
without a cable, just the plug opened and shorted with a short wire).
If you still get the hum noise it is coming from the PA itself
otherwise it may come from the mixer or from bad shielding/grounding .
3.Are you getting sound from the unit?
How strong is the hum noise relative to the soun?
TIP 31-32.jpg
 

73's de Edd

Aug 21, 2015
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.



Jeepers . . . . .Creepers . . . .newjeeper of March 1969 . . . . .

I'm now placing heavy reliance, upon your analytical statement of :


i shut the power off and the noise is there ( till the caps drain) and then [it makes a low to high frequency noise]. the best way i can describe the {noise is like a old video game sounds of a motorcycle}.....it made me jump when it started lol.

Along, with my also taking secondary significance of (XXX) and tertiary significance of the statements [XXX] and {XXX} portions.

Consult your page 1, of your supplied schematic, refer to its bottom left quadrant for Caps of C6 and C7 which are being the black cased units of your picture page 5.
Dig thru your junque box and locate a set of electrolytics of . . .hopefully 4700 or 3300 ufd capacitance at the prescribed 50VDC rating.
Test lead/Clip them in, SHUNTING ACROSS the existing units .

Power up and see what the hummmmmmmmm situation is being then ?
There is being some leeway on the mentioned substitution, shunting caps, you can certainly go higher on the voltage ratings and the cap could be as low as 1000 ufd and even those would tell us something.
(Matter of fact, if you end up needing replacement of those main units in the repair , step up to 63VDC (European) or 75VDC (Amerikanski) . . .since your developed + and - 47VDC readings are nudging against their present ratings specs. )

Feeding by for standback . . . . . . . . oooooooooooops . . .my dern "dsyleixa" kicking in again . . . . . . make that . . . . . .standing by for feedback !





73's de Edd




..
 

newjeeper369

Aug 30, 2015
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thanks 73's de edd

i was just wishing a minute ago i had a scope at home to see what the power supply caps are doing. i had my own speculations of them being up to the task.
your idea of adding capacitance it a good one. i will let you know what it find.

as for what Dorke was talking about, i tried your test and still got noise possibly more than without the a grounded line in signal. and for the Q 8 and 9 thanks for that data sheet, you were correct i was thinking it was EBC. i do have the correct 1.1v and -1.1 on the bases of those TRs.
 
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