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Peavey XR600b with 400b/g low volume output

Started by nosaj, July 14, 2016, 09:54:22 PM

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nosaj

I've got a Peavey xr600b   that had a shorted output transistor on it.

I replaced all 8 TO3's and the 4700/63v and the 1000/35v caps.
The + - 52 rails are good and the -+15rails are good.

Running some music in the AUX and Monitor inputs or any of the 6 channels
The volume is real low.  But if I lineout the amp to another amp it's decent sounding with  a little hiss when cranked up  ...  I diid look over the solder joints  nice and silvery.

The sand resistors tested ok on the board.  Any suggestions on how to proceed?

Thanks,
nosaj

Enzo

Probably didn't have to replace all those transistors, but that is water over the dam now.

Monitor input?  AUX in I know, MAIN out, Graphic IN and OUT, PA in, FX out, Footswitch jack, and MONITOR out.  Where is monitor in?

meanwhile, isolate the problem.  You ran a line from I assume Main out or possibly graphic out to another amp and it is strong.  But did you try a patch cord from graphic out to PA in?  Or Main out to graphic in?  Plug a signal into PA in to check the power amp itself.  You can also bypass the graphic by patching MAIN out to PA in, just for test.  Oh, and also try running a signal into Graphic in.

All those jacks on the front have cutout contacts, and if they get dirty, you lose signal.  Exactly the same as dirty FX loop jacks in guitar amps.  But we need to isolate the problem to the power amp, the graphic section, or elsewhere.

nosaj

Will do on that/
also from another thread i read..  I get dc on the speaker jack if a speaker is plugged in(or a dummy load. if nothing is connected to a speaker jacks  I get 0vdc.
Same on the mon and main out.

Thanks,
nosaj

Enzo

I don't follow.  You get DC on the MAIN and MON outs if you plug speakers in?

The MAIN, MON, and other front panel jacks are all line level jacks, no speakers to be connected there ever.  You can plug a cord into each jack and check at the free end for DC voltage across the jack.  There should be none.  If you get 15v, then there is a shorted diode, and might be the cause of your low levels.

How much DC on the speaker are we talking?

If this thing no longer blows fuses, get rid of the bulb if you are using one.

nosaj

#4
scratch all that.   meter was on ohms instead of voltage.
8ohm load .1 to .01  you can hear what sounds like a heartbeat real low  I'm guess something is motorboating.
no load  is .03vdc.
no combination of patch cable increase volume.

So I would venture to say it is in the power supply.

Dim bulb is no longer being used.
Also the compressor light is constantly on.
Thanks,
Nosaj

Enzo

The patch was ONLY going to tell us if one of the jacks had an issue.  If a circuit has a problem, patching won;t cure it.

But did we put signal into the PA IN jack?  If that is weak, then the power amp is faulty and we can certainly fix it.

Power SUPPLY?  If you have both 50v rails and all the 15v rails, then power supply is OK. 

The ICs are in sockets, right?  Assuming the power amp is bad.  Isolate it.  Pull that five wire cord from the corner of the 400 board to the master board up front.  Just operate the power amp alone.  You can use clip wires to input signal to it right on the now bare connector pins.

There is the small IC, the 87478.  Pull it.  Does the sound recover?  That IC is the compressor/limiter.  Try a different TL074.  If you don;t have any, steal one from the front panel boards for test purpose.   And there is also a single 4558 on the 400 board, it too is a limiter, pull it.

nosaj

Will do. It will be Sunday evening before I get back into town before I cando any of it.

nosaj

nosaj

Quote from: Enzo on July 15, 2016, 11:32:06 PM
The patch was ONLY going to tell us if one of the jacks had an issue.  If a circuit has a problem, patching won;t cure it.

But did we put signal into the PA IN jack?  If that is weak, then the power amp is faulty and we can certainly fix it.

Power SUPPLY?  If you have both 50v rails and all the 15v rails, then power supply is OK. 

The ICs are in sockets, right?  Assuming the power amp is bad.  Isolate it.  Pull that five wire cord from the corner of the 400 board to the master board up front.  Just operate the power amp alone.  You can use clip wires to input signal to it right on the now bare connector pins.

There is the small IC, the 87478.  Pull it.  Does the sound recover?  That IC is the compressor/limiter.  Try a different TL074.  If you don;t have any, steal one from the front panel boards for test purpose.   And there is also a single 4558 on the 400 board, it too is a limiter, pull it.

Inserting signal at the 5 wire plug connector on the 400b/g board signal is still low.  Pulling the 87478 no change in vol, tried a new tl074 no change  and u3 for the 4558 looks like it was never installed.
Re verified power rails  50v and 15v.

One thing I did notice  (by the 400bg silk screen logo)the white/blue/red cable connector has -51v on two wires  Whereas the other white/blue/red cable only had +50v on one of the wires..
Tomorrow I can be more specific on the wire colors with the voltage  it's pouring here and shed is separated from house which is where the internet is.

Thanks,
nosaj

Enzo

Wire colors mean nothing to me.

Oh, I know what you mean.  You are talking about the two three-wire cable from the main 400 board over to the power transistor board.

it is important to know what you are looking at.  Look art your schematic.   There are two sections of output transistors, with dashed lines around them.  Q3,4,5,6 is one such grouping.  Think of it as one big transistor for a moment.  it has three terminals, the top is V+, the left end is the base, and the bottom goes to the output bus.   What voltages would we expect there?  V+ is V+, so +50 at the top.   Normally the output sits at zero volts, so we expect zero volts there.  And the base?  According to the drawing it ought to sit at about +0.58v.  That is close to nothing if you are only looking for 50v at a time.

The other group?  This is a quasi-complementary circuit.  So the bottom grouping is also NPN.  But look at it.  The bottom is now V-.  So -50v there.  The top is the output bus, so zero there.  And the base? According to the drawing, there is that same 0.58v bias there, but in this case, that base is 0.58v less negative than V-.  SO -50v less 0.58v means roughly -49.4v there.  And if you are only looking for 50v, that looks close enough to be 50.

So first, even at 50v, half a volt matters, as it is written on the print.  And second, the circuit tells you what to expect there.   I have no idea which wire is which color, I just look at what is connected to what.

I will believe U3 was omitted, that same board is used in a variety of models, and not all features are installed on all boards.

So we eliminated the easy stuff.  If you do not have a large DC on the output, I will assume the DC voltages are probably OK in there.  But the print is full of them, so see how your voltages compare to the print.   I don;t care if the 50 ought to be 49 or 51.  I do want to see both V+ and V- the same voltage.  All of them are more or less, not precise.

Follow the signal. Apply a steady signal and see what happens to it.  What does it look like leaving U1-14?  How about the collector of Q9?

nosaj

on the schematic  the top row of transistors  E -51.22 and B -51.22  C -.07
bottom row E  -.09          B    -.09            C    52

Q9 E -51  C  -.16  B -50

Signal coming  out of U1 pin 14  is larger than the input it is amplifying there.

This is just a guess since i don't quite understand whats happening 
Q9's B should be .96 not -51
Q2  E -.07  B .37  C 51.

Thanks nosaj

Enzo

Um, I think you have top and bottom row reversed, let us stick with Q numbers.  Top row to me is Q3,4,5,6.  C is +50, B and E ought to be within a volt of zero.   Bottom row is Q13,14,15,16.  And C ought to be around zero, while B and E are about -50.  That agrees with your numbers.

Look at the drawing fo Q9, that 0.96v has arrows by it showing that is 0.96v between that base and the V- rail, not ground.   In other words, R58 has 0.96v across it.  The C of Q9 has roughly zero on it, so I;d expect roughly zero on my speaker outs.

Maybe I am confused what we are chasing here, your voltages look normal.

nosaj

Low volume is what I'm getting not even as loud as a portable radio.  I'll verify Q9 shortly.

Thanks,
Jason

nosaj

i understand on a tube amp where I can inject signal but on a Solid state amp where would one inject signal at?

Thanks,
nosaj

Enzo

I have never had to inject a signal anywhere but those Molex pins in the corner.

You can remove Q10 just for grins, it is a limiter.

You can lift diodes CR10,CR15 to disable Q1, Q7.  They are also limiters.

The collector of Q9 is connected to the dual diode on the heatsink.  Whichever is more convenient, compare the signal level you find there with the speaker output level.  Are they about the same?

Look straight up from Q9, see the 50/50 cap C8?  For some reason I am always suspicious of that one.  Do you have 26v atop it (or whatever half of V+ is in your amp)?   And try clipping another cap in parallel.

nosaj

c8  50/50v has 24vdc on oneside and 1.09vdc on the other
c21 2uf/35v has -13 on one side and -3.88 on the other
c11 2uf/35v has 4.5vdc and 14.65 vdc one the other

I'm gonna have to see what I have on hand tomorrow. Would a 4.7 uf work in c21 and c11. i may have a 47uf/250 to test with.
Will have to pull the requested parts tomorrow.

Injecting signal at q9 collector was louder than injecting at the molex pin.

Nosaj