Peavey xr-600

[cbass]

Replaced bad transistor still no change

[Jazz P Bass]

Ping Pong Post

No change as in what.
As I understand it you had Vdc on the 1/4" input jacks.
Then you found a bad +15V buss.
Fixed that.
After that fix you found voltage on the output jack.
Broke & replaced a transistor.
So, you still have voltage on the output?
The power rails & the + - 15 are o/k?
You still have voltage on the input jacks?

[cbass]

Sorry I thought i had mentioned that replacing the bad resistor on the supply rail seemed to have fixed the voltage on the input jacks.After rereading my posts i failed to mention it.
After replacing that resistor i discovered that i had dc on the speaker outs that was not there before.

Yes I still have dc at the speaker outs -20 to be exact

The +- 15 rails are in check now.I'm getting 15 volts on pins 4 and8 on the 4558's.I do not have the mixer board hooked up now just the board with EQ,effects loop and power amp in jack.

I have a very small amount of dc on the power amp in now about .2volts

[cbass]

schematic

Thee power amp for my mixeer is on the last page 6

[Enzo]

OK, power amp then. -20vDC on output. Looking at drawing for 400R.

Top, left of center, see two 5w resistors, 2k and 1k? Is one of them open? Ther are DC reference voltages all over this schematic. At the junction of those two resistors, it calls for +16v. Do you have roughly 16v there?

There is a 430 and a 431 driving the strings of output transistors. I am going to wager you have about -20v on the bases of those two transistors. Do you?

See the lower left 430? The one with the 18 ohm resistor in its emitter lead? If that is turned on hard, it will pull the outputs negative. Is there about 0.6V across that 18 ohm?

[cbass]

There is almost no voltage at the juction of the 1k and 2k 5watt resistors.they both test good
There is +50 on the other side of the other side of the 2k.But there is -21Von the other side of the 1k.
I hope that maks sesne I really appreciate the help.

[cbass]

I am getting about .4vdc across the 18 ohm resistor .I think I found the right one i didn't see any other 18ohm sistors

[cbass]

Yes I am getting -20 on the bases of the 430 and 431

[Enzo]

As I knew you would. And since there is -20 on the downhill end of the 1k, there better be -20 on the base of that 430, since they are connected together.

If there is +50 on one end of the 2k, and zero on the other, either it is open, or something is dragging it down. 50v across 2k ohms is a watt and a quarter. That resistor would be getting pretty warm. Does it get warm? If it gets warm, it is conducting current.

The bias string is through those 2k and 1k, down through the dual diode, then the single diode, through the 430 and through the 18 ohm. From your numbers, you do seem to have about 20-25ms flowing through that. Perhaps that 430 is just doing what it is told.

Did you go through the DC voltages all over?

Top left, left of the 2k. Diode, then 330 ohm provides 48v for the differential pair. Is that there? The diffy pair itself, is there roughly zero volts on those emitters? And down below them, getting about 1.1v on each collector?

[cbass]

Both the 1k and 2k 5W resistors are getting hot.

The 330ohm drops the voltage from around 52 to about 50.But on the other side of the 22k i'm seeing -10vdc


The emitters on the diffy pair are seeing about -10vdc.
Bases are -10 and -1.5vdc
collectors -50vdc

[Enzo]

OK, the 330 is not open, good.

I wish these older drawings had included part numbers, so much easier than calling something the "third one from the left" or some such.

So it looks like that bottom 430, with the 18 ohms, is conducting hard. I'd probably just change it, if it were mine. It may be bad, or it may be a victim of excess drive, but a new one would at least eliminate the question. I think everything to the right of it is working. It all is just being told to slam over 20v negative.

So the base of that 430 connects over to the left to the collector of the left diffyQand ther is a dual drop diode down to V- (-50v). According to the print, the base of the 430 chould be about 1.1v more positive than the V- rail. Sicne the emitter is up from that same rail 0.6v through that 18 ohm, we can figure ther ought to be about 0.5b from base to emitter at the 430 itself. Explore all that.

The output bus samples through the 68k to the base of the right diffyQ, that negative voltage pulls the transistor on harder, so more current flows through it, increasing the V drop across the 22k. That tends to reduce the bias to the left diffyQ so it conducts less. That in turn would mean less current through the base of the 430, and that should make the 430 conduct less. And if it conducts less, then it won;t drag the voltage down at its collector as much. What a glorious self-correcting feedback loop these amps are. Not only that, but the increased current through the right dQ raises the V drop across the 1k in its collector lead, and that in turn turns on harder that other 430 we have been ignoring, and it drags more current through teh 18 ohm, raising the V drop across it,and that also reduces the bias to the main 430, also causing it to want to conduct less.

When they are working. That is how it is supposed to work, yours is not.

Y'know, not only that 430, I'd probably shotgun the thing and replace the diffy pair and both 430s, just because parts are cheaper than my labor. SO don;t neglect to at elast test the second 430. Find that small 10uHy choke and make sure it has not opened. It could possibly have a resistor there instead of the choke, by the way. It will look like a resistor regardless, most likely.

Since the whole system is one big loop, any one of those components can upset its balance.

But what do you want for $5?

[cbass]

Thanks Enzo i'll try peplacing that 430 and the diffy pair.

I figured for $5 what the hell.If i can gett it working i plan on running a pre amp into it and using it for a practice bass rig.
If i cant't get it going i figure i would use the shell for a head cab for a 4x6v6 trainwreck type amp i built.

[Enzo]

I can't imagine it is non fixable.

[cbass]

maybe for you.

[cbass]

Guessi'll have order the 2n5400s Is there anything i could sub for it?
I'll go ahead get some extra tip31 and 32s.

I went ahead and pulled the diffy pair and the 430's to tst them .They alltested ok but i guess that doesn't mean they willwork right with voltages applied to them