31 volts on the output

[Axtman]

I can't figure out why I am getting +31 DC volts on the speaker output of the Peavey 260D power module of the Peavey Standard Mark III. I have checked all the usual suspects (electrolytic capacitors, ouput transistors, and driver transistors) and they test okay. Nothing is burned or look damaged. All the solder joints are okay. What am I missing here?

Here is the schematic. 260D schematic.pdf (THIS SCHEMATIC IS INCORRECT. U1 OP AMP IS WRONG. DO NOT USE!)

[Mick Bailey]

What's the base voltage of Q9/Q10? seems like they're getting turned on due to unwanted voltage.

[Axtman]

I just checked the speaker output of the amp today and it has crept up to +38 DC volts. (BTW the amp is plugged into a 100 watt light bulb limiter. When the amp is turned on the 100w bulb does not light up.)

Q9/Q10 base voltages are +40 DC volts. Q5/Q6 base voltages are +38 DC volts.

[g1]

Do you have a load on the amp, or are you getting DC at the output with amp unloaded?
It may not power up properly with load connected if using limiter lamp.

[x-pro]

..Базовые напряжения Q9/Q10 составляют +40 вольт постоянного тока. Базовые напряжения Q5/Q6 составляют +38 вольт постоянного тока.

Mick Bailey asked about the base voltage of these transistors. In this case you need to measure correctly, between the emitter and base of the transistor, in the appropriate polarity.
Repair of any amplifier (not on tubes) is done initially with no load (speaker ).
The tuning is done with a resistive equivalent at the output of the amplifier.
After that, when everything is in order, the speaker can be connected.

[x-pro]

I can't figure out why I am getting +31 DC volts on the speaker output of the Peavey 260D power module of the Peavey Standard Mark III. I have checked all the usual suspects (electrolytic capacitors, ouput transistors, and driver transistors) and they test okay. Nothing is burned or look damaged. All the solder joints are okay. What am I missing here?

Here is the schematic. [ATTACH]n977516[/ATTACH]

It does not say anything about supply voltages.
This is usually the starting point for repairs.

[Axtman]

I put an 8 ohm dummy load on the speaker output and measured +16 vdc. Q9/Q10 base voltages are +16vdc to ground. Q5/Q6 base voltages are +13vdc.

Also, the dim bulb limiter is now dimly lit.

[x-pro]

From the beginning without a resistor on the output. The input of the amplifier is shorted. Are all supply voltages present?

[Axtman]

The power supplies are good. +/-15vdc and +/-40vdc.

[The Dude]

What is the voltage on the base of Q1 (or the output of the op amp)? By the way, it appears pins are labeled wrong on the op amp. If the op amp shorts, it will upset voltages throughout the rest of the amp depending on which rail it's shorted to.

[g1]

Is there DC on output when amp is powered up with no load?

[Axtman]

I can't believe this! Peavey made a mistake on the schematic!!

My friend and fellow amp tech, Mark Black, noticed this on the schematic.

There was a 5534 single op amp in the amp originally when it blew while I was testing it. I tested the op amp and it was bad. At one point I noticed this 5534 and replace it with a 4558 dual op amp just like it indicated on the schematic! I assumed that the circuit used only one of the 4558 op amps (much like sometimes only one triode is used of dual triode 12AX7 tube). It makes sense because the assemblers would just install one kind of chip for U1 and U2.

Here is the correct schematic: Correct 260D schematic.pdf

(BTW with the incorrect 4558 op amp I was getting 1.4 volts on the base.)

[Axtman]

Since changing out the IC there is no voltage on the output (loaded and unloaded). I put a signal through the amp and got a clean sine wave output reading on the oscilloscope. Everything appears to be fine.

It really pisses me off all the time I spent chasing my tail on this one only to find out it was a drafting error!

From now on I am going to, "Trust, but verify."

[The Dude]

Ah, that explains my previous statement that the pins were labeled wrong. I guess it wasn't the pins, but the part number that was wrong.

[Axtman]

The Dude,

Yes thanks for mentioning that the pins were labelled wrong (for a 4558). My bad for not noticing it.