Ampeg B115 DC on output

[harleyeg]

Gonna repost this cause it looks like the schematic didn't attach.

Hello,
Trying to figure out how 10 vdc is getting in the 0 vdc rail. This condition was exhisting when the amp was given to me. When I opened it up I found the bias transistor bad so it got replaced along with Q204 and Q205. I also replaced the power transistors with 2SA1962 and 2SC5242, all coupling caps. All diodes were pulled and checked.
When the amp is first turned on, with no load, my DVM shows 58 vdc at the speaker jack, then quickly drops to 10 vdc. All other test voltages line up, except for the zero volts at R223.
Any help would be greatly appreciated.
Thanks.

[Enzo]

Hi Harley, welcome to the forum.

The way to do that is not start a second thread, just post under the original. Unfortunately, you are going to have to either zip your file or alter it in some way. We had a server crash a while back that emptied all our files. But the index remained. So you are posting a schematic that already existed, so when clicking on it, we get a blank page.


Verify you have both power rails, and also verify you have good 15v rails right at the ICs.

ANy time a transistor is burnt out, always check any associated resistors, they often open. Especially those fractional ohm power resistors in the emitter leads of the output transistors.

[Jazz P Bass]

Schematic.

B115 B410.zip

The driver transistors usually take a hit when the output is blown.

Q302 & Q303.

And, yes, what Enzo said.

[harleyeg]

Thanks Enzo, I will remember that.

This is where I got the schematic.. Free Ampeg Diagrams, Schematics, Service Manuals :: Schematics Unlimited

I do have both power rails, they are a little high, should be 56.5 vdc and I'm reading closer to 60 vdc.
I also replaced the IC's (IC #1) since they were gone as well. I was getting 19 to 20 volts there at the input of IC #1. So at that point, I took out the 680 ohm 5 watt resistors and put in 750 ohm 5 watt and new zener's.. That got me down to 17.2, plus and minus. Not sure how sensitive those IC's are.

On the power board, I have pulled every component off that board and checked it. Some of the carbon comp res. have really drifted and they have been replaced. All the fractional resistors check out too, however, R227 and R228 are not .33 as the print calls for, they are .47's. I would also like to note that when testing, the pre amp boards are totally isolated from the power amp side. Anyway, like some people, I don't have that much experience with SS...this thing is kicking my butt!!

[harleyeg]

Hey P Bass, thanks for posting the schematic, very cool.

Q302 and Q303 are new and still show good on my meter in diode mode. But how can I be really sure unless current is flowing through them.. right?
Also, the four driver's show good as well and are pulled out of the circuit when testing.

[Enzo]

I would also like to note that when testing, the pre amp boards are totally isolated from the power amp side.

I don't recall this model inside, but if you are disconnecting the power amp from the preamp, some amps have to have their input side grounded via the input connector. Note on the schematic that the preamp grounds are all the symbol looking like a garden rake, while the power amp grounds are the triangular symbol. The two ground systems join in the middle via P1. If the input ground is not terminated, the amp can float to a DC voltage. Simple enough to check, plug in the preamp.

except for the zero volts at R223

Unless you have zero at both ends, you don't have zero "at R223", you have zero at one or the other END of it. Specify. Since you have 10v on the output, which is wired directly to R223, that contradicts zero. if you have zero at the other end, then the resistor seems to be open.