Acoustic 140 question

[pontiacpete]

Could someone tell me what power transistors are used in the 140. I have one here that someone had changed out Q303,304,305 and306 with NTE130.
Just what to know if the NTEs are suitable replacements.
So far all I"m getting is a very low distorted output in both channels

[Jazz P Bass]

2N3055 is the cross.
Enzo recommends MJ15003 (cause that's what he has)
http://music-electronics-forum.com/t36028/

[pontiacpete]

Could someone help with this? I'm trying figure out why there is no voltage on the collectors of Q305 and 306.
Q303 andQ304 have about 71vdc on their collectors. On the schematic it looks like there is suppose to be 37v is that right? If so there is only .5v.
Does this means the bottom half is not conducting?
Does voltage come down from R312?

Thanks in advance

[52 Bill]

On the schematic it looks like there is suppose to be 37v is that right? If so there is only .5v.
Does this means the bottom half is not conducting?
Does voltage come down from R312?

If you look at the power amp schematic the top two power transistor collectors connect to the 75 volt bus and the bottom two transistor emitters connect to ground (through the ballast resistors). Also connected across the 75 volt bus and ground is the string of resistors R312, R314, R313 and R315. Disregarding the thermistors for now, with the resistors on the top half and the bottom half being equal, the center point of the circuit should be at 1/2 of the 75 volt supply voltage.

Do all of those resistors read close to their marked values?

[pontiacpete]

Lifted the legs of each and all looks good.

[52 Bill]

Lifted the legs of each and all looks good.

Then something is either shorting the voltage to ground or your voltage readings are off.

Check Q305 and Q306 for shorts as well as C310.

[pontiacpete]

I just replaced Q305 and 306 and changed out C310 as well.
There's 14ohms to ground at the collectors of Q305/6. I wonder if something is going on with the sockets.

[Enzo]

The collectors of Q305,306 is your output. C311 there heads to the speaker. Should be 37v there, half of 75v.


If that point is shorted to ground, something will be getting hot. If Q305,306 are on hard, they will be hot. If those collectors are shorted to ground then Q303,304 will be hot.

Your problem pretty much has to be on the right side half of the power board.

Does this means the bottom half is not conducting?

No, if the lower were not conducting, then those collectors would be up at 75v.

I probably wouldn't even analyze it. Four transistors, check them. Then measure base to emitter resistance while they are IN CIRCUIT. The circuit is some smallish resistance of the drive transformer coil, 6.8 ohms, then the 0.5 ohms 5w. So if any of those series things are open, so will your reading be. And you have the other half circuit to compare.

In circuit we have R312, R313 and the two 6.8 ohm in series all across 75v. You could even remove the power transistors. Now are the voltages present? On those 10w 400 ohm jobs, 75v on the top, 37v on the bottom of R312, roughly 37v on the top of R313 and ground at the bottom.

You have the bias caps on each base, C309, 310. If one shorts, that turns off its transistors. C309 shorts, poof, top transistors are stuck off, and you have your symptom.

It doesn't especially matter what the transistors are. As long as they meet the minimum specs, they will work.


I highly recommend against NTE parts, but not because I think they won't work. They generally cost multiple times what the real parts they replace cost. And they are not the same parts. If you replace Q301 or Q302, I wouldn't care much, because each acts alone. But your outputs work in pairs, they have to share currents, so we want them all to be the same.

Yes, god help me, I would prefer to see all four of them NTE130 rather than three real parts and one NTE.

[52 Bill]

There's 14ohms to ground at the collectors of Q305/6. I wonder if something is going on with the sockets.

Is the speaker connected? Is C311 shorted?

[Enzo]

14 ohms?

Is there a missing mica insulator under one of Q305,306?

What voltage is on the emitters of Q303,304?


You either have a serious load down or you have an open.

[pontiacpete]

The speaker was connected. Now it's not. Took the NTEs out and put in 15003s in all places earlier today.

Now, I pulled all the output transistors and the collectors of Q305/6 are now 1.2v. Q303/4 has 58v on limiter.

[g1]

The speaker being connected should not have caused the 14 ohm reading from Q305/6 collector to ground. The speaker is coupled through C311.
Do you still have the 14 ohms there? Is C311 shorted?

Edit: I see 52Bill already asked about C311.

[Enzo]

The problem SHOULD be apparent. The output transistors are socketed, are they not? If we pull them we are left with the bias/drive circuit. And that is little more than two 400 ohm and two 6.8 ohm all in series across the 75v. By measuring at each junction we ought to get expected voltages. If we do not, then either the voltage is not getting to the top of R312 in the first place, or something is open along the way. Oh, also possible a wrong value part has been installed. For example a 6.8k instead of 6.8 ohm. Each output base is fed by a yellow wire from the drive transformer. Whatever voltage is on the 6.8 ohms ought to be on BOTH ends of the drive tranny wires. So, does the bias voltage reach the base conections?

[pontiacpete]

All the electrolytics are new on the power board and C311, all but the big main one at the back of the amp.

With all the transistors out of the sockets I'm getting 15 ohms on the collector center tab.

I lifted the one leg of R312 at the junction of R314 and R316 and there is about 60v with the limiter, put it back in and there's 1.2v
Lifted one leg of the thermistor, lifted all the legs of the resistors in this stage and all measure in spec.
I don't get it.
Someone has worked on this before and I keep thinking there is something not connected right, but what. There's not much to this part of the amp.
Is it possible that transformer has a winding shorting to ground?

[Enzo]

Is it possible that transformer has a winding shorting to ground?

You have an ohm meter, 10 seconds and you will find out.


My advice: Never think up reasons not to check something.

If you think a transformer winding is shorted to ground, just find out, don;t try to decide ahead of time if it "could" happen.


If you are still getting 15 ohms to ground at the output (+ end of C311), track it down. Circuit paths radiate from there. follow down them with your ohm meter to find the 15 ohm path to ground. Each branch of circuit will either get closer to ground or farther from it.