Acoustic 370 grounding issue, ch2 buzz

[PRNDL]

I've got an old Acoustic 370 Solid State bass amp that has a minor issue with the low-gain channel, which buzzes when used. The 1st channel is fine.

The first issue seems to be the amp ground. Someone put in a 3-prong cord, but the green wire was cut and not connected anywhere. I took out the "death cap" and soldered the AC ground to the chassis where it was connected.

It still has issues with the 2nd channel buzzing. The scope shows a sine wave on (what I believe is) the board ground when the function generator is connected to channel 2, but not with channel 1, which makes me believe that the issue is the grounding scheme.

Both jacks are 3-terminal with two of the lugs tied together and wired to the board ground to form a voltage divider with different resistors in series (10K and 22K).

I'm not sure what to do next.

Could it be an improper connection to the chassis with one of the jacks?
Should put a ground wire from the AC ground to the jacks?

Any suggestions are welcome!

Here's a schematics page
http://acc.homeunix.net/schematics.html

[PRNDL]

Fixed. Volume is the next problem.

It turns out that someone wired the input jacks to the wrong places on the preamp board connector. It was a simple fix.

The next issue is the overall volume is low. At 300 watts, this amp should be LOUD.
I have it connected to a 16 watt Marshall speaker.

Any ideas?

[PRNDL]

Something's burning

I saw smoke ... some puffs between the power board and preamp, but I couldn't tell exactly where it was coming from. The chassis under the power board is very hot.

Perhaps one of the output transistors has shorted.

Since they are all in parallel, do you disconnect all of them to test them separately?

[cbarrow7625]

Do R322, R323, L301,R328,C404 look healthy? That would be the first place I would start. Those are all series elements on the output. If any one of them open up your volume will drop.

How do the rails look? Right voltage or sagging? Without seperate emitter/collector resistors on each transistor I worry about the health of all of the output transistors. They can't possibly share equal current for the life of the amp & not have one go down.

How do all of the reference bias voltages look? Looks like you are pretty much DC coupled from C301 through to the output transistors. There is one adjustment pot (R304) that could adjust the DC throught the output stage.

Let us know.

[cbarrow7625]

You can test with all of them connected just to determine if one of the 3 is shorted. If something is shorted then you need to pull them all individually to determine which one it is.

[PRNDL]

Do R322, R323, L301,R328,C404 look healthy?

Yes, and the output transistors tested fine with ohms.

The smoke came from the other side of the board, which might be the preamp board.

The difficulty is the hex screws for the pots is not one that I have in my set.
I'll have to go to Ace Hardware.

[cbarrow7625]

You can inject a test tone (sine wave) just ahead of C301 & isolate whether the volume problem is in the preamp or power amp. With around 0.5-1.5vac injected at that spot you should get something approximating full output if the power amp is fine.

[PRNDL]

R329 is sizzling and leaking.

It's a 22 ohm 2W connected to the speaker out (before the coupling cap C404) to the rectifier return (ground) via C313, another coupling cap.

I measured it as 72 ohms.

[Enzo]

SO is C313 shorted? Pull it and see.

The output on this single ended power amp sits at 45v, so this stability netwrok has to endure that. Not usually a problem, but if that cap fails, then the 22 ohm resistor finds itself across 45v. That is 92 watts. If C313 is only leaky, then a few less watts.

Replace them both, I'd say.

Then check the output of the amp for RF.

It is possible C313 is fine and the amp is oscillating at high frequency, like 50kHz or something. Scope the output and see. Such oscillation could burn up that resistor as it tries to shunt those HFs away.

[cbarrow7625]

PRNDL, I'd replace both the cap & the resistor just to be sure. Check the output with a scope like Enzo said. If C313 does not measure as leaky or shorted then you likely do have a high frequency oscillation. Even after replacing the parts they will just get cooked again.

How about the rest of the DC reference voltages? Have you had a chance to check & see if this thing is even running at the right voltages?

[PRNDL]

I replaced the resistor and found a cut trace on the preamp board that's for the ground (It's sort of odd that I missed that earlier).

The amp still doesn't work, although I believe it is a preamp issue because the signal at the monitor jack is in the mV range and looks hacked instead of a sine wave.

It's somewhat of a difficult chore to trace out the preamp since it is mounted upside down. There seems to be enough wire to flip it around.

[PRNDL]

I'm still working on the amp.

I replaced the 22 ohm resistor with two 10 watt 10 ohm resistors (Yes, it's overkill, but the local shack doesn't seem to care about electronic parts, and this was the only thing close).

The ground trace on the preamp board burned out about 1/2 an inch with a lot of scorching on the rest of it. So there's a ton of current going through there.

This is because the entire amp is grounded through the preamp trace to the chassis at the input jacks.

I've cut the jumper between the power board and preamp board grounds and plan on adding a ground wire from the power amp board ground to the chassis. That will protect the preamp trace with a risk of adding a ground loop, but will allow me to test the amp without risk of burning the preamp board trace even more.

I did see some hair (high frequency) on the power supply rails, but it wasn't very high in amplitude.

[PRNDL]

It's HF oscillation in the power amp section.

The preamp signal before R301 (27K) is a sine wave (as expected), but it has high frequency oscillations afterward. It's bursts of HF with no signal in between, so there's some kind of switching with it.

The power rails look OK -- +90 volts DC from the big cap and +45 DC before the speaker cap. I also checked the DC test voltages and they seemed fine too.

I'm not sure what too look for ... any suggestions are welcome!

[cbarrow7625]

What about the other DC reference voltages on the schematic? Is there 11VDC at the base of Q301 & Q302? 11.6VDC at the emitter junction of those transistors?

Is there 43.7VDC on the collector of Q304 & 41.7VDC on its emmitter?

How about the voltages around Q310 & 311?

How about the voltages around Q316 (bottom of the schematic)? .3VDC at the base & 10.4VDC at the collector with 9.8VDC at the junction of R333 & R334? That is a protection circuit. If more than about .6VDC is present at the base of Q316 it will "close" that transistor, pull down the 10.4VDC & cause Q315 to shunt the input signal to ground.

What is S301 in the amplifer? Is that some kind of reset switch for the protection circuit?

Find out some of the voltages & let us know. Seems like you are closing in on the problem slowly but surely.

[PRNDL]

I did measure all those voltages this morning and they seemed fine (it took a while to trace out the board).

I'll measure again with the DMM and write them down this time.

S101 is an AGC defeat button that is used when setting the trim pot R304.