Acoustic 320, low output (again!)

[Jeppe]

Google brought me to the very enlightening thread between W2L, Enzo and others on this forum concerning an Acoustic 320 with low gain in the power amp (first hit on "acoustic 320 low gain"!).

My first hope that my own low-gain-320 could have aquired the same disease was however disappointed. The AGC (the limiter circuit) does not seem to be the culprit - I tried to remove the jfets Q1 and Q2, and output is still low. As Enzo has convinced me, this proves that the AGC is not involved.

Unfortunately I don't own an oscilloscope, only a multimeter. But my ears tell me that the amp doesn't distort, it just plays low. I tried feeding the power amp directly with my SansAmp D.I. - still low.

There is one other thing that perhaps could be a clou: The Acoustic has a "Power Boost" button that used to make the amp play even loader (but less "hifi"). But now it just cuts a little off of the treble, the gain is unaffected. When I look at the schematic, turning power boost on, grounds C3 and R15. Grounding C3 must create a low pass filter on the input, right? This corresponds to what I hear. Perhaps that could indicate that the power button only has effect on C3 but on in the R15 area (btw, I checked R15 and it matches its color code)...

Test points: There are some test points on the scematic.

between C6 and R10: Expected 21v, meassured 20.6v
between Q4 and R19: Can't meassure, the amp oscillates if I touch here (ouch!)
between R5 and R43: Expected 10v, meassured 6.3V
supply: Expected 93v?, meassured 93v
output line, left of inductor: Expected 42v?, meassured 44.6v
base of Q11: Expected 1.45???, meassured 1.5v

The schematic I found on the internet is barely readable, thats why some of the values have question marks. I anyone can point me to a readable version, I would be grateful.

I'm pretty stuck at this point, any suggestions on how to move forward is appreciated! I already got one suggestion from the back of the amp: "Refer servicing to qualified service personnel", so no need to repeat that


Jeppe

[Twist]

You may have already tried this but have you plugged a jumper cable from the combined output jack to the power amp input jack? Also try another output from a good amp to the power amp input of the acoustic.

[Enzo]

Or even just plug the guitar into the power amp in jack.

That is the first step - isolate the problem. Is it really the power amp, or some earlier stage?

Also, put a signal into the front input, and send the preamp out signal to some other amp for a listen. Is it strong?

Your power amp voltage all look OK. That 6v instead of 10v is suspicious, but with Q1,2 removed, that shouldn't matter.

Apply a good steady signal to the powr amp in jack. Set your meter to AC volts. How much signal is on that inductor L1 (in parallel with R46). Now measyer the same AC volts on the other side of the output cap - that 3900/75v cap in series with the speaker. They should read about the same.

What DC voltage is at that coil L1? SHould be about half the B+, or 42v on the print.

I wouldn't think of the boost as a power boost, but if that is how they label it... I think of it as a bass boost. The switch grounds two places. One is C3. C3 is a simple shunt across the input. Grounding it will lop high freqs off the input signal. Likewise R15. Grounding it essentially enables C8 into the circuit. C8 is at the base of Q4, which is the feedback from the output for the amp input diffy pair.

I doubt they are involved.

[Jeppe]

Thanks for the feedback so far.

I tried feeding the power amp with a SansAmp line driver, still low output. I also tried feeding another power amp I have with the output of the Acoustic preamps (preamp combined output) and they seem to be ok. So I'm pretty confident that the problem is located in the power amp.

I have now tried to measuere the signal on both sides of the 3900uF capacitor with my multimeter set to AC. The outside goes to around 1.5v if I play the loudest I can (bass through preamp). However, the inside measures 97v! Probably my cheap multimeter that cannot handle ac with a dc offset..

So I tried something else. I put a capacitor in series with the multimeter, in order to avoid the dc offset (0.1uF was the largest I could find). Now the signal is not even detectable on the outside (0.0v), but on the inside of the 3900uF cap, I still get 0.3v!

So there is a difference between outside and inside. Should I assume that the 3900uF capacitor is broken? Is there a good way to confirm that if I dismantle the capacitor?

The DC voltage at both sides of L1 is 44.6V.

About the power boost button: I have owned this amp for more than 20 years, and earlier the power boost did infact increase the volume. But surely, there was also a filtering function. I would describe its function as "loud and ugly". I never used it.

Jeppe

[Enzo]

Yes, the cap on series with the meter takes care of the DC. I would not worry much about the .3v difference around that cap. If you are getting .3v of signal on the coil side of that cap, then no wonder nothing comes out the speaker. Since you have 44v on the choke side, that is about half B+ so the DC balance on the output stage is OK.

Keep that cap on your meter and set for ACV. Follow signal through the amp. Signal comes in to the base of input xstr Q3 (Q4 is the feedback transistor), whose colector drives voltage amp Q5. Got signal at the collector of Q5?

The outputs by way of their drivers - Q8, Q10 - have their bases tied together for bias by Q6. If Q6 opened, the two polarities would both turn on full and the amp would blow fuses. SInce it balances out at 44VDC I have to assume Q6 and everything to the right of it is OK. (If Q6 shorted, the amp would work fine except for a little crossover distortion.)

SO Q5 collector grabs that stack centered on Q6 and shakes it up and down with the signal. The drivers and outputs then just follow along.

Trace the signal down that path. Q3, Q5, Q6, drivers, outputs

I'd be thinking open Q5 right now.

[Jeppe]

I may need to purchase a better multimeter. BUT what I can measure (with a 0.1uF capacitor in series with the multimeter) is that the AC voltage at the collector of Q5 is actually a little higher than the AC level on the inside of the 3900uF capacitor (0.4v vs. 0.3v).

I did an in-circuit conductance test of Q5. It is conducting base-to-emitter and base-to-collector, but not vice versa (i.e., if I put plus on the base, it is conducting). The voltage drop is around 0.9v in both cases.

[Enzo]

Everything to the right of Q5 is a current amplifier, bosting the current for the speaker. There is no voltage gain there. SO finding the same signal at Q5-C as the output makes sense. What signal is present at the collector of Q3? Or the base of Q5 if that is more convenient - or the top end of R11 - they are all hooked together. That is what Q5 amplfies.

I'd pull Q5 and test it out of circuit. .9v doesn't sound so healthy for a silicon transistor.

[Jeppe]

I pulled out Q5, and I still measure the 0.9v drop on it. I'll pick up a replacement monday, and see if that makes a change (I have one of the few oldfashion electronic component shops that are left 5. min walk from work).

[Jeppe]

Well, I tried replacing Q5 with a new transistor (the exact type of Q5 is not really revealed by the schematic, but I was adviced to try out a BD159).

Unfortunately that didn't change anything - still clean, but low output. I also tried replacing the output capacitor - no change either.

I measured all the diodes on the print, and they all read around .7v voltage drops, that seems reasonable. However, Q3 and Q4 (and Q11) all have voltage drops around 1.2v, isn't that pretty high? Strange that they are so consistent though..

I can't really detect any signal levels behind Q5 with my low-price multimeter. Guess I need to find a scope somewhere.

[Enzo]

1.2? No that sounds wrong. I would at this point start doubting the meter. A meter that can't separate out the AC from DC sounds pretty low end, no offense. I think we are running into its limitations.

[Jeppe]

It was C8!!!

As you pointed out, my measurements indicated that the error had to be located left of Q5. So it seemed likely that Q3, Q4, a resistor or capacitor was out of spec in that area. I borrowed a quality multimeter, and it showed voltage drops around .6v for Q3, Q4.

Then I exchanged C8 with a new electrolyte of the same value - the amp made a monotone hum, no signal amplification at all. I putting back the old one, back to normal. Strange, maybe I had got a bad replacement. I tried putting the new cap back in - a couple of seconds of hum, some noise, and then suddenly: Back to full power!

Don't know exactly what happened, perhaps some of the other old components needed a wakeup call, or something. I'm considering replacing the other electrolytes on the board as well. But the main thing is that it works now!

Thanks for all your help Enzo!

Jeppe

BTW, I noticed that R44 is running very hot (too hot to keep your finger on). But if you look at the schematic, its a 15k resistor with appr. 92v accross it which gives 0.56w on a 0.5w resistor! No wonder that it broke (after 30 years) in W2L's amp! (the other Acoustic 320 thread on this forum).

[acousticcontrolcorporation]

320 Parts and Schematics!

www.acousticcontrolcorporation.com

[Bruce / Mission Amps]

Add some content and less commercials

www.acousticcontrolcorporation.com

If you are going to take the time to type... why don't you participate instead of leaving your website URL?
How about just sharing some info like the rest of us.
Otherwise your just another annoying free commercial here and that is not in the true spirit of AMPAGE.

[tubeswell]

Bruce - I admire your perseverance

[bnwitt]

Maybe the admin can change his grade from Member to Troll.