Acoustic 320, low output, what to do?

[W2L]

Hello,

I am trying to repair a bass amp head Acoustic 320.
Preamp is working fine but the power amp provides very low output when fed with direct line input.
Have checked the output transistors with a multimeter, all eight of them were OK. Voltage supply is OK. What is the most likely cause?
I have access to an old scope with test signal but dont know where to start the trouble shooting.
The (unreadable) schematic found on Internet says it could be due to defective input or driver transistors or leaking diodes.
I have tried to search for replacement types (pref. european) for 2N4360, 2N4248, 2N6474, 2N2484 but no success.
Greatful for any suggestions and/or a readable schematic, thanks in advance!

W2L

[Amp Kat]

Does it have an effects loop ? Try patching a cord from send to return and also try a preamp out signal straight into the return and see if it works.

[W2L]

Preamp is working fine but the power amp provides very low output when fed with direct line input.

There is no contact problem what so ever. First thing I checked.
The problem is located to the power amp.

//W2L

[Amp Kat]

Could be a driver transistor or op-amp/transistor but usually when they go it brings the supply rail voltage down with it. Could be a solder joint so your going to have to trace the signal with a scope thru the drivers and find out where it stops. You could also use a DMM set to AC voltage.

[52 Bill]

Depending upon the vintage, Acoustic used solid wire jumpers to connect the output transistor sockets to the boards. These wires will fatigue and fail at stress points, cutting off voltage or signal to the transistors.

After checking all of the normal things, I would check to see if any of these jumpers have broken.

[W2L]

Need a readable schematic for Acoustic 320, please!

Hello all,

Thank for all the respons so far.
I have checked all connections all over again and everything seems OK.
Among other things the AGC (automatic gain control) needs to be checked.

Can somebody please help me with a readable schematic for Acoustic 320?
The one I found on the Internet is very blurry and the connections and test points etc is not readable.
Over at www.ampix.org there is a readable schematic for model 140 (guitar amp) but there seems to be some differences compared to model 320 (bass amp).

//W2L

[Enzo]

Is it clipping really early, or is the output simply very low? Scope it and find out. Apply a sine wave. I use 100Hz myself to reduce ear fatigue, but 1kHz is the common test signal.

is the signal clean or distorted?

This is a single supply amp, so there is a large cap in the output line to the speaker. Is the signal level about the same on both sides of this cap? Ignoring the DC difference of course.

Like most SS amps, there is a thick wire inductor in the output line. This one has a 22 ohm resistor in parallel. Measure that resistor in circuit. If it measures 22 ohms, or anything other than zero ohms, then the inductor is open. Most likely one end is broken off the board then.



Is the full + rail there?

If you think the limiter - the AGC - is involved, just remove the two JFETS, Q1, Q2.

In fact the circuit could be OK and one of those is leaky. I'd consider removing them anyway to see.

Q5 is the voltage amp, how much signal is at its collector?

The power amp is board 170079, I don't know what is in the 140.

replacement types (pref. european) for 2N4360, 2N4248, 2N6474, 2N2484 but no success.

I rarely use the BCxxx and similar transistors, so I cannot advise. I do know that Asian small transistors have a different pinout than the USA types. You can use one in place of the other but you have to mount the part sideways and bend the legs to fit the holes on the board. I never know which legs are which on the euro types. Watch that.

2N4360 are P channel JFETs - take your pick, most any will work.
2N4248 are just audio transistors PNP I'd pick some higher voltage types
2N6474 is a 4 amp 120v NPN TO220 like an FT317A, but I'd use a TIP41C or something I think.
2N2484 are low current 60v NPN - they act as limiters, so most anything will work there.

AMps like this are extremely flexible as to what transistors you put in there. It is nothing like selecting which brand of 12AX7 you prefer in your Marshall.

Today, small signal transistors tend to all be TO92. There is no reason to find types in the older packages some of these parts represent. Q5 is a TO39, and a replacement should be the same.

[W2L]

Thanks Enzo!

I will check these things out with my old scope asap.
The AGC circuits in model 140 and 320 seems to be identical so the test point values probably are the same too.
Is Q1/Q2 acting as part of a voltage divider between signal and signal ground?
In that case input signal can be reduced a lot if the AGC-circuit output is to large, am I right?
Will compare input signal with signal at Q3 and Q5.
Thx for the valuable info concerning replacement transistor types; hopefully I wont have to replace any of them.....

//W2L

[Le Basseur]

Over at www.ampix.org there is a readable schematic for model 140 (guitar amp) but there seems to be some differences compared to model 320 (bass amp).

The schemo you meant @ ampix is for the 220 model,uploaded a few days ago.
For the 140,please look here: (I just uploaded it,as soon as I just got it today)
http://www.ampix.org/albums/userpics...oustic_140.pdf
Regards,

[Enzo]

Q1,2 are mutes. If they turn on, it shunts the input signal. The little circuit monitoring output current controls them. Too much output current, and those JFETs turn on, squaching the signal.

[W2L]

The schemo you meant @ ampix is for the 220 model,uploaded a few days ago.
For the 140,please look here: (I just uploaded it,as soon as I just got it today)

Dont know why I wrote model no 140; it says clearly model 220 in the pdf-manual, sorry, my fault.

About Q1, Q2 and AGC: I think I get the picture now, thanks again Enzo. Hopefully I will get this amp up and running again.....


//W2L

[Le Basseur]

Dont know why I wrote model no 140; it says clearly model 220 in the pdf-manual, sorry, my fault.

No prob! Maybe you looked at bobsamps' request here:
http://music-electronics-forum.com/s...49759#poststop
I'd also like to thank Enzo for his explanations and guiding.Very valuable infos,hats off!
Regards,

[W2L]

Hello,

These limiter circuits (Q11, Q1/Q2) is driving me nuts..........
With no signal applied there is 0,35 VDC at the drain of Q11 where it should be 10 VDC according to the schematic for model 320.
Source of Q11 reads 0,3 VDC and Q11 gate reads 1.0 VDC.
I tried replacing Q11 with another NPN all purpose; same result.
The resistor - diod arrangement around Q11 is hard to understand.

I checked the input resistance, it is 4kohm, that is to say Q1/Q2 almost clampes input signal to ground. That must be the cause of the low output.
(Control voltage at gate of Q1/Q2 is around 0,35 VDC, se text above)

Rail voltage and output coil is OK.

Can someone explain the need of these limiter circuits in th first place?
What happens if this function is removed? Is it possible to permanently remove Q1/Q2?

//W2L

[Enzo]

OK, Q11 is a bipolar transistor, so we use the terms collector, base, and emitter.

What happens if you remove the protection circuits? What happens if you remove the seat belts from your car? They only matter when you need them.

This circuit is not right and it is messing your amp up, yes, but that doesn't mean the circuit shouldn;'t be there.

First, if you remove Q1,2 does the amp work normally and produce full power? We want to make sure the amp is 100% before we worry about the protections. I want to know my car will start before I worry where the seat belt is hiding.

Now the circuit. Q1,Q2 are JFETs. JFETs are ON until we turn them off at their gates. You get 10v on those gates and they should open up and let signal pass. Q11 turns them off and on. R43 runs from the +90v rail to the collector if Q11, and on to the JFET gates. R43 and R5 form a divider that knocks the +90 down to the lower voltage. R44,R45 and R42 also divide the +90 down to bias the emitter of Q11 up +1.2v. R45 is selected for proper circuti action, more on that later.

R37-41 serve to bias the base of Q11. R45 value is adjusted to bring the emitter of Q11 up enough to keep Q11 off until enough output is flowing to trigger it.

R36 is a 0.03 ohm (!!!) resistor used as a current monitor. The current of the output stage runs through it. The more current flowing, the greater voltage drop across it. Normally that voltage is very low, vlose to ground potential. That means the cathode of D7 is essentially grounded. That means R38 can pull the base of Q11 down enough to keep it turned off. That is why we had to raise the emitter of Q11 that small amount. If we had grounded the emitter of Q11, then D7 would not have been able to turn off the base.

If the current through R36 rises enough, the voltage across it rise, and eventually S7 can no longer gold Q11 off. When Q11 turns on, it pulls the collector voltage down. That in turn removes the voltage from teh JFET gates. The JFETs relax to on and the inut signal is shunted. This prevent the amp from being over driven.

SO what is wrong here? If Q11 is shorted, the JFETs stay on. Check Q11 with a meter. But if you have only 0.3v at the emitter of Q11, then I am suspicious. A shorted Q11 wouldn't lower the voltage at its emitter. So I wonder if R44,R45 is open, or not connected to +90 somehow. R36 won't be open, or the amp wouldn't pass signal. But an open D7 would leave Q11 stuck on. An open R38 would cause D7 to lose out also.

REmoval of Q11 SHOULD make R43 turn the Q1,Q2 off.

That is how it works. I'd fix it, I wouldn't remove the seat belts from this car.

[W2L]

Thanks Enzo for your comments about how this thing works!
I am hobby musician and not a pro when it comes to electronics.
Sorry about my poor english and lack of knowledge about the terminology.

About removing Q1/Q2: I did not know if these circuits were just for protection. Are they also ment to add some form of compression at high power levels?

About the amp:
I have tested Q11 with a digital voltmeter and it is OK.
I also tried to replace it with a brand new BC547b (only 45 volts, universal NPN), all voltage readings around Q11 were identical.

If Q11 is off and its collector is connected between R45 (6.8M) and R5(1M) it should read approx 10 VDC at this point (= 1/8 of 87 volts)

And another thing: Lets say R44/R45 in parallell is about 10-15k and connected to 87 VDC, then it would result in at least 6-8 mA current through R42. R45 is factory select.
7 mA through R42 gets 1,26 V at the Q11 emitter; schematic says 1,2 Volt, that would be OK.
Now I get 0,3 V over R42; that is 1,7 mA.
There must be something funny with R43, R44, R45; think I will have to recheck the soldering.

Please let me know if I am totally wrong about this and thanks again for your support.


//W2L


EDIT:

When I returned from work I checked the resistance value for R43, R44 and R45 with my meter.
I had to loosen one end of the resistors to get a correct reading.
R43 measured 6,9M (OK) and R45 68k (OK).
R44 should have been 15 k according to the color code and but it was broken!!
I replaced it and voila, that did the trick. The Q11 collector measured 10.5 VDC as it should.
The amp sounds very good and very loud.
Thanks again for your kind interest and support.


//W2L