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Author Topic: Acoustic 370 Output Level Issue  (Read 8065 times)

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #15 on: March 25, 2021, 07:56:45 PM »
I removed Q303 and replaced with 6.8k and voltage on C404 rose to 28V. Replaced Q 305/310 with usual alternatives:2N3440 and 2N 4033 . Now voltage at C404 is at ps rail 84 V. Also had removed all other transistor in output. The amp is connected to a light bulb limiter which is not showing any excessive current draw. I'm stumped. I should be looking at complementary pa configuration with a single ended supply rail. It's as if Q305/314 are not connected?

Enzo

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Re: Acoustic 370 Output Level Issue
« Reply #16 on: March 25, 2021, 09:54:33 PM »
The parts list shows 1N4731A OR 1N5229.

From here it looks like the right half of the PA is working.  Your bias transistor Q304 has a couple volts across it at all times if it is working.   And it runs the whole power stage.  Within a couple volts, whatever voltage is on Q304 is what will appear on C404.

Q303 is not a resistor, and a resistor will not operate as a transistor.

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #17 on: March 25, 2021, 11:55:50 PM »
Replaced Q303 with original transistor since there was no change when a substituted in a 2N3440. Even with Q303 in place I get 83V at C404 and 81V at the collector of Q304.

Enzo

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Re: Acoustic 370 Output Level Issue
« Reply #18 on: March 26, 2021, 12:38:05 AM »
OK, so Q303 does seem to control the voltage on the output

You have the 80 at the collector of Q30, and may I assume also at collector Q303?

So wwhat is happening at the base of Q303?

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #19 on: March 26, 2021, 02:44:11 AM »
Yes Q303 collector has 83V and base is sitting at 53V. Assume Q301 is biasing Q303?
« Last Edit: March 26, 2021, 02:47:06 AM by gbono »

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #20 on: March 26, 2021, 07:36:01 PM »
Okay now the amp is doing this: voltage at C404 is 4V and voltage at collector of Q304 is 2.4V and emitter is 2V .... The longtail pair has 10.2V on base of Q301 and of course base of Q302 is 2.4V. Only Q305/10 are connected in the output all the other transistors are out of circuit. The amp does not draw excessive current. I am completely stumped! The bias circuit has an issue but where? Could the filter cap be bad? It looks like the zener diode in the base of Q305 is conducting so I assume that is the 4.2V on the collector of Q304 ???
« Last Edit: March 26, 2021, 08:05:26 PM by gbono »

Enzo

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Re: Acoustic 370 Output Level Issue
« Reply #21 on: March 26, 2021, 09:31:00 PM »
OK, remember, whatever is on the bias transistor will be within a couple volts of the output.  The bias transistor circuit controls where the output goes.

It sounds to me like the bias circuit works, whatever you have done to this thing, the output always tracked the bias transistor within a couple volts.  That is how they work.

Zener diode "seems like" it is working?  It is a protective device here, you can unsolder and lift one end if you suspect it.

The output can sit at most any voltage and not draw excess current, because there is no load on the output to draw that current.

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #22 on: March 27, 2021, 08:31:26 PM »
Lifted the zener and no change to voltages. Also noticed that the emitters of the long tail pair are at 3.6? Base of Q301 is around 10V and base of Q302 is under 3 volts.

Yes Q304 is keeping 2 volts between VC/VE but what happened to the 40 plus volts that should be at the collector?

Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #23 on: March 27, 2021, 11:26:29 PM »
With PNP transistors in a diff pair, the transistor with the lowest base Voltage "wins" and pulls the common emitter point down.

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #24 on: March 28, 2021, 04:44:19 AM »
I thought that if the base voltages vary then the collector currents would also vary but the emitter current (total) would stay the same.
With 3.6V on the emitters of the dif pair then Q301 would be off and Q302 would be on - correct?

How is Q303 biased? Isn't Q301 supplying bias to Q303?

Still not able to explain why the voltage at C404 isn't at VCC/2?

Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #25 on: March 28, 2021, 06:04:45 AM »
I thought that if the base voltages vary then the collector currents would also vary but the emitter current (total) would stay the same.

That's the way it's supposed to work, but only when the two base Voltages are pretty close, like within +/- 0.2 or 0.3 Volts. When one transistor takes control, the total or "tail" current can change slightly. In this case, tail current comes through R310 and R309.

With 3.6V on the emitters of the dif pair then Q301 would be off and Q302 would be on - correct?

Yes.

How is Q303 biased? Isn't Q301 supplying bias to Q303?

Yes, Q301 controls how Q303 is biased. In the state you describe, Q301 is trying to turn Q303 OFF which SHOULD allow the collector Voltage on Q303 to rise, something is preventing that from happening. The experiment I suggested above was an attempt to find what is not allowing the Collector Voltage of Q303 to rise. (Note: sometimes Enzo misses things in earlier posts when he comes late to the party.) Since the experiment kind of worked, I would conclude that Q303 is leaky or defective.

Still not able to explain why the voltage at C404 isn't at VCC/2?

If the Base Voltage of Q303 is zero, then it's not obeying it's input OR something else is drawing current around it.
« Last Edit: March 28, 2021, 06:11:44 AM by Loudthud »

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #26 on: March 28, 2021, 10:35:07 PM »
Voltages around Q303
VC=.8
VE=2.5
VB=.2

note: Q303 previously replaced by 2N3440 250V/1A/TO-3 very similar to 40409/8

Voltages around Q310
VC=.8
VE=2.5
VB=1.9

Voltages around Q305
VC=82
VE=3.7
VB=4.3

I had replaced Q310 and Q305 with  2N4033 and 2N3440 respectively -Q310 and Q303 are off??. VE on Q310 should be around 40V. Issue with replaced Q310?
« Last Edit: March 28, 2021, 10:40:22 PM by gbono »

Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #27 on: March 29, 2021, 01:44:43 AM »
Voltages around Q303
VC=.8
VE=2.5
VB=.2

note: Q303 previously replaced by 2N3440 250V/1A/TO-3 very similar to 40409/8

These numbers don't make any sense. Please double check them.

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #28 on: March 29, 2021, 02:07:40 PM »
Rechecked voltages around Q303
VC=1.9
VE=.18
VB=0

I have replaced this transistor several times with 2N3440...

Voltages around Q204
VC=4.37
VE=1.9
VB=2.67


Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #29 on: March 29, 2021, 06:46:37 PM »
Rechecked voltages around Q303
VC=1.9
VE=.18
VB=0

I have replaced this transistor several times with 2N3440...

Base Voltage is ZERO, the transistor should be OFF. The Emitter Voltage should be ZERO indicating NO current is flowing. Do the math: I=V/R....    0.18V/18Ohm=10mA. That current has to come from somewhere. It's either coming through the transistor you replaced, or through some other path, a conductive circuit board, flux on the circuit board, a little sliver of solder, a dirty finger print. Remove the transistor, do you still have 0.18V on the Emitter ?

Is the tip of your soldering iron grounded ??? Check resistance to ground and/or AC Voltage at the tip when it's plugged in.

Any static electricity where you are ? Boil some water to raise the humidity or try this: solder naked.

Does this amp have a two wire Mains cord with or without a Ground Switch ? Unplug it when you solder anything in the amp.

Voltages around Q204
VC=4.37
VE=1.9
VB=2.67

I assume you mean Q304, it's not the problem. As Enzo said above, the Voltage drop across Q304 should be about 2V all the time. It's job is to sense temperature and reduce it's Voltage drop slightly when it gets warm. This part of the circuit is called the "Vbe multiplier".

This output stage is called the "Quasi-Complementary" emitter follower. It uses all NPN transistors on the top side, a PNP driver and NPN power transistors on the bottom side. The output stage has a Voltage gain slightly less that one. Google it.