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Solid State Amplifiers => Amplifier Discussion => Topic started by: gbono on March 14, 2021, 11:34:03 PM

Title: Acoustic 370 Output Level Issue
Post by: gbono on March 14, 2021, 11:34:03 PM
I'm stumped why the output of this amp is limiting or at least I think that's the case.
DC tests:
The bases of Q301,302 are both at 12v and Q304, the bias transistor, has VC=48.3 and VE=46.1. The limiter circuit has the collector of Q316 at 8.55V and the voltage at node R333/4 is 8.48V? The base of Q316 is .259V
I replaced Q315,Q316 and Q303 and the tantalums.
CR301-304 are neither open or short.
Attached waveforms are voltage at R301 (sinewave) and the voltage waveform at the base of Q310

What am I missing?
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud on March 15, 2021, 01:14:59 AM
Which side of R301 is the waveform from ? One side is essentially the same as the Line outputs, the other might be limited by the JFET compressor circuit.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 15, 2021, 01:28:34 AM
The waveform shown is on the line input side of R301 if that makes sense. The waveform on the other side of R301 is almost a square wave (if I remember). How does the limiter work on this amp?
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud on March 15, 2021, 02:08:55 AM
Normally the diodes CR301-304 don't conduct. If they do, the input signal is too big. When the signal at the output of the power amp gets high enough, the JFET turns on shunting the input which in turn reduces the output.

A too big signal on the input makes me think the gain of the power amp is too low somehow.

The gain of the power amp is set by R314, R313, R315 and C306. R312 helps set the DC gain (operating point) of the output. The pot R304 is used to adjust the DC (45V) on the output. The low frequency corner is about 30Hz, set by R313, R315 and C306.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 15, 2021, 03:51:18 AM
What does Q316 do? I noticed that the schematic shows about a .6V difference between the collector and node between R333/334. This amp has the collector of Q316 at 8.55V and the voltage at node R333/4 is 8.48V? The base of Q316 is .259V. The resistors in the limiter appear to test okay. More head scratching..
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud on March 15, 2021, 05:26:18 AM
Positive Gate Voltage turns Q315 off. This comes in through R330. Q316 is normally off, but it has a slightly positive base Voltage, so it is ready to conduct. When signal comes in via R335 or R337, Q316 starts to conduct lowering the Gate Voltage on Q315 which turns it ON lowering the Source to Drain resistance. C315 gives the whole thing a fast attack - slow release action.

R331, C314 unknown function, probably for fast attacks.

Note: When replacing semiconductors in an amp this old, double check the Base Emitter & Collector connections. Sometimes the original parts had non-standard pin-outs. Same with the JFET.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 15, 2021, 06:04:33 PM
Thanks for the explanation. It looks like there was a SPST switch in the limiter circuit but this amp has it depopulated. It looks like the switch was there to pull the base of Q316 to ground? Still wonder why the collector voltage on Q316 is so low and there isn't a .6V difference between VC and the voltage at node R333/4? Looks like no current through R330 so FET stays near IDSS (i.e. on) ??

I'm looking at removing the JFET and seeing if that isolates my problem.

I usually use the same MPN for replacement of semiconductors and did so with both Q315/6.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 20, 2021, 11:24:44 PM
I removed the JFET (Q315) and input limiting diodes (CR301-4) and now have a good signal to the base of Q301. Unfortunately, the output waveform is still distorted. All caps were previously replaced and I've checked out resistors in signal path. I'm missing something???
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 22, 2021, 03:19:20 PM
I'm assuming I should see the undistorted signal at the collector of Q301 (long tail pair) - even if the feedback loop was having issues?
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud on March 22, 2021, 05:34:59 PM
No, you might see what the feedback is doing to the signal in an attempt to make the output look like the input.

Since the feedback seems to be working at DC:
11V on the input of Q301, Q302
45V on the plus side of C404

The amp should be able to make a clean sine wave on the output with NO LOAD. Get it working with no load first, then try it with dummy load. Try 8 first, if good, 4 and 2.

Check Voltage across R341, should be about 120mV by my calculation. Check CR310 with DVM diode check out of circuit. It should look like whatever your meter reads with two diodes in series. These components sort of set the current limit on the minus side.

Check CR308(?), CR306 and CR307 with DVM. I'm coming up blank on 1N473A, I'm assuming it's a low Voltage zener, they can be pretty flakey. It sets the current limit on the plus side.

Check the Voltage drop across R339 and R340. They should have similar Voltage drop. Check resistance of all the big low value resistors as well as you can with your DVM. Usually these will be open when they fail.

Good Luck !
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 22, 2021, 07:38:40 PM
I get 180mv across R 341 the voltage across R340/339 is 0v though. CR310 is two diodes in series both check out okay. All other diodes are good. All low value resistors all test good. I have been testing the amp with no load. frustrating....
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 24, 2021, 06:43:12 PM
need sanity check here. The amp now shows 12.6 V on collector of Q 304 and 10.2V on Q304 emitter(assume Q304 okay by the 2v difference and it was replaced with drop_in alternative) . That explains why there is only 12v at C404 which should be VCC/2. But why? Base of Q305 is sitting at 12.6v though DMM doesn't show anything wrong with Q305. Before I remove Q305 what am I missing?
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud on March 24, 2021, 10:29:09 PM
Q304 doesn't do much except bias the output stage. It's voltage drop should be about 2V all the time. First look at the input stage, Q301 and Q302. Is the base of Q302 near +11V ? If it's low like it should be (~4V) with 12V on the output, then Q301 should be off and Q303 should be off (base = 0V) allowing the output to go high. What might be pulling the output down ? The base of Q310 pulling current through R340. 1V drop across R340 would pull the output down to 12V.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 25, 2021, 03:11:07 AM
The bases of the long tail pair are both at 10.5V. The voltages around Q310 are VB=10.1 VE=10.7 and VC=0 Q310 is off..

What does the zener do on the base of Q305 - shouldn't it keep Q305 at 4.1V? (1N4731A)
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud on March 25, 2021, 07:42:02 AM
CR308 looks like it says 1N473A. Info I found says it's 4.7V +/- 5% 200mW. Google keeps coming up with 1N4731 when you search for 1N473A, but that's not the correct part.

CR308 normally shouldn't  conduct. When it does it sets the current limit on the plus side (which flows through R322) by clamping the base of Q305 to the output (my math says @ 35A) . It also sets the current limit on the minus side when it conducts in the other direction through CR306 and Q304 (my math says @ 20A). This is where CR310 comes into play because it sets a current limit on Q303 so it doesn't pull too hard when a minus current limit is reached.

Back to the other issue. Look at the feedback network consisting of R314, R313 and R315, and R312. Do the math on this Voltage divider. At DC, C306 should not affect anything. If the output is stuck at 12V, the base of Q302 should be around 3.3V (without doing the math I guessed 4V). If R312 is not doing it's job, you might see 10V at the base of Q302.

Experiment: If Q303 is off (base at 0V), remove it from the circuit and substitute a 6.8K resistor from emitter to collector. That should allow the output to float up to 45V. That would prove that there is not an issue with the output stage, Q305 to Q314.

If that experiment doesn't work, remove all the power transistors (mark them so you can put them back in the same place) and try again. Q305 and Q310 can be bad, but may partially work at less than full Voltage.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono 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?
Title: Re: Acoustic 370 Output Level Issue
Post by: Enzo 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.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono 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.
Title: Re: Acoustic 370 Output Level Issue
Post by: Enzo 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?
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 26, 2021, 02:44:11 AM
Yes Q303 collector has 83V and base is sitting at 53V. Assume Q301 is biasing Q303?
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono 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 ???
Title: Re: Acoustic 370 Output Level Issue
Post by: Enzo 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.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono 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?
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud 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.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono 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?
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud 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.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono 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?
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud 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.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono 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

Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud 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.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 30, 2021, 05:43:29 PM
Rechecked the board and found Q3010 was replaced but not soldered.
Voltages around Q303 are:
VC=4.3
VE=1.9
VB=2.6

I'm still seeing LT 3V at C404.

Soldering station is grounded and RH in Northern California isn't usually an issue (40% here) - always check mains connections at the start and ground/neutral/hot all properly connected.

So to summarize: bias transistor Q404 is functioning - Q303 is biased on and differential pair is also functioning but unbalanced BUT amp will not pass signal beyond Q301. With no signal applied: DC voltages around bias transistor and output cap are 4V (VC of Q304) and 2V at C404.

frustrating...
Title: Re: Acoustic 370 Output Level Issue
Post by: Enzo on March 30, 2021, 06:17:04 PM
Yes, I DO miss things, it gets harder and harder to focus any more.  Sorry in advance.

Q303 VE 1.9v?  Yet it should be at ground.  Where are you grounding the meter?

HAve we checked the positive rail is available through R316/317 ,top center?  Open up there and there is no voltage to pull Q303C back up to anywhere.

Your output seems to be tracking the collector Q303, which is what it i ssupposed to do.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 31, 2021, 12:07:28 AM
One side of R316 is at 83V and between R316/7 I get 43V but on the other side of R317 it's sitting at 4V. I have lifted one side of CR308 (zener) which doesn't change voltage at node - what is dropping the voltage?  All measurements referenced to minus terminal on C403.
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud on March 31, 2021, 01:51:02 AM
One side of R316 is at 83V and between R316/7 I get 43V but on the other side of R317 it's sitting at 4V. I have lifted one side of CR308 (zener) which doesn't change voltage at node - what is dropping the voltage?  All measurements referenced to minus terminal on C403.

This is all consistent with Q303 being ON as well as the measurements at the top of the page.

Voltages around Q303 are:
VC=4.3
VE=1.9
VB=2.6

I'm still seeing LT 3V at C404.

This leads us back to the input pair, Q301, Q302.

What are the Voltages around those two transistors now ?

Q303 is what is called the "VAS" (Voltage Amplifier Stage) transistor or stage. It provides most of the Voltage gain needed to drive the output, because all those transistors, Q305 to Q313 have a Voltage gain slightly less than one, but have a huge current gain. Probably less than 1/10 of a milliamp can cause over 20 Amps to flow in the speaker. The Diff pair, Q301 and Q302 compare the input to a portion of the output (provided by the feedback network) and send a correction to the VAS stage.

Current for Q303 starts at ground, flows through R341, and Emitter to Collector in Q303. A small portion flows into the Base of Q310. The current continues through Q304 Emitter to Collector, and a small portion flows into the Base of Q305. The current continues through R317 and R316 at to the +83 supply. The diodes CR306, CR307 and CR308 only turn on when too much current flows through R322 and/or R323.

C307 is called the "Bootstrap" capacitor. It's job is to keep the Voltage across R317 relatively constant for AC signals. Don't worry about it, you have to get the amp running at DC first.


Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 31, 2021, 03:38:37 PM
Voltages around Q301/2

Q301
VB=9.1V

Q302
VB=2.8

VE=3.5

So Q302 base is getting the lower DC voltage through the voltage divider set up by R314, etc. Question still remains why is voltage at collector of Q304 so low??? So if there is 4V at node R317/339 then there is a fault (over current) flowing through R323/3? So I could have a short in some of the output transistors Q307-314??
Title: Re: Acoustic 370 Output Level Issue
Post by: Enzo on March 31, 2021, 06:27:20 PM
The voltage at C of Q304 is so low because the voltage at C of Q303 is so low.  Q304 is working, as it is keeping the 2v difference across itself.  Q304 simply follows whatever C of Q303 does.  The reason C of Q303 is so low is what Loudthud is trying to help you with at Q301/Q302.   They control Q303.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 31, 2021, 07:23:14 PM
Understood but if the node at R317/339 is at 4V how does this condition apply to the differential pair Q301/2?
As I understand it the difference in base voltages at Q301/2 are why VE is measuring at around 3V. I'm missing the connection (no pun) between the diff pair and the voltage at R317/339?
Title: Re: Acoustic 370 Output Level Issue
Post by: Enzo on March 31, 2021, 08:19:54 PM
I see R317, connected to C ot Q304, but I don't see R339 anywhere, help me out.

The voltage at the bottom end of R317 is controlled by Q303 (including the 2v through Q304), which in turn is controlled by the diffy pair.  It seems that the diffy pair is turning on Q303 too hard.
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud on March 31, 2021, 08:28:43 PM
Q301 should be OFF because it's Base Emitter junction is reverse biased. However, Q301 is feeding current to R308-R307 and thus turning Q303 ON. The low Voltage on Q303's collector is fed back to the Base of Q302, but Q302 is unable to turn Q301 OFF. Looks like Q301 is bad.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 31, 2021, 09:04:14 PM
UPDATE - I found that Q303 was mismarked and has been replaced with NOS 40408

Voltage around Q303
VC=7.2
VE=.15
VB=.75

Q304
VC=9.5
VE=7
VB=7.7

Q301
VB=7.6

Q302
VB=7.8

VE=8.2

I have the amp on a light bulb limiter and at turn on you see a bright flash (charging caps) and then a very very slight glow. I haven't tried to drive a signal into the board yet and it still looks like Q303 collector is no where near VCC/2.
Title: Re: Acoustic 370 Output Level Issue
Post by: Loudthud on March 31, 2021, 09:48:30 PM
Read my lips:

Looks like Q301 is bad.
Title: Re: Acoustic 370 Output Level Issue
Post by: gbono on March 31, 2021, 10:17:12 PM
Replaced Q301/2 with 2N4248

voltage around diff pair

Q301
VB=7.5

Q302
VB=7.8

VE=8.2