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Author Topic: Acoustic 370 Output Level Issue  (Read 8147 times)

gbono

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Acoustic 370 Output Level Issue
« 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?
« Last Edit: March 14, 2021, 11:35:56 PM by gbono »

Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #1 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.

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #2 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?
« Last Edit: March 15, 2021, 01:30:06 AM by gbono »

Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #3 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.

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #4 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..

Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #5 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.

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #6 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.
« Last Edit: March 15, 2021, 06:06:05 PM by gbono »

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #7 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???

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #8 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?

Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #9 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 !

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #10 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....

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #11 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?

Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #12 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.

gbono

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Re: Acoustic 370 Output Level Issue
« Reply #13 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)
« Last Edit: March 25, 2021, 03:58:07 AM by gbono »

Loudthud

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Re: Acoustic 370 Output Level Issue
« Reply #14 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.