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Messages - Loudthud

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Amplifier Discussion / Re: The Sunn Solos II
« on: April 15, 2021, 06:33:44 PM »
Man, it's been so long since I posted that schematic, I forgot about it. That's a pretty rare amp.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 31, 2021, 09:48:30 PM »
Read my lips:

Looks like Q301 is bad.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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:

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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 29, 2021, 06:46:37 PM »
Rechecked voltages around Q303

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

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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« on: March 29, 2021, 01:44:43 AM »
Voltages around Q303

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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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?


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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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 !

Amplifier Discussion / Re: Old SS Project Rebuild
« on: March 18, 2021, 07:24:09 AM »
I think you have it right. The only problem I see is the diodes from the output to the rails should be 1N400x.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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.

Amplifier Discussion / Re: Acoustic 370 Output Level Issue
« 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.

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