Amp Camp Amp - ACA

There is a possible latchup mode caused by leakage of a new output cap when no speaker is connected. A brand new cap may have excessive leakage for a short time. Connect a dummy load resistor or a big woofer to the speaker terminals. Verify the polarity of C1 and C3 is correct.

Of course cap polarities are ok, I will try with a load resistor following your advice.
I was also thinking of a problem on ZTX450, what do you think?
 
Of course cap polarities are ok, I will try with a load resistor following your advice.
I was also thinking of a problem on ZTX450, what do you think?

Any problem with a component or soldering could cause the amp not to bias properly. I was just thinking of a couple of reports from builders who couldn't bias their amps at first and then the amp started working with no logical explanation. My thinking is that leakage of C1 with no load could come through the feedback network and reverse bias C3. R14 tries to prevent this, but a brand new C1 might have high leakage before the electrolyte forms it's insulative layer. This would cause the DC bias point to go low like you reported. Check that the DC Voltage at the output (R14) is less than 1 Volt or so and that won't be an issue.

Center the pot before you apply power. Q1 and/or Q2 should start to warm up after 10 seconds or so. Depending on where the bias is settling, one may get hotter than the other. The Voltage across R1-R4 should be in the half Volt range. These resistors will get warm, but too much heat indicates a problem. If none of the parts get warm, that might indicate Q3 or C2 is shorted or leaky.

If you can't find the problem, measure the Voltages on all the transistor pins and post the results.
 
Any problem with a component or soldering could cause the amp not to bias properly. I was just thinking of a couple of reports from builders who couldn't bias their amps at first and then the amp started working with no logical explanation. My thinking is that leakage of C1 with no load could come through the feedback network and reverse bias C3. R14 tries to prevent this, but a brand new C1 might have high leakage before the electrolyte forms it's insulative layer. This would cause the DC bias point to go low like you reported. Check that the DC Voltage at the output (R14) is less than 1 Volt or so and that won't be an issue.

Center the pot before you apply power. Q1 and/or Q2 should start to warm up after 10 seconds or so. Depending on where the bias is settling, one may get hotter than the other. The Voltage across R1-R4 should be in the half Volt range. These resistors will get warm, but too much heat indicates a problem. If none of the parts get warm, that might indicate Q3 or C2 is shorted or leaky.

If you can't find the problem, measure the Voltages on all the transistor pins and post the results.

Appreciate it!
I will do, and report back.
 
Any problem with a component or soldering could cause the amp not to bias properly. I was just thinking of a couple of reports from builders who couldn't bias their amps at first and then the amp started working with no logical explanation. My thinking is that leakage of C1 with no load could come through the feedback network and reverse bias C3. R14 tries to prevent this, but a brand new C1 might have high leakage before the electrolyte forms it's insulative layer. This would cause the DC bias point to go low like you reported. Check that the DC Voltage at the output (R14) is less than 1 Volt or so and that won't be an issue.

Center the pot before you apply power. Q1 and/or Q2 should start to warm up after 10 seconds or so. Depending on where the bias is settling, one may get hotter than the other. The Voltage across R1-R4 should be in the half Volt range. These resistors will get warm, but too much heat indicates a problem. If none of the parts get warm, that might indicate Q3 or C2 is shorted or leaky.

If you can't find the problem, measure the Voltages on all the transistor pins and post the results.

Ok, I made some measurements:
Across R14 I have less than 0.1VDC.
R1-R4 are in the 0.4V range and are uniformly warm, none is hotter than the others.
Q1 and Q2 are equally hot and no one gets "strangely" hot.
Bias on central Q1 pin still about 14.5V, and there is no way of setting it to 10V: 14V.... and tutning the pot down at a certain point sets to 0.5V.

Now my question is: can I go ahead using the board like this or am I getting troubles?
 
The board probably won't make a sound.

It seems from your description that there is some problem with Q4. When the bias point goes to 0.5V there shouldn't be enough voltage feeding back through P1 to turn Q1 on ... unless Q4 is supplying it through some fault.

Voltage readings on Q4's pins in both 14.5V and 0.5V modes will help confirm this.
 
The board probably won't make a sound.

It seems from your description that there is some problem with Q4. When the bias point goes to 0.5V there shouldn't be enough voltage feeding back through P1 to turn Q1 on ... unless Q4 is supplying it through some fault.

Voltage readings on Q4's pins in both 14.5V and 0.5V modes will help confirm this.

I'll check it.
Another strange thing: it happened that I power the board and the voltage on Q1 goes slooooooowly from about 6V up to 14V. I mean it takes maybe 15 minutes. Veeery slowly but up to 14/14.5V.
Anyawy I will remove and substitute Q4 and check again all the voltages.

BTW: all my previous mesurements and checks were taken in 14.5V "mode".

Thanks so much!
 
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Could be a good thing to have extra mosfets and jfets. To not deter anyone from building these amps I must mention my ACA, F5 and B1 was pretty much plug and play. Need only to readjust the bias and maybe replace the resistor in the F5 PSU that drains the caps. I now use 2.2K and it keeps blowing fuses if I turn the amp on and off within 1 minute. I will try 10-22k.
 
I'll check it.
Another strange thing: it happened that I power the board and the voltage on Q1 goes slooooooowly from about 6V up to 14V. I mean it takes maybe 15 minutes. Veeery slowly but up to 14/14.5V.

This is fairly normal for this circuit. C2 charges up slowly through R7 until eventually Q2 starts to conduct. When Q2 is conducting enough current, Q3 turns on and holds the current at that level. However in a normally operating amp, this process should take about 10 seconds.

I have attached a pic to identify the names of the transistor terminals where I would like you to measure Voltages. The capacitor polarities are shown just so you can double check them.
 

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This is fairly normal for this circuit. C2 charges up slowly through R7 until eventually Q2 starts to conduct. When Q2 is conducting enough current, Q3 turns on and holds the current at that level. However in a normally operating amp, this process should take about 10 seconds.

I have attached a pic to identify the names of the transistor terminals where I would like you to measure Voltages. The capacitor polarities are shown just so you can double check them.

I will definitely do the measurements the day after tomorrow when from an easter trip and report here.
;)
 
Nice and compact build. But that is going to run hotter than a whorehouse on nickel night. Have some fans ready to be installed. I found that 2kg of heatsink per block is enough.

Total weight per monoblock is 4,5 kg. I can hold my hand on heatsinks as long as i want after 5h of work. Heatsinks are termally coupled with 10mm Al plate, and with front and side being 10mm thick as well the entire enclosure act as heatsink.
 
This is fairly normal for this circuit. C2 charges up slowly through R7 until eventually Q2 starts to conduct. When Q2 is conducting enough current, Q3 turns on and holds the current at that level. However in a normally operating amp, this process should take about 10 seconds.

I have attached a pic to identify the names of the transistor terminals where I would like you to measure Voltages. The capacitor polarities are shown just so you can double check them.


Here are the measurements on the faulty biard with a 19V SMPS.
Btw, I heve already installed the 2.21K mod.

An externally hosted image should be here but it was not working when we last tested it.
 
Something seems wrong around R6. The Source of Q4 is 4.6V connected to one end of R6 but the Gate of Q1 connected to the other end is 0V. Normally the current flow through R6 is so small it can be ignored. Either R6 is defective or there is a break in the track on the PCB somewhere. With the power off measure the Ohms between the Source of Q4 and the Gate of Q1. It should read 100 Ohms +/- a few percent.
 
Something seems wrong around R6. The Source of Q4 is 4.6V connected to one end of R6 but the Gate of Q1 connected to the other end is 0V. Normally the current flow through R6 is so small it can be ignored. Either R6 is defective or there is a break in the track on the PCB somewhere. With the power off measure the Ohms between the Source of Q4 and the Gate of Q1. It should read 100 Ohms +/- a few percent.

Unfirtunately it reads exactly 100.1Ohms... :worried:
Not only R6, but between Q4s and Q1g. Same value.

I will check again voltages in that area.

SORRY: I checked again and I have the same voltage on Q4s and Q1g, about 4.7V. And all the traces are good in the area. R6 is correctly 100Ohms.

Here it is

An externally hosted image should be here but it was not working when we last tested it.
 
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