When the voltage goes negative, what is the voltage on the base and collector of the regulator transistor?
Collector voltage follows the positive rail voltage pretty closely.
The Base tracks the emitter voltage but is +.6V behind it as it goes negative.
The Base tracks the emitter voltage but is +.6V behind it as it goes negative.
With no RCAs plugged into the amp, do you read ~1000 ohms between the shield and chassis grounds?
Is the voltage on both sides of R90 dropping along with the base/emitter voltage?
What is the voltage across D90?
Is the voltage on both sides of R90 dropping along with the base/emitter voltage?
What is the voltage across D90?
YesPerry Babin said:
Left side is 0.9-0.8V
Right side is 22.2V (basically rail at time of measurement)
steady 5.9V
Are R22 and R25 both within tolerance?
Is the voltage drop across R22 and R25 the same? Check twice if the voltage is changing on the emitter.
Does the voltage on the emitter of the negative regulator transistor remain at negative 18v?
Is the voltage drop across R22 and R25 the same? Check twice if the voltage is changing on the emitter.
Does the voltage on the emitter of the negative regulator transistor remain at negative 18v?
If the resistance or R22 and 25 is the same and they have the same voltage across them, that means that the current flowing through them is the same so the voltage on the positive regulator should be holding also.
Does R90 read 10k ohms?
Are you using a PZTA06 for Q90 (the positive reg transistor)?
Does R90 read 10k ohms?
Are you using a PZTA06 for Q90 (the positive reg transistor)?
Leave the PZTA56 in the circuit.
You said you have output transistors. Which NPN outputs do you have?
Does R90 read 10k ohms?
You said you have output transistors. Which NPN outputs do you have?
Does R90 read 10k ohms?
Do you have any small, flexible wire that you could use to connect the large transistor to the pads on the board? The wire has to be flexible enough to prevent damaging the pads. The individual conductors from a computer ribbon cable would work well. You'd have to insulate the tab because it would have rail voltage on it.
It seems like the transistor in the circuit now isn't up to the task. The larger transistor should be better. If the larger transistor works, you know the transistor is the problem.
If you do this, when you power up the amp, you should watch R22 and R25 closely for the first few seconds. If there is a shorted op-amp, the resistors may burn. Be prepared to disconnect the power supply quickly.
It seems like the transistor in the circuit now isn't up to the task. The larger transistor should be better. If the larger transistor works, you know the transistor is the problem.
If you do this, when you power up the amp, you should watch R22 and R25 closely for the first few seconds. If there is a shorted op-amp, the resistors may burn. Be prepared to disconnect the power supply quickly.
Ok, system powers up. I get around 10V input and system starts to oscillate. By oscillate, current and voltage analog meters bounces up and down. I pushed to it 12V for a second and it was steady then oscillated. However, it does play music in both channels and sounds ok other than oscillation making the output bounce with power input current.
Well, it actually only oscillates when I tried to read voltage on the positive emitter. Started the system up cold, and played music. Once I crank the volume up, it starts to oscillate and doesn't settle down.
Checked for DC on outputs and that appears low, in the millivolt range.
-22V on the negative emitter
.548V across resistor
+16V on the positive emitter
.699V across resistor
Well, it actually only oscillates when I tried to read voltage on the positive emitter. Started the system up cold, and played music. Once I crank the volume up, it starts to oscillate and doesn't settle down.
Checked for DC on outputs and that appears low, in the millivolt range.
-22V on the negative emitter
.548V across resistor
+16V on the positive emitter
.699V across resistor
No. It's an Adcom Preamp. I tried grounding it.
Then I tried a Sony deck I had. Works for a bit then it starts to oscillate. Maybe it's my power supply?
Idle current draw is almost 2A.
Then I tried a Sony deck I had. Works for a bit then it starts to oscillate. Maybe it's my power supply?
Idle current draw is almost 2A.
Connect the input RCA shield frame (sheetmetal that goes into circuit board) to the chassis ground of the amp. Does it still oscillate?
After connecting the frame of the RCA jacks measure the voltage on the emitters of the regulators again.
After connecting the frame of the RCA jacks measure the voltage on the emitters of the regulators again.
Still oscillates with shield connected to ground.
It seems that on cold start it'll be stable at 13V until I do something. This time I measured the positive emitter at 16V and then the Negative emitter at -23V and then it started to oscillate.
Possibly the negative transistor is leaky?
It seems that on cold start it'll be stable at 13V until I do something. This time I measured the positive emitter at 16V and then the Negative emitter at -23V and then it started to oscillate.
Possibly the negative transistor is leaky?
Also, I tried to up the voltage back to 13V after shutting down and it not only doesn't go past 12V, it's starts to draw excessive current (+4A) the more I try to push the voltage up. Cold start seems fine until I do something.
From the numbers you posted, it appears that the neg reg transistor is OK.
Is the non-striped end of D91 show ~0 ohms to the RCA shield? Disconnect the jumper to chassis ground.
Does the amp power up normally with no RCAs plugged into the amp?
Reconnect the jumper and measure the DC voltage on both ends of D91 and D94. Post the voltages.
Is the non-striped end of D91 show ~0 ohms to the RCA shield? Disconnect the jumper to chassis ground.
Does the amp power up normally with no RCAs plugged into the amp?
Reconnect the jumper and measure the DC voltage on both ends of D91 and D94. Post the voltages.
yesPerry Babin said:
Yes, it seems to be powering up normal on cold start either way.
D91
K(left side)= 17V
A= 175mV
D94
K(left side)=300mV
A= -18V
RCAs are to my left
I would have expected the 175mv/300mv points to be the same but I believe they're OK.
What is the current rating of the 12v power supply you're using?
Is the voltage across the B+ and ground terminals dropping when the amp is oscillating?
Did you check the shield ground on the Sony head unit to confirm that it was not open?
If you want to reduce the idle current slightly, measure the voltage across the test points for each channel and make a note of the voltage. Then turn the bias pots CCW 'slightly'. That should reduce the bias current and therefore the idle current.
Does changing the crossover, gain, equalization have any effect on the oscillation?
What is the current rating of the 12v power supply you're using?
Is the voltage across the B+ and ground terminals dropping when the amp is oscillating?
Did you check the shield ground on the Sony head unit to confirm that it was not open?
If you want to reduce the idle current slightly, measure the voltage across the test points for each channel and make a note of the voltage. Then turn the bias pots CCW 'slightly'. That should reduce the bias current and therefore the idle current.
Does changing the crossover, gain, equalization have any effect on the oscillation?
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