The op amp in question is on the second photo in my first post. Op amp in the top left corner of the photo.
1: -13.64
2: -13.67
3: -13.51
4: -14.92
5: 0.001
6: 0.004
7: 0.004
8: 14.32
There are 5 more opamps on the board. They have something like this: legs 1-3, 5-7: almost 0 leg 4: -14.92, leg 8 14.32V
Today when I switched the amp on, current limiting lamp started to glow, switching between HPF and Full did not help this time. Switched the amp on and off couple of times and then the current draw dropped.
1: -13.64
2: -13.67
3: -13.51
4: -14.92
5: 0.001
6: 0.004
7: 0.004
8: 14.32
There are 5 more opamps on the board. They have something like this: legs 1-3, 5-7: almost 0 leg 4: -14.92, leg 8 14.32V
Today when I switched the amp on, current limiting lamp started to glow, switching between HPF and Full did not help this time. Switched the amp on and off couple of times and then the current draw dropped.
Remove that op-amp, clean up the solder pads and measure the voltage again. Do you still have significant DC on pins 1, 2 or 3?
With the opamp out, measured voltages on solder pad 1, 2 and 3. No significant DC present 0.002V. When measuring solder pad 1 for the first time, multimeter showed for a split second 11V but after that almost zero.Tried, how it produces audio: better than yesterday, a bit louder and much less noise, some noise is heard if music is not playing. When amp is powering up, then for a second cone gets pushed out, after that returns to center.
Check the other op-amps again to see if they have any significant DC on any pins other than the power supply pins.
Had some TL072CP-s replaced the defective opamp with that. Measured voltages on legs 1, 2 and 3: 0.004, 0.004, 0.002V.
Other op amps are as they were.
Did not draw current, turned the gain and hpf pot, switched hpf on and off. Still no current draw.
Tried if it produces audio. It produces audio but volume is still very low, still best result since I got the amp but still garbage.
Then some new symptoms:
Switched amp off to check the RCA connections, potentiometers solder pads, potentiometers resistance. Everything checks out OK.
Then tried to switch the amp on, it started to draw current and pushed speaker cone out. Measured DC on speaker terminals: 3,4V. Power Led barely glowed, when switching amp off, it flashed protection LED, it has not done this before.
Switched it off, did not touch it at all. After ten minutes I switched it on, no current draw. Started to measure again voltages on op amps, transistors, did not find anything strange. Then I heard some screeching noise from somewhere near rail caps and transformers, switched amp off, touched them to check if they are hot, nothing gets hot. Then tried to switch amp on and again it started to draw current.
Other op amps are as they were.
Did not draw current, turned the gain and hpf pot, switched hpf on and off. Still no current draw.
Tried if it produces audio. It produces audio but volume is still very low, still best result since I got the amp but still garbage.
Then some new symptoms:
Switched amp off to check the RCA connections, potentiometers solder pads, potentiometers resistance. Everything checks out OK.
Then tried to switch the amp on, it started to draw current and pushed speaker cone out. Measured DC on speaker terminals: 3,4V. Power Led barely glowed, when switching amp off, it flashed protection LED, it has not done this before.
Switched it off, did not touch it at all. After ten minutes I switched it on, no current draw. Started to measure again voltages on op amps, transistors, did not find anything strange. Then I heard some screeching noise from somewhere near rail caps and transformers, switched amp off, touched them to check if they are hot, nothing gets hot. Then tried to switch amp on and again it started to draw current.
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Earlier, when the dead opamp was still on the board, then I tried to yank the pots and sometimes yanking gain pot made speaker to make popping noises. Yesterday I tried that also but could not replicate that popping noise or make the amp draw more current.
I guess I can resolder the pots and switches to be 100% sure.
Is it normal that + voltage is a bit lower than - voltage?
I forgot to mention yesterday that when amp drew too much current I measured voltage on output transistor legs. Last week I measured 21V on each leg, yesterday I measured 5-6V on every leg and 3V on speaker terminals. If it does not draw current then voltages on transistor legs are similar to working channels.
Maybe I have a problem with power supply?
I guess I can resolder the pots and switches to be 100% sure.
Is it normal that + voltage is a bit lower than - voltage?
I forgot to mention yesterday that when amp drew too much current I measured voltage on output transistor legs. Last week I measured 21V on each leg, yesterday I measured 5-6V on every leg and 3V on speaker terminals. If it does not draw current then voltages on transistor legs are similar to working channels.
Maybe I have a problem with power supply?
I think I had a little breakthrough today with this amp. Yesterday I resoldered RCA plugs, switches, pots, rail caps, transformer legs, smaller transformer legs, caps which are near the small transformers. Added solder to each pad so that I can be 100% sure that everywhere is good electrical contact.
Nothing improved, still erratic current draw, sometimes pushes speaker cone out, sometimes not, sometimes makes noises, sometimes not.
Now I think I have narrowed the search down. I removed rectifiers and erratic current draw is still there. And now it seems that current draw goes with noise from near the transformer. Noise has got much louder, than it was earlier.
Do I have a shorted transformer?
Nothing improved, still erratic current draw, sometimes pushes speaker cone out, sometimes not, sometimes makes noises, sometimes not.
Now I think I have narrowed the search down. I removed rectifiers and erratic current draw is still there. And now it seems that current draw goes with noise from near the transformer. Noise has got much louder, than it was earlier.
Do I have a shorted transformer?
Tried it.
For ten minutes it ran normal, no current draw, no noise. (I have no heating in the room I have my amps in, it is -5 degrees celsius (23 F) outside, maybe the defect occurs only when transformer gets warm? It does not get that warm that I would feel it with my fingers though.)
Then it started to draw a little current and frizzled a bit. The longer the amp ran the more current draw and more noise. Twisting/pulling had some effect but not really. At first twisting would make it stop to draw current for a second, but afterwards it drew current and I could twist all I want, I could not stop it.
For ten minutes it ran normal, no current draw, no noise. (I have no heating in the room I have my amps in, it is -5 degrees celsius (23 F) outside, maybe the defect occurs only when transformer gets warm? It does not get that warm that I would feel it with my fingers though.)
Then it started to draw a little current and frizzled a bit. The longer the amp ran the more current draw and more noise. Twisting/pulling had some effect but not really. At first twisting would make it stop to draw current for a second, but afterwards it drew current and I could twist all I want, I could not stop it.
I cant tell, I was using 12V 50W lamp as a limiter and I can tell if it draws current by checking if the lamp lights up or not. And how brightly it lights up, when the noise starts then lamp barely glows, but afterwards it is at maximum brightness.
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If possible compare the DC voltage on the gates of the transistors in the good supply and in the bad supply when the amp is drawing excessive current?
How low is the voltage dropping across the B+ and ground terminals?
How low is the voltage dropping across the B+ and ground terminals?
I am ready to give up on this 🙁
I wanted to measure DC on gates of IRFZ44Ns before current draw and after current draw. Before current draw result I did manage to get: 3,55; 3,55; 3,61; 3,61 on the OK side. 3,62; 3,62; 3,64; 3,64 on the bad side.
11,57V DC between + and GND terminals.
Then started to wait for current draw, 10 minutes and nothing happened, tried to twist, push, pull, poke the transformer, rail caps, other transformer no result. Then I went outside to barbeque some meat, spent about 45 minutes outside, every 10 minutes I checked on the amp. 45 minutes, no noise, no current draw.
Then I reinstalled rectifiers and I am again on square one, no current draw when speaker is not connected, draws current when speaker connected to channel 1 or 2. No noise from the transformer.
When amp drew current with speakers connected it had 8V between + and GND terminals. 2,2V on each power supply FET gate.
Now some new symptoms: every op amp has only 3 and -3 V on their power supply legs. Small transistors in audio section have 7, 21, 7V on their legs.
I managed to fry power supply chip? Cant read the number on it clearly, maybe it is K9094 or K909A?
I wanted to measure DC on gates of IRFZ44Ns before current draw and after current draw. Before current draw result I did manage to get: 3,55; 3,55; 3,61; 3,61 on the OK side. 3,62; 3,62; 3,64; 3,64 on the bad side.
11,57V DC between + and GND terminals.
Then started to wait for current draw, 10 minutes and nothing happened, tried to twist, push, pull, poke the transformer, rail caps, other transformer no result. Then I went outside to barbeque some meat, spent about 45 minutes outside, every 10 minutes I checked on the amp. 45 minutes, no noise, no current draw.
Then I reinstalled rectifiers and I am again on square one, no current draw when speaker is not connected, draws current when speaker connected to channel 1 or 2. No noise from the transformer.
When amp drew current with speakers connected it had 8V between + and GND terminals. 2,2V on each power supply FET gate.
Now some new symptoms: every op amp has only 3 and -3 V on their power supply legs. Small transistors in audio section have 7, 21, 7V on their legs.
I managed to fry power supply chip? Cant read the number on it clearly, maybe it is K9094 or K909A?
It's likely a kia494.
Post the DC voltage on all of the pins of the IC if you want to troubleshoot it further.
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:
Post the DC voltage on all of the pins of the IC if you want to troubleshoot it further.
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:
01: 1,4
02: 4,2
03: 0,1
04: 0
05: 1,5
06: 3,5
07: 0
08: 10,2
09: 3,7
10: 3,7
11: 10,2
12: 9,7
13: 4,9
14: 4,9
15: 4,9
16: 4,2
I noticed that 220/3W resistors near power supply chip are getting very hot, I burned my finger. This did not occur earlier.
02: 4,2
03: 0,1
04: 0
05: 1,5
06: 3,5
07: 0
08: 10,2
09: 3,7
10: 3,7
11: 10,2
12: 9,7
13: 4,9
14: 4,9
15: 4,9
16: 4,2
I noticed that 220/3W resistors near power supply chip are getting very hot, I burned my finger. This did not occur earlier.
The IC appears to be producing output. Are you sure that the power supply isn't producing any output?
Where does the 15V come from? Through 220/3W resistors from rail voltage?
I have 21V on one end of the resistor and 3V on the other end. And they are getting hot.
I have 21V on one end of the resistor and 3V on the other end. And they are getting hot.
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