Bought this amp as defective. It has 4 channels, output transistors: 2SB688, 2SD718. Power supply MOSFETs - IRFZ44N.
At first it draw too much current, checked output transistors, one in channel 1 was shorted. Removed output transistors from that channel. Then powered amp up, when idling it does not draw current anymore. Tested if channels produce audio, channels 3 and 4 produce audio and everything seems to be OK with these channels. Then tested channel 2, with speaker connected to channel 2, amp starts to draw current, pushes speaker cone out and makes squeaky noises for a few seconds after powering amp up. Channel produces audio, I dont have a scope but seems quite clean audio.
Checked output transistors, soldered them out, to be sure, they are OK. Removed MOSFETs, checked them: OK. Checked rectifiers: OK.
Measured voltage on solder pads of MOSFETs 1: 3.5V, 2:12,2V, 3:0V.
Rectifiers are in correct locations. Then took output transistors from channel 2 and installed them in channel 1, result is the same: amp draws current, pushes speaker cone out.
Measured voltages to ground terminal on legs of output transistors. Working channels: 1: 0V, 2:21V, 3: 0V. Problematic channels: 1:21V, 2:21V, 3:21V.
I dont know what causes this... 🙁
At first it draw too much current, checked output transistors, one in channel 1 was shorted. Removed output transistors from that channel. Then powered amp up, when idling it does not draw current anymore. Tested if channels produce audio, channels 3 and 4 produce audio and everything seems to be OK with these channels. Then tested channel 2, with speaker connected to channel 2, amp starts to draw current, pushes speaker cone out and makes squeaky noises for a few seconds after powering amp up. Channel produces audio, I dont have a scope but seems quite clean audio.
Checked output transistors, soldered them out, to be sure, they are OK. Removed MOSFETs, checked them: OK. Checked rectifiers: OK.
Measured voltage on solder pads of MOSFETs 1: 3.5V, 2:12,2V, 3:0V.
Rectifiers are in correct locations. Then took output transistors from channel 2 and installed them in channel 1, result is the same: amp draws current, pushes speaker cone out.
Measured voltages to ground terminal on legs of output transistors. Working channels: 1: 0V, 2:21V, 3: 0V. Problematic channels: 1:21V, 2:21V, 3:21V.
I dont know what causes this... 🙁
Attachments
It appears that you may have a defective driver or output in the channel that pushes the cone out.
Was the voltage read with the outputs in or out of the circuit?
Did you check the outputs for open junctions and for leakage (meter on resistance)?
How can the channel with 21v on it produce clean audio?
Is the DC (pushing the cone out) only present for a few seconds?
Was the voltage read with the outputs in or out of the circuit?
Did you check the outputs for open junctions and for leakage (meter on resistance)?
How can the channel with 21v on it produce clean audio?
Is the DC (pushing the cone out) only present for a few seconds?
Voltage on output transistor legs was measured with outputs in.
Now I removed the transistors and measured voltage on their solder pads:
1: 21.1V that should be base?, 2: 21.1V collector? 3: 0.5V
I measured resistance between legs. One transistor was shorted, that pair I threw away already. The pair in the other problematic channel did not show signs of being shorted and there were no small resistances between legs. There was a difference in htfe one transistor had htfe 69, other 120. Datasheet says that minimum htfe should be 50 and maximum 160.
I measured voltages on driver transistor legs. Those are near emitter resistors, one leg connected to outputs base? They have 21V on each leg, on both of the problematic channels, does not matter if outputs are in or out. 21V is present on the next row of small transistors as well. Only on the third row of small transistors (almost in the center of the board) I measured voltages 1:0,0 2: 21V 3:0,0V on transistor legs.
DC is present all the time, not for a few seconds. I dont know how it produced audio. I had only 1 RCA connected to channel 1, one speaker connected to channel 1, cone was pushed out and audio was played, although very quietly.
So, what to do next remove small transistors and check them all?
Now I removed the transistors and measured voltage on their solder pads:
1: 21.1V that should be base?, 2: 21.1V collector? 3: 0.5V
I measured resistance between legs. One transistor was shorted, that pair I threw away already. The pair in the other problematic channel did not show signs of being shorted and there were no small resistances between legs. There was a difference in htfe one transistor had htfe 69, other 120. Datasheet says that minimum htfe should be 50 and maximum 160.
I measured voltages on driver transistor legs. Those are near emitter resistors, one leg connected to outputs base? They have 21V on each leg, on both of the problematic channels, does not matter if outputs are in or out. 21V is present on the next row of small transistors as well. Only on the third row of small transistors (almost in the center of the board) I measured voltages 1:0,0 2: 21V 3:0,0V on transistor legs.
DC is present all the time, not for a few seconds. I dont know how it produced audio. I had only 1 RCA connected to channel 1, one speaker connected to channel 1, cone was pushed out and audio was played, although very quietly.
So, what to do next remove small transistors and check them all?
Are both defective channels on the top in the photo?
There are eight 5.1k and eight 2.7k resistors (2 of each each per channel). Compare the voltages on the 2.7k resistors in the good channels to those in the defective channels. Four of the 2.7k resistors should have approximately 15v on one end. The other four should have approximately negative 15v on one end. Is this what you have?
There are eight 5.1k and eight 2.7k resistors (2 of each each per channel). Compare the voltages on the 2.7k resistors in the good channels to those in the defective channels. Four of the 2.7k resistors should have approximately 15v on one end. The other four should have approximately negative 15v on one end. Is this what you have?
Pulled driver transistors A1013 and C2383 next to big emitter resistors on both defective channels. Checked for 0 ohms between legs, did not find it. Soldered jumper wire to leg 1 and 3, set meter to diode check, measured resistance between legs 2 and 3 one way and then the other way. Each transistor had open circuit one way and 1500-1600 ohms of resistance the other way. Measured hfe, 2 had about 180, one had 190 and one had 200.
I understand that they are OK?
I understand that they are OK?
The jumper is only required for enhancement mode FETs. These are BJTs (Bipolar Junction Transistors).
Re-check them with your meter set to ohms (no jumper) and make sure that they read open 1-2 (no matter the orietation of the probes), and one way 2-3 and 1-3. Then set the meter to diode check and confirm that you read open 1-3 and 2-3 and with the probes reversed, you read ~0.6 1-3 and 2-3.
The following is one example. It's important is that you confirm that you have two good junctions (base-collector and base-emitter) that pass current in one direction but block current in the other direction. It's also important that no current flows from collector-emitter in either direction.
Re-check them with your meter set to ohms (no jumper) and make sure that they read open 1-2 (no matter the orietation of the probes), and one way 2-3 and 1-3. Then set the meter to diode check and confirm that you read open 1-3 and 2-3 and with the probes reversed, you read ~0.6 1-3 and 2-3.
The following is one example. It's important is that you confirm that you have two good junctions (base-collector and base-emitter) that pass current in one direction but block current in the other direction. It's also important that no current flows from collector-emitter in either direction.
Attachments
Checked the 4 driver transistors again as seen on image you attached.
Results:
Numbers (1, 2, 3 etc) are probes positions according to your image. 1 means open circuit.
Transistor 1
1: 1
2: 1,496 (diode check)
3: 1
4: 1
5: 1,481 (diode check)
6: 1
Transistor 2
1: 1
2: 1
3: 1
4: 1,517 (diode check)
5: 1
6: 1,510 (diode check)
Transistor 3
1: 1
2: 1,482 (diode check)
3: 1
4: 1
5: 1,467 (diode check)
6: 1
Transistor 4
1: 1
2: 1
3: 1
4: 1,515 (diode check)
5: 1
6: 1,508 (diode check)
My multimeter goes only up to 2Mohm so with that setting I get open circuit from everywhere.
Results:
Numbers (1, 2, 3 etc) are probes positions according to your image. 1 means open circuit.
Transistor 1
1: 1
2: 1,496 (diode check)
3: 1
4: 1
5: 1,481 (diode check)
6: 1
Transistor 2
1: 1
2: 1
3: 1
4: 1,517 (diode check)
5: 1
6: 1,510 (diode check)
Transistor 3
1: 1
2: 1,482 (diode check)
3: 1
4: 1
5: 1,467 (diode check)
6: 1
Transistor 4
1: 1
2: 1
3: 1
4: 1,515 (diode check)
5: 1
6: 1,508 (diode check)
My multimeter goes only up to 2Mohm so with that setting I get open circuit from everywhere.
Yes. Outputs have different leg configuration 1 base, 2 collector, 3 emitter. Using diode check I get 1,2xx between 1-2, 1-3 one way, open circuit the other way. Open circuit between 2-3, does not matter which way the probes are.
When the cone gets pushed out initially, how forcefully does it get pushed out?
Or... how much DC is present on the speaker terminals when the cone is pushed out?
Does the DC completely drop to ~0v after a few seconds?
Do both channels produce clean audio after the initial DC has gone?
Or... how much DC is present on the speaker terminals when the cone is pushed out?
Does the DC completely drop to ~0v after a few seconds?
Do both channels produce clean audio after the initial DC has gone?
It does not get pushed out all the way. I cant measure right now, all the transistors are out. I think it was the same 21V.
I did not express myself clearly DC does not go away. As long as amp is switched on DC is present on speaker terminal.
I did not express myself clearly DC does not go away. As long as amp is switched on DC is present on speaker terminal.
If I send you a generic schematic diagram, can you insert the circuit board designations for your amp? It should be pretty easy to do with the traces silkscreened on the top of the board. I'm assuming that you don't have a schematic diagram for the amp.
Email me if you want the generic diagram.
babin_perry@yahoo.com
Email me if you want the generic diagram.
babin_perry@yahoo.com
No, I dont have a schematic diagram. I would do anything I can to get this amp working and hopefully learn something from it.
As I understand this will not be a easy fix.
As I understand this will not be a easy fix.
Drew the schematic for Perry, he asked me to measure voltage on some of the transistors bases. I put all the transistors which I had removed for testing back to the board. I did not replace any transistors only put the old ones back in but somehow voltages on transistor legs are similar to the OK channels transistors. E: 0v, C: 21V, B:0V when idling.
Connected speaker to output, cone does not get pushed out anymore.
The board is quite thin and while I was drawing the schematic I turned the board around quite a lot of times, and the board bended also quite a lot. I suspect that somewhere was a bad connection and that bending temporarily fixed it.
However the problematic channel does not produce quality audio. Sound is very quiet and there is a hissing noise present. When I turn the gain and hpf frequency pots, then hissing goes away but soon returns.
Maybe I should solder a jumper wire from one of the OK channels gain/frequency op.amps output to the problematic channels input and hear how it sounds then.
If that way I get good audio then resolder legs of problematic channels opamps, potentiometers etc?
Connected speaker to output, cone does not get pushed out anymore.
The board is quite thin and while I was drawing the schematic I turned the board around quite a lot of times, and the board bended also quite a lot. I suspect that somewhere was a bad connection and that bending temporarily fixed it.
However the problematic channel does not produce quality audio. Sound is very quiet and there is a hissing noise present. When I turn the gain and hpf frequency pots, then hissing goes away but soon returns.
Maybe I should solder a jumper wire from one of the OK channels gain/frequency op.amps output to the problematic channels input and hear how it sounds then.
If that way I get good audio then resolder legs of problematic channels opamps, potentiometers etc?
You should be able to do that. To the left of the schematic diagram you marked up, there is (would be) a muting transistor. One leg (likely the center leg) connects directly to the non-bridging speaker terminals. The left leg likely connects to a resistor. This resistor is driven by the signal source (op-amp). Disconnect one end of the resistor in the defective channel and connect the two leftmost legs of the muting transistors (good channel and defective channel).
I did not try that yet but installed new outputs to channel 2, that channel was problematic as well. On the legs of new outputs I measured 0, 21, 0V.
But that channel does not produce audio. When turning on the amp, speaker cone pushes out and goes back to center.
With speaker connected to channel 1, amp did not draw current anymore but with speaker connected to channel 2 amp started to draw current. Interesting was that if I switched HPF off, current consumption dropped to minimum. Tried to switch between Full and HPF without speaker connected, sometimes with HPF it started to draw current, sometimes not.
Measured outputs, inputs of the opamps, there are 6pcs on the board near inputs. The top one, near defective channels had 13,4V on output1, is that normal?
But that channel does not produce audio. When turning on the amp, speaker cone pushes out and goes back to center.
With speaker connected to channel 1, amp did not draw current anymore but with speaker connected to channel 2 amp started to draw current. Interesting was that if I switched HPF off, current consumption dropped to minimum. Tried to switch between Full and HPF without speaker connected, sometimes with HPF it started to draw current, sometimes not.
Measured outputs, inputs of the opamps, there are 6pcs on the board near inputs. The top one, near defective channels had 13,4V on output1, is that normal?
If the op-amp has 9 pins, it's likely normal. Otherwise it's not normal. What is the DC voltage on the other pins?
IC#
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
IC#
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
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