DC Voltage speakers terminals

Thread Starter

VicSD

Joined May 13, 2019
12
Hello everyone,

I am new member on the forum, so this is my first post.
I have embarked on the power amp repair, and looking for some guidance on the basic amp repair. The unit's speakers terminals shows 70 V DC (rail power). To start, I have checked the the PS voltage (the same 68-70 V) , no shorts. Checked capacitors and resistors. The R17 resistor was replaced as original showed signs of deterioration. Found three burned output MOSFETS and replaced them, but the problem still persists.

Not sure what to check as a next step.

Thank you,
 

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Audioguru

Joined Dec 20, 2007
11,248
You do not say if the output is +70V or is -70V. The output should be 0VDC. So either the N-channel Mosfet is shorted and pulls the output to the positive supply or the P-channel Mosfet is shorted and pulls the output to the negative supply.

Also, maybe a rail voltage has no voltage.
 

Thread Starter

VicSD

Joined May 13, 2019
12
You do not say if the output is +70V or is -70V. The output should be 0VDC. So either the N-channel Mosfet is shorted and pulls the output to the positive supply or the P-channel Mosfet is shorted and pulls the output to the negative supply.

Also, maybe a rail voltage has no voltage.
The Power Supply PCB has +70V and -70V on Fused_P+ and Fused_P- connections going to the iZone Channel board.
Also, both CH1_OUT and CH2_OUT showing + 70 Volt.
 

BobTPH

Joined Jun 5, 2013
8,813
When you replaced the MOSFETs, did you get exact replacements? If not, that could explain the problem. Also, there are adjusting pots for the biasing of the output transistors (P1 and P2), have you tried turning these? If you can get them to zero the output, it might be a good thing.

Bob
 

Ylli

Joined Nov 13, 2015
1,086
The pots shown in the schematic are idle current adjustments. Lots of fuses in this thing, check them all with an ohmmeter. If they are all good.... Let's concentrate on Ch1.

First, lets be sure the drive has power. Measure the voltage on U1 pin 13/14 and pin 15. These should be the same as the rails, so be careful and don't short anything or knock yourself off the chair. Post the results.

Assuming Pre+ and Pre- are OK, move on and measure the voltages on pins 2 & 3 of U1. Post the results. Now measure the voltages on U1 pins 5 & 6. Post the result.

Is there a part number printed on U1?
 

Thread Starter

VicSD

Joined May 13, 2019
12
When you replaced the MOSFETs, did you get exact replacements? If not, that could explain the problem. Also, there are adjusting pots for the biasing of the output transistors (P1 and P2), have you tried turning these? If you can get them to zero the output, it might be a good thing.

Bob
Hello Bob,

I am aware that MOSFETS have to match. I have used pooled 2SK1058 and 2SJ162 pairs of same series from another amp. Checked them with DMM and they seemed OK. Unless, I made mistake with checks. I was able to turn on and turn off every N and P Channel transistor with DMM. Not sure about required resistance between D & S on these.20190513_192955.jpg

Since the new parts are not available for this unit will try get Exicon's ECX10N20-S and ECX10P20-S as a replacement.
P1 and P2 are not preset in this version.
 

Thread Starter

VicSD

Joined May 13, 2019
12
The pots shown in the schematic are idle current adjustments. Lots of fuses in this thing, check them all with an ohmmeter. If they are all good.... Let's concentrate on Ch1.

First, lets be sure the drive has power. Measure the voltage on U1 pin 13/14 and pin 15. These should be the same as the rails, so be careful and don't short anything or knock yourself off the chair. Post the results.

Assuming Pre+ and Pre- are OK, move on and measure the voltages on pins 2 & 3 of U1. Post the results. Now measure the voltages on U1 pins 5 & 6. Post the result.

Is there a part number printed on U1?
Thank you. I will measure the voltage later today and will post the results.
 

BobTPH

Joined Jun 5, 2013
8,813
Not only do they have to be matched, they have to have the same threshold voltages as the originals, otherwise they will not not bias correctly. The pots may or may not be able to adjust for the difference.

Bob
 

Thread Starter

VicSD

Joined May 13, 2019
12
The pots shown in the schematic are idle current adjustments. Lots of fuses in this thing, check them all with an ohmmeter. If they are all good.... Let's concentrate on Ch1.

First, lets be sure the drive has power. Measure the voltage on U1 pin 13/14 and pin 15. These should be the same as the rails, so be careful and don't short anything or knock yourself off the chair. Post the results.

Assuming Pre+ and Pre- are OK, move on and measure the voltages on pins 2 & 3 of U1. Post the results. Now measure the voltages on U1 pins 5 & 6. Post the result.

Is there a part number printed on U1?
Thank you for helping me out with this. Here are the voltage values on the U1 pins.

Pin13 + 68.2
Pin14 + 67.8
Pin15 - 68.3

Pin2 +67.5
Pin3 +67.6

Pin5 +67.7
Pin6 +67.8

The U1 labeled LM4702CTA.
 

Ylli

Joined Nov 13, 2015
1,086
I would strongly suspect the LM4702 is bad. Even with 70 volts on the output, the feedback voltage should not be more than 35 volts, and if the input pins are neat 70 volts, that must be from an internal fault.
 

Thread Starter

VicSD

Joined May 13, 2019
12
Ylli,
you were correct about the LM4702 being faulty. The high voltage feed stopped after the U1 replacement. In addition, I've replaced the all pairs of output MOSFETS with Exicon's ECX10N20-S and ECX10P20-S. These now available as as a matched pairs.
I had amplifier on yesterday on for about 2 hours. Working with no issues.

Thank you all for your help.

This was my first repair :).

I will be starting an thread on another B&K repair later.
 
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