troubleshooting definitive technology prosub 60

[khajr]

Hello,

My prosub 60 is blowing fuses. I bought replacement 1.25 amp 125v/250v slo blo fuses. When I replace the fuse it blows immediately along with a loud buzzing/hum from the sub. Attached are the schematics from definitive technology. There was one bad resistor, R19, and I have replaced this with the correct 1/2watt 1ohm resistor. When I plugged the unit it that resistor pops and the fuse blows. I have also replaced the following: caps C2 and C4, the bridge rectifier D1, op amp U1D, and the power mosfets Q1 and Q7. I have also checked R18, R20, and R21 and they checkout fine (removed from board to check). I have also checked power thermistor R7 and it came back 17 ohms. I removed Z1 and Z4 and checked them and they read a 0.7 drop. I have also verified the subwoofer is viable per def tech by checking the resistance across + and - and it reads 59 ohms. It is supposed to be +/- 15% from 64 ohms. I have also gently applied even pressure on the sub and it moves without any noises. I am at a loss now. Anyone have any thoughts?

I should mention that this happens w/o the amp being connected to the receiver.

Thanks in advance.

[R_J]

If Q1 & Q7 were shorted and R19 open, it is very likely the gate drive circuit is also damaged, Z1, Z4, R1, R18, Q3, Q4, Q5 etc. Z1&z4 are zener diodes and may check like a diode but zener at the wrong voltage.
If Q1 or Q7 shorts drain to gate, it applies 170vdc to the gate components, so it may have damaged U3d as well

[khajr]

Originally Posted by R_J

If Q1 & Q7 were shorted and R19 open, it is very likely the gate drive circuit is also damaged, Z1, Z4, R1, R18, Q3, Q4, Q5 etc. Z1&z4 are zener diodes and may check like a diode but zener at the wrong voltage.
If Q1 or Q7 shorts drain to gate, it applies 170vdc to the gate components, so it may have damaged U3d as well

Thanks BadCaps. The new Q1 and Q7 are shorted source to gate. All pins were shorted on the ones I took out. I did replace U3d and more zener diodes are on order. I will check Q3, Q4, and Q5.

[R_J]

Originally Posted by khajr

Thanks BadCaps. The new Q1 and Q7 are shorted source to gate. All pins were shorted on the ones I took out. I did replace U3d and more zener diodes are on order. I will check Q3, Q4, and Q5.

You can use a 60~100 watt incandescent lamp in series with the a/c input, this will limit the current and may help prevent the amp from blowing up during troubleshooting.

[khajr]

Originally Posted by R_J

You can use a 60~100 watt incandescent lamp in series with the a/c input, this will limit the current and may help prevent the amp from blowing up during troubleshooting.

I had to look that up. That is a great idea. I have access to a flir camera can I use this in conjunction with the flir and identify hotspots...or am I supposed to unplug the sub when I notice the light doesn't go out?

[khajr]

Originally Posted by khajr

I had to look that up. That is a great idea. I have access to a flir camera can I use this in conjunction with the flir and identify hotspots...or am I supposed to unplug the sub when I notice the light doesn't go out?

Let me rephrase this...I understand that the bulb is drawing the current but if use a 100 watt bulb will it still be safe enough to leave on long enough to get a good thermal image w/o compromising any parts?

[R_J]

If you have a variac, that would work the best so you could increase the a/c voltage slowly. If you don't have one then you can use a lamp, but the wattage of the lamp is not that critical. It is NOT the lamp that is drawing the current, it is the unit under test. If there is a short in the amp, the lamp will light up full brightness, if the amp is not drawing a lot of current, the lamp will usually light for a second (as caps charge etc.) then dim.

Just a note that if you used a very low wattage lamp (25watt) even a working amp may not operate so you might assume a problem when there was none.

[khajr]

I don't have a variac so I am going to use the lamp....I had all the parts in my garage to build one. I hope it stays on long enough to get a good thermal image of the board so I can pin point a problem.

[khajr]

Attached is the IR image of the board. Nothing blew. R7 is hot. The camera was set to auto scale temp range based on field of view. so it was 169 degrees F but it got as high as 200 degrees F when looking at it for a few more seconds before I turned the power off. Is it supposed to be that hot? I will have to order one but I am not sure what to order to replace it. The markings are SCK 202 and on the schematic it says 20 NTC SL20002 US Sensor. But I can't seem to find anything on mouser that matches that. D1 shows hot but it is directly down stream from R7. I will also buy another D1 (DF08) and I have plenty of 1 ohm 1/2 watt resistors for R19. I think Q4 and Q 1 are fine but I will get a couple more. C14 showed warm when I took this pic (lower right of image) but that was errant. It never showed warm again.

Badcaps thanks so much for your suggestion for the dim bulb tester.

[R_J]

R7 is a ntc thermistor, it gets hot and the resistance goes down, thats how it works. it will not be the problem. The cold temperature is 20Ω It is just for inrush protection. they can fail open but this one is working. The bridge rectifier is also likley ok, check if you have + & - 170 volts, this voltage is likely lower with the lamp in line. but both + & - voltages should be close to equal.
I see Q1 and Q4 (or should that be Q7) and R19 are quite hot, it could be that the mosfets are turned on hard.
What is the voltage on the speaker line? that needs to be near 0 volts

[khajr]

Originally Posted by R_J

R7 is a ntc thermistor, it gets hot and the resistance goes down, thats how it works. it will not be the problem. The cold temperature is 20Ω It is just for inrush protection. they can fail open but this one is working. The bridge rectifier is also likley ok, check if you have + & - 170 volts, this voltage is likely lower with the lamp in line. but both + & - voltages should be close to equal.
I see Q1 and Q4 (or should that be Q7) and R19 are quite hot, it could be that the mosfets are turned on hard.
What is the voltage on the speaker line? that needs to be near 0 volts

Yes. It’s Q7. Don’t know why I had Q4 in my head. All three pins on Q1 are shorted together. I will check speaker line voltage and the bridge rectifier. But I am going to guess the speaker line voltage is not going to be zero as it makes a hum.

My original replacement were the two mosfets, R19, the bridge rectifier, C2 C4. and the op amp. Would we still be concerned about the zener diodes at all?

[khajr]

Originally Posted by R_J

R7 is a ntc thermistor, it gets hot and the resistance goes down, thats how it works. it will not be the problem. The cold temperature is 20Ω It is just for inrush protection. they can fail open but this one is working. The bridge rectifier is also likley ok, check if you have + & - 170 volts, this voltage is likely lower with the lamp in line. but both + & - voltages should be close to equal.
I see Q1 and Q4 (or should that be Q7) and R19 are quite hot, it could be that the mosfets are turned on hard.
What is the voltage on the speaker line? that needs to be near 0 volts

The + and - voltages on the bridge rectifier were off by an order of magnitude and the there was current through the speaker wire.

[khajr]

Where should I be measuring the bridge rectifier?

[momaka]

Originally Posted by khajr

Where should I be measuring the bridge rectifier?

Measure across the four caps after bridge rectifier D1, if that is more convenient. C1 and C2 are in parallel, so you should see the same voltage across them, and hopefully it should be around 160-170V DC, depending on how much voltage your dim bulb is dropping on the AC. Likewise, C3 and C4 are in parallel and should also have the same voltage across them. Lastly, placing your red (positive) multimeter probe on the positive lead of cap C1/C2 amd the negative (black) multimeter probe on the negative lead of cap C3/C4 should give around 320-340V DC. Alternatively, you can measure the DC voltage across the bridge rectifier (+) and (-) pins. Whichever you chose, just be safe and pick the one where you're not likely to touch any of the metal parts in that area, as those DC voltages are quite high and can be dangerous.

[khajr]

Originally Posted by momaka

Measure across the four caps after bridge rectifier D1, if that is more convenient. C1 and C2 are in parallel, so you should see the same voltage across them, and hopefully it should be around 160-170V DC, depending on how much voltage your dim bulb is dropping on the AC. Likewise, C3 and C4 are in parallel and should also have the same voltage across them. Lastly, placing your red (positive) multimeter probe on the positive lead of cap C1/C2 amd the negative (black) multimeter probe on the negative lead of cap C3/C4 should give around 320-340V DC. Alternatively, you can measure the DC voltage across the bridge rectifier (+) and (-) pins. Whichever you chose, just be safe and pick the one where you're not likely to touch any of the metal parts in that area, as those DC voltages are quite high and can be dangerous.

Thanks for the help momaka. I am waiting on new Q1 and Q7 mosfets to come in. Then I will check those voltages. Yeah...I am trying to be very careful. I took Q1 out and pins 2 (drain) and 3 (source) were closed. 1 (gate) and 2 (drain) were open. and 1 (gate) and 3 (source) were open. I am going to take Q7 out and check too.

[momaka]

NO problems.
You can check those voltages without Q1 and Q7... and in fact that might be better so you can confirm if your power supply is OK with the output transistors removed. Then once you fit back Q1 and Q7, you can see how they affect the power supply if something is still not quite right.

[khajr]

Originally Posted by momaka

NO problems.
You can check those voltages without Q1 and Q7... and in fact that might be better so you can confirm if your power supply is OK with the output transistors removed. Then once you fit back Q1 and Q7, you can see how they affect the power supply if something is still not quite right.

Attached is a pic of the board where C1/C2 and C3/C4 are. It looks like you either have C1 and C3 or C2 and C4. Since this is the large case I have C2 and C4.

I am using a 100 watt bulb in the dim bulb tester. With Q1 and Q7 removed the lit dimly for a moment and went out. Across C2 I get 168v. Across C4 I get 168 v. Across C2 and C4 I get 360v. Across the + and - of the bridge rectifier I get 336v.

[khajr]

The new mosfets Q1 and Q7 ought to be here tomorrow or Friday. What is concerning is that I already swapped those out along with C2, C4, R19, D1, and U3. And the it still blew the fuse and R19. Maybe I got a bad component. My soldering sucks (its really pretty good). I know now to test the new mosfets before I install them.

[R_J]

If the drive to the mosfets is bad, then the mosfets will be turned on, when they are turned on they will act as a short. If the circuit is working, the mosfets will be turned OFF and only turn on with the audio signal, basically Q1 and Q7 will alternate turning on and off according to the audio signal, and the voltage on the speaker will vary +/- producing the audio.
You need to be very careful working with these double sided boards, If you remove a through hole component and destroy the feed through, you will no longer have the circuit connection between one side and the other.

[khajr]

Originally Posted by R_J

If the drive to the mosfets is bad, then the mosfets will be turned on, when they are turned on they will act as a short. If the circuit is working, the mosfets will be turned OFF and only turn on with the audio signal, basically Q1 and Q7 will alternate turning on and off according to the audio signal, and the voltage on the speaker will vary +/- producing the audio.
You need to be very careful working with these double sided boards, If you remove a through hole component and destroy the feed through, you will no longer have the circuit connection between one side and the other.

I tested to voltage at the connections for the mosfets and found the following:

Q1
P1 and P2 = +169V
P2 and P3 = -169V
P1 and P3 = -300mV

Q7
P1 and P2 = +163V
P2 and P3 = -169V
P1 and P3 = -6V

I will try and figure what they should be.