troubleshooting definitive technology prosub 60

[R_J]

Re: troubleshooting definitive technology prosub 60

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]

Re: troubleshooting definitive technology prosub 60

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.

[khajr]

Re: troubleshooting definitive technology prosub 60

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.

[R_J]

Re: troubleshooting definitive technology prosub 60

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]

Re: troubleshooting definitive technology prosub 60

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?

[khajr]

Re: troubleshooting definitive technology prosub 60

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?

[R_J]

Re: troubleshooting definitive technology prosub 60

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]

Re: troubleshooting definitive technology prosub 60

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]

Re: troubleshooting definitive technology prosub 60

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]

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.