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Old 09-26-2020, 02:11 PM   #1
khajr
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Default troubleshooting definitive technology prosub 60

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.
Attached Files
File Type: pdf hightlevelamp.pdf (48.0 KB, 16 views)
File Type: pdf lowlevelamp.pdf (59.0 KB, 11 views)

Last edited by khajr; 09-26-2020 at 03:40 PM..
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Old 09-26-2020, 07:48 PM   #2
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Default 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

Last edited by R_J; 09-26-2020 at 07:56 PM..
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Old 09-26-2020, 08:30 PM   #3
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Default Re: troubleshooting definitive technology prosub 60

Quote:
Originally Posted by R_J View Post
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.
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Old 09-26-2020, 09:14 PM   #4
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Default Re: troubleshooting definitive technology prosub 60

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Originally Posted by khajr View Post
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.
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Old 09-26-2020, 10:26 PM   #5
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Default Re: troubleshooting definitive technology prosub 60

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Originally Posted by R_J View Post
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?
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Old 09-27-2020, 09:05 AM   #6
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Default Re: troubleshooting definitive technology prosub 60

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Originally Posted by khajr View Post
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?
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Old 09-27-2020, 10:31 AM   #7
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Default 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.

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Old 09-27-2020, 01:07 PM   #8
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Default 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.
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Old 09-27-2020, 08:44 PM   #9
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Default 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.
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Old 09-27-2020, 10:17 PM   #10
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Default 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

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Old 09-27-2020, 10:38 PM   #11
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Default Re: troubleshooting definitive technology prosub 60

Quote:
Originally Posted by R_J View Post
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?

Last edited by khajr; 09-27-2020 at 10:46 PM..
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Old 09-28-2020, 08:18 AM   #12
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Default Re: troubleshooting definitive technology prosub 60

Quote:
Originally Posted by R_J View Post
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.
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Old 09-28-2020, 10:11 AM   #13
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Default Re: troubleshooting definitive technology prosub 60

Where should I be measuring the bridge rectifier?
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Old 09-29-2020, 06:57 PM   #14
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Default Re: troubleshooting definitive technology prosub 60

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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.
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Old 09-29-2020, 08:27 PM   #15
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Default Re: troubleshooting definitive technology prosub 60

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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.

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Old 09-29-2020, 10:57 PM   #16
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Default Re: troubleshooting definitive technology prosub 60

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.
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Old 09-30-2020, 08:48 AM   #17
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Default Re: troubleshooting definitive technology prosub 60

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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.
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Old 10-04-2020, 06:16 PM   #18
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Default Re: troubleshooting definitive technology prosub 60

Quote:
Originally Posted by khajr View Post
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.
Looks like your power supply, at least on the primary side, is OK.

Also, I don't see a schematic of a switching PS anywhere after the 200V caps and instead the two 200V caps provide two rails: P170V and N170V. Does this mean this amplifier IS NOT ISOLATED FROM LINE VOLTAGES?
That would be a very bad design.
If it really is not isolated by design, I HIGHLY suggest you only use a GFCI plug when you test voltages inside the amplifier.

I also find it equally appauling that the two 15V power supplies, P15V and N15V are derived directly from the P170V and N170V supplies simply through a resistive drop circuit with Zeners used for regulation. Not only is this inefficient, but it means even the low-voltage rails are not isolated from mains? Am I the only one that's catching this BS, or is the schematic drawn incorrectly?

*EDIT*
Looks like there are two P15V and N15V power supplies??? Because one appears to be generated from P170V and N170V on the "line side", as mentioned in the schematics.
But then there is another set of P15V and N15V power supplies on the "safety side", and these appear to be generated from P32V and N32V power supplies, which come from a transformer, meaning they are actually isolated from the line side. That makes more sense then. Otherwise, I don't see how it would be even possible to sell this amplifier and still meet safety inspections. But then this suggests there are two P15V and N15V power supplies?

Quote:
Originally Posted by khajr View Post
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.


We can try to figure out what they should be as well... but I'm not sure what you mean by those "P1", "P2", and "P3" test points. It would be easier if you use the component pin designators - i.e. Gate, Drain, Source.

Quote:
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Attached is an thermal image without the mosfets installed. The second pic has the labels. Not sure if this tells us anything. Took the picture with the sub hooked up with the dimbulb test with a 100 watt bulb in it. The bulb dimly lights up for moment and turns off.
It's normal for the bulb to light up for a moment and then go out when plugging in a device with large input primary caps (i.e. the two 200V caps.) Basically, since the 200V caps are uncharged, they pull a substantial "surge" current as they become charged when the amplifier is plugged in, which is what makes the bulb glow. Once fully charged and at their normal voltage operation levels, the surge current stops and the bulb goes out.

As for the thermal images... looks like your hottest component are R8 and R6, which shouldn't be surprising at all. These are the resisitive droppers that provide P15V and N15V power supplies. And like I noted above, this is a very inefficient way to do that, so that explains why they are running hot.

That said, seems like they are running around ~130F (54C), which isn't really that hot. So everything looks OK there.

However, looks like Q4 is running hot, while Q2, Q3, and Q5 aren't (?)
In that case, I suggest you remove Q4 and test it out of circuit. Better yet, replace it if you have spares.
Also check the values of all resistors that connect to Q4. I don't believe Q4 should be running hot at all with the main transistors (Q1 and Q7) removed.

While at it, also check the voltages across C21 and C27, then report back what they are.
And if you can, check voltages across C5 and C6 as well (but be careful around these, since they are on the primary side.)

Last edited by momaka; 10-04-2020 at 06:37 PM..
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Old 10-05-2020, 04:27 PM   #19
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Default Re: troubleshooting definitive technology prosub 60

Quote:
Originally Posted by momaka View Post
We can try to figure out what they should be as well... but I'm not sure what you mean by those "P1", "P2", and "P3" test points. It would be easier if you use the component pin designators - i.e. Gate, Drain, Source.
Hey momaka...Sorry I was referring to the pins for that mosfet. 1 is gate, 2 is drain, and 3 is source.

Quote:
Originally Posted by momaka View Post
However, looks like Q4 is running hot, while Q2, Q3, and Q5 aren't (?)
In that case, I suggest you remove Q4 and test it out of circuit. Better yet, replace it if you have spares.
Also check the values of all resistors that connect to Q4. I don't believe Q4 should be running hot at all with the main transistors (Q1 and Q7) removed.

While at it, also check the voltages across C21 and C27, then report back what they are.
And if you can, check voltages across C5 and C6 as well (but be careful around these, since they are on the primary side.)
It only appears that Q4 is warm. I will test as I don't have any spares. I tested R12 and it was fine. I will have to figure out where R5 is. Most of the resistors are soldered right next to the board and I can read the screen printing underneath them but I can figure it out.

I will also check C21 and C27 and C5 and C6. Thanks so much for your help.
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Old 10-06-2020, 07:50 PM   #20
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Default Re: troubleshooting definitive technology prosub 60

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Originally Posted by khajr View Post
Hey momaka...Sorry I was referring to the pins for that mosfet. 1 is gate, 2 is drain, and 3 is source.
Ah OK.

In that case, Q4 and/or the sub-circuit around Q4 are definitely suspect, because you have 6V on the Gate of Q7, which means it's going to conduct as soon as a MOSFET is installed.

Quote:
Originally Posted by khajr View Post
I will also check C21 and C27 and C5 and C6. Thanks so much for your help.
No problems.

Just to be clear, we are looking for the voltages across these caps (but if you want to take out and check the caps with a cap meter in addition to measuring the voltages, that would be fine too.) An unbalance in the P15V and N15V voltage rails could also be the reason why there is a voltage on the Gate of Q7.
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