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Old 10-04-2020, 06:16 PM   #24
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Default Re: troubleshooting definitive technology prosub 60

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?

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?

Originally Posted by khajr View Post
I tested to voltage at the connections for the mosfets and found the following:

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

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.

Originally Posted by khajr View Post
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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