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Old 7th March 2012, 07:47 PM   #11
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Join Date: Mar 2012
Location: PVR Mexico
Default some progress...

Thanks again!

Here is some progress.

I decided to put both transistors in the heatsink of the amplifier. I have some sil-pads laying around, so i drilled a bit and they are ready to install. I have checked again the amp before assembling and i noticed that the blue resistor besides B649 gets very warm to the touch, in like 20 seconds is enough hot so i checked i made a bad solder conection or bridged something. Needless to say, the amp works as before, im just not trust about that part getting hot. I will have to ask, is this normal too? im afraid to burn something else.

I greatly appreciate your help.
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Old 7th March 2012, 08:17 PM   #12
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Location: Louisiana
Both R47 and R45 should be the same temperature. It's normal for them to get hot.

With no power applied, confirm that both are within tolerance (±5%).

With power applied, confirm that the voltage across ZD4 and ZD5 is the same.

Deburr the holes you drilled so that the transistor can lay flat against the heatsink.
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Old 8th March 2012, 02:32 AM   #13
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Im back with the readings:

Voltage across ZD4 and ZD5 is nearly equal

ZD4 15.39

ZD5 15.34

Resistor Values=

R45 = 3240 OHM

R46 = 323 OHM

R47 = 323 OHM

R48 = 3020 OHM


R47 gets hot really fast. I checked my psu and got 11.02 volts, so "just in case" i changed for another PSU, which gets 12.30 with the amp running, and the only change was getting hotter more fast R47. I´m going to recheck my soldering points of the wires of the regulators to see if one is bridging. Thanks.
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Old 8th March 2012, 03:03 AM   #14
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That may indicate that you have a leaky capacitor. C221 should be checked or replaced.

What is the DC voltage across R46 and R47?

Does the voltage across those two resistors change significantly if you unplug the preamp board?
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Old 8th March 2012, 03:36 AM   #15
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Thank You.

Voltage readings from R46-R47 with the amp powered up, preamp board installed:

R46=17.22
R47=25.5

Without preamp board, the values change very much:

R46=8.6
R47=15.8


I´m going to search the internet to check how to test properly a cap, since rarely i test one, and im going to be back with the readings of C221. It is a 4.7 milifarad 50V, or at least its what it reads.

Thank you very much.

Last edited by Desafinado; 8th March 2012 at 03:36 AM. Reason: wrong values.
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Old 8th March 2012, 09:08 PM   #16
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Join Date: Mar 2012
Location: PVR Mexico
I think will be better to go grab some capacitors from the electronics store. I have a question:

While im there, it will be better to get some other parts that might be replaced too? if so, which ones? some resistors, diodes, some upgraded part? so i can get everything i need from my trip.

And the other question it is if i need to get the exact replacement of the caps? (4.7 milifarad 50V). Currently, there are five at the store:

Capacitor electrolítico chico, no polarizado, radial, de 4.7 uF (micro Faradios) a 50 Volts

Capacitor electrolítico radial, de 4.7 uF (micro Faradios) a 100 Volts

Capacitor electrolítico radial, de 4.7 uF (micro Faradios) a 160 Volts

Capacitor electrolítico radial, de 4.7 uF (micro Faradios) a 350 Volts

Capacitor electrolítico radial, de 4.7 uF (micro Faradios) a 63 Volts


Hiperlink:

Capacitores electrolíticos 4,7 uF en Steren

Thanks in advance.
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Old 9th March 2012, 12:52 AM   #17
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You likely have a 4.7 microfarad cap, not a millifarad cap.

The voltage isn't important as long as it's equal to or greater than the original capacitor. The higher voltage capacitors are likely to be larger.
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Old 10th March 2012, 07:29 PM   #18
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Hello again.

Went and brought from the store a pair of 4.7 microfarad 63V caps (there werent 50v ones). I removed both C121 and C221 and tested. R47 is still getting very hot. R46 is normal. Audio quality is not affected.

Q16 and Q15 are not getting hot, that`s good news. (Maybe they got hot because they were initially near of R47)

Like you said, maybe R47 resistor should be hot with normal operation. I have plans of assembling it and tested a day or two, but im affraid of blowing something because the high temp of R47.

Thanks in advance, i keep you posted.
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Old 10th March 2012, 09:42 PM   #19
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Location: Louisiana
I would expect the regulator transistors to get hot if not clamped to the heatsink.

I would also expect both R46 and R47 to have the same voltage across them.

The only thing that I'd expect to fail is R47 unless something else is already damaged and ready to fail.
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Old 10th March 2012, 10:10 PM   #20
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R46 = 17.08
R47 = 16.50

Well, i started to think im bothering a lot with this amplifier, so i assembled it an played for a few minutes, 10. Both R46-R47 gets very hot.

Plays beatufully, everything working.

I talked with my friend about this, and he said to me that his brother (he was told the real story...) when was using the amp, he plugged a speaker wire to the outpots of the amp TO ANOTHER WORKING AMP, accidentally (a USAMPS xt2000d), instead to a subwoofer (because he was thinking that wire was routed to a speaker...) much like like "amplifier chaining" .

The amp didnt stopped working that day. One month or so after that incident, they were using the 500.2 with 4 6x9 speakers at low volume, playing for like an hour, the amp sounded what he described to "very much pink noise" (i think a wire of a speaker got grounded to the car chasis)
and stopped working. They removed the amp, tested in another car but played fine. They decided to remove from the car it was, and sat like 2 or 3 years, when finally the amp fall into my hands.

Maybe im being too ambitious trying to repair this amp (because i can have it if i can repair it...) without the proper knowledge. I am going to give another shot to the board if i can detect something.

I truly appreciate your valuable help Mr. Babin. Thank you.
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