DIY Audio Projects Forum

Hello everyone. My name is Emil, I just found this site earlier today and was hoping someone could help me out with diagnosis and repair of this power amp. It is an Audiosorce/MTX model 300. It was given to me non working. According to the original owners(a gym where it was in service for several years for hours and hours every day), there was lightning storm when the amp stopped working...it is unclear if there actually was a power surge, or if the amp was actually on during this storm, but it is a possible cause for failure. While I am not completely tech iliterate, I will confess to having very limited knowledge regarding the repairs of this amp.

The amp shows no signs of life. The inline fuse at the rear power cable connector pops as soon as the power switch is turned on. The amp was extremely dirty, both inside and out, so a quick cycle in the dishwasher and an air dry were in order. While this might be an unconventional way of cleaning up an amp, it is highly affective, you should try it.

I'll start with a few before and after shots of the amp getting cleaned up...
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So far I have noticed that 1 or 2 of the 4 large caps have very minor bulging on the top end, and there is "goop" on the bottom, against the board...however, I think the "goop" is an adhesive placed there by the manufacturer as opposed to the caps leaking. I have also noticed a dark spot on this board near the IC's and resistors. Could be something is wrong there, or maybe it's just something getting hot but still within spec?
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And the caps, you probably won't be able to see the slight bulge.
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So, what do the experts here think I should check first? And how should I go about doing it? I have a DMM and a soldering iron. If you lead the way, I'm sure we can get this guy up and going again. I will include some more photos for refernce.
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Hi Emil,

if you're still out there... Let's give it a try. Can't go wrong, but you'll need a lot of patience...

Have fun .

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Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.

Hey Tom. I'm still here. I was beginning to wonder if anyone was going to reply. Do you think you might be able to help me with this?

Hi Emil,

looks like R03 and R04 are damaged. At least R04 shows signs of charring. IC01 and IC02 are linear regulators with fixed 15 V output. IC01 is a positive regulator (replacement for LM7815) and IC02 the negative counterpart (replacement for LM7915).
As the fuse blows when you turn on the device, the problem is most likely with the power supply. You should measure the resistance across the two resistors R03 and R04. If those are okay, test pins 1 and 2 of IC01 and IC02 with the continuity test of your DMM. Those regulators might have latched up and now short the power supply.
Which kind of fuse is the device rated for? If the device needs slow (time-delay) fuses that might have been the problem. I had a similar problem myself last weekend when I tried to get an oscilloscope working. In the end I found the shop where I bought the fuses had given me the wrong kind. I burned several of the fast fuses before I realized the mistake.

Good luck and be careful when working with line voltage.

Yes, i believe that we (not only me) can help you with your problem. Ferdinand made a good start already - nothing to add here!
I also think that the Power Supply is a part of the main issue and the first step to start with. I also had a transformer short in mind (due to a possible arc in the windings).

Take yourself enough time to check some things - step by step. I believe we can follow the circuit from the start to the end and then shoot out the failure..
That's what i can offer. You'll need time and patience. It might turn into a fiddly job when the fault is not only in the power supply section.

_________________
Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.

Hello Ferdinand, thanks for the reply.

I did notice a slight discoloration on R04, and wondered if this might be a sign of it being bad. As far as I know, it is OK to test resistors while they are on the board, and they do not need to be removed from the circuit. Is that correct?
Measuring the resistors on the board, R03 and R04 both read the same at 1.485 kOhm.

Checking all the resistors on the board I determined there is one that reads 0 ohms, and that is R01. All others had a value. I am a bit confused as to the specs of R01 as the online charts don't recognize this color sequence.


When checking ICs like the ones on this power supply, do they have to be removed from the circuit or is it OK to measure them on the board?

When looking at what I assume is the front of the IC(the face with the type on it), is pin 1 on the left, pin2 center and pin3 on the right?

If measuring on the board is OK, and assuming I have my pinout correct, IC01 measures 1.490v with the positive probe on pin 1, negative on pin 2. Switching polarity yields a reading of .598v. IC02 yields almost identical readings.

Now, when you mentioned the fuse, something occured to me. I realized I made a big "rookie" mistake here. I was using slow blow fuses, but I made a mistake as to it's rating. The original fuse was difficult to read, but it looked like it said 1.0A 250v. I quickly glanced at the back of the amp case and thought to have confirmed this rating, but just now double checking I see it was actually supposed to be a 10A fuse.

I went down to the local Shack which is the only place near me that carries anything resembling electronic components, and the closest I could get was a 6.3A slow blow, 250v. Should I try turning on the amp with that fuse? Or is more troubleshooting required before I power it on?

That looks alright. They should measure 1.5 kOhm +-5%.
As long as we're looking for a short you don't need to remove them from the circuit. But always turn off the device and disconnect it from the mains. Also give the caps some time to discharge.


R01 and R02 should read 1 Ohm. The last band (the white one) gives the temperature coefficient of the resistor (100 ppm/°C in this case). I found this calculator very helpful: http://www.okaphone.nl/calc/resistor.shtml?ohm=1&tol=5&tmp=100.


Again as for the resistors, even without removing the parts from the board we should be able to measure a short, so that's fine. You got the pins correct. Can you please check again with the continuity or resistance mode of your DMM. Just to be sure.


Well, looks like Murphy's law got you there. Before you reassemble the amp please take some pictures of the top and bottom side of the power supply PCB. Then we can blend them together later if more troubleshooting is necessary.
10 A is quite a lot, so I'm not sure if I would give it a try. But maybe Tom has an opinion on that, as he's more proficient in devices of this power rating.

OK, so R03 and R04 are probably fine, but R01 checks bad.

I checked the ICs using the continuity, or diode, setting on my meter. Checking them for resistance, they both measure 1.485 kOhms in both directions. Is it safe to assume these as being good?

I know that to fully test a capacitor's spec it needs to be removed from the circuit, but I thought I could at lease check for continuity and resistance with it on the board. Doing so, I found 2 that read 0 on both continuity and resistance. C09 and C10 are small, ceramic disc type caps marked 104. Can those be assumed bad?

In the photos below, the 2 blue wires from the transformer go to C09 and C10, which then go to what I assume is the power on relay(white square thing branded GOODSKY).
The brown wire on the left goes to R01, then to what I think is a rectifier(GW RS202M), then to a small ceramic cap (C01), then back to the black wire in between the 2 browns. The cap checks OK, at least for continuity. I don't know how to check the rectifier. It has 4 pins. The face is marked with a + on the left and a - on the right and "AC" in the middle.

R03 and R04 are fine. They seem to be connected across the input and ground terminals of the linear regulators. So these are also ok.


These caps are most likely connected across the terminals of the main rectifier. The part with the heatsink is the main bridge rectifier. I think it's quite unlikely that they are the problem as they would have survived the inrush current.


The part marked GOODSKY is indeed a relay, but I think its purpose is to short out R01 and R02 once four large caps have reached a certain voltage. That way the inrush current is reduced to tolerable levels.
To test the rectifier try the following: turn you DMM to diode mode; connect the positive lead to one of the terminals marked AC and the negative to the terminal marked plus, you should read a voltage of about 0.7 V, try it with both AC terminals; connect the positive lead to the terminal marked minus and the positive to one of the AC terminals, you should again read a voltage of about 0.7 V, again try both terminals marked AC. You can also test if the rectifier is shorted at the inputs (marked AC) or outputs (marked plus and minus).

Please take one photo each of the complete top and bottom side of the power supply PCB.

Let me take this opportunity to say thank you for your assistance. I really, really appreciate it.




OK, I have 3 terminals marked AC. They are labeled AC-B(blue wire), AC-A(blue wire) and AC-G(black wire). As you instructed, positive probe on AC terminal, negative probe on rectifier + leg, I get the following readings...
AC-B = OL
AC-A = OL
AC-G = I get a reading, it starts very low and then slowly rises to 2v when the meter goes back to OL. I assume the rise in voltage is due to the caps charging up?

For clarity, there are 2 regulators that I see, the smaller black one(GW RS202M), and a larger one with a heat sink(KBPC25-04W). I did the same test on both, and got the same results.

Question... What is the possibility here that the fuse is the only thing that went bad in the first place, and me checking the amp with the 1A, as opposed to the 10A fuse, is whay it always popped. Could it be that with the right fuse this thing will power up?