I have a Rockford Fosgate Power 750x that has been happily married to my Empeg for the last 6 years. I got in my Jeep the other day and when the Empeg fired up, what I heard was not music to my ears. The right channel seems to almost no significant sound coming from it. The left channel has noisy, crackly program from the Empeg. I tried the Empeg in the dock at home and it seemed to pump out the tunes just fine. I flipped up my back seat and plugged an iPod directly into the amp and got the same results as before: almost nothing on the right side, and crackly distorted program on the left. This held true for both the front and rear sections. I also did find at this point that if I turn the iPod volume all the way up, the distortion nearly disappears; as I back off the volume, the distortion comes back.

I thought I might crack the amp and have a look around inside. I imagine I could somewhat easily fix it, if I could only determine where the problem is. I have a general understanding of electronics and amplifiers, but not a whole lot of practical experience repairing them.

My first guess is that something in the input stage of the power amp is failing/has failed, where if I crank up the signal going into that stage, it's enough to compensate to a degree. Anyone else have any thoughts or advice on diagnosing?

Cheers,
-Randy

I would look first for bad/cracked solder joints around the external connections. It's almost always that sort of thing that goes first.

Cheers

Hi,

Before you look for the poor solder connection that Mark referred to, make sure your install doesn't have problems.

1) Make sure you have a good connection from the Amplifier to the frame of your vehicle. This includes a large diameter wire (4 AWG or larger) to the vehicle frame local to the amplifier. This connection can become corroded with environmental conditions over time. It's best to clean both the lug and the chassis frame connection area initially to provide a good bond, then seal the connection to prevent corrosion. Check your positive and negative connections to your Amplifier and or Bulk Capacitor (sometimes referred to as a power stifferer capacitor).

2) Make sure your RCA connectors are not corroded. Amplifiers often have gold plating on their RCA connectors. It's best to use gold plated RCA connectors on your cables. It makes a long term gas-tight connection. If you have not used gold on gold, you may have galvanic corrosion from dis-similar metal migration. This is often seen as a black dot of oxide where the surfaces meet. You can clean the connection and it will work for a while, but the problem will return later. Just use gold-on-gold to eliminate the problem.

Inside the amplifier, look at the following...

1) Look for an open ground on the input. This may be a poor solder connection at the connectors where they are stressed by cable/connector movement, or a burned open trace on the circuit board.

2) Clean the input level controls (if you can get to them) with De-oxit D5 to clean them, then Pro-Gold to maintain them.

3) Look for burned resistors in the front end low level stages of the amplifier.

4) Look for radial capacitors (tall cans) that are bulged (the top is not level or puffed up from outgassing), overheated capacitors (the plastic wrapper around it has shrunk and doesn't partially cover the top of the can), or have leaked electrolyte onto the circuit card. The ones usually suspect are located near heat producing components like power resistors, power transistors on small heatsinks, or capacitors that have to handle high ripple currents in the power supply. The same thing happens in TVs too. Always look for capacitors around heat producing devices like the flyback transformer, power resistors, vertical amplifier, audio amplifier, pincussion circuitry, power supply, etc. Sometimes they take out small fusible resistors less that a few ohms too.

If this is the problem, it would most likely be a smaller capacitor <100 uF in the signal path somewhere. They can be used to AC couple signals between stages or can be used to set the gain of circuits when they bypass an emitter resistor too.

5) It could be a bad op-amp in the front end too. Always check the voltages on the op-amp before declaring it deceased (common pins are pin 7 (+V) and pin 4 (-V) for 8 pin op-amps like the 4558) to ground. The voltage could be +/- 6 to 15V, but should be equal on both pins to ground (like +6V and -6V to ground). If the voltages are not equal, check for 2 small regulators that may or may not be on the main heatsink. They will supply the low level supply voltages. If one of them are bad, look for other problems too.

6) Check that you have proper voltage to the output devices. The outputs are rows of black devices mounted on the main heatsink assembly. They will likely have 2SAxxxx, 2SBxxxx, 2SCxxxx, or 2SDxxxx, labled on them. If it is a FET output stage (digital amplifier), they will be 2SJxxx, 2SKxxx, IRFxxx, or BUxxx. Don't confuse these with the power supply MOSFETs in the power supply. They will be locaed away from the power supply section. These are easy to measure as the devices are likely TO-220 or TO-247 packages. The center pin will be the supply voltage. They will be arranged as paralleled sets of 2SA/2SC or 2SB/2SD for the positive and negative rails. The amplifier will be split this way and if you measure from the center pin of a 2SA and a 2SC device to ground, you should see a similar voltage + to ground and - to ground within a volt or so. Be careful measuring this and don't short anything out. The voltage will probably exceed 30 volts for each of these voltages. If this is there, the problem is likely back in the input stage again.

This should get you started.

Ross

Thanks for all that great info. I rechecked all my connections and they appear to be fine; gold on gold connectors, 4AWG power leads, good ground, no corrosion.

I've just pulled the amp out of my Jeep and have it on the bench now. Upon removing the cove, I can't see any components that show visual signs of failure. I'll keep updating as I go.

-Randy

Just finished testing all of the op-amps. I've included the results in the attached .pdf. On all 11 of the op-amps, the pin 4 and pin 7 voltages did not correspond to each other; pin 7 voltages were all low. Also, when probing pin 3 of op-amp U300, there was a high frequency noise coming from a component in the power supply area, possibly a capacitor. I'm about to investigate the voltage regulator section. It consists of a LM339D Quad Voltage Comparator and a TL494C PULSE-WIDTH-MODULATIONCONTROLCIRCUITS. Any ideas what I should be checking for?

Results on the voltage regulator:
output 1: +6.0 v
output 2: +14.06v
output 3: +6.0 v
output 4: +6.0 v

Results on the output stage MOSFETs:

According to Ross's trouble shooting info, I should have seen something other than 0v on the pin 2 of all of the MOSFETs. From my testing it looks like set Q2xx is the only one getting voltage, while sets Q1xx, Q3xx, and Q4xx are not. Hmm. I think that will end my poking around in the amp for tonight. Guess I'll wait and see what the knowledgeable folks here have to say.

Cheers,
-Randy

One more thing… I did notice that 7 of the 8 big, black capacitors were slightly convex on the top surface. In the photo above you can see it just a little from the reflection of the camera flash. We're probably talking about a rise of 3/64" from outer edge to center. I don't have anything else to compare them to. Could a curvature of this degree in the top of a cap be suspect?

-Randy

Big caps like those are normally *flat* on top. A bulge is often the sign of a bad cap.

I've seen quite a few cases with large diameter electrolytics where it's just the plastic top that 'harmlessly' bulges out.

Randall should be able to push down on the tops and confirm they're flat underneath.

It seems as though there is some pressure in those large capacitors; that is when I push on the tops of them, there is a fair amount of resistance. I'm not exactly sure what to make of that fact along with the other data I've collected. I guess I'll attempt to trace through the circuits to find the problem, though not having any experience at this I'm not too optimistic.

I've also started reading up on the basics of amplifiers to gain some insight. http://sound.westhost.com/amp-basics.htm seems to be a really good resource.

It's likely that if I can't make some headway on this project this weekend, I'll probably start looking for another amp- driving with no music all of the time just isn't fun. This is especially a problem since I was planning on going on a 3000mi road trip in the next week or two to NYC, Washington D.C. and Miami.

-Randy

What bothers me is that IF all those caps are bad, and they look to be so to me, then something caused them to go bad. I can see one going bad on it's own, not 8 or so. You may be throwing money and time away trying to fix something that will be more expensive in the end than just buying a known good amp. Just my 2¢.

If you're unsure about whether the caps have bulged then remove the plastic cap to get a better look.

Hi,

Sorry I didn't get back to you sooner, real busy.

The measurements you provided tell a lot about the health of the amplifier.

First, the Main PWM Power Supply is operating and providing +/- 36 Volts to the MOSFET Amplifier Output devices. This is the TL494 PWM, all of the circuitry on the Right side of the picture composed of the Power MOSFETs, dual diode bridge rectifiers, Transformer, etc. We don't know if there is any ripple without oscilloscope waveforms (to verify health of the large bulk storage capacitors that other list responders are concerned about are okay). I was concerned with the DC Voltage measurements on Q210, Q223, Q218 & Q211 though, can you re-measure them.

Second we see that the low voltage regulators are operating as the TL072 Dual Op-Amps have +/- 12 Volts on them. The pinout is different from the 4558 that I mentioned before, but, you have the correct voltages on pins 4 and 8. Again, we don't know if there is any ripple without oscilloscope waveforms.

I didn't see if you had checked for open ground traces or solder connections on the RCA Connector ground pins as Mark had mentioned. Did you do this and see anything there? With the other items being good, this is still a good suspected problem. I also didn't see that you had cleaned the controls, but noticed that these are likely sealed versions anyway. Have you rotated all of them through their range a few or more times to remove dirt and oxide build-up? The slide switches are also suspect, move them through their positions a few or more times too. Those could be cleaned too.

Once you have access to the back side of the amplifier board (Solder Side), check for open traces and poor solder connections as I mentioned and see if there are any of them in the Input and low-level amplifier stages.

At this point, it is time to provide an input and troubleshoot with an oscilloscope to locate the bad Op-Amp or missing signal. If you still want to fix it, we can go through that and find the problem. I would think that we might want to take that off-line though, then others won't have to read the long emails and may not be interested in how to fix this kind of problem.

Good luck,

Ross

There was a big thing awhile ago about industrial espionage regarding a electrolyte used in electrolytic capacitors. An incomplete formula was stolen and it was missing a key ingredient that if left out would cause the capacitors to start bulging after a period.

Another brand had similar bulging issues but it was because they were incorrectly filled and had too much electrolyte inside them.

In both cases, you could repair the device if you replaced all of the failing capacitors (and any others of that type just to be safe) with brand new capacitors.

Hi,

Then there is always the issue with counterfeit devices too.

This happened especially with power transistors.

Ross

Ross, thanks for the time, effort, and the offer. (Thanks to all the others that offered comments as well.) As per Ross's suggestion, I'm going to continue dialog off the board, however I'll post the results of this endeavor and perhaps some updates along the way incase anyone is interested.

Cheers!

In 2007 we got to swap out 30 Dell motherboards (under a warranty that, to Dell's credit, was extended because of this "bad cap" issues). Lots of manufacturers like MSI and others were affected , so if some od these bogus caps made it into AV devices, that is no big surprise. I had a Soundstream amp in 2003 that just *sucked* the amps; I wonder if it had a cap problem.

I didn't read the whole thread, but when was this amp manufactured?? I think the bogus caps stopped being distributed to manufacturers sometime in 2004.

http://en.wikipedia.org/wiki/Capacitor_plague

Damn, that's good to know. I knew there were some issues with Dell, but did not realize the issue was so wide spread. In fact, the only bad caps I've personally seen were on a video card that overheated because the fan was jammed. I never saw a single instance otherwise, and I deal with a LOT of hardware.