replacing filter caps?

[jondog]

Hi, I know this is a long shot and the correct answer is take it to a tech but I'm going to ask anyway. I recently bought an older USA Crate powered PA head. There's a solid thump when it turns on and the mixer portion works fine, all fuses are good, but there is no output to the speakers. The typical diagnosis for this symptom is old filter caps that need replacing. I have basic soldering skills and a multimeter. Can anyone point me to a good tutorial on how to do this? Or is the answer still play it safe and take it to a pro?

 

[nashvillebill]

I doubt it's filter caps. Especially if the mixer portion works, that would imply that it does have a functioning power supply.

Don't continue to plug a speaker into it until you do this simple test: Plug a speaker cable into the output of the head, but not a speaker. Now use your meter, look for the presence of DC on the output, by putting one of your meter's leads on the tip of the speaker cable and the other lead on the speaker cable's shield. Normally you would expect to see a few millivolts; anything in the order of tens of volts means the output transistors are shorted. If this is a stereo head, repeat for the other channel. However, usually when amps have shorted output transistors, they trip the fuse inside the amp...unless there's a protection circuit that is keeping the amp from doing harm to itself...

Take it to a tech, sorry.

 

[Kevin aka Kebo]

Trust me, its not your caps, they have nothing to do with it... Do us all a favor - This is for a qualified tech to look at.. Folks get hurt noodling around..

 

[beans-on-toast]

Some transistorized amps have a relay or some other form of protection that disables the output until voltages and stabilized. It acts like a standby and avoids the thump. If this circuit is not working properly, such as the relay or whatever type of switch is used is stuck on, there can be a loud thump. No audio output to the speakers complicates things. A tech would trace the audio signal with test equipment to see where it is being blocked.

 

[BassmanPaul]

If the filter caps are failing you would expect more mains hum coming out of the speakers. Your thought about a "typical diagnosis" is just simply wrong. With the amp on you should still hear a hum or hiss from the connected speakers. This indicates that the power amp is still working but not receiving any input signal. If the offset voltage that Bill asks you to check is tiny you'll have to trace out how the signal from the mixer output is routed to the power amp just as Beans advised.

 

[will33]

How would you know it's the power supply? Have you checked its various "outs". There'll be more than one and the one feeding the mixer frontend will likely be the lowest voltage.

Does the mixer have an "out" jack for connecting it to an external poweramp? Check for signal there. If you have it, the problem is between there and the speaker jacks, so follow the signal through the circuit until you lose it.

 

[bassliner50]

Sounds like the power amp section is hosed. If this amp has been transported a lot and seen action, it may be a bad solder connection on the pc board which is failed. Usually the power amp section has large transistors mounted on rather heavy heat sinks or occasionally they use the external steel case as a heat dissipater or both.. Problems are frequent where these transistor and heatsink assemblies are phyically mounted to the metal exterier of the amp. Athe same time the transistor connection leads are soldered to the printed circuit board. The printed circuit board is physically mounted to another portion of the external case usually at right angles to the heatsinks. Over time constant flexing of the amp enclosure causes the transistor connections to pull free of the PC board. This may not be visible without a magnifying glass. Sometimes moving the heatsink assembly with the eraser end of a pencil will reveal an intermittent connection. If not, remove the PC board and examine under good light and with a glass if you need. You may find a lifted land, or a lead which appears to be soldered, but is really free floating in the surrounding solder. Check for continuity with an ohmeter. This classic problem is very very common. Without a schematic is it one of the first places to check....

 

[jondog]

I fixed it! I followed Bill's instructions and found the correct few millivolts at the output. So I decided to try plugging in a speaker again and it worked! The only cause I can attribute to this "repair" is that last week when I couldn't get any sound out of it I opened the case and doused every pot, jack, etc. with a ton of Deoxit. I guess whatever was dirty is now clean and it's passing sound. Lots of the pots are still scratchy, but I'm psyched it works!

 

[beans-on-toast]

Good work! Nice when you can fix things yourself.

When you apply deoxit, you sometimes have to scrub the surfaces to get them clean. Follow the product instructions carefully. I use an inter-dental brush available at a drug store.

 

[nashvillebill]

Great!

One possibility is that your mixers power amp has a jack, maybe called Power Amp In. These jacks are typically what's called a "switching jack" and here's how they are designed to function: when nothing is plugged into the Power Amp In, the jack has an internal set of contacts that closes a connection between the Preamp Out (or Mixer Out) and the actual circuit that connects to the power amp. So whatever comes out of the Preamp or Mixer gets internally sent to the power amp. When you plug something into that Power Amp In jack, however, the jack opens the internal contacts from the Mixer or Preamp side, and instead accepts whatever signal comes through the plug and sends that signal to the power amp instead.

Often, these internal contacts get a little bit of corrosion just like a pot. The result will be just like opening the jack's internal contacts, meaning the signal from the preamp/mixer side is cut off...but there's no signal from anywhere else going to the power amp. Dead silence, or sometimes a muffled signal. Deoxit can fix this too, but sometimes it takes working the jack several times to help clean the contacts.

 

[agedhorse]

Hint: when using DeOxit, use only a tiny amount... more than that can cause problems by washing lubricants and debris that may be sticking to the inside of the backshell not causing problems onto the resistive element where it will now cause more problems.

 

[jondog]

Yes my guess is that it was the Power Amp In jack. I've had this can of DeOxit since 2010 - one short spray cleans most things, but on a powered PA head like this there are at least 40 knobs and jacks so it all adds up to one ton!

 

[beans-on-toast]

I use the 100% concentrate Deoxit that comes in a needle applicator bottle. You can apply just the right amount where you need it. Cleaner and more economical than a spray.

As was mentioned by Bill, the jacks can oxidize to the point where the junk interrupts the signal pathway. Good to keep those jacks clean with a regular maintenance schedule, at least once a year.