Peavey CS-800 Static

[gaweir]

Hi Folks;

I'm new to this so I'm just going to dive in. I picked up four Peavey CS-800's and a CS-1000X, all in various states of dis-repair, for $200 thinking it would be an interesting project to get them working again. The 1000X was just a bad resistor on the output and a thermal switch that failed open. Three of the 800's are working now by changing output triacs and finding a couple of failed components on the output board. The fourth one is a container for the various failed parts from the other three.

Of the three working units, two are as quiet as one could expect from one of these amps but the third is the one that has me puzzled. When I turn it on, it runs quietly for about 30 - 35 minutes and then produces static on 'B' channel that sounds like static you would hear from a radio. In fact, my wife heard it and the first thing she asked was if I had a radio hooked up to it. By mixing and matching components, I've narrowed the problem to the output board, but I'm not sure where to go from here. How do you test a component that only fails after running for a half hour or so? I can't detect any abnormal amounts of heat on any of the components after shut-down, and when cooled it works fine. Having an input signal or speakers connected makes no difference and there is no change with the sensitivity control or the DDT system in or out.

Any suggestions??

Thanks in advance for your assistance.

 

[Enzo]

That is what freeze spray is for. Most general electronics supply houses sell the stuff. Circuit freeze spray, circuit chiller, each brand has its own name. If you have a circuit that warms up and starts doing something wrong, you can spray chiller on parts of the circuit until you find something that goes back to normal when chilled. Obviously this doesn;t always work, but it works often enough. SOmething doesn't have to get hot to have a heat problem. Everything warms up to its normal operating temperature after a while. The difference between 70 degrees in the room and 90 degrees in the part may not feel like much, but it is enough.

The other side of freeze spray is heat. They make heat guns. They blow super hot air. One major purpose is to shrink up heat shrink tubing. But they are also used to heat circuit components for troubleshooting. NO heat gun? A hair dryer works pretty well too. Instead of waiting 30 minutes, sometimes hitting a part with hot air will wake up the problem.


When you say output board, you mean the large board with the heat sink and all the power transistors? Not the smaller driver card that plugs onto the underside. I wouldn;t normally expect anything on that board to go noisy. All that is on it are power transistors and a few resistors. Normally if an amp went to white noise (off air radio station sound) I'd first suspect an op amp IC on the driver card. Or on the litttle volume control board.

 

[unclejed613]

if you can't find freeze spray, "canned air" or "duster" is the same stuff, but without the little hose that goes to the bottom of the can. if you turn the can upside down, the propellent comes out as a liquid, and it is VERY cold (avoid getting it on your skin, as it can cause instant frostbite. a few drops are ok, but don't get enough on you to freeze the skin). use the little hollow tube to freeze one component at a time. if you don.t find a component causing the problem, next go to where component leads feed through the board. flaky solder connections can also be noisy. when you freeze the diff amp transistors (the two transistors that make up the input stage) freeze them as simultaneously as possible, as freezing one may cause DC offset at the output.

 

[gaweir]

Hi Guys;

Thanks for the replies and suggestions. I'll pick up a can of freeze spray today and give it a shot (so to speak) and see what happens.

I also have a heat gun, but I don't have a tip to concentrate the airflow to a single component. It has a display and rheostat that allows me to select whatever temperature I want, so the question is; What temperature do I want? From what Enzo said, I'm thinking 90 to 100. Is this a good setting?

And yes, Enzo, I should have said "drive board" not "output board", my bad... I've swapped the drive board from channel B to channel A and the problem followed. I swapped them back and the problem went back to B.

I've also replaced the op-amps on the drive boards of all the amps "on spec" when I found a couple of them with broken pins on the chip and one board had a 5534 in place of a 4558. I've also replace the 4558 on the input boards while I was at it. When this problem came up, I went so far as to reinstall the original chips, but with no change as a result. The new ones are back in.

I've also re-flowed (heat until it melts) the solder on all the connections on both the drive board and the output board. Side bar question; Does re-flowing the solder correct cold solder joints, or is removing the old solder and re-soldering required?

I checked the resistors and capacitors on the output board and all seems well there. Do resistors and capacitors break down with heat?

Thanks again for your replies and suggestions. Much appreciated.

 

[Enzo]

Yes, ANY component can warm up and get noisy. In my experience, noise usually comes most often from semiconductors, then resistors. Caps can but usually don't make noise. They can leak or dry up, resulting in unwanted DC or hum from lack of filtration, but that is different from just making noise.

When I have a circuit that warms up and fails, if I want to hurry the process, I am not too woried about directing heat, at least not at first. If I just warm the whole area up, it will usually be enough. I do direct the chiller at smaller areas. It can be hard to hit ONE resistor for example, but if you need to, you can take a piece of stiff paper of something and use it as a spray shield. If I want to spray just an IC, I can fold some paper into a U shape and hold it around the IC. That keeps overspray off the nearby parts.

I have no idea what my heat gun puts out temperature wise. It is DARN hot though.

I always add a little solder for reflow, if nothing else for the flux it adds.

 

[gaweir]

Hi Guys;

The spray worked!! I gave a shot to Q1, 2N5461 Jfet, and the noise went away. There was a bit of frost on the item, and as I watched it evaporate away the noise came back. I've just ordered the parts and I'm hoping they'll arrive soon. I ordered extras' as I know there are two of these failed on the remaining boards for amp #4.

That one should be a treat to fix as the boards have obviously had someone at them a number of times over the years. Finding out what works and what doesn't will be the next thing to get at.

Thanks again for the great suggestions and info. I'll keep you posted on #4, but I'm sure I'll be back with more questions.