PV 4080HZ amp issue......

[e_c_mccollum]

Got in a PV 4080 that had channel B shorted out. Driver Q113 was shorted, one output on that polarity was shorted, FET Q132 was shorted. All have been replaced. Emitter resistor associated with shorted output exploded. All emittter resistors were replaced on channel B due to stress. Replaced driver emitter resistor and base resistor for piece of mind. Amp works, but had a "ring". Found R163, 2.4 ohms, to be shorted out. Would only show it when heated a little. Now amp works pretty good, but noticed power supply caps C1, C5 don't discharge like the rest after powering down. The schematic shows they are in parallel and for pos high rail. Any ideas why?? Also like to include the 30A thermistor had a crack in it. Was replaced. Any replies would be apperciated.

[Enzo]

R163 was not shorted, in fact if it doesn't measure as shorted, you have a problem. A shorted resistor is a DARN rare thing. I don't recall seeing one in the last 50 years.

Look at the schematic. R163 is in series with the output of the channel, but it has inductor L100 in parallel. Inductor is just a coil of wire, so it has a DC resistance like wire - next to zero. If the coil is open, or more likely broken free at one end, or has a broken lead that still touches sometimes, your resistor will be left alone to measure as itself. But if the coil is intact and connected, it will be across the resistor and make the resistor appear "shorted."

Generalaly, most of the output current flows through that coil, so if it is open or not connected, then that poor R163 is now in series with your load. And the amp loses the stability the coil adds. A broken coil could have been the reason the amp failed.

Two 12,000uf caps is probably a lot cheaper than one 24,000 cap. And probably takes up less space.

For the cap betwen low and high rail ro discharge fast, there would have to be something discharging it.

[e_c_mccollum]

I left out that fact that I did check that 2.4ohm resistor with one lead off the board. I know their's a coil in parallel that will make it look like a short. Heat makes components do funny things especially when there bad, so when I desoldered the resistor, I put a meter to it. Read zero ohms. ???? I kept the leads on their and observed as it cooled the resistance went up and finally back to 2.6 ohms. I already bought some new ones and checked them with a little heat. Went up a little in resistance. As I stated before, their was a "ring" coming from channel B. Gone since R163 was replaced. Just curious why those 2 caps will not discharge. The other supplies will, but the pos high rail supply keeps a charge for a whole day. I was thinking maybe their was a still a problem like a load being put on the +/- low and - high rail causing it to discharge fast.

This customer of mine blew up a Crown mt1200. All the low side outputs were shorted and were tested out of circuit. We nicknamed him smoky.

[e_c_mccollum]

I think I figured out the cap discharge issue. R166 is a 7watt 3.3k resistor connected in series with C116. C116 is connected to - high rail, while other side of R166 connected to ground. The + high rail side has a different config. That's why the positive side is'nt discharging, right? Or at least alot more slowly. The main caps are 10,000uf each, so that is 20,000uf each rail. The schematic says 12,000 each, but mine is 10,000 each. ???? Peavey ran out of parts????

This guy is very hard on his equipment. He says amp ran cold, but I figured it out. Channel B regulator for the fan broke one lead. I also noticed on the top cover it had more dirt on channel B than channel A. Something was going on. Fan probably stop working that day or did'nt speed up. The extra dirt to me says that channel ran hotter than the other when the fan was actually running.

Those 7watt radial resistors get very hot. I think they should have installed 10watt. Going to do that. Your right Enzo.... that resistor on the output across the coil is rare to mess up. Last time I saw that was in a Crown mt2400 that ran in parallel mono mode. The high and low side output solder joints were nasty looking. That mode makes them run hot!!! But keeping them biased properly is very important. I've known some people to connect 8 speakers per side to one amp and complain why is'nt it working anymore. I had a guy connect a small power amp outputs to a PV 2000 inputs for more power. A good one is when I got in a QSC ex2500 that stopped working. The guy said he was using it for subs. Had 4 per side. Said they were all 4 ohm dual 18's. Let me see..... does'nt that make 1 ohm per side??? Poor amp!!!!!

[Enzo]

Wow.

Well, at least part of the QSC setup procedure is to drive the output into a dead short to adjust the current limiters.