Test complete....VAS is OK, drive board is fine.
Something on the power board is messed up. I found a cap in an output snubber circuit that has no capacitance, its open. Im not sure if that would cause this issue though....unless this thing is that picky! The snubber is a 5.6 ohm resistor in series with what was supposed to be a 0.1uF film cap, this is connected directly across the output.
Schematic...open cap highlighted in red.
Something on the power board is messed up. I found a cap in an output snubber circuit that has no capacitance, its open. Im not sure if that would cause this issue though....unless this thing is that picky! The snubber is a 5.6 ohm resistor in series with what was supposed to be a 0.1uF film cap, this is connected directly across the output.
Schematic...open cap highlighted in red.
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Oh fun...another "that ain't right!" moment.
The 5.6 ohm 5W resistor in parallel with the output inductor is open circuit. The PCB beneath is charred! I've never seen an open resistor in parallel with an output inductor. (Then again...I'm used to repairing much smaller amps. Apparently things get weird once you get near 1kW.)
I KNOW this didn't happen while I owned it....I'd would have smelled that! Anyway this resistor is probably pretty important for stability....but what could have caused it to get so hot that it went open? 5W sand resistors are hard to kill! The other resistors on the board are in tolerance. The diodes are OK.
A quick pic of the damage I found...the loose resistor is the burned one.
That Radio Shack component tester is awesome...it figures out transistor leads automatically. Its a handy little gadget they should have never discontinued.
The 5.6 ohm 5W resistor in parallel with the output inductor is open circuit. The PCB beneath is charred! I've never seen an open resistor in parallel with an output inductor. (Then again...I'm used to repairing much smaller amps. Apparently things get weird once you get near 1kW.)
I KNOW this didn't happen while I owned it....I'd would have smelled that! Anyway this resistor is probably pretty important for stability....but what could have caused it to get so hot that it went open? 5W sand resistors are hard to kill! The other resistors on the board are in tolerance. The diodes are OK.
A quick pic of the damage I found...the loose resistor is the burned one.
That Radio Shack component tester is awesome...it figures out transistor leads automatically. Its a handy little gadget they should have never discontinued.
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My best guess is that sometime before I got this amp; channel A went into oscillation. The high frequency resulted in a voltage drop across the output inductor, thus causing excessive current in the resistor.
A similar fate befell the capacitor....excessive current due to the excessive high frequency output. The capacitor will conduct more as frequency increases. The cap has a lower power handling ability than the resistor in series so the cap failed sparing the resistor.
Sound plausible?
A similar fate befell the capacitor....excessive current due to the excessive high frequency output. The capacitor will conduct more as frequency increases. The cap has a lower power handling ability than the resistor in series so the cap failed sparing the resistor.
Sound plausible?
How are the solder joints on the output inductor? The output signal is supposed to go thru it not the resistor, in normal operation a 5W resistor won't last long.
Craig
Craig
Solder joints are rock solid. That got inspected way before I found this open This resistor is the only resistor that cannot be accurately checked without removing from the circuit on the power board. The burn mark didn't show on the other side....I was rather surprised to find it.
Remember; the cap in parallel with the output is open circuit as well....something BAD has to happen to do that.
The question is, could a messed up output snubber cause the whole works to destabilize? I cant find anything else wrong unless I perhaps have a leaky transistor on the power board.
Remember; the cap in parallel with the output is open circuit as well....something BAD has to happen to do that.
The question is, could a messed up output snubber cause the whole works to destabilize? I cant find anything else wrong unless I perhaps have a leaky transistor on the power board.
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The resistor in parallel with the output inductor can ONLY burn up if the inductor leaves the circuit. The solder may be intact, but that doesn;t mean one end of the coil hasn't broken off inside the terminal.
Measure that 5.6 ohm resistor while it is in circuit. If it measures 5.6 ohms, then the coil in parallel with it is open. With an intact coil, the resistor reads like a short due to the low wire resistance. If the resistor reads open, then the coil must also be open.
When the coil opens, the full power of the amp tries to flow through that resistor to the load. Burns up.
That other 5.6 ohm 5w in series with the cap is your stability network, also known as a zobel network. Look up the zobel for further explanation. It prevents the amp from oscillating. Or tries to. If the amp goes into high freq oscillation, that resistor burns up. Not uncommon in amp repair. The caps are almost never bad, but any cap can fail.
Without a functioning zobel netwrok, the amp will be unstable, and start cranking out RF for all it is worth. The resulting excess current draw COULD be involved in your burning 4.7s.
Measure that 5.6 ohm resistor while it is in circuit. If it measures 5.6 ohms, then the coil in parallel with it is open. With an intact coil, the resistor reads like a short due to the low wire resistance. If the resistor reads open, then the coil must also be open.
When the coil opens, the full power of the amp tries to flow through that resistor to the load. Burns up.
That other 5.6 ohm 5w in series with the cap is your stability network, also known as a zobel network. Look up the zobel for further explanation. It prevents the amp from oscillating. Or tries to. If the amp goes into high freq oscillation, that resistor burns up. Not uncommon in amp repair. The caps are almost never bad, but any cap can fail.
Without a functioning zobel netwrok, the amp will be unstable, and start cranking out RF for all it is worth. The resulting excess current draw COULD be involved in your burning 4.7s.
Firing up the soldering iron again.....
The resistor in circuit reads .01 ohms. The inductor is fine. It looks like it's been repaired before....fresher looking solder on the leads.
I failed to recognize last night that that was a Zobel network.....that might be the issue.
Bias diode tests good.
The resistor in circuit reads .01 ohms. The inductor is fine. It looks like it's been repaired before....fresher looking solder on the leads.
I failed to recognize last night that that was a Zobel network.....that might be the issue.
Bias diode tests good.
Progress....Zobel cap replaced, inductor snubber resistor placed.
Running amp at less than 10W output with 1 pair MJ15024 installed. Test will run for 20 minutes followed by a shutdown and installation of additional pairs if the test is successful.
No overload light and the heatsink is cold. Its survived one song....it wouldn't last 10 seconds before. 😀
Running amp at less than 10W output with 1 pair MJ15024 installed. Test will run for 20 minutes followed by a shutdown and installation of additional pairs if the test is successful.
No overload light and the heatsink is cold. Its survived one song....it wouldn't last 10 seconds before. 😀
Hate to tell ya this...but that amp is only 200w per channel at 8 ohms. its not really that big of an amp power wise. But yes things do get weird when you start dealing with high voltages and lots of power. up over a couple kilowatts per channel, they get real weird. I worked on 3.6kw amp recently that arced through the layers of the board from the top traces to traces on the bottom side, tunneled through the middle of the board to various points. arced over from the bottom of the board to the aluminum heatsink below and melted a nickle sized hole in it. AND it still ran!
and it's scary as hell to work on the really big amps that are 5kw++ where you have 320V from top rail to top rail and several 10's of KW of current at a dead short that could literally blow a hand or finger off should it find it way to a place it shouldn't be!
Yep...200W at 8 ohms, 400W at 4 ohms, 800W bridged into 8 ohms.
Not the best solution for an HT sub, but it works and if all goes well it will be CHEAP. Less than $150 has invested in the whole project! 🙂
Tests with one pair complete! No blown speakers....no smoke alarms beeping...no red lights. 😀
Yeah...320V rails could be NASTY. Does that big amp run on single phase 240VAC?
Not the best solution for an HT sub, but it works and if all goes well it will be CHEAP. Less than $150 has invested in the whole project! 🙂
Tests with one pair complete! No blown speakers....no smoke alarms beeping...no red lights. 😀
Yeah...320V rails could be NASTY. Does that big amp run on single phase 240VAC?
Now running with all 5 pairs of MJ15024 outputs...... no smoke, no blown speakers, no red lights. 😀
Torture test begins MAUAHAHAHAHAHAH!
Torture test begins MAUAHAHAHAHAHAH!
I hate to be overly cautious, but that resistor parallel to the coil burning has me worried. It may measure OK, but do me a favor and tug up on it just to be sure. Broken wire can remain touching and fool the meter. If it is OK< I'll shut up about it. Just in all the years I have run a Peavey shop, that resistor has never burnt up without the coil broken off at one end.
And 1200 W bridged into 4 ohms. You're not "supposed to do this", but it will tolerate this better than modern amps in the same power class (5 pair of 15024 compared with 3 pair of C5200). And it won't get mad and shut down half way through the show like an RMX1450 will.
I checked that possibility...both with the wiggle test and the crank it to 11 test. It would be burnt by now....Ive been having some fun with this amp!
I'm guessing the inductor was repaired previously. It appears that this amp has been serviced 3 different times in its life judging by the various date codes on obviously changed parts.
1976 cs800b
The 1976 CS800B schematic in full detail is already on diyaudio.com, just search cs800. It's a zip file. These early models are selling about $140 each in triplets on craigslist in this market. By contrast, the 1998 CS800S I bought this spring goes more like $200 (in package deals) to $400. By 1998 the op amps have been upgraded to JRC4560, quieter than my room background noise. Current transformers have been wired around the outputs, and run through a board to a microprocessor, where presumably they measure SOA violation instead of clipping on the cannon shots of 1812 overture like cheap voltage peak protection. I tried playing 1812 all the way up outdoors on July 4, no obvious clipping. The output transistors have been changed to full complimentary, for what that is worth, and the power rail voltages lowered. Download the cs800s schematic from eserviceinfo.com if you're interested, the Peavey team learned a lot between 1976 and 1998. Mine sounds as good as anything I have ever owned.
The 1976 CS800B schematic in full detail is already on diyaudio.com, just search cs800. It's a zip file. These early models are selling about $140 each in triplets on craigslist in this market. By contrast, the 1998 CS800S I bought this spring goes more like $200 (in package deals) to $400. By 1998 the op amps have been upgraded to JRC4560, quieter than my room background noise. Current transformers have been wired around the outputs, and run through a board to a microprocessor, where presumably they measure SOA violation instead of clipping on the cannon shots of 1812 overture like cheap voltage peak protection. I tried playing 1812 all the way up outdoors on July 4, no obvious clipping. The output transistors have been changed to full complimentary, for what that is worth, and the power rail voltages lowered. Download the cs800s schematic from eserviceinfo.com if you're interested, the Peavey team learned a lot between 1976 and 1998. Mine sounds as good as anything I have ever owned.
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For some odd reason I cant find deals like that....the Series A from 77 goes for almost $300 here. Its good in the heat I guess. 🙄
Well the economy is picking up. Anything on craigslist "allegedly" works, bring your own 8 ohm 200 W resistor and an 80 VAC meter. Also, I'm retired, I have time to look at CL every day, though I don't always get the bargains I bid on. For a cheap source of huge elcetrolytic capacitors with a recent manufacture date and high quality, make the acquaintance of a maintenance man in a food plant. Large numbers of 460 VAC three phase motor drives are blown up by the sanitation team with water hoses- usually the output SCR board, not the capacitors. These usually have 10 each 400 uf 400 VDC electrolytics of extreme quality in them, if it is an AB or TB Woods drive. All conveniently soldered to a board in gangs. You can cut lands on the TB woods boards and make two each 2000 UF 200 VDC gangs out of them. Also a great supply of massive heat sinks and tiny 12 V fans if you want to brew your own amp. The maintenance man will have to dumpster dive to get them, so you might have to make it worth his effort.
jeeez how many of them do you want? people here have started using them as Semi truck Wheel Chocks! LOL! I see them in various states of working and non working and I have even sold many in the $50 to $150 price range! I think there is 2 sitting in the scrap pile now.