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So I have this Peavey on the bench... It came in and the standby switch was open the amp would come on but when you click the standby switch the green light wouldn't come on... So I measured the switch and it was open on both positions so I ordered another one and replaced it... So I go to turn on the amp to test it out, yay and verily the green light comes on and then I get this humming sound coming out of my speaker and it smells like it is burning... I open it up and find that the Molex connector that's connected to the standby is on fire... Literally... It burned up and it burned a spot next to the other one and made a mark on the 400 ohm resistor... Any idea why it does that other than Molex being a very poor choice in this particular place on an amp? Thank you for looking...
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I would have to wonder if the cause of the original switch failure and the Molex connector failure are caused by excess current. Maybe try a bulb limiter, pull the tubes, etc, to figure out what is going on. -
That's a fine idea... I was thinking along the same lines... Also, when the amp first came in and the switch wasn't working, there were zero burn marks or black carbon scoring anywhere... there was no indication that anything other than a bad switch was present... Now I see that something was quite amiss and had caused the switch to die...Originally posted by glebert View PostComment
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Connectors burn when they are either too small size to handle the current they are conducting, or they have been exposed to heat by the wire from a nearby bad connection. In your case maybe the switch got bad and started to heat up and that led to the connector going bad also losing its conductivity. Once you replaced the standby switch the Molex connector had lots more voltage drop over it and firing it up.
Edit: I now see you posted that there really was no sign of heat on the connector prior to the switch replacement.Comment
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Well your B+ has a lot of voltage but not very much current, most any Molex I have come across has a very high leakage resistance and the pins are stout enough to carry a fair amount of current, certainly more than this amps B+. I suppose the Molex insulating properties could have degraded somehow but that does not help much because you will need to replace that connector or jumper around it making a captive connection just to troubleshoot the problem. You still have the problem of reading what's going on there. I think inserting an ammeter in line would give you some indication, you should not have much more than 1/4 amp of B+, if so then you need to go on a short hunt... while doing so, follow the schematic and don't ignore the filter capacitors between B+ and ground, a leaky one can kind of look good if you don't actually do leakage tests on it but it will ruin your day none the less.... That's $1.00 for the chalk mark and $49,999.00 for knowing where to put it!Comment
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If that Molex connector only hooks up the standby switch to the board you might as well get rid of it entirely and solder the wires directly to the PCB. You will not not lose servicabilty that much because if you ever need to lift the board you simply let the switch go with it.Comment
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On these amps, often you find heater connections can be degraded and cause heat damage and oxide buildup. However, the molex connectors which connect the B+ to the power supply board suffer burning and damage because they are too close to the blue resistor (I can't remember the exact value off the top of my head), which gets extremely hot. I usually remove the Molex pins and solder directly to the board, unless I have the appropriate molex to replace it. You're probably gonna want to check the filter caps as well as they can also suffer over exposure to heat.If I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.Comment
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You know, come to think of it... the damage on the board is mostly under the 400 ohm blue resistor... Yes the molex connector is black from heat, the PINS aren't messed up... no warping, no arc marks....neither the male nor the female pins are marred in any way... yet, the 400 ohm resistor has a shiny look in the black spot on it...Almost like metal sublimated and flashed it with a silvering... But.... said resistor still reads 402 ohms.... nothing crazy... hmmmmmmm....Originally posted by SoulFetish View PostComment
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I know, it’s strange, they’re pretty robust. Often Ive expected them to have failed an they test okay. In cases where there’s alot of heat damage, Ive changed and moved the resistor to use the chassis as a heatsink.Originally posted by Danglin' Fury View PostIf I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.Comment
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That's not a bad idea... Well, Im gonna change the caps in the power supply....Im gonna troubleshoot the amp but I just tried to roll a capacitor slightly to see the value and it spun around... very easily... so I tugged on the lead slightly and it broke off the can... Lol! It was the cap closest to the 400 ohm resistor... There was also some sticky brown liquid under the cap... could be from the cap or it looks like somebody spilled something in the amp and tried to clean it up just enough to sell it on Reverb...Comment
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Good luck.Originally posted by Danglin' Fury View PostIf I have a 50% chance of guessing the right answer, I guess wrong 80% of the time.Comment
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Thank you much....Comment
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Ok... So... I changed the power supply caps and took all the bad moles connectors off and hard soldered them, now it comes on and it sounds great! Plenty of volume! But... (sniff sniff) I smell heat.... OH NO THE POWER TUBES ARE RED PLATING!!!!! Any ideas? Lol!Comment
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Yes, pull the power tubes and measure the voltages in the sockets, in particular the bias on pin 5 of each.Education is what you're left with after you have forgotten what you have learned.Comment
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You changed some caps in the power supply. If the bias filter caps were among those, make sure they are correct polarity, + end goes to ground for the bias caps (C6 & C7).Originally posted by Enzo
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