Hi all, I am teaching myself soldering and how to fix my own amplifiers. I bought a pair of these QSC RMX 1450 VERY used off of facebook marketplace knowing the right channel in both did not work. (suspecting something someone did with bridging at some point in time). I knew these were rough, but they seemed like a good project to start learning on for the price.
When I bought the amps, the fuse for the right channel would blow instantly when powered on (12A) I tested the bridge rectifiers and they were failing so i replaced all 4 (2x on each amp) with the closest I could find to the original rectifier GBPC2502W-ND https://www.digikey.com/en/products...2?s=N4IgTCBcDaIOICEAKBhMBWADGA6gWgDkAREAXQF8g
After I replaced those, the fuse would still blow. then I tested the Bipolar Transistors (8 per amp channel, 16 total per amp) a few of them were failing the test on the bad channels, so I order 16 of them (I did not replace the other 16 on the good channels). 757-2SC5200-O(Q) https://www.mouser.com/ProductDetail/Toshiba/2SC5200-OQ?qs=EEns8I54Y6BCHV7Kwts0vw==&countryCode=US¤cyCode=USD
Success! I can turn the amp on and it no longer blows the fuse! however, after hooking a speaker up to the right channel I got a really loud humming noise coming out of it and then a cap started sizzling and the top popped off before I got the amp unplugged. I let the amp set for a couple days then just unsoldered the 4 caps off of the bad channel and am a little defeated, but again its a learning project and my first time doing something like this.
The question(s) Are:
1. what produces that humming sound I heard?
2. should I go ahead and replace ALL of the caps? (total of 16 caps between both channels on each amp)
3. if so is there anything else I should replace aswell?
4. should the 2 good channels have there 8 Bipolar Transistors replaced too?
5. what caps should I go with and why? I know theres got to be better ones out there since these were made.
Cap Specs: 100V, 3300uf, Nichicon, 25mm x 50mm. the new ones cannot be more than 25mm wide, but could be taller if need be.
I saw this post and thought it was interesting that I could go bigger https://www.diyaudio.com/community/threads/bulging-capacitors-on-qsc-mx1500.367169/post-6513907
Thank you for helping a beginner!
When I bought the amps, the fuse for the right channel would blow instantly when powered on (12A) I tested the bridge rectifiers and they were failing so i replaced all 4 (2x on each amp) with the closest I could find to the original rectifier GBPC2502W-ND https://www.digikey.com/en/products...2?s=N4IgTCBcDaIOICEAKBhMBWADGA6gWgDkAREAXQF8g
After I replaced those, the fuse would still blow. then I tested the Bipolar Transistors (8 per amp channel, 16 total per amp) a few of them were failing the test on the bad channels, so I order 16 of them (I did not replace the other 16 on the good channels). 757-2SC5200-O(Q) https://www.mouser.com/ProductDetail/Toshiba/2SC5200-OQ?qs=EEns8I54Y6BCHV7Kwts0vw==&countryCode=US¤cyCode=USD
Success! I can turn the amp on and it no longer blows the fuse! however, after hooking a speaker up to the right channel I got a really loud humming noise coming out of it and then a cap started sizzling and the top popped off before I got the amp unplugged. I let the amp set for a couple days then just unsoldered the 4 caps off of the bad channel and am a little defeated, but again its a learning project and my first time doing something like this.
The question(s) Are:
1. what produces that humming sound I heard?
2. should I go ahead and replace ALL of the caps? (total of 16 caps between both channels on each amp)
3. if so is there anything else I should replace aswell?
4. should the 2 good channels have there 8 Bipolar Transistors replaced too?
5. what caps should I go with and why? I know theres got to be better ones out there since these were made.
Cap Specs: 100V, 3300uf, Nichicon, 25mm x 50mm. the new ones cannot be more than 25mm wide, but could be taller if need be.
I saw this post and thought it was interesting that I could go bigger https://www.diyaudio.com/community/threads/bulging-capacitors-on-qsc-mx1500.367169/post-6513907
Thank you for helping a beginner!
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On old amps the electrolytics usually go first.
They have a shorter life than other components.
It sounds as though bridge rectifier has gone and let AC through and blown the outputs and electrolytics.
They have a shorter life than other components.
It sounds as though bridge rectifier has gone and let AC through and blown the outputs and electrolytics.
Usually when output drivers blow they take other driver transistors with them. Have You checked them also ?
When the rectifier failed the first time it took the caps and output transistors with it. You just didn’t notice that the caps were bad till you fired it back up (Without the fuse instantly blowing).
Next time fire it up on a dim bulb or variac to prevent spectacular things from happening, until you’re sure everything is fixed.
If one channel is good, there is no need to change out caps or output transistors in it. The two channels run off completely separate transformer windings so what happened will not affect the other channel. This is perhaps the most desirable feature of old-school QSC amps.
Next time fire it up on a dim bulb or variac to prevent spectacular things from happening, until you’re sure everything is fixed.
If one channel is good, there is no need to change out caps or output transistors in it. The two channels run off completely separate transformer windings so what happened will not affect the other channel. This is perhaps the most desirable feature of old-school QSC amps.
A "dim bulb" is a tungsten incandescent light bulb for mains service, placed in series with the AC input of the amp. An amp of this capacity, a 100 W bulb is the minimum. I have my light bulb socket in a grounded steel case with a mesh top, so if a wire pops off the bulb base it blows the breaker instead of setting fire to my work table. If the bulb stays on more than a second or two, there is a problem and you need to probe DC voltages to find it.
As you are rather new, here are some more basics: Never measure with two hands, use an alligator clip lead on one probe. Voltage >24 across the heart can stop it. Never touch metal unless you have measured it at below 25 v to ground. Note discharging the rail caps after a trial with a 470 ohm or so resistor & a couple of clip leads may be necessary to achieve this state. Wear safety glasses, exploding transistors or other parts can bounce off the ceiling or your eye. Unsoldering, the solder can splash into the eye. Don't probe around the AC inlet area, mains voltage can burn the end off a probe and cause molten copper to splash. Don't work alone and don't work distracted by media or conversation.
As far as the undamaged channel, you can assume if it puts the rated wattage onto resistors after you are done, that it is working okay. Note if the rail caps are >25 years old they may short and take out the bridge rectifier on that side, too. I like e-caps rated at >3000 hours service life, 10000 if I can get it. I don't like replacing ecaps over & over 3 or 4 times the way I did on my first amp with TV parts store caps. My last order, mouser wouldn't divulge the service life rating of the caps they sell. Digikey and Newark will in the selector table.
If you want this amp to be reliable, and not fail all the time, after it is working properly you will replace all the other electrolytic caps. They are all the same age and same quality of sealant. You can enjoy an amp a bit at home without upgrade, but the other 198? caps will inevitably fail one by one. Change only 2 at a time & test, people make a lot of bad solder joints including me. If it performs worse after a parts change, you know just where the problem is: what you just did.
Other time/hours stressed components are are volume pots, input & speaker connectors, and possibly the fan. Certainly clean any dust off the heatsink, thick pads of it will eventually kill the transistors of an amp.
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thank you guys for all the great info!
ill do some reading on how I can add a bulb in series to my table. if you don't mind, what are the other things i should be concerned with testing on this amp, and how? i want to go ahead and get new caps ordered. are there any recommendations you may have for the caps? size, voltage and capacitance? the only Caps i can find that meet these exact specs are cheap, non-audio grade caps. how much will that matter in this project? over explanation is super welcome as I am trying to soak up all i can. eventually I would like to upgrade my caps on a few emotiva XPR 1's that i have, and I'm learning the methodology on a much cheaper project like this.
"audio grade" electrolytic caps are a marketing program IMHO, not a design feature. If you buy >3000 hour service life caps from panasonic, nichicon, rubicon, vishay, kemet, you are getting a premium cap. Standard grade is 500 hour or 1000 hour. all sell brands them for repairmen that want the customer back in his shop in 5 years with a dead amp.
Don't buy "low esr" caps, those are special for switcher power supplies. This amp has a transformer power supply. Just match the capacitance and voltage rating on the exisitng caps. A little higher voltage or capacitance is okay. 3 v or 6 v caps will be replaced by 25 v.
Note after meltdowns, the emitter resistors attached to the output transistors are suspect, should be checked. Also the drivers, the MJE15032/33. Gross meltdowns can go further back than that to predriver transistors or even the input op amp.If the op amp has gone, check the 50 v ceramic caps around it. I have found a few of those opened.
Near completion check the DC out of the speaker line (at ouput inductor or speaker terminal) at <100 mv with no sound. With no sound check the idle current on the output transistor emitter resistors. no emitter resistor should have more than 25 ma on it. If wrong check R131 bias adjust pot. If driver transistors damaged check that pot, the bias diodes d106 d108 and the temp sense thermistor r134 on the heat sink.
Don't buy "low esr" caps, those are special for switcher power supplies. This amp has a transformer power supply. Just match the capacitance and voltage rating on the exisitng caps. A little higher voltage or capacitance is okay. 3 v or 6 v caps will be replaced by 25 v.
Note after meltdowns, the emitter resistors attached to the output transistors are suspect, should be checked. Also the drivers, the MJE15032/33. Gross meltdowns can go further back than that to predriver transistors or even the input op amp.If the op amp has gone, check the 50 v ceramic caps around it. I have found a few of those opened.
Near completion check the DC out of the speaker line (at ouput inductor or speaker terminal) at <100 mv with no sound. With no sound check the idle current on the output transistor emitter resistors. no emitter resistor should have more than 25 ma on it. If wrong check R131 bias adjust pot. If driver transistors damaged check that pot, the bias diodes d106 d108 and the temp sense thermistor r134 on the heat sink.
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