if that works for you, then fine....
in my amp, i wire the speaker ground return wire direct to the psu 0v where the filter caps are....
my reasoning is that the speaker return wire is also a signal wire, so that returning them straight to the psu zero point is what i do...
I would remove R53/54 and use a bulb limiter in series with the mains voltage to the power supply instead of the variac. The bulb will allow you to observe current flow when adjusting bias. You should be able to smoothly control the brightness of the bulb with the bias pot. If the bulb suddenly goes from dim to full brightness, the amp is likely oscillating.
R53/54 are a nice idea for testing, but they are bypassing the rail fuses. When the fuses do blow they continue to supply power to the damaged amp until they go up in smoke, not ideal.
R53/54 are a nice idea for testing, but they are bypassing the rail fuses. When the fuses do blow they continue to supply power to the damaged amp until they go up in smoke, not ideal.
check for shorts on the output devices and between the rails.OPP's brain fart. I do not have the board fuses only the resistors. Now I understand why the small 22 ohm 1/2 watt resistor fried. I was wondering what F1 and F2 meant on the board. I looked at the BOM and sure enough fuses and holders are listed but I just glanced at it and assumed it meant the amp fuse and not fuses on the boards. I will get fuses and holders and I think it should bias up fine then. Does everyone use fuses being that 4 amp 250 volt fuses is 1,000 watts. Just wondering if say 22 gauge or smaller wire may serve the same function? I have the fuses in stock but no PCB fuse holders. Where do you buy them?
One other question, I assume from instructions about the LTP current you measure across the R14 resistor looking for 8.25V. Is this correct?
Thanks for responding guys.
One other question, I assume from instructions about the LTP current you measure across the R14 resistor looking for 8.25V. Is this correct?
Thanks for responding guys.
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what i do with busted fuses, i solder 10 ohms or thereabouts to the fuses to monitor current, experienced builders would know what those 22 ohm resistors are there for..
my reason is so that i can monitor idle currents whilst adjusting bias....
but i still use series lamp testers when powering up...this trick has saved me those output power trannies many times over, as i also make mistakes, the recent ones was when i interchanged npn's and pnp's in one channel, had i not used a series lamp tester, then i could have lost a lot of power trannies...
first thing i do when the series lamp tester glowed bright then dim is to adjust the output offset voltage, closest to zero, and then the output idle bias is adjusted next...
when all these are done, then i replace the regular fuses and remove the series lamp tester...
my reason is so that i can monitor idle currents whilst adjusting bias....
but i still use series lamp testers when powering up...this trick has saved me those output power trannies many times over, as i also make mistakes, the recent ones was when i interchanged npn's and pnp's in one channel, had i not used a series lamp tester, then i could have lost a lot of power trannies...
first thing i do when the series lamp tester glowed bright then dim is to adjust the output offset voltage, closest to zero, and then the output idle bias is adjusted next...
when all these are done, then i replace the regular fuses and remove the series lamp tester...
4 amp fuses can handle much more, the question is what is the width and/or duration of the signal hence the energy imposed. i think that fuses are there to prevent fires in the event of output trannie failures, that is what they are there for... you can solder pigtails on fuses if you do not have holders..
R14 is to monitor tail current and it showed 3.75mA, which you can set for 4mA giving 2ma per half of the ltp...no biggie...
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I found fuse holders and fuses on Amazon and bought them. The 4 amp 250V fuses I have are the old long types that would not fit in the space provided. I have both the lamp bulb tester and variac. I normally use the light bulb tester in all my builds but used the variac this time not knowing if I had the pot adjusted the right way. While turning the variac I monitored the bias to be sure.
Thanks Tony, having this forum and helpful members has enabled me to complete many builds.
Thanks Tony, having this forum and helpful members has enabled me to complete many builds.
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Whelp. It was playing beautifully the last several days, getting slightly warm... and then it appears as though the kids had turned it on (stereo stuff is all at kid height unfortunately) and left it on, for who knows how long, and all four fuses were toast. Seems like some output transistors are gone, but I will have to dig into it. I don't see any burt resistors or other components. It had been adjusted out to spec per the build guide, and was sounding great. So, will pull some transistors off and see how many went. Power supply is fine, as is the DC speaker protect. Boo.
Just ran out there and messed with it, on one channel, one bad pnp output, and bad npn output. pulled the bad ones, and cleaned up the heatsink mount holes, and it fired up, and put out a clean signal. I am going to say it was a short to heat sink that caused the issue. Clean up your heatsink holes, and drill the edge of the hole down a tad so the threads don't pull a little bit of aluminum up when you tighten them. I think that is my lesson....
"Hey
As a beginer I have question."
Why make it so darn complicated? As a beginner, he would most likley run the amp with +/- 56 to +/-60V dc rails.
Just say 500VA and up for a stereo amp.