Lm3875 Help

[Tristanc1]

Hi,

Finished off my second Peter Daniel LM3875 Kit today, and powered it up and nothing happened....Checked and re-checked my wiring and as far as I can tell everything is good, but there is no sound whatsoever.

I put the boards together a few months ago, but never wired them up until yesterday, so maybe something happened while they were just sitting there? Not sure, is there any way I can find out if the boards are working or not? I'm thinking I might have fried them, whilst being a little over zealous with my fancy new soldering iron....

Also, if it turns out I have to take all the wiring apart and take the amp apart is there anything I should watch out for? I know caps can retain a charge for awhile...but not sure how long I should wait, or how to discharge them(if that's the term for it)

Thanks in advance,
Tristan

[AndrewT]

Hi,
get a multimeter and very carefully check that you have voltages in the right places.

[Arx]

Originally posted by Tristanc1

Also, if it turns out I have to take all the wiring apart and take the amp apart is there anything I should watch out for? I know caps can retain a charge for awhile...but not sure how long I should wait, or how to discharge them(if that's the term for it)

Those caps are pretty small, and won't be particularily dangerous or anything. Waiting won't necessarily work, in some circuits it'll hold a charge for a long time.

to discharge it, I would just grab a big power resistor 10-100 ohms or something, and then touch its pins to the pins of the capacitor for a few seconds. 10 ohms will kill it almost immediately.

It's probably more responsible to do the math and calculate a resistor which will drain the cap over a period of a few seconds. (you should definitely do this for very large caps)

Either way, you're doing a lot better than the people who short them with a screwdriver.


Hopefully you shouldn't need to disassemble it anyways.

first thing to do if it's completely together is probably to check if you've got proper voltages from the power supply. If not, fix that first.

-Nick

[Tristanc1]

Thanks for the replies so far, and for the tip on how to discharge caps that will come in handy some day i'm sure

I'm not getting any reading on the output voltages with the multi-meter....Well that's not entirely true, it goes up to 12 and then slowly falls down to zero over a period of about a minute or so.. It's weird, it's wired exactly the same as my working one, so one or more of it's parts must not be working. I have a plitron 25V 225va transformer wired for 115VAC, so I should be getting 35V or so rectified right?

So what do you guys think? Should I take it apart and check it over again? If I do take it apart is there a way to test the individual parts to see if they're working properly? could be a bad solder joint or something creating a short somewhere or something, but if that was the case the fuse would've blown correct?

Thanks again!
Tristan

[Tristanc1]

Took it apart and put it back together again using a different set of boards, still isn't working but now i'm getting 0.74 on V+ and -1 on V-. Measured the DC offset at the speaker terminals and got 0.

Any Ideas? I know stuff like this is a pain in the *** for everyone to answer, but any help would be much appreciated.

Thanks!
Tristan

[Tristanc1]

Did some more tinkering, i'm now getting the correct voltage on both outputs from the rectifier board(stupid newbie mistake I assumed seeing as it's a monoblock that i'd only need to hook up on set of secondaries not both...apparently I was mistaken )

Anyways, i'm still not getting sound, every few seconds with my cd player/pre hooked up I get a really quite hiss of distorted music out of my sacrificial speaker. DC offset is 1.4 mv on the + post and 2.7 mv of the -. I unfortunately can't take pictures right now, but I have a feeling it's something really stupid that for some reason i'm not seeing...

Tristan

[Arx]

Originally posted by Tristanc1
Did some more tinkering, i'm now getting the correct voltage on both outputs from the rectifier board(stupid newbie mistake I assumed seeing as it's a monoblock that i'd only need to hook up on set of secondaries not both...apparently I was mistaken )

Yep.. Still need both rails.

Anyways, i'm still not getting sound, every few seconds with my cd player/pre hooked up I get a really quite hiss of distorted music out of my sacrificial speaker. DC offset is 1.4 mv on the + post and 2.7 mv of the -. I unfortunately can't take pictures right now, but I have a feeling it's something really stupid that for some reason i'm not seeing...

Tristan

Offset sounds really good, but I'd suggest checking your wiring yet again.

Maybe play a sine wave into the input. Measure with your voltmeter set to AC. See if you get any significant voltage at the input of the chip. (maybe you've got it wired backwards, or it's shorted to ground or the - in or something)

Just guesses. Good luck

[Tristanc1]

Offset sounds really good, but I'd suggest checking your wiring yet again

I keep thinking it's something with the wiring aswell, but i've checked and re-checked a ton of times and i'm 100% sure the wires are in the right places.

There is a new symptom though, the lm3875 is incredibly warm...well actually I shouldn't say warm, it burnt my thumb when I touched it after powering down the amp, and it had only been on for, at the absolute most, 2 minutes.

So now i'm thinking the fault isn't in the wiring, but something I might have screwed up assembly the board....Any way to check just the amp board for faults? I know the PS is okay, and the Transformer's hooked up correctly so by the process of elimination it must be the board itself.

Slightly off topic, but i'm kind of glad that I screwed this one up...the last one worked fine, and while that was great I didn't really learn anything, it was more like connecting the dots than anything. I think Peter Daniel, Brian GT etc should purposefully sobotage all their kits when they suspect newbies, such as myself, are involved. That way we'll actually learn about this 'electricity' thing that everyone keeps saying is the bees knees

Any other advice, now that I know it's the board itself?

Thanks again!
Tristan

[Arx]

You may want to double check that you've got your rails the right way around. Powering things backwards tends to make them very hot.

If not, (or possibly if so) I fear that you may have cooked your amp chip, protection and all :P

You may now be in that dreaded position of having to try another part, potentially blowing that one too, if there is still a problem.

I had a similar experience with a 21" sony monitor. it had several blown parts. Every time I replaced them, I found more, but was never sure if I was finding the true source of the problem, or if stuff was just busting because I kept replacing the other parts that blew first.

I wasted about a hundred bucks before I gave up and tossed it. Fortunately, you've only got a couple parts to blow.

I don't know what you could do to damage the board.

Perhaps you've got the resistors in the wrong places or improperly soldered and are getting severe oscilation?

-Nick

[baboon81]

I have done the same thing with one of my LM3876T chips. I had it built using the Jaycar 50W design and hooked it up to my sub, had oscillations and the chip's heatsink got very hot!! I fried the chip.

Firstly if yours is the LM3876T (not the LM3876TF) de-solder it from the board, have it sitting on the table and measure the resistance from Pin 1 to the bit of metal on the chip that is attached to the heatsink. It should measure 1-3 Megaohms! The one i fried measured 7.3ohms.

Also measure resistance on pin 3 to the metal back and it should measure 1-3Megaohms. If it measures around 1 ohm the chip is fried.

Finally measure pin 4 to the metal backing and it should measure less than 1 ohm.

If alls ok then check that V+ gets to pin 1 and V- gets to pin 4 when soldered onto the board. Careful not to short between pins. If volts arent getting to the chip check rest of circuit.

Hope this helps.