Help. Accidentally dropped needle on record. Blown amp result

Ok I did as you suggested. I bought a power supply and set it at 12V 40mA and both of those relays kicked in. I also went through and VERY carefully checked all the inputs into the board for AC voltage (Red, blue, orange and brown). They all tested working. I also tested (again) the bridge rectifier coming off the blue connector which puts out 12V DC. I put my multi meter to the + and -- terminals coming off it while the amp was powered up and got no voltage coming off it. I think I may now know why the amp isn't working properly and the speaker relays were not working.

There isn't any 12V power to the board due the bridge rectifier being stuffed (I'm pretty sure).

On a scarier note.....I had my power supply set to 12V 40mA to test the relays and then upped the amperage to 950mA to test elsewhere. For some reason I went to double check the relays again and when I put the leads on the circuit to test they didn't click and I saw some smoke! I turned the power supply off immediately and wept (well felt like crying) as I thought I'd now stuffed the amp up due not turning the amperage down (Although won't the circuit only take the amperage required? I wasn't forcing amps through it?)

So I can't see and damaged parts and I have no idea where the very small amount of whispy smoke came from...only the general area. I hope I haven't stuffed something else up :(

Anyhow... I just shouldn't have mucked around putting voltage through parts of the 12V circuit just because I thought I knew what the problem was. I tested the small diodes in blue and the relays I've circled in Red.

u3J7Rh.jpg


The suspect Bridge Rectifier is D519 part number S1NB60-4062

http://au.mouser.com/ProductDetail/Shindengen/S1NB60-4062/?qs=/ha2pyFaduhQpnubSiXYNQT21Sy4XtCsah0%2b%2bxP96emZJvwlDhAcfg==

Of course it's not available :( :( :(

On the mouser page, on the unavailable listing, click on the "show similar" button. Do any of those smd rectifiers have the same dimensions and specs?

Edit- I just sorted by size and this looks the same (but you should check all the specs to verify):

http://www.mouser.com/ProductDetail...=sGAEpiMZZMtQ8nqTKtFS/FNrAZmebG6GtHiaWinZERY=
 
Last edited:
Well the new bridge rectifiers finally arrived along with a Chip Quik SMD-1 Removal Kit. Upon taking the old one off I discovered a leg broken. I must have done it while trying to remove it for testing previously. Unfortunately it tested ok and with the new one in place my previous theory of this being the problem vanished.

Once again, it powers up and then tries to power up the main amp with the clicking and then the amp going into standby mode and the volume know flashing red. It definitely is on the blue circuit. I don't know where to go from here now.

Help much appreciated!

Cheers......
 
Harry, this is a class-D switching amp, so there is no bias setting equivalent to that in a Class A or AB amp. There is a DC offset setting, which I believe adjusts the balance between the high and low outputs.

The speaker selection relays RL501 and RL502, and speaker detection (and protection?) relays RL503 and RL504 don't appear to otherwise affect the amplifier circuitry, so it seems you can test for amplifier operation independent of speaker operation.

With an input signal at the inputs -- I'd use a signal generator, or signal generator app on a mobile phone or tablet -- you should see a square wave at the gates of output MOSFETs Q521 and Q523 (good opportunity to use your oscilloscope.) Keep your hands off everything whilst testing, because a short or misconnection could put your 'scope input at power supply rail voltage. Set up your 'scope accordingly -- highest volts/div to start with, and work down -- to avoid damage!

The square wave should be constant amplitude but vary in frequency and pulse duration as you vary input volume and frequency. If this appears to the case, it's probably a problem with the protection circuitry, and if you're willing to gamble everything on a wildly risky attempt, you can try forcing the speaker relays closed to see if it either works or sends the amplifier and speakers up in flames. If you're going to do this last bit, do it outdoors and keep a fire extinguisher handy, and be prepared to throw away the corpse and chalk it up to a learning experience. Make sure to video this part, so if there's great immolation you can put it on YouTube.

If there's no square wave at the gates of the output MOSFETs, it's probably a problem with the amplifier drive circuitry.
 
Can I power the 5V only with a power supply and test that way?

Since the fault is on this circuit?
You can, but are you sure the fault is powered by the 5V rail?

Does the amplifier circuitry otherwise work properly if the 5V rail isn't powered, i.e., if you force the relays closed?
 
It works fine with the 5v unplugged through headphones. All tone controls work perfectly ok and the volume works as it should.

When I plug the 5V blue connection in to the board that's when it turns on and the relays click and then the amp goes into standby with the volume knob flashing red.
 
It works fine with the 5v unplugged through headphones. All tone controls work perfectly ok and the volume works as it should.

When I plug the 5V blue connection in to the board that's when it turns on and the relays click and then the amp goes into standby with the volume knob flashing red.
This doesn't guarantee that the power amplifier circuitry is working, because the headphones are driven by the output of the preamplifier, before the power amplifier.

It might be a failure in the speaker detection/protection circuitry -- which is powered by the blue 5v supply -- but it might also be the speaker protection circuitry working correctly.
 
Forgive my probably stupid questions but how exactly do I do that?

Treat me like a child and give me precise step by step instructions please :)

Thanks guys.
 
Get you a good function generator (audio range sine/square) and a scope and the schematic, start chasing it down one section at a time.
 
Get you a good function generator (audio range sine/square) and a scope and the schematic, start chasing it down one section at a time.

Thanks Mark,

I'll have a look for one and then educate myself (as best as I can) on how to do as you say.

Cheers
 
Back
Top Bottom