LM3886 Power Transistor help needed

[jimmy74]

Ok the only problem is that I can't attach one speaker at a time, the tweeter and speaker are both attched to the same 4 pin plug

[J M Fahey]

That shouldn't be a problem to a Tech armed with nippers, pliers and a soldering gun 

[jimmy74]

ok I mounted the amp inside the speaker cab, tried it first with the tweeter/horn and it was hissing but the amp doesn't go in protection and volume makes the hissing get louder. I then tried with the speaker only and the same result only difference is that the noise isn't a hiss but a crackling sound with a continuous airplane noise in the background. Tried with both horn and speaker and still the amp didn't go in protection mode so noise apart I think we've got it on the right road.

what's next?

Merry Christmas to all and have a wonderful New Year!!

[Roly]

volume makes the hissing get louder.

Sounds like a bad case of snakes.   

More seriously, that's a fairly good sign that the power stages are now operating, but given the wild voltages previously applied to the preamp op-amps I wouldn't be surprised if one (or more) have died.

Feed a signal source such as an MP3 player in the front end, and lacking a CRO use a powered computer speaker to trace the signal forward from the input, and each op-amp output, until it vanishes to localise the problem.

[jimmy74]

is there anyone in the EU that sells the right sort of insulator bushings for the LM3886? I think the TO-220d type bushing should be correct but searching ebay the only seller is in China.

[J M Fahey]

For odd shapes often the best is to buy a small piece of Silpad rubber sheet (warning, it's relatively expensive) *once* and cut small customm pieces as needed.
It's a good investment.
Other solution is to buy 2 big mica pada (for TO218/247 and larger) and use 2 side by side.
You'll have to punch your own bolt hole.

[jimmy74]

Thanks for the reply but I don't need the mica spacer, the original ones are still fine, I need that plastic but that separates the chip amp mounting screw from the base of the chip amp so that there's no connection between the heatsink and the base of the chip amp:

http://www.ebay.co.uk/itm/Insulation-Bushing-for-TO-220D-Transistor-Washer-x-50-/130579772765?pt=LH_DefaultDomain_0&hash=item1e6729355d

these are the only ones I could find

[phatt]

Hi jimmy,,
Trash bins mate that is where you find all those hard to get bits. :dbtu:
Old TV sets/ stereo units/ dead computer supplies,, plus the hundreds of other electrical gizmos folks just throw out in this throw away world.  All the better for us tinker types. <3)

Phil.

[Roly]

If you really can't find the right thing take a plain fiber washer and a short length of heat shrink, put the washer on the screw and follow it with the short length of heatshrink.  But as phatt says, go for a scrounge in an old TV or amp.  You shouldn't need to go to China, electronics suppliers here sell such washers/bushes, as well as nylon screws and nuts, and I'm sure you'll find someone closer, e.g.;

http://au.mouser.com/catalog/English/102/APAC/1771.pdf

[jimmy74]

Pulled apart some old burnt up PC power supplies I had lying around and sure enough there were some of those insulators... thanks for the tips!!

[Roly]

Pulled apart some old burnt up PC power supplies I had lying around and sure enough there were some of those insulators... thanks for the tips!!

Yo!  That's the ticket.  Scrounge.  Every tinkerer needs a well stocked junk bin.   :dbtu:

[jimmy74]

well after a very long delay waiting for he new preamp opamps, I swapped ou the BA6110 and one of he BA4560's and sure enough that got rid of the crackling noise. Still usin the current limiter I hooked up a guitar and everyting seemed to be working fine, no noise or problems so I mounted everything back up and playd through it for around 10 mins without the current limiter, eveytn seemed perfect, when all of the sudden it just went dead, vey low sound coming from the horn, it doen't go in protection mode though.. one of the amp chips dead?

thanks

[Roly]

Oh dear.  Sorry to hear.   

Well you are just going to have to go for a dig again, but at least this time you have a much better idea of what you are looking at/for.  It may be the chip amp, but it could be lots of things, broken track, dodgy solder joint...

Since the "protection" is an automatic level control that operates in the preamp and isn't coming on it suggests that this time the problem may be in the preamp ahead of the protection circuit ... fingers crossed.

[jimmy74]

here are the voltage readings I got:

http://www.flickr.com/photos/73835524@N00/12134847064/

the +ve and -ve 29.9 readings follow back to the rectifier bridge, and after its 21 and 22vAC

thanks

[Roly]

Well those voltages, particularly the pin 3's at zero, suggest that the three power amp chips are okay.

very low sound coming from the horn

You're only going to get signal content above 2.5kHz from this path, so I would expect it to sound pretty quiet from a guitar.  This also suggests (along with the limiting LED not lighting) that the gain reduction/protection U6  is not kicking in, which leaves the path between there and the main amp input, U4A&B.

If you inject some sort of a signal to the join of R45 and C34 you should get some sort of output confirming the main amp is okay.  If so look back to U4, if not you have a break or short after there before U8 (cracked track at U8?).

I still think you have a dry solder joint or cracked track somewhere, and if it's a crack it's a fair bet it  will be very close to one of the solder joints (but may be well hidden).

HTH