LM3886 Power Transistor help needed

[Roly]

LM317
Pin 1 adj = 34.8
Pin 2 out = 36.4
Pin 3 in = 36.4

The input is close enough, but the others clearly aren't right.

This regulator works by maintaining 1.25V across the resistor between Output and Adj, which defines the current through both resistors and therefore the output voltage.

The differential between Out and Adj is 36.4 - 34.8 = 1.6 which may be right within measurement error.

This gives an R54 current of;

I = E/R = 1.6/220 = 0.00727273 or about 7mA

7mA through R52 2k2 gives a voltage drop of;

E = IR = 0.007*2200 = 15.4 Volts, which would be about right.

But that isn't what you've got.

So first guess is that R52 2k2 isn't actually finding its way back to ground/common because of an open resistor, dry joint, cracked track - something.

Second (less likely) guess is that the regulator is boofed.

At this point what I would do is remove it from the board and lash it up externally with 220 and 2k2 resistors and prove that it is (or isn't) actually regulating.  If it is then you have a fault on the board, and if it isn't...

While you're at it I would do the same with the other regulator.

[jimmy74]

When you say ground/common you mean real ground or something else? I've tried following the schematic from the mains ground wire to D4,D5,R51 and C39 and across to CONN 11 and up to here everything seems fine. CONN 11 then meets the junction of C46 and C47 and that's fine too. But I get no continuity to the junction of R52 & R53 nor to the junction of C48 & C49 not to the junction of C50 & C51. However there is continuity from the junction of R52 & R53 through to the junction of C50 & C51.

Could this be what's causing all the trouble?

[Roly]

If you look at the circuit just to the right of the main filter caps C46 and C47 you will see two "ground" symbols; the first implies mains ground/chassis, the second (triangle) implies amp signal common.

I get no continuity to the junction of R52 & R53 nor to the junction of C48 & C49 not to the junction of C50 & C51.

Well there should be, and yes this will cause the problems you are seeing, so you have to find out why the ground common doesn't carry through from the main filter caps to the regulators and beyond by tracing/inspection.

If this is a double-sided PCB (i.e. has copper traces on the top component side) then something like a regulator leg may need to be soldered on both top and bottom sides.

[jimmy74]

It isn't a double sided circuit so all that is possibly tracable is underneath. What exactly should I look for?

thanks

[J M Fahey]

What exactly should I look for?

That circuit is very simple and follows the datasheet example, so you must check that all torn/cracked/missing tracks and pads are there and each 3886 leg is connected where it should.
If *one* connection is missing, it won't work.

[phatt]

I don't wish to dampen the enthusiasm here but I've recently spoken to a chap who works for a local Sight and Sound company who run quite a few gigs around these parts and they purchased several of those Db powered rigs and half of them blew up in the first 6 months.
Hence they sold them off real fast. :grr :grr

My concern is small bridged chips claiming humungous wattage :lmao: :lmao: is a real worry and even if you are able to fix it it may well blow again.

On the bright side *IF* the unit can't be fixed you can just make a cover panel and use it as passive speaker.

You have a big heat sink, PSU and likely some other bits to make a complete Amp rig.
The PSU is the most expensive part of building power amps.
I'll bet the Do-nut transformer is quite capable of driving a 50~100 watt Amp and the Filter caps will also save you money.

You can likely pickup a barebones Amp kit for $50~$80 au.

But meantime you are gaining experience by fault finding this unit and heaps of professional help makes life easier.
Hope it works out for You, :tu:
Phil.

[Roly]

What exactly should I look for?

There should be continuity from CONN11 to the join of C46 and C47, then to R52 and R53, then to C48 and C48, etc.

You say you measured NO continuity from R52, R53, C48 and C48, to ground.

You now have to find out why, and restore it.  Start at the join of R52 & R53 and trace back towards C46 and C47 until you find a broken track, missing jumper, whatever.

[jimmy74]

Ok I actually tracked from the junction of C46 & C47 to the next point joined by a jumper track(I think all jumper traces are on top of the board so it is a double sided board) and probably a cold solder point or faulty trace which I bypassed with a short piece of wire. Now I get continuity to pins 7 of all the chip amps, and to the ground points that were "left out" before, through to CONN11.

voltages on the regulators now are:

LM317
Pin 1 adj = 12.4v
Pin 2 out = 13.7v
Pin 3 in = 37.7

LM337
Pin 1 adj = -12.6v
Pin 2 in = -38.1v
Pin 3 out = -13.9v

looking better?

[jimmy74]

new voltage chart for the chip amps (not installed):

http://www.flickr.com/photos/73835524@N00/11291983513/in/set-72157638111984374

[Roly]

voltages on the regulators now are:

LM317
Pin 1 adj = 12.4v
Pin 2 out = 13.7v
Pin 3 in = 37.7

LM337
Pin 1 adj = -12.6v
Pin 2 in = -38.1v
Pin 3 out = -13.9v

looking better?

Not ideal perhaps, but +/-14v is quite satisfactory.


If the link isn't an actual wire then the holes at each end will be "plated-through" and one of them has given up - normally due to abuse of some sort.  If there is one such then be aware that you may strike this again around the main chip amps.

[jimmy74]

Only the jumpers are done this way, all the other traces are on the underside of the board. Have you taken a look at the chip amp voltage chart?

[Roly]

Have you taken a look at the chip amp voltage chart?

I have, and I can't see anything that looks out of order there.

I'd suggest that you now try fitting one of your LM3886 chips in the treble amp position, clip your meter to its speaker output terminal (speakers disconnected), and give it a short burst of power via the limiting lamp (which should flash and then go dim).

If the voltage settles to zero (or within half a volt), that looks good.  If the voltage goes anywhere else (most likely full +ve or -ve, or perhaps the chip shorts the two supply rails together) then I'd mark that chip as dubious, remove and place to one side, and try another.

If it goes well however I'd fit another chip in one of the other positions and repeat.

The aim here is twofold; to identify any of the power chips that may have been damaged, and to try and get three good ones in place.

If you get to this point you can try connecting the speakers and gently feeding a signal in and see what happens.  With the previous wild preamp supply voltages it is quite possible there is still dead silicon to be found in the preamp section, but having good supply voltages at least is a good sign.

[jimmy74]

would it be wise to take the voltage readings of the opamps first? (BA4560's and BA6110)

As for the "treble amp" position, do you mean U5? And if so that would mean clipping the meter to J5 and J6 right?

[Roly]

would it be wise to take the voltage readings of the opamps first? (BA4560's and BA6110)

When it comes to electronics it's wise to measure (and write down) everything you can, and you can give the preamp a going over before tackling the output stages if you want.

Since the preamp runs on split (+/-15V) rails a simple test for op-amp health is that their outputs should all be somewhere close to ground for DC.  Possible exceptions are U3A and U3B; but what you are looking for is that the outputs are not stuck at +ve or -ve rails.  Since the preamp is AC coupled to the main amps it is possible for the pre to be damaged and not working, but be no threat to the main power output chips.

Yes, U5 appears to drive the tweeter; again, a healthy chip should bias its output to something very close to ground, zero volts, and stuck at + or - 40V is a bad sign.  The meter check is to ensure that there is no (less than half a volt) DC voltage across either of the speakers.

Once you are quite sure that all three of the chip amp outputs (pin 3) are within half a volt of ground you can try reconnecting the speakers and driving the rig (gently) with a signal from your MP3 player or whatever, and see what happens (still via the limiting lamp of course).

If all seems well, no nasty distortion, smoke or fireworks, then you can try using a higher powered lamp, and if everything is still okay apply full power and test.  Note that while using the limiting lamp the amp will only operate correctly at low levels; if you try and crank it the lamp will light and the amp will distort.

But first you have to check out each of the chip amps as in my previous post.

[jimmy74]

sorry for the late reply, finally got down to getting the readings off the opamps, from U1 to U4 which are BA4560N's getting -13.8 off pin 4 and +13.7 off pin 8 so that looks normal. U6 BA6110 here's what I'm getting:
Pin1 = 0.5v
Pin2 = 0.5v
Pin3 = -12.7
Pin4 = -12.9
Pin5 = -14
Pin6 = 2.8v
Pin7 = 2.8v
Pin8 = 2.7v
Pin9 = 13.7v

Pins 6 & 7 are soldered together. Everything look normal here?