elac design flaw lm3886?

Hi.
I been helping out and repairing several elac active speakers.
they have a lm3886 which amplifing the tweeter and tip41 and 42c for the rest,
there is also a tda180 circuit which i dont really understand,
but the problem is that the resistor gets really hot connected on the negative rail to ground.
also had a lot of capactors bad which is replaced.
the negaive vee supplying the lm3886 and tip42 plus tda180 which connect directly to ground with a 2w 500ohms resistor. get burning hot, and it measures 25 volt over it.
I looked at other circuits but cant see any using 500 ohm, on the postive side I measure 9k ohm to grund.
any idea?
best regards after midnight in norway...

Niklas.
 

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is a dual rail with positve and negative,
I have about 5 of these small amps and the all get hot,
all suffered from that several capactors was blown.
the 500 ohm get very hot , so do the resistors connecting to each lm7812 and lm7912,
so when playing for about 1.5 hour in the box, the whole amp is hot.
but i know its difficult not to have access to scematics.
im was hoping some could say that the 500 ohm connect on the negative rail was totally wront.. .:)
but i will keep on digging
 
resistors over cap.

My guess would be, that there was a problem with the negative rail not dropping fast enough and causing a problem when powered down. (common practice to put a couple K resistors across power supply caps for that reason)

but here is a reistor conected right to ground with 500, and none to the positve side,
Im just thougt it was odd..
when i build my for exp, gainclone i typical has 2.2k connected on each side on the rail.
 
is a dual rail with positve and negative,
I have about 5 of these small amps and the all get hot,
all suffered from that several capactors was blown.
the 500 ohm get very hot , so do the resistors connecting to each lm7812 and lm7912,
so when playing for about 1.5 hour in the box, the whole amp is hot.
but i know its difficult not to have access to scematics.
im was hoping some could say that the 500 ohm connect on the negative rail was totally wront.. .:)
but i will keep on digging

A component may get hot due to its size and cooling (which in this case is translated to the wattage rating), but whether it heats up the rest of the chassis depends only on how much heat it dissipates. In your case it is only a watt.

For example, if you replace it with a 10W resistor with metal housing, the resistor won't feel hot at all. But it doesn't change the temperature in the chassis. Most of the heat is coming from the LM3886.
 
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It seems best to draw a schematic and improve this design flaw. It should not be necessary to create useless heat. Probably a cheap OEM board. Does it have an output (speaker protection) relay ? Maybe they wanted to skimp on that one and used a resistor for fast turn off or something stupid like that.

Can you show a picture of the PCB ?
 
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Cheap OEM work with a known german brand name on it. Where is the resistor you meant ?

*I see a few other large resistors bent against the electrolytic caps. I would bent them the other direction to keep the caps from cooking. Cheap low quality caps too.

Let's first find out the meaning of the resistor. Please indicate with an arrow in the picture where you see it.
 
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Joined 2002
It is hard but possible to draw a complete schematics. Is ELAC not helping out ? Second possibility is to take an already defective module and remove the 500 Ohm resistor and measure what happens with no load connected (rude approach but also possible). If you still have access to multiple units I would try the latter. Or replace it for a 4k7 one and check what happens.

Some first observations:

- it is uncommon to bleed 25V with 500 Ohm. Either a serious design error or a wrong value.

- I see the designer drops the voltage to the 78XX/79XX regs with 330 Ohm resistors to avoid the need for a power transformer with multiple secondary windings. These probably also will be hot.

- In any case please replace all electrolytic caps for ones of known good quality after root cause analysis + repair. Panasonic FC/FM is reliable and 105 degree rated.

- bend resistors and electrolytic caps from eachother to prevent the caps from overheating and thus to avoiding them drying out.

- If you want to solve flaws and improve the device while you are at it please think of replacing the Chop Suey relay for a Schrack or similar A brand with hard silver contacts.

- You could carefully drill holes for cooling in the PCB under the 330 Ohm resistors to allow for a somewhat lower temperature.

It all depends if you just want the speakers to work for a while or if you have the time to eliminate obvious flaws and make it a better product that does not fail anytime soon. Personally I can not choose an other option than that :D

Sent an email to Elac Deutschland if they can provide a schematic or to provide technical assistance to solve this design flaw. Please stay tuned.
 
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As it is wise to replace all caps anyway you could solder the new small caps in the vicinity of the hot resistors at the other side of the PCB (if space allows !).

If you do please observe polarity, you wont be the first one to mirror them wrong ;) Don't think of drilling holes if you mount the caps at the other side !
 
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Lo and behold. I got all schematics from ELAC. I did ask very politely in german ;) I think it is not meant for publication. Also I can confirm that the 560 Ohm is straight from V- to GND. Its purpose is not described. ELAC is sorry for the failures and want to help out. They pointed me to a few issues and even want to send the parts. They also recommend to replace all caps. Let's handle this by means of PM as I do feel they are responding like an A brand should.
 
Lo and behold. I got all schematics from ELAC. I did ask very politely in german ;) I think it is not meant for publication. Also I can confirm that the 560 Ohm is straight from V- to GND. Its purpose is not described. ELAC is sorry for the failures and want to help out. They pointed me to a few issues and even want to send the parts. They also recommend to replace all caps. Let's handle this by means of PM as I do feel they are responding like an A brand should.
Nice! Sent you a PM.
Thanks,
Duong
 
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Joined 2002
AM180 schematics

Here they are. Advice from Elac is to replace driver IC and ALL electrolytic caps.
 

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