Completed LM 4780-but fuses blow?

[Dominick22]

Finally,

With the help of this site, my father in law and I have completed this amplifier...LM4780 (almost). We fired up the transformer by itself, and it was running fine, but when we connected it to the chip amps (with no signal input), there was a strong hum followed by both fuses blowing within 5 seconds. Its pretty obvious we are either missing something or we just made a mistake.

Some ideas would be appreciated.

We have one though...the PSU schematic shows R1, R2, C3, and C4 as parts not included in the chip amp kit for the parallel wiring.

http://www.audiosector.com/lm4780.shtml
*Click on the PSU schematic at this site to view it.*

We wired in parallel configuration without these parts since there values are unknown. We are thinking that maybe these missing parts will make the circuit stable?????Is this True or are those part optional???? If true, what values do we use for those parts?

Also, tell us what you think about the overall construction. The base is Maple and the ends are Cedar which I think is very attractive looking.

Thanks to all who contribute to this site.











Dominick in New Jersey

[BobEllis]

Hi Dominick,

Where is the hum? From the speaker or the amp itself?

R1, R2, C3, and C4 are for an optional snubber. That will not prevent the amp from working in the parallel mode.

Are you sure that you hooked up the positive PS leads to the positive terminals on the chip amp boards, etc? (most likely problem - bought that t-shirt myself a few times) Unfortunately it usually toasts the chip and caps if you get it wrong. Are the 1500uf caps bulging on top? (are they oriented correctly?)

Try hooking up just one chip amp at a time to the PSU. That way you can further isolate your problem. Is the problem with both or just one?

You can also try putting a 100R 1/4W resistor between your PSU and the rail connections - it will act as a current limiter if there is a fault. If it goes up in smoke you're only out a few cents. Once you don't blow the resistor, then you can fire up without it (remove one at a time to ensure that you get the PSU wiring correct.)

Hope this helps.

[Dominick22]

Thanks for that,

It think the hum actually came from the fuses untill they blew.

The caps are not buldging and we were extra careful to ensure proper connecting of everything on the board. We already double checked so its good unless we just missed it.

Thanks for trying to help--I will try the single chip testing.


Dominick in New Jersey

[Dominick22]

Just to clarify,

I did NOT have any audio input or speaker hooked up.

[Peter Daniel]

Those extra parts are not needed for proper operation.

The holes in the boards are aligned in such way that if you not cross the cables the proper voltage is delivered from rectifier board to amp board.

I see you have some bars over rectifiers, are they isolated from the diodes?

Are the chips isolated from the chassis?

For parallel configuration dont forget to install the jumper: http://www.diyaudio.com/forums/showt...571#post636571

[BobEllis]

Don't forget to look very carefully for solder bridges on the amp boards

[chipco3434]

I see you have some bars over rectifiers, are they isolated from the diodes?

... do you have pads under those diodes to keep them from shorting out each other through the aluminum sinks????

[Dominick22]

No,

The aluminum piece was just simply attached for a heat sink.

New update! LOL

I disconnected the rectifiers from the amps andtook off the aluminum heat sinks.

This time the fuses were fine and I did not get a hum...but after 10 seconds, a capacitor scared the crap out of me as it blew up!
Only one on one rectifier board blew.

Ill replace it...Does this help anyone solve the problem?
I'll attach a close up of the rectifier board...









I have the ac1's and ac2's connected to the ac1's and ac2's on the other rectifer board. Is that correct?

Is the blown capacitor a sign of to much voltage?

Thanks guys,

Dominick in New Jersey

[jeff mai]

Originally posted by Dominick22
Is the blown capacitor a sign of to much voltage?

Possibly, but it is much more likely that it was installed backwards. Observe the correct polarity when you install the new one.

[Greg Erskine]

Hi Dominick22,

I think the diodes are in backwards. As a general rule, all components except resistors need to be orientated correctly.

If you put the heatsinks back on the diodes, they must be insulated.

EDIT: The black line on the PCB represents the metal tab of the diode. Also, you can get away without that capacitor for testing, so try again.

I think you may have applied reverse voltage to one chip.

regards
Greg