No output from a HK Audio Lucas Nano 300 speaker system.

Thread Starter

Yami

Joined Jan 18, 2016
352
I have got a HK audio lucas nano 300 in for repair. The issue was the unit not powering up, no indicator lights. Opening up the unit I found that something was shorting out +15V and -15V power rails. The culprit was 2 tantalum capacitors on the preamp board. There were actually 4 capacitors and it seems to be that 2 capacitors are in parallel with the 15V and -15V rail.
After removing the two shorted out capacitors I powered on the unit. This time the unit sprung to life but the amp is on mute mode. The indicator light is red and the LED on the main board labeled 'mute' is also lit. I measured the power rails and everything was ok and stable.
During troubleshooting one of the remaining tantalum capacitor on the preamp board shorted. So I have replaced all the capacitors now, there are no shorts now but the amp remains in mute mode. I tried playing back audio thinking that amp was in idle mode or something. No change, no sound but I heard very faint audio from the small satellite speakers.
I couldn't find any service manuals online, which makes the troubleshooting that much harder. I emailed the manufacturer but they are not willing to hand out the service notes unless its their authorized re-sellers, even for their legacy products. Any help on getting the service manual would be really appreciated.
The amplifier chip is LM4780. Nothing seemed to be shorted there. One thing I would remark is that the -VEE pin is tied to ground. I thought it would be connected to the -15V.
I traced the mute pin track to a SOT26 package transistor, the markings on it is '1Ft'. I couldn't find the pinout, knowing this would also help to figure out why the amplifier chip is mute.
Any help on this would be really appreciated.
Thanks in advance.
 

Attachments

Thread Starter

Yami

Joined Jan 18, 2016
352
I have figured out why the amp is mute! :)The output of this 555 timer needs to be pulled low in order for the amp to un-mute. I manually pulled it low and the amp was working fine as long as I have it pulled low.
Can you please help me figure out how this 555 timer is used (which configuration) and what might be the possible reason for the output not going low. I suspect R25 and C20 to be the culprit. The marking on R25 is '30E' what would the resistance be? I am guessing the 555 timer is used as a delay to start up the amp.

Thanks in advance for the help.
 

Attachments

Last edited:

tonyv

Joined Jun 2, 2021
1
Hi Yami, any joy with the HK Lucas Nano 300 amp muting, I have a unit in for repair with the exact same problem. The same components had failed on the pre-amp board (must have been a batch of bad caps). Your assistance would be appreciated. Thanks Tonyv
 

Dodgydave

Joined Jun 22, 2012
10,050
I have figured out why the amp is mute! :)The output of this 555 timer needs to be pulled low in order for the amp to un-mute. I manually pulled it low and the amp was working fine as long as I have it pulled low.
Can you please help me figure out how this 555 timer is used (which configuration) and what might be the possible reason for the output not going low. I suspect R25 and C20 to be the culprit. The marking on R25 is '30E' what would the resistance be? I am guessing the 555 timer is used as a delay to start up the amp.

Thanks in advance for the help.
Measure the voltage on pins 2 and 6, from gnd and see what they are?
Pin 6 should rise up from zero to approx 10V.
 
Top