No output from an HK Audio Lucas Nano 300

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.


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I have figured out why the amp is muted:). 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 had pulled it 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 to 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.


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lucas nano 300 testing the tripleamp 200

Hi everybody,
I plugged a line output (mixer) to the input 2 of my lucas nano 300, but leaving the switch in the instrument position. Then… no more sound, no more light to the status indicator, speaker out leds still lighting red.
I have completely dismantled the subwoofer. I tested the + 15V and -15V pins on the tripleamp 200 board with a multimeter by turning it on.
1 when the preamp cable is connected, the -15V pin drops to -1.5V, the + 15V remains stable.
2 when I unplug the cable, the + 15V and -15V pins are OK
I guess the preamp is out of order.
HK audio offers reconditioned spare parts. But before ordering a preamp, I would like to be shure that the amplifier operates normally. But without wiring diagram, I do not see what to do. Does anyone have any advice?
Thanks in advance.
I have a service manual, but at 7MB it exceeds the limit. PM me

Dear Kevin,

I sent you message to ask you if you could send me the sevice manual for the Lucas nano 300. I do not know if it arrived to you . In this case ignore this new message....
Recently I got problem with my nano300 and I try to find doc to guide me to troubleshoot the amp. If you could help my bysending me the service manual it would be great.
Here my email address:
Thanks a lot ,
Hi, this is the first time I write on this forum, I have at home a lucas nano 300 from my sister that has the same problems, at first there was a shorted preamp tantalum capacitor, replaced that one worked for a month or so, now it remains in protection immediately, I also removed the final of the satellites but the block remains, the signal arrives, both on Ch2 and Ch3 of the satellite amplifier, so the pre amp works, but also by lowering the Pin 2 of the 555 nothing changes, the mute led stays on, if you have solved the problem, can you suggest something? thank you

I have the same problems with mine. I want to replace the four 22uF/25V tantalum caps. Are they type (size) B, C or D? I'm about to order a bunch of components, but the amp is at my parents place a few hours away, so I can't check myself.
Also, does anyone have the schematics and/or a parts list for the power amp? I seem to only have the preamp schematics.
Всем привет! тоже нуждаюсь в четырёх танталовых конденсаторах. Ищу усилители. Пока решил проверить те, что указали в этих комментариях.

:cop: This is an English language forum, please use a translator to post in English.
Это англоязычный форум, пожалуйста, используйте переводчик для сообщений на английском языке.

Translated by Moderation :
Hi all! I also need four tantalum capacitors. I'm looking for amplifiers. So far I decided to check the ones indicated in these comments.
Nano 300 Power amplifier schematic.

It does not exist. These modules, along with others used by many Pro Audio manufacturers, are made by B&O; service is by replacement module only.

Even B&O main dealers do it this way. My boss had a mega expensive B&O television which went on the blink, I could find no service data for it, and so the dealer was contacted, and a service call arranged. I wanted to speak to the technician to find out how to get the service data. Said technician arrived with a massive flight case containing one of every board, along with a diagnostic unit.

I asked him what happens if it is just a fuse blown? The answer was that every board the diagnostic unit condemned is replaced without question.

I have seen some schematics of these modules drawn out and placed on the web. The boards are often multiple layers; unless you have the requisite X-ray equipment, how can you draw it out?

Also, even if you manage to draw out the circuit, how do you place a value on the ceramic capacitors?