Trying to repair an HK Audio Lucas Nano 300 speaker system

[Yamin]

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.

[Yamin]

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 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.

[Pete66]

Replace Q1 even if it tests good in circuit and out of circuit.  I had an issue recently in an LED controller circuit where the output of the 555 needed to be pulled low manually for the LEDs to light up.  Even though the transistor tested good I replaced it and everything worked.

So replace Q1 and let me know if that works for you.

[Ettio]

Hi, I also have the same problem, I have already repaired this amp once by replacing the tantalum capacitors, after a few months it has a new problem now it remains silent and therefore does not turn on, have you solved the problem ??

[onin111]

Hi, any chance of an update on this?

I've got the same problem having it muted after some time of use. When I first turn it on, it mutes for a second or 2 then turns green, after like 10mins or if turn up the volume, then it mutes again.

I've already replaced a 100uf 63V capacitor which was a bit bulging, and also cleaned and reapplied thermal paste on the heat sink and components hoping it was some kind of overheating problem, but still not working after a while.

Thanks

[okw]

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.

[Juka77]

I require more information in order to provide the desired solution. Kindly provide me with additional details to better assist you. Thank you.

[KevinA]

Hi OKW,

You will never see a schematic for the E5400011 power amplifier. This is manufactured and supplied by B&O, and their service policy is module replacement only.

As an example, my old boss had a MMEEGGAA expensive B&O television which went faulty. I was unable to find any service data so a service call was arranged with the retailer. On the arranged service call date, I was waiting for the service tech to ask about the service data. He arrived with a large service case on wheels, inside was a diagnostic unit, along with every board for the television. I questioned him as to what the action would be for a blown fuse? Replace every board the diagnostic module tells me to replace was the answer.

[Audiorepair]

I believe B&O power amp modules were/are branded Icepower and were fitted to various manufacturers' powered speaker systems.
All the ones I have seen are clearly marked Icepower.

Elektrotanya have some Icepower schematics.
https://elektrotanya.com/showresult?what=icepower&kategoria=All&kat2=All


Likewise, the likes of RCF use power amp modules branded Digipro.
These are also usually clearly marked as such.
Elektrotanya has a couple of these also:
https://elektrotanya.com/showresult?what=digipro&kategoria=All&kat2=All


So it sometimes pays to have a careful look at the PCB and take the hint provided, these manufacturers sometimes just buy modules in.