[Wotan1]

Hi Guys,

I'm new in the DIY forum, but I quess the is the place to be with technical questions.

The primary electrolytic capacitors of my portable amp (Lisa III) blown after receiving the amp and using the PSU for the first time. Same happened again after repair by the producer of this amp.

Two different PSU's were used in both occasion and were according to specs: 24v (measurured 23.8) and used 22v. The amp needs about 500mA and the PSU delivers 1200mA. The amp contains a batterypack with internal chargers. I made 100% sure that the polarity was right (centerpositive).

So
a) why are the capacitors blown twice. Why do the blow at all?
b) can repair beeing done by a local repair shop or is this difficult and must this be done by a specialist, knowing the amp's circuits inside out.

Your suggestions and remarks are very welcome

It's a great amp and my bad luck that I have issues. Would recommend the amp to everyone.

 

[TzeYang]

can you take pictures of the blown capacitor?

I really doubt the thing blow off.

 

[n_maher]

Just from the information provided I would have to guess that even though you may have gotten the polarity correct something inside didn't. The only things that I'm aware of that will blow an electrolytic cap quickly are large over-voltages (unlikely in this case) and reversing the polarity.

If it were me I would return the amp for a refund having had the same problem twice.

 

[Wotan1]

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif Just from the information provided I would have to guess that even though you may have gotten the polarity correct something inside didn't. The only things that I'm aware of that will blow an electrolytic cap quickly are large over-voltages (unlikely in this case) and reversing the polarity. If it were me I would return the amp for a refund having had the same problem twice.

Thanks Guys,

I had the same problem with 2 different adapters. What are the changes having 2 faulty PSU's?

Could the polarity issue be inside the amp?

Quote:

Originally Posted by TzeYang /img/forum/go_quote.gif can you take pictures of the blown capacitor? I really doubt the thing blow off.

TzeYAng,
I will see if I can get some pics, but I'm not a technical person and I do not whan to get things wors by opening the amp. Why do you doubt that the caps are blown.

 

[TzeYang]

The caps should not just blow like that. It should not at all!!

Because the caps should tolerate at least 24V and since the voltage remained stable all the time i doubt it would. Plus, it's a properly designed PCB. Everything remains fixed and unchanged. UNLESS the PCB traces are faulty from the factory itself.

So i think something else inside broke down.

If you're using a hammond case, just remove the back panel, and slide open the top. Just take pics from the above. It's safe this way and nothing should screw up. You're going to need to replace the 9V batteries too right? It was designed in a way (everything is soldered on board) to ease the disassembly.

 

[applegd]

Quote:

Originally Posted by Wotan1 /img/forum/go_quote.gif Thanks Guys, I had the same problem with 2 different adapters. What are the changes having 2 faulty PSU's? Could the polarity issue be inside the amp?

1. Check the DC input jack, the center pin of the jack should be +V. Make sure the polarity is correct on the circuit board power rail too.

2. Use Multi-meter meansure the output of the PSU. Pay attention to the voltage reading and polarity.

 

[n_maher]

Quote:

Originally Posted by Wotan1 /img/forum/go_quote.gif Could the polarity issue be inside the amp?

Yes, most definitely. Could be that the builder wired the DC jack incorrectly. But I would have thought that the amplifier would have had some sort of reverse voltage protection, maybe not though.

And I don't think that anyone doubts the caps are blown it's just that we can offer much more informed opinions when we've actually seen what has happened and some of the internal circuitry. Maybe we can spot a wiring error from pictures. That said, if you haven't opened the amp how do you know the caps are blown?

 

[Wotan1]

Quote:

Originally Posted by TzeYang /img/forum/go_quote.gif The caps should not just blow like that. It should not at all!! Because the caps should tolerate at least 24V and since the voltage remained stable all the time i doubt it would. Plus, it's a properly designed PCB. Everything remains fixed and unchanged. UNLESS the PCB traces are faulty from the factory itself. So i think something else inside broke down. If you're using a hammond case, just remove the back panel, and slide open the top. Just take pics from the above. It's safe this way and nothing should screw up. You're going to need to replace the 9V batteries too right? It was designed in a way (everything is soldered on board) to ease the disassembly.

The supplier told me that after repaired the amp was tested on 26/27v. This is the max it can carry.

I will see what I can do with the pics.

Quote:

Originally Posted by applegd /img/forum/go_quote.gif 1. Check the DC input jack, the center pin of the jack should be +V. Make sure the polarity is correct on the circuit board power rail too. 2. Use Multi-meter meansure the output of the PSU. Pay attention to the voltage reading and polarity.

I did, especialy after the amp returned after repair. Polarity 100 correct and voltage of the 24 adapter was measured at 23.8 and I used 22 to be save.

Quote:

Originally Posted by n_maher /img/forum/go_quote.gif Yes, most definitely. Could be that the builder wired the DC jack incorrectly. But I would have thought that the amplifier would have had some sort of reverse voltage protection, maybe not though. And I don't think that anyone doubts the caps are blown it's just that we can offer much more informed opinions when we've actually seen what has happened and some of the internal circuitry. Maybe we can spot a wiring error from pictures. That said, if you haven't opened the amp how do you know the caps are blown?

See above about the blown caps. As history repated it self 100% I presume the same outcome.

So what about the AC/DC and AC/AC? I did not know there were different PSU's. I will look into this and let you know. But I think need and used an AD/DC.

 

[n_maher]

If the amp required an AC power supply there would be no need to specify "center pin positive" as AC, by definition, does not have positive and negative voltages, they are alternating.

 

[Wotan1]

This one is recommended by Triadadio for the Lisa www.triadaudio.net

http://www.alliedelec.com/Images/Pro...ts_8790090.pdf It says 2,5-140.4 WATT AC. (please note tha Triad advised 20-24 and that amp can take max 27volt.

The Ansmann PSU i used says: 100-240 AC and 12-24 VOLT DC

Could it be that I'm stupid and used an AC/DC and should have used an AC/AC power supply

 

[Wotan1]

Ooopps

 

[TzeYang]

i've seen the LISA III's PCB and design before. It is meant to take DC only. You're right, and you didnt screw up. The ansmann psu you used it adjustable? why the 12-24DC Volt rating??

n_maher, you're right, the LISA III does not have a reverse voltage protection diode

 

[Wotan1]

Quote:

Originally Posted by TzeYang /img/forum/go_quote.gif i've seen the LISA III's PCB and design before. It is meant to take DC only. You're right, and you didnt screw up. The ansmann psu you used it adjustable? why the 12-24DC Volt rating?? n_maher, you're right, the LISA III does not have a reverse voltage protection diode

A pity I did n ot screwed up. It would at least have solved my problem with the trade oof of beeing stupid

The Ansmann has an adjustable volt regulator..12, 14 m 16 op to 24. I used the 22 or 20 volt option. Not sure if this is the aswer you were looking for.

 

[holland]

Which caps blew? The 100uF/20V rail to rail caps?

Does the LED work?

I don't believe the LISA III has a built in voltage regulator for the power rails. If the schematic I am looking at is accurate, it is indicating a 20V cap, then the problem is there. Increase that to 35V rating.

Please measure the rail to rail voltage when you plug in the adapter. Maybe there's a regulator in the triad design to control rail to rail voltage. If so, it's possible the regulator is bad.

 

[Wotan1]

Quote:

Originally Posted by holland /img/forum/go_quote.gif Which caps blew? The 100uF/20V rail to rail caps? Does the LED work? I don't believe the LISA III has a built in voltage regulator for the power rails. If the schematic I am looking at is accurate, it is indicating a 20V cap, then the problem is there. Increase that to 35V rating. Please measure the rail to rail voltage when you plug in the adapter. Maybe there's a regulator in the triad design to control rail to rail voltage. If so, it's possible the regulator is bad.

Thanks for helping. Not sure, according to Triad the primary electrolytic capacitors in the amp were blown the first time. Does this help? So as history was repeating it self I presume the same outcome. I have not looked inside the amp myself

What do you mean with increasing to 35V? And what is rail to rail voltage?

Sorry, I'm not so technical...

Again thanks for helping..

It's a great amp and I have not heard of others having issues. It's my bad luck and I would recommend the amp to everyone.