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Old 11-19-2012, 01:19 PM   #21
midibob
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Default Re: Vox mini 3 amp

Hi Maissani,

I don't understand all of your message (gancia?) do you mean running the amps too loud? Tests I've done show that even at full volume they don't blow as the output chips should be overload protected.

I gather you are hoping to use an LM380 as a replacement. Watch the gain figure as the LM380 is set at 34dB whereas the original chip is only 12dB. Also the original is a class D device and runs on 5v with extremely low quiescent current.

Good luck
Midibob
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Old 11-21-2012, 07:44 AM   #22
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Default Re: Vox mini 3 amp

if I meant to ganancia.lo we're arming with a friend is not to replace the LM380 IC3 if not make an audio output on a pcb mount apart and then we will replace the amp output working amplificador.estamos in that when you have the issue armed with photos and post it all here or try to give you the link for you to see.
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Old 01-27-2013, 03:36 AM   #23
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Default Re: Vox mini 3 amp

Hi midibob,
Did you get anywhere with you Vox mini 3 daughter-board idea please? I've got a few of teh Vox amps turning-up soon with I suspect the same output problem needing repair. Little did I know what I was getting into....

Regards
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Old 01-27-2013, 06:27 AM   #24
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Default Re: Vox mini 3 amp

Hi Piwi,

I assume that was the last lot sold on Ebay?
I was watching it but didn't bid. Still you've got a good deal there I think.

As for the daughter boards, the project is on hold for the moment due to no heating in my garage/workshop. I've got as far as the PCB draft but no etching yet. The cost of making them might also be too high ie components, time etc for an economical repair which is another reason why I haven't progressed.
Watch this space as I will report back if I make any progress.

I'm pretty sure all yours will have the same problem that the IC has popped. I never did get to the bottom of why these should fail unless it is due to people shorting the speaker leads to ground.

Midibob
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Old 02-07-2013, 04:29 PM   #25
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Default Re: Vox mini 3 amp

Hi Midibob,
Sorry for the delay in replying. Yes, they are the e-bay amps and they are in very good condition. I've not had a chance to look at all of them yet but the first one produces an output from the aux input but not from the main input sockets. Will try to investigate over the weekend when I have a bit more time. I'm sure that at least one will have the blown o/p ic though...

Piwi
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Old 02-08-2013, 03:17 AM   #26
midibob
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Default Re: Vox mini 3 amp

That is the most common fault (so far), sound OK out of the phone jack but not the speaker. Most amps derive the headphone output from the main output device but these don't.
I've only ever seen one other fault and that was a broken track in one of the pots.
Still you have all the info I've posted so you should be able to confirm the o/p chip demise pretty quickly.
Let me know how you get on.

Good luck.
Midibob
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Old 02-10-2013, 04:11 PM   #27
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Default Re: Vox mini 3 amp

Thanks Midibob. Will advise on what I find (and when I find it). It's a shame that Korg will not release any information at all about these amps.

regards
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Old 05-14-2013, 12:04 PM   #28
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Default Re: Vox mini 3 amp

Hi Midibob,
I'm glad i've read your post, but also a little disappointed that it is that hard to repair.
I just received a Vox mini 3, one i've bought at an auction, missing the power source, so I could not test it. but after attaching a 12VDC it did not function through the speaker. Everything else works fine through the headphone output. So I first thought it was the headphone interuption which was malfunctioning but it was not, then I read somewhere on the internet that the headphone is before the amp, so I tought that the output amp was fauty, but without a scheme on a smd board, I did not knew where to start. But I think i've got the same problem on my amp as all off you in this post. But I do not have a clue how to repair an smd board like that, so I hope for an other solution :-)
Thanks already for sharing all this
Grtz
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Old 05-14-2013, 01:09 PM   #29
midibob
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Default Re: Vox mini 3 amp

Hi Vidji,

I always like to share knowledge if it helps somebody as some day hopefully someone will do the same for me.

As I've stated previously I would warn against anyone trying to repair these boards without the correct tools as they would most likely damage them beyond repair.

My progress of the daughter board has stalled for the time being. As soon as I have made them I'll post here but it could be a while yet as other things have taken precedence.

Watch this space.

Midibob
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Old 05-20-2013, 01:46 PM   #30
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Default Re: Vox mini 3 amp

Hi Midibob.
Thx for the info, I was thinking if it is possible to put another (prefabricated, like from old computerspeakers) amp in, taking the input signal from the headphone, here you've got some space to solder and output directly to the speaker. Or could give this some other kind of problem?

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Old 05-20-2013, 02:19 PM   #31
midibob
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Default Re: Vox mini 3 amp

That is possible although the current consumption would likely increase which may or may not be an issue depending on battery life.
All been discussed here previously.


Midibob
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Old 05-20-2013, 04:42 PM   #32
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Default Re: Vox mini 3 amp

Yes you are right. consumption would increase.
I still find it very cool that you reached in repearing a broken amp like that, resoldering such a microscopic part. I did not get any further than basic electronics. But still thanks for your info
Grt
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Old 06-27-2013, 01:53 PM   #33
midibob
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Default Re: Vox mini 3 amp

OK, at long last I have produced the daughter board. Making the printed circuit has been a very long process but that's another story.

I've attached a picture of the board and am testing it at present as I only finished it this afternoon. This is the 2nd attempt as the chip had too much gain so I had to redraw and etch new boards to hold the extra components.

One slight snag so far, there is a cap across the chip power rails and I've found the ESR of it isn't low enough which causes break up at max volume. Other than that it appears to work fine.

The plan was to have a board that would replace the original o/p chip without having to desolder it and not to have to modify the board by drilling holes, also not needing any special tools to accomplish it.
All it takes is the removal of the two electrolytics C17/18 and lifting the speaker connections off the old board and adding the power rails via holes that are already in the main board.

Hopefully all I need now is to find some lower ESR 10uF caps and I think it's finished.

Midibob
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Old 07-15-2013, 02:43 PM   #34
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Default Re: Vox mini 3 amp

Hello Bob , I would be really interested in one of your 'daughter boards' . What was the chip you eventually used ? did you originally use the TPA 2034 and did the gain on that seem OK . What gain is your daughter board chip rated at ? I hope it has been successful for you , I am impressed . Would you be interested in selling an example or selling the wiring spec. ? I would really like to get my amp going ,( I have one that works fully and its very good ). I did message you several months ago and thanks for your replies , Mic .
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Old 07-16-2013, 02:25 AM   #35
midibob
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Default Re: Vox mini 3 amp

Hi Mic,

I've made a number of boards up but I need to get hold of some low ESR caps as mentioned. I'm looking at Panasonic FP series and it appears Farnell is the only place that stocks them. Trouble is with minimum orders and postage it makes them really expensive so I'm having to wait until I need some other bits.

The chip used is the SSM2305 which has a gain of 18dB over 12dB of the original which is why I had to add some gain reduction with resistors. As it's an 8 pin MSOP package it's really easy to replace if it ever blows.

I'll PM you.

Midibob
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Old 07-16-2013, 07:08 AM   #36
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Default Re: Vox mini 3 amp

Hi BOB , just seen your message , hope you got mine , Mic
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Old 07-24-2013, 09:50 AM   #37
midibob
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Default Re: Vox mini 3 amp

The Panasonic FP series I eventually found at Radiospares and these have proved to be a perfect replacement as their ESR is rated at 0.85ohms.

Just waiting now for reports to come back from a volunteer who is going to install one to confirm they are as easy to fit as planned.

Midibob
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Old 08-18-2013, 09:30 AM   #38
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Default Re: Vox mini 3 amp

NOW WORKING !!! Thanks Midibob for those daughter boards for the Vox Mini 3 amp . I have now fitted them to the two amps I have .( both had that microscopic and blown amp chip ! ) What a great way to bypass that original chip with a minimum of fuss .They were really well made by you and your instructions to install them very clear and easy to follow . It was only my clumsy soldering that caused me a problem !. They now sound identical to the original working model I had and have the same max. volume . BRILLIANT ! So much better to be able to fix such things which may have once been scraped . Many thanks , I will be in touch , Mic . Nice little amps .
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Old 08-18-2013, 07:48 PM   #39
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Default Re: Vox mini 3 amp

Man, this kind of stuff almost brings a tear to my eye. It's amazing what a dedicated community can accomplish. Good job! Long live Badcaps.net !
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Old 08-19-2013, 11:57 AM   #40
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Default Re: Vox mini 3 amp

Quote:
Originally Posted by Th3_uN1Qu3 View Post
Man, this kind of stuff almost brings a tear to my eye. It's amazing what a dedicated community can accomplish. Good job! Long live Badcaps.net !
We need to build power supplies!!!
I desperately want to do this xD

Yes, good job folks, that is a nice layout on the daughterboard. I guess you chose no ground plane since the parts are hand-soldered, right?
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