Repairing Alesis M1 Active mk2 Monitor Speakers

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

R3+4 ok, both 47k and about 157v over each. (Orginal 24k)
C8 looks good(don't know how to measure) and replaced again.(330uF/50v)
D5 ok, R5 (5ohm) ok.
U1 was a 3842 and is replaced 2 times by a UC3842AN

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

What did you replace C8 with (brand / series)?

I don't suppose you happen to have acces to a(n isolated) bench power supply, do you?

Also, if the PWM's supply is so low, Q3/4 are starting to look suspicious (too)...

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

C8 is replaced by(sorry China again ;-)):
https://nl.aliexpress.com/item/10pcs...311.0.0.lsgS44

And no sorry, I don't have a bench power supply.

Q4 was damaged and is replaced. In my speakers Q3&4 are resp. 2N5401 & 2N5551
Is there a way to measure these?

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

I have Q3&4 (2N5401&2N5551) available so just have a try by replacing?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Elna RJ3 seems to be "just" some 105C general-purpose series; you'd ideally want a low-ESR cap in that position (because the power coming from the auxiliary winding of the main transformer is at a few dozen kHz).

If you want small quantities of good quality caps, try this seller - i've ordered a few times, after seeing him(?) recommended here, and shipping took under 2 weeks.

To be on the safe side, yes, i'd replace those two transistors; maybe even Q5 as well, at least to eliminate some variables.

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Of course, a low-ESR cap, why didn't I think about that ;-)
I'm wondering about all the knowedge you have about this, I don't have any idea were you're talking about. In some cause the value doesn't matter so much, in other causes you need more details then ever given in the documentation. Without help of guys like you I never could fix this, so I'm very pleased with all the help!!!

Do you think this wrong choosen cap could be the course of my problem(low voltage on U1)? I will exchange Q3+4, Q5 I don't have available jet(it's really a 2N4403, or can I exchange this by a 2N5551 too, like they did with Q4)

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

And what should I use for value for C8 220/330uF 35/50volt? (orginal 220uF/35v/105C)

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Time and experience I wasn't born knowing all this, and i still don't know even half as much as some other guys on this forum, at least about switchmode power supplies

The minimum would be the original value; the chip doesn't take anything over 36v anyway (nominally its supply should be around 15v), and 220uF should be plenty.

I was just battling with a UC3842-based power supply earlier this week, that wouldn't power on by itself; 47uF, which is a value i've most often seen, turned out not to be enough. taking on a second one in parallel with the first cured that, so it ended up with a 100uF cap there.

A pack of these should sort you out, with spares left for later projects
http://www.ebay.com/itm/20-Pcs-NCC-K...item440e967662

Speaking of which, it might be time for me to make a "shopping list" and re-stock on some values...

[Micwest]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Is Elna RJH serie good?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

https://cdn.badcaps-static.com/pdfs/...444a92c721.pdf

Just don't get them from aliexpress or other shady places, hmm?

[dannykap]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Nice to see that there are still some people interested in these lovely speakers. I posted a few pages ago about a pair that I acquired, but only managed to get get one working. Well that one has blown its fuse again after a few months. I think they are now beyond my ability to fix. So, I'd like to make an offer here that if anyone would like to fix one for me, they can have the other one to fix for themselves for free. I'm in West London so more than happy for someone to collect or I could deliver within a reasonable distance. Alternatively I could post at your cost, and then if you fix it I'd pay for my postage back to me plus the postage costs you paid to deliver it to you. I also have a bag of diodes, resistors etc that I will include. Any offers, please pm.

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

What voltage (if any) is present between pins 7 and 5 of U1?

I've done a few measurements and between 5 and 7 I get 0.77 V roughly.
Not sure if this is bad or good, if bad what do I measure next?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, 0.77V is nowhere near enough to turn on the PWM chip, that's for sure.

Is C8 still the one from the factory?

[diydidi]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Would be really cool if someone could come up with a step by step repair manual with all the tests and common faults to get these guys working again.
I have two to repair, and its proving really difficult.

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, 0.77V is nowhere near enough to turn on the PWM chip, that's for sure.

Is C8 still the one from the factory?

No, i replaced that one earlier as it being one of the most common problems.
I did measure it before and matched it to the still working speaker of mine, the values were the same.

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Bad part is, not all capacitors are made equal. While they may have the same nominal capacitance & voltage ratings, you want a low-ESR / low-impedance cap, where switching power supplies are concerned. A "general-purpose" one won't cut it (or if it does, it, won't last long).

What brand / series cap did you use?

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Ah, interesting. I was having a hard time understanding the part specs and after finally getting the right components there is more to keep in mind :O
For the C8 part I ordered this one:
https://www.conrad.nl/nl/elektrolyti...j-1505587.html

It doesn't state anything about ESR values, funny since they seem important.

Do I understand correctly that this part might be 'not good enough'? Of is might there be another culprit?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, Conrad aren't exactly one of the (super) "professional" electronic suppliers, i think.

Anyway, Chemicon KY actually is indeed a low-ESR series, so that means the problem is somewhere else...

[markovitch]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

voi perkele, excuse my finnish

I could cross measure some parts with the working speaker I have, what would be a good starting point for this?

[Khron]

Re: Repairing Alesis M1 Active mk2 Monitor Speakers

Well, for starters, with the power off, assuming your meter has a diode-check mode, that could be a start.

Red probe on pin 5 of the chip (careful how you count them), keep it there, and black probe first on pin 7 (Vcc), and then pin 8 (Vref). Normally, you'd expect something around a diode voltage drop (5-600mV or so); anything less, and the chip may well be toast.