MK sub not working

[odonnks]

Hi All,

I have a MK MX-2000 sub that isn't working. No audio at all
The two internal fuses are not open and the DC rectifier is providing 45 vdc to the amp board.

Is the plate amp worth repairing?
Or would I be better off buying a new amp?
Any recommendations for a replacement?
As its a push pull unit is there a specific type of amp needed?


Any feedback appreciated.

Thank you

[Cal Weldon]

I've moved this to solid state as it seems to deal with your amp.

[Zero Cool]

I would bet that there is something like +/- 15vdc that is being fed to the crossover section. do you have + and - Voltages at the opamps? no voltage to that section would cause it not to work.


Zc

[odonnks]

Hi Zero,
I checked the Op Amps.
There is a pair of LM348's and both have the + and - supply. (measured + and - 13 vdc)

Without having a schematic I'm scratching at what to check.

Question,
Being a quad op amps I assume that one is for the Rt and the other is for the left channel.

If I were to scope the input pins of each (4 per IC) should I see the audio input signal from the speaker input terminals.


Thanks for the suggestions!

[godfrey]

Hmm... hard to help without knowing anything about it but here's some thoughts anyway...

How dead is it? - Not even a faint buzz or hiss if you listen closely? Not even a click/pop/thump when you switch on or off?

How did it die?
e.g. "It was working fine and just suddenly died in the middle of a party" probably means death by overload - either the amp's output stage fried or the speaker burned out.
alternatively: "It used to work but it's been in storage in the attic for the last 15 years and now it doesn't work" could mean it's just some passive components e.g. capacitors that have died of old age.

btw: Are you sure it's the amplifier that's dead, not the speaker?

You mentioned finding 45V DC supplied to the amp. I'm wondering if it's supposed to be just a single 45V supply. Maybe it's supposed to have +45V and -45V (a more common arrangement), but the power supply is half dead?

Another thing to check is if there's a relay connecting the speaker to the amp. If so, it's possible either the relay or the protection circuit is malfunctioning and leaving the speaker disconnected.

Probing around with a scope as you suggested might give some clues. It could even be the proverbial "loose wire" or a bad connection. It's probably worth checking the signal from input to circuit board and then from circuit board to any front-panel controls and back etc.

Is there no obvious physical damage e.g. burned resistors, bulging capacitors etc.?

Not having a schematic is a bit of a handicap, but assuming the power amp is blown, one could start the diagnosis at the output stage and work back from there. Checking the voltages on and across all the transistors on the heatsinks would be a good start.

Enough for now!
Good luck.

Regards - Godfrey

p.s. Sorry for the OT, but I can't resist sharing my all-time favorite description of a dying amp; this one from Carlos
Dx Blame ES .... based into the Blameless, i am trying a new amplifier

[odonnks]

Thanks for the ideas.
It was passed to me from a family member who is known for running his equipment very hard. Its been in my basement for a few years and I figure its time to give it a go or can it.

When powered up there is no pop or hum. It has no output at all.
As a push-pull unit I'd have thought it the speakers wouldn't go at the same time.
Checking the rear one I have resistance, (around 20 ohm), at the speaker coil so its not opened up.

Knowing what I do form its use I'd say the amp is somehow cooked.

I'll start checking with a scope as you suggested from input to amp ans try to follow it through.

Thanks again !

[odonnks]

OK. did some scoping around.
I may have overlooked the obvious. Looks like the speakers are open.

The one I have easy access to is open for sure. Connecting a very small test speaker I had laying around I get output. With a lever change when I run the control up and down. Not much but about what I'd expect from a small 2 or 3 in speaker.

So I'm off to fine a 12-in replacement woofer.

Thanks for getting me to think.!