Mackie SWA1501

[crombie]

Enzo your the man

I am sure all do appreciate all that you do for the comunity at large,

[HadYourPhil]

The second amp is now repaired, and it was the preamp in this case. T-14 was broken off at the emitter lead. That may well have been all that was wrong, but as I inadvertantly plugged the ribbon cable connector in backwards, I ended up with more problems. I have an excuse: The connector was mounted backwards on the cable.
At any rate, I also had to replace U-1, T-5 and R-84, all most likely damaged by my reverse plug-in. Also, the 1A fuse (F-2) was blown on both amps.

So, my advice on these things is to check T-14 first! That transistor being loose caused (IMHO) every other problem I had with each amplifier.

Thank you again Enzo for being the catalyst in this procedure.

[Enzo]

Catalyst. Is that the same thing as a cowboy? Catalyst?




Thank you, thank you, I'll be here all week.

[HadYourPhil]

No, but the word "bull" comes to mind here... (LOL)

[Enzo]

I wouldn;t steer you wrong, but your name was a pun before my comment, don;t you heifer say otherwise. I'm telling mom, you started it.

[HadYourPhil]

To beef or not to beef...
(goes to make a pot o' coffee)

[crombie]

Resistors still getting hot with preamp unpluged

I unpluged the preamp but still get a hot resistor situation, r17 and r 64 still start to smoke. could q5 and q18 be the culprits??or is it further in?

[HadYourPhil]

I'd check all the semiconductors in that area, specifically D40,41,13,14. Also, D9 can be a problem. I wouldn't suspect Q5 or Q18 to be the problem here.
Save yourself a lot of grief and check all the semiconductors on that board. I assume you know how to properly check semiconductors...

[J M Fahey]

Hi friends, why don't you post the schematic, so we can all share the fun?
After some time, all this unreferenced C13 Q21 IC2 talk becomes a little boring
Happy new year.

[crombie]

burnt resistors

Diodes 13 14 40 41 check out fine in the circuit, looks like a forward voltage of around .520v I did check around with the fluke but I am not sure what reading I should be getting with all the componets in the circuit.
Do you recomend takin each component off the board to check?

[Enzo]

JM, just for you...

[J M Fahey]

Enzo, thanks a lot.
You're too kind and helpful, as usual.
Happy 2010.
JMFahey.

[HadYourPhil]

Thanks, Enzo... I was scanning my copy and saw you had beat me to it.

[J M Fahey]

Well, now that I have 5 minutes free to sip a delicious green tea cup, I have this beast on my screen.
To begin with, I find the output transistors quite wimpy for what is expected them to do.
+/- 70V (or 140V total as was previously stated) are no-kidding voltages.
The 2SC5200 shows that they crumble (their SOA, that is) above 50V, where all their curves bend downwards sharply.
The short-circuit protection provided is very primitive Zener diode current limiting (to 10A Pk), which would mean milliseconds of survival.
I know, I know, they are powered subwoofers and thus isolated from the cruel "outside" world, but they have the worst load to drive: *sub*-woofers, with their complex impedance, lots of kickback (EMF) voltage , heavy cones attached to long coils immersed into powerful magnetic fields, a bipolar transistor nightmare. (MosFets handle that much better).
Besides, the dissipation capabilities are marginal, I would have used 4 TO218 class devices there, not two.
Maybe somebody can suggest beefier transistors there.
What the designer did not spend on protection or dissipation, was wasted on what I think is a floating (maybe "intelligent"?) center tap for the power supply , courtesy of those IRF640s and all the complex schematic to the right of the output transistors.
The schematic to the left is a relatively conventional power amp.
R64 and R17 smoking probably mean base-collector shorts in Q5 and Q18, (*if* the output transistors are alive); otherwise their base-emitter diodes would not allow more tan 1 V (very worst case) across them, *or* R63 and R16 might be open.
OK, I'll go to work a little.

[Steve Conner]

I think there's more going on than meets the eye here. The power supply has a switchmode chopper of some sort in it, and there are two sets of high-current rails.

To me it looks like some horrible combination of Class-D and Class-H. The power amp normally runs off +/-25v, and the chopper kicks in on signal peaks to pump up the outer pair of rails (connections "1" and "2" on right hand side of page 1) to the required voltage.

This explains why they can get the rated power from one pair of transistors: dissipation is reduced because they're running in Class-H, G or whatever.

I don't see any Class-D modulator in the circuit, though, so maybe it's just what Douglas Self called "syllabic Class-G". The outer rails aren't adjusted up and down, they're simply turned on when they're needed.

This also explains the "intelligent centre tap", it means they only need one chopper in the power supply instead of two, so it saves money.

You might think this would generate lots of HF distortion, and it probably does, but why would that matter when it's driving a subwoofer in a bandpass cabinet.

Mackie don't generally make bad stuff.