Help needed understanding this circuit

[Bigdrums]

This is the power supply section of a Mackie HD1501 sub woofer where the line voltage enters, shortly thereafter is rectified, then fed to "T1" the high voltage supply transformer. Yet, as we all know transformers don't work for DC. So whats going on here? The problem is I have no AC or DC voltage at TP27, TP28, TP29, or TP30.

[The Dude]

It's a switch mode power supply. If you have no experience with this type of supply, I wouldn't attempt repairing it. At the very least, do some reading about SMPS first so you understand the concept. Beware, primary ground is not chassis ground, so be careful when making measurements. It's quite easy to create more damage and smoke if you ground the wrong circuit points.

[Enzo]

That is why they are called switchmode supplies. That DC high voltage is switched off and on through the transformer primary at high freqquency.

I agree with Dude, if you don't know what you are doing, SMPS can be VERY dangerous to service. Yes, even lethal. The mains are rectified and filtered directly, so you have 300vDC with all the current in the world behind it. Use an isolation transformer.

Look in your schematic. On the primary side see the triangular ground symbols? They have a little sine wave inside them? That indiccates the "ground" there is not ground at all, it is the high voltage common return. It sits at about -170vDC. You clip a scope ground to it and BLAM...

[Jazz P Bass]

In looking at this schematic I am confused on where VCC comes from.
Any takers??

[Enzo]

These are complicated. Your dead test points are the outputs to the circuits of the amp. But your main switcher isn't switching. There are secondary smaller supplies that start up and then allow the main switcher to function.

[Enzo]

VCC comes from page 2 section C. VAC just above it comes from the mains bridge through the three 150k resistors. They kick start the controller IC U7. U7 drives switcher Q11 who in turn switches T4. B winding of T4 then creates the VCC supply. Also from T4B, D20 makes a supply for U7. That supply takes over from the kick starters.

[Mick Bailey]

Without an isolation transformer the safest way to work on this is to do is static component tests with the power disconnected. Bear in mind that C6 & C7 may be holding a charge, so discharge them first. Check for continuity from L&N through to BR1 and do a diode check on BR1. Q1 and Q2 are the main switching transistors so check them for shorts, though if they do short the fuse usually blows and sometimes there is other damage. I've had two these power supplies dead because D9 & D10 have failed, so its worth checking those as well as D11 to D14.

Apart from this you're down to the legwork of working through the schematic looking for failed components - especially open resistors or shorted semiconductors. The issue is that the circuit is a closed loop and with many fault conditions it either works 100% or doesn't - no inbetween. Printing out the schematic and using a coloured marker to identify the functional blocks is useful.

[The Dude]

Another thing I will often do when troubleshooting SMPS is scope a couple of the output supplies and see if they try to start. If you see the supply try to start and then shut down (sometimes repeatedly), there is usually a short on the secondary and the supply is shutting down to protect itself. If the supply doesn't even try to start, the problem is more likely in the primary circuit. This helps narrow down the search a bit.

[Mick Bailey]

Also, the power LED will often flash slowly if the supply continually restarts.

[Marshtech]

I'm an electronics tech and recognized this as a switch mode PS. I've only seen one - in a bass amp. I admit I can't explain how it works any better than the posts above.
I have to ask this - did you check the 10 Amp fuse? The original question stated no AC or DC power (as well as 'what's going on here?' for circuit operation).

[Bigdrums]

I have now resolved all the PS issues I think and can power up without worry. But the limit light stays lit, even without a signal, and output is quite low. What might be the cause?

[glebert]

I have now resolved all the PS issues I think and can power up without worry. But the limit light stays lit, even without a signal, and output is quite low. What might be the cause?

Usually there is something that causes a power supply to fail, and that may have damaged parts of the amplification circuit. Maybe one failure caused the other. Time for the next round of troubleshooting. Have you checked the speaker with a different amp to make sure it is working right? Also, make sure there is no DC going to the speaker.

[Bigdrums]

Yup, the speaker coil was shorted to 2 ohms and needed replacement. Noticed that as I put a spare home subwoofer amp to it and it kept shutting the amp down prior to my last post. There is no DC from the speaker outputs, I'm just not familiar with the limit circuit.

[Bigdrums]

Update: Don't know what happened but when turned on all is normal except the "Limit" light comes on and the output is limited. News though is that after 5 seconds or so the protect relay kicks out and the fans stop. Then 5 seconds later they kick back on again. FYI If I unplug cable "to power amp" the on and off cycling ceases and just stays on.

Looking at pg 29 of the attached service manual, and checking the ribbon cable solder joints marked "to power supply", B & 8 cross on schematic, I measure 15v at pin j2-8 marked as "limit" but further up marked "thermal" When the relay kicks out the 15v drops to .1v.

Then at 1 & C schematic, ribbon header marked "to power amp board" there are no measurable voltages except pin marked +75V-sense measures 1.6v fluctuating.
I've attached also a pic of the affected area of pg 29 with voltages written down.

If anyone cares to comment on this dilemma I would be grateful

As far as the squashed output and limit light being on, I think this is because U110 pins 1 and pin 7 are sitting at -13.52v. These are the outputs so I don't think they should be at -13.52v.
But I can't figure out where it's coming from. U110 is not shorted between pin 4 (negative supply volts) and pin 1 or pin 7. and I believe the diodes D104 & D105 should prevent any voltage from Q102 and its sitting at +14.88v anyway. But there is no other circuitry attached to pins 1 & 7 of U110. So where is it coming from? Anyone? mackie_hd1501_sm.pdf

[Enzo]

On the SMPS, we found VCC, which came up once the full output is enabled..Page 2 of SMPS shows VCC in lower right quadrant turning on optoisolator U5, whose output grounds off LIMIT, LIMIT then leaves the SMPS via that connector. Over on the page 29 board, LIMIT enters the board and is renamed THERMAL.

+75v sense sounds to me like it is looking for +75v, when that line gets to the power amp on pin 4, sure enough it connects to the +75v rail through a resistor.

U110 has a reference voltage on pin 2, the audio out from the amp is fed to pin 3. Whenever pin 3 goes more positive than 2, it yanks pin 1 positive.. Essentially it rectifies the signal and sends peaks through that diode at pin 1 and from there up to Q100. Note that little 1uf cap there that has to charge up, so it doesn't trigger the thing for every tiny peak. U110 is doing its job.