Mackie FR M1400i blowing main fuse

[stoneattic]

After you guys helped me out with my Peavey CS800S I thought I could tackle anything (just kidding), but I'm trying to solve a Mackie FR M1400i that blows the main fuse as soon as it's powered on. Unlike the CS800S, which is modular and I was able to narrow it down to a single board before really digging in, the Mackie has almost everything on a single board, except the controls and outputs. All of power and both channels of power amp are on a single board. I've attached the service manual with schematics.

Here's what I've looked at so far:

1. Powered on with current limiting bulb - glows super bright.
2. Unplugged the power transformer - powered on with current limiting bulb - no light. I assume this mean at least the primary x-former windings are not shorted?
3. Measured voltage on the secondary side of x-former - 67 VAC on yellow, 60 VAC on red. I assume this means that the x-former is good?
4. Removed the four 10A fuses that I believe will isolate the power supply from the power amp (at least the power transistors?) - and using the light power it back on - still really bright.

I was hoping this was narrowing it down to the power supply so I tried to test what I could reach. D19 through 22 tested good. I tried to test the rectifiers D7 & D8 but get really confusing measurements. I saw the schematic and figured I could test them like 2 diodes tied together but results don't seem to indicate that. I've attached the datasheet for these.

I can't test any of the caps in the power supply without pulling the board. I tested the 4 power transistors I could reach (the rest are obscured by a heat sink which require removing the board to remove) and they appear fine. At this point I think I have to pull the board and dig deeper, but was hoping someone might be able to tell me how to test the rectifiers and if my logic regarding pulling the 10A fuses points to something in the power supply.

Any suggestions would be greatly appreciated.

[The Dude]

I used to be a Mackie service center. I don't recall ever seeing shorted outputs in those amps- at least very rarely. The problems I saw were ribbon cables- not the issue in your case. The other was shorted diodes- usually the 1N5404's off the rails. There was also a Mackie amp (maybe this one?- I don't recall) that had triacs on the output connector boards like the crowbar protection in Peavey's. Those triacs would sometimes short, but rarely. You can tell if the triacs are shorted simply by measuring across the output binding posts. I'd start by checking those larger 3A diodes.

[g1]

The fuses don't narrow it down a lot. Just eliminate issues with the +/-80V rails at the power amp modules.
All the other places the +/-80 and +/-90V go are not fused.

[stoneattic]

I used to be a Mackie service center. I don't recall ever seeing shorted outputs in those amps- at least very rarely. The problems I saw were ribbon cables- not the issue in your case. The other was shorted diodes- usually the 1N5404's off the rails. There was also a Mackie amp (maybe this one?- I don't recall) that had triacs on the output connector boards like the crowbar protection in Peavey's. Those triacs would sometimes short, but rarely. You can tell if the triacs are shorted simply by measuring across the output binding posts. I'd start by checking those larger 3A diodes.

Thank you for your response. I tested the 1N5404 diodes on the 80V rails (D1, D6, D9 & D12) and they all tested fine, ~450 in one direction, open in the other using the diode test mode on my DMM. I checked the outputs for shorts and they both read ~15k, which I assume it good (at least they are not a shorted).

The fuses don't narrow it down a lot. Just eliminate issues with the +/-80V rails at the power amp modules.
All the other places the +/-80 and +/-90V go are not fused.

Thank you for your response. I thought that by pulling the 10A fuses it would eliminate at least the power transistors from being the short. Am I wrong there?

I'm not sure what to do next besides pulling the main board and just testing everything. I was hoping to narrow it down first. Any thoughts on how to test the rectifiers?

Thanks again everyone.

[The Dude]

......Any thoughts on how to test the rectifiers?......

Are you talking about the dual diode rectifiers? Not sure what you mean. If that's your question, you check them like any other rectifier. It's just 2 of them in one package, so test both of them.

[J M Fahey]

I checked the outputs for shorts and they both read ~15k, which I assume it good (at least they are not a shorted).

"15 k????"
You donīt use a resistance scale to check transistors, but the diode one (guess why )
IF you find a short, meaning a low voltage drop, say,less than 500mV (which is what the diode scale checks) , then you pull it out to separate it from other possible shorts and remeasure.

Thank you for your response. I thought that by pulling the 10A fuses it would eliminate at least the power transistors from being the short. Am I wrong there?

Not wrong but maybe incomplete.

I'm not sure what to do next besides pulling the main board and just testing everything.

Maddening way to repair things, can become an endless trip and you are never certain of success.
And in the process you ruin the PCB

I was hoping to narrow it down first.

Sure, thatīs the way.

In my view, best is functional testing, meaning you turn amp ON , and try to make it work, checking voltages and searching for wrong ones.

But ... but ...fuses blow!!!

Ok, first check gross offenders , such as power transistors, rectifiers, main caps, the transformer on its own connected only to mains.

IF shorted or visibly dead (cracked/blown/molten/etc.) replace as needed, reassemble and turn amp ON again, maybe itīs already repaired (or near it).

Only you play it safe, so slowly ramp up a Variac while checking current or plug amp into a bulb limiter.

For a large amp such as that, a 60 to 100 W bulb is fine.
Not larger even if amp is 1400W because you first want to make it work idle, no load or signal, and that is relatively low power.

Measure voltages again and if, say, rails are collapsed even with guaranteed good transistors, voltages will tell you whether drivers are dead, they are overbiased, etc.

Only when all DC voltages are fine and stable you start dynamic tests, connecting a speaker and feeding it some signal.

[stoneattic]

Are you talking about the dual diode rectifiers? Not sure what you mean. If that's your question, you check them like any other rectifier. It's just 2 of them in one package, so test both of them.

That's what I thought, but when I test it that way I get "weird" results.

According to the data sheet I included in the first post the center leg is a common cathode or anode, depending on which suffix the rectifier has. So I expect that at the DMM's diode test setting I would read ~.6V between the center leg and an outer leg and after flipping leads, infinity. But I'm getting ~.35 on one, 0 on another, ~.6 on one and 0 on another, for expample.

I know that sounds like something is shorted, but I have another one of these amps that works fine and I get similar results testing the rectifiers on that one. All tests were done in circuit so I'm guessing that's the reason for the confusing results? When testing diodes in the past I have always gotten good results in circuit, but maybe that was just dumb luck.

[stoneattic]

"15 k????"
You donīt use a resistance scale to check transistors, but the diode one (guess why )
IF you find a short, meaning a low voltage drop, say,less than 500mV (which is what the diode scale checks) , then you pull it out to separate it from other possible shorts and remeasure.

Maybe I misunderstood The Dude's post. I measured the + and - binding posts of each channel for a short.

Ok, first check gross offenders , such as power transistors, rectifiers, main caps, the transformer on its own connected only to mains.

I THOUGHT by removing the 10A fuses I eliminated the power transistors, but since you mentioned that is an incomplete way, then maybe I haven't eliminated them.

I'm struggling checking the rectifiers (see previous post).

I can't access the main caps for testing without a complete disassembly, which is not easy and once I disassemble it I have to reassemble to power up and test. So I was hoping to narrow down the problem before tearing it apart.

I did test the transformer and that appears fine.

IF shorted or visibly dead (cracked/blown/molten/etc.) replace as needed, reassemble and turn amp ON again, maybe itīs already repaired (or near it).

Nothing appears cooked.

Only you play it safe, so slowly ramp up a Variac while checking current or plug amp into a bulb limiter.

For a large amp such as that, a 60 to 100 W bulb is fine.
Not larger even if amp is 1400W because you first want to make it work idle, no load or signal, and that is relatively low power.

I have been using a 100W bulb. I do have a Variac, but I feel more comfortable with a bulb limiter.

Measure voltages again and if, say, rails are collapsed even with guaranteed good transistors, voltages will tell you whether drivers are dead, they are overbiased, etc.

Only when all DC voltages are fine and stable you start dynamic tests, connecting a speaker and feeding it some signal.

This is probably the path I should be taking. I do need to spend more time measuring voltages, and since I do have a functioning one of these amps I even have something to compare against. I tend to think more along the lines of testing components and not testing the circuit.

[J M Fahey]

Quote Originally Posted by J M Fahey View Post
"15 k????"
You donīt use a resistance scale to check transistors, but the diode one (guess why )
IF you find a short, meaning a low voltage drop, say,less than 500mV (which is what the diode scale checks) , then you pull it out to separate it from other possible shorts and remeasure.

Maybe I misunderstood The Dude's post. I measured the + and - binding posts of each channel for a short.

But you used the wrong (resistance) scale.
Which is shown by your stating you measured 15K

So I expect that at the DMM's diode test setting I would read ~.6V between the center leg and an outer leg and after flipping leads, infinity. But I'm getting ~.35 on one, 0 on another, ~.6 on one and 0 on another, for example.

then you pull it out to separate it from other possible shorts and remeasure.

I do need to spend more time measuring voltages,

I tend to think more along the lines of testing components and not testing the circuit.

Let me give you an analogy:
"testing components" is the endless and uncertain job of fully disassembling a car engine, measuring every part against blueprints, trying to find which is out of tolerance.
You will become crazy replacing slightly out of tolerance parts which are not the real problem, maybe slipping past some which are, in the end reassemble and still have a non functioning engine.
The only guaranteed approach is to replace *everything* ... which is more expensive than straight buying a new engine.

Now on a "functional" test, you try to start the engine.
Then youīll quickly find the battery is flat or no fuel reaches it or ignition fails or it binds/stalls (no oil) or one cylinder does not fire or it vibrates too much or it shoots oil through a burnt seal or .. or .. or ..
In a nutshell, "the defect shows itself" and you can FOCUS on possible causes instead of UNFOCUSING in 10000 different ways.

Here you try to try to start the amp and see what happens.

Say: instead of +/-70V rails you get +/-45V (remember, bulb limiter) but you find +44V DC at the output instead of 0V ; or + rail is +40V but -V is -0.7V = massive short on that side , and so on.

[stoneattic]

Sorry I haven't responded in a while to this thread. I had a lot going on and a desperately need vacation that put this on the back burner. I'm back and still trying to figure this out.

But you used the wrong (resistance) scale.
Which is shown by your stating you measured 15K

I'm not sure what scale I should be using. I was checking between the binding posts for a short as suggested by The Dude. I used an auto-ranging DMM and a non-autoranging DMM and got the same result. Which is ~15K ohms initially and then starts climbing. I have two of these amps so I can test them side by side and both act the same measuring resistance across the binding posts.

then you pull it out to separate it from other possible shorts and remeasure.

Just for clarification, you are saying that I need to remove the rectifiers to test?

I'm really hoping to not pull the board until I have some idea what's wwrong because there is only one board with EVERY component on it, which requires a complete disassembly to get out and then a complete reassembly for further testing. I would like to do as much troubleshooting as possible before disassembly. I realize that is going to limit what I can test, but I want eliminate as much as possible first.

I still suspect something in the power supply so I started measuring voltages there. With the four 10A fuses still removed, and the 100w current limiting light, I measured voltage on the hot side of fuse clips of ~1.5VDC on the "good" amp and the "bad" amp ~25mVDC . This is consistent on all four fuses on both amps.

This leads me to believe it is either one of the caps (C3, C4, C10, C13, C14, C17) on the hot side of the 10A fuses or one of the rectifiers. I already tested D19 through D22. Does this make sense?

Any other thoughts?

As always, thank you for any help!

[stoneattic]

So I've spent some more time with this amp and found out that apparently by DMM's diode setting is not working. I used a different one and now I appear to be measuring a short on half of D7 (center pin to one of the outer pins), which is one of the rectifiers. The other half (center pin to the other outer pin) reads ~400, and infinity with the leads reversed. The other diode, D8, measures ~400 and infinity on both legs, as expected. I measured these two rectifiers on the spare, functioning amp (same model) and both D7 and D8 read like D8 on the bad amp, with D7 having the opposite polarity of course.

So now that I believe I have at least part of it figured out, is there likely anything else that went with this? Also, I'm not having any luck finding a replacement. The SF304 rectifier appears to be obsoleted and I haven't found a substitution.

Maybe this is a substitute:

FFA60UP20DNTU

https://www.mouser.com/ProductDetail...2Bnw0GaA%3D%3D