Mackie 1640i PSU issue

[mikeyB]

Hi everyone, got an issue with my Mackie 1640i. The NTC R1 in the PSU has quite a hole blown in the side of it. Flash and cracking sound at last turn on, mixer continued to work!
It is a SCK103, none I can find here in UK, ebay from Germany, or long lead time from China.
Looking at alternatives, but not familar with NTCs so asking a little advice.
Attached are psu schematic, data sheets from SCK, then alternatives I think that may be suitable from RS and Mouser, hopefully RS is suitable as I have a counter just down the road, (can buy 1 off and no minimum order) and I want to get the mixer operational asap.
So RS sheet has limited info, but from the photo I can determine that Mouser carry the same brand and they have a much more comprehensive data sheet!
Here's the comparison:

Parameter SCK103 Mouser/RS
Cold res 10R 10R
Current 3A 3A
Resid res 0.297R 0.273R
Therm dis cst 17mW/C 12mW/C
Max Power 2.4W 2.5W
Temp range both -40 to 170C
Recmd capac 330uF 220uF (Mackie has actually got 220uF!
Therm time cst 43s 56s

Should I be concerned about the thermal dissipation of 12mW/C and the themal time constant of 43s vs 56s?
All the other parameters look closely matched.
As always, thanks in advance.

 

[Khron]

Usually, when a component's blown in such a manner that it has a hole in it, it's good to determine the exact (or at least most probable) cause for that. Otherwise, if you replace that (same goes for fuses btw) and it blows again, what good will that have done?

Worth checking if the R2 varistor / TVS i shorted out or not. Failing that (no pun intended), is this a switchmode power supply? I'm betting it is, going by R111, D100 and C9, most likely coming off the auxiliary primary winding of the flyback transformer this likely employs.

 

[mikeyB]

Usually, when a component's blown in such a manner that it has a hole in it, it's good to determine the exact (or at least most probable) cause for that. Otherwise, if you replace that (same goes for fuses btw) and it blows again, what good will that have done?

Worth checking if the R2 varistor / TVS i shorted out or not. Failing that (no pun intended), is this a switchmode power supply? I'm betting it is, going by R111, D100 and C9, most likely coming off the auxiliary primary winding of the flyback transformer this likely employs.

Thanks for the quick reply Khron. It is indeed a switchmode supply, i'll attach the full psu schem. When I get to the bench, i'll photo the damage and post and do a bit of measuring/testing as per your suggestion.
Many thanks Khron.

 

[mikeyB]

Had a little time to do some testing. Just a quick reminder, the unit was functioning as normal with the exception of the flash and crack on power up! All power rails seem ok and passing audio ok.
All diodes/bridge measured ok, most caps are in spec with a couple just outside tolerance, no signs of swelling bulging on any caps.
R2 NTC is not shorted and R1 (with the hole! measures 18R, not 10R). Hopefully this NTC is the only problem and hopefully caught early before damage to anything else.
I will do more testing/measuring tomorrow, may lift a few smaller caps to measure out of circuit for peace of mind, will probably replace elecs for longevity.
Any tips/advice on areas to check from the masters of the forum? - before I put an order together for spares.

Thanks in advance.

ps pic of R1 and hole!

 

[cyrano]

The flash and crack on power up means "up for a recap" to me...

But I'll admit that more often it's "use till fails".

At the very least, check if there's an X-rated cap on the power switch. These tend to fail a lot and they're sometimes nasty smelling.

 

[mikeyB]

Cheers Cyrano,
I will probably go for a refit of all "wear" parts, Capacitors, and any other suspect devices! The schematic states the load currents so I should be able to test and soak on dummy load to avoid risk of damage to the mixer circuitry.
Thanks again!

 

[Bo Deadly]

Don't get emotionally invested in this. SMPS can be very tricky to diagnose.

Just to give you a completely contrived example, imagine that there's a bad diode causing current to double back into the transformer shorting it out momentarily and stressing things enough that it might seem like you fixed it but after 10 power cycles it does the "flash and crack" thing again.

These mixers are not designed for longevity. The parts are spec'd for a certain lifespan so that you "just buy a new one".

When power silicon parts get too hot, they get burned up inside and start to fail in strange ways (they don't turn on or off completely or do so slowly).

The ideal solution would be to find a replacement SMPS but of course that is not likely in this particular case.

Alternatively, replace all silicon parts like rectifiers, diodes, scrs, triacs, transistors, mosfets, etc which carry any kind of vaguely high power and all electrolytics. This approach is known as "shotgunning" where you just replace everything and not try to diagnose anything at all.

But then of course you could find that there is a problem with the downstream mixer circuits like all of the electrolytics are dried out and leaking like a pasta strainer. You might want to jerry-rig a temporary supply and measure the current consumption of each rail and then somehow try to deduce if it's normal.

 

[mikeyB]

Cheers Bo, yeah I got enough stuff lying around to "bench" it. May even consider going the external linear route. I have a few hefty rack mount supplies I could try out. I do like the compactness and versatility of the mixer, so want to get it back in service soon as! A chunky external supply capable of supplying 3 or 4 mixers mounted outside of the control room may be on the cards, I'm toying with the idea of linking them for solo, monitor duties and option to combine (or not) subs and aux sends. Maybe a few other additions like aux return solos and mutes. I'm just studying the service manuals before I post my ideas, hopefully sometime this week. I'll keep you posted.

 

[mikeyB]

Bollox!! My colleague offered the loan of his 1640i whilst I repair mine, he said he'd had no need to use it for a while.
Powered it up and ***POOOOFFFF!!!!*** whisps of smoke emitted!!
Well Bo Deadly, I think it may be external linear supply x 2 time, easy to build, easy to maintain!(Hopefully).
Checked mains supply, no surges or anything, all other studio/workshop equipment working fine!

 

[JohnRoberts]

May be a common weak link... If it still works perhaps the inrush current limiter failed short circuit... I would keep trying to source a replacement(s).

JR

 

[mikeyB]

Cheers John, yes, I have some on order. I'm just in the process of tidying my workshop/bench, hopefully at that point I'll be in a good enough mood to diagnose the 2nd mixer supply fault! I have a few linear supplies that I can cobble together so that I can still test the 2 units. Got a good dummy load stack so as to the test/soak the mackie smps supplies as well, it's just finding the time! Still trying to find time for the mixer combining project I want to realise. (2 or 3 1640i and a 1604VLZ - common solo bus and option to sum/split sub and aux busses).

 

[cyrano]

Mmmm...

The only Mackie I've ever had to repair, an older 56 channel one, also had a blown PSU. No SMPS, a heavy linear one. Unfortunately, it burned a two inch hole in the board. In the end, the owner sourced the last new one from a Mackie importer somewhere in Europe. Afaik, the board is still running today.