Behringer 1222FX power supply UB-SPSU2 - giving me double voltages ?

[chazpope]

Hi

bid while not looking and ended up buying a broken 1222FX mixer on ebay Found that power supply is knackered, mixer works ok with another psu from another mixer (yes I broke into that just to try it).

What it does is output double voltages on all un-Zennered outputs - +12 is +19, +-15 is +-28 while +5 and +48 are ok.

This is teh schematic - it seems to be a common psu for many Behringer mixers

Looks like the transformer is just giving double AC - but I guess measuring with meter is dodgy because of HF. The most weird is at T1 - collector is +16V while emitter is +28V !

So I suspected D6 as it also seems to me to affect some feedback via +UR?.

Pulled D6 it out (tricky) looks OK. Put it back in - and now the whole thing it is kind of pulsating every second, cannot find what I did wrong - it seems to have been just on one side of the pcb and I was being very careful...

Any ideas how to fix this b****** ?

thanks

[soundguruman]

Hi

bid while not looking and ended up buying a broken 1222FX mixer on ebay Found that power supply is knackered, mixer works ok with another psu from another mixer (yes I broke into that just to try it).

What it does is output double voltages on all un-Zennered outputs - +12 is +19, +-15 is +-28 while +5 and +48 are ok.

[ATTACH]20262[/ATTACH] This is teh schematic - it seems to be a common psu for many Behringer mixers

Looks like the transformer is just giving double AC - but I guess measuring with meter is dodgy because of HF. The most weird is at T1 - collector is +16V while emitter is +28V !

So I suspected D6 as it also seems to me to affect some feedback via +UR?.

Pulled D6 it out (tricky) looks OK. Put it back in - and now the whole thing it is kind of pulsating every second, cannot find what I did wrong - it seems to have been just on one side of the pcb and I was being very careful...

Any ideas how to fix this b****** ?

thanks

framing hammer?

[Enzo]

A "pulsing" SMPS is probably just one that is trying to start over and over, and each time it does, it faces a short on a secondary and shuts down. Power up cycle repeats until mains power is removed.

There is only one transformer, so it is real unlikely it is spitting out double voltage on just two secondaries.

Since R and R D6 and you have the new symptom, my first suspect is that work. Remove D6 again, carefully. Now does teh SMPS still "pulse"? Or does it fire up but without that voltage rail? Any chance D6 is now shorted? Was it in backwards? Any chance a solder bridge was made?


As to all those voltages being off, a couple things. One is that you have more than one ground here, the digital and the analog, on the output side. All voltages must be measured against their own ground. So check to make sure you don;t have a burnt open spot on a ground trace. But in terms of circuit, UR is the feedback upon which the SMPS regulates itself. It feeds over to the opto and the TL431 regulator on the primary side - though still isolated from the primary. You have a good SMPS to compare to still? My spider sense is tingling that maybe your 431 is getting nothing at its gate and of course the system ramps up to try and elevate the missing voltage back into place, or that it is itself defective. Lower on my list is a bad opto.

You have a bunch of unrelated voltage supplies all running way high, that leads me to look at controllers rather than the supplies themselves. All of them are functioning, they all are reading high because they are not regulated individually, they are regulated by the overall system.

It is also possible the controller IC is freewheeling, maybe from lack of control by the opto, or maybe R16 C10 C26 have issues.

[nickb]

Hi Chazpope,

This looks like a flyback converter. The +12V rail goes to the voltage regulation circuit formed by IC3, IC4 and associated components. The other rails follow in proportion to this regulated rail. The +/- 15V outputs just have noise filters so they also follow the +12V. The 48V and +5 have their own regulators. The power supply may need a load on the +12 output to regulate properly. My guess is that the problem lies in the IC3/IC4 area. Take care working on the HV side as it is connected to mains power.

You need to fix the pulsing first.

[J M Fahey]

Disregarding soundguruman's useless-contribution-of-the-day, these "multi-primary-voltage" PSUs, roughly work by being able to supply *a lot* more on the secondaries and throttling it down as needed.
They usually monitor *just one* secondary, and *assume* all others are proportional .
In this case they monitor +Vr.
I *think* there's some monitoring flaw which allows for +16V (almost spot-on) on the "official" test point, but full voltage elsewhere.
Obviously, if the regulator *thinks* everything's fine, it will throttle down nothing or very little.
Just on a hunch, (wouldn't call it an absolute truth as SGM would), please check that C12 isn't open, or that one of the tracks leading to it isn't cracked or open ...... or if possible scope that point
If it were open, your meter would strill read *average* 16V, although the peak value (no longer DC there) would be much higher .... . and all other voltages would also be very high (as much as the winding will supply).
Good luck.

[chazpope]

Fixed !

Yes, J M Fahey, C12 was bust and this is exactly what was happening!

This now explains why I was getting such a big voltage difference across T1, a scope would have revealed it but I don't have one.

When I replaced it (I only had a 680/25 cap small enough to fit it there) it all started to work. So no idea why it kept pulsating when it did but it does not now. I had noted the 2 different earths and was measuring accordingly. I spent hours looking at it with a 10x loupe and did not find any shorts. You really can't see anything on the top layer - unless you have a 'tight orifice cam' also the board is painted dark blue and you can't see the tracks




so now it is all screwed in and working, thanks everybody who helped !

[J M Fahey]

Ok, glad to help.
As of the "pulsating" voltages, maybe the unchecked voltage did rise beyond some capacitor's rating, current consumption increased abnormally and some over current sensor on the regulator, on the primary side, tripped.
Of course, you had lost secondary side sensing (or it was quite compromised)
Good luck.

PS: glad you couldn't find your framing hammer first

[chazpope]

Arrrrghhhh ! It is pulsing again! After working OK for sevral hours it is broken again. This time no double voltage just pulsing normal voltage - starts pulsating at 1hz about 10 seconds after normal start.

Spent 1 hour trying changing caps on the secondary side - no joy. Found more busted ones (I only have ohmmeter but it shows and some were bulging). This is actually quite hard as original caps are much smaller than normal. Pulsing seems to become worse as I change caps - will go to shop tomorrow to buy some new ones hope they are small. Outlook is not very good at the moment I must say ...

[Enzo]

Try these things.

First, disconnect the loads from the SMPS, does it still pulse at you? This wilol tell us if the troublpe is in teh SMPS itself or something wrong with the mixer.

If the SMPS has the issue, first thing I suspect is rectifier diodes on the secondary side, rather than caps. Look for shorted ones.

[S2udio]

first thing I suspect is rectifier diodes on the secondary side, rather than caps.

Well yes as long as you....?...replaced all those tired looking cooked electos
As stated it has worked, ?
Just replace the caps first...all except the bulk DC ,unless 30 yrs old
Its intermittent, and frequency/duty cycle/temp related.......hope it blows a fuse.
You wont find a "short"...as either its a leaky diode or depleted fet,bad cap,usally the associated with the pwm ref circuit ....just replace ..... feck we are only talkin cheap bits.
Sometimes no howmuchever searchin and measuring .....just replace or chase the tail for weeks........caps first, chaps.
The answer is usually...I Cant belive it was just that.

[J M Fahey]

Must agree.
If I find a 30 y.o. cap in a Behringer SMPS, that's also the first part I replace

[nickb]

...and don't overlook C13/D7 while doing that.. The switcher chip will go into auto restart mode if no current flows into C10.

[Enzo]

I have to admit, if I found 30 year old caps in a Behringer anything, I;d be changing them. In particular since this SMPS came out on 2006. Maybe they age fast, like "dog years."

[S2udio]

I was just remarking in genaral about the main cap,not just about behringer gear,but yes they do tend to use
"dog brand" caps !!

[J M Fahey]

Bean counters rule the world