Neumann KH120 studio monitor power supply repair

[beatnik]

Hi everybody

I am trying to rescue a dead Neumann KH120 studio monitor that likely suffered from a power surge.

The SMPS has blown up and it is proving very difficult to repair it.

My first findings were disintegrated mains fuse and shorted switching mosfet. also one of the diode rectifiers on the transformer secondary was short circuit.

I have replaced the mosfet and the diodes, and also the pwm controller ic. I have checked all diodes, resistors and capacitors and also the opto transistor in the feedback path. Everything seems to be fine

When turned on the power supply fails to start, the voltage is trying to come up cyclically and there is a clicking noise at every interval.

My main worry is a shorted turn in the transformer, that would be impossible to replace.

Schematics aren't available for this unit as it's still in production.

Before I give up on this I was wondering if someone with more experience on SMPS repair can chime in and give some advice on possible things that can be checked.

Any help would be immensely appreciated.

 

[scott2000]

Is there a model number or any markings on the supply???

I'm just asking because some of them are pretty inexpensive?? The one in an Apogee Rosetta is like $10 for example iirc...

 

[beatnik]

That would have been great, but unfortunately the power supply is embedded on the same pcb as the amplifiers.

Seems to be a nicely designed board but unfortunately not very repair friendly. There isn't even a components overlay, not that would have been much useful anyway, since the manufacturer doesn't provide diagrams.

I have had a few smps for repair recently and didn't have much experience before. I have been reading a lot and watching youtube videos, there is a lot of information out there.

Unfortunately there is still something I am missing. It seems that the primary side of the transformer is sorted out, possibly there is something on the secondary that is clamping down the voltage as the power supply is trying to come up.

There are test points labeled +15V / -15V, could be worth trying to feed these through an external power supply and watch for current draw ? Next to these pads there are two more labeled +UB and -UB, I cannot guess what these are

 

[beatnik]

I have tried attaching a picture of the board but I get an error message saying my file cannot pass security checks. Weird. Yeah we know smps power supplies have dangerous voltages !

 

[scott2000]

Just a quick look I found this discussion about UB+....??

https://www.proz.com/kudoz/german-to-english/electronics-elect-eng/1257001-ub.html#2994859

Maybe someone can jump in who knows???

That stinks....... If it's your monitor I suppose creative thinking outside the box could be in order or a possibility if you can't get it sorted....

 

[beatnik]

Thank you. That likely means there are two separate +/- voltage rails

There is a 7815/7915 pair to regulate the +/- 15V rails. Maybe the +/- UB is the non regulated voltage before the regulators

The next thing I have to try is taking these regulators out and test them, it could be they have failed since there was a shorted diode on the secondary of the transformer

 

[pvision]

Did you get anywhere with this? One of my KH120As just failed with a blown fuse

The Mosfet is shorted but I haven't found any other bad components yet. I have a couple of mosfets to replace the bad one & plan to do this tomorrow

Nick Froome

 

[beatnik]

Very ashamed to say, but I finally gave up and sent it to Sennheiser UK to repair !

In my case the mosfet was shorted, and also one of the diodes on the transformer secondary. I also replaced the pwm controller ic since I had no means of testing it, but knowing from previous smps repair that being a common point of failure.

Check also the 7815/7915 regulators on the output.

I think mine had a problem in the feedback that sends the voltage back to the controller, or possibly a short in the DC-DC transformer

I managed to get the power supply starting without blowing the fuse, but the voltage would not come up and it just kept cycling on and off.

I really hope you can be more successful than me. It would be nice if you post any updates here.

 

[pvision]

I'm going to throw a Mosfet in today and test. Will report back

What did Sennheiser charge for the fix?

Nick Froome

 

[beatnik]

They have a 240 gbp flat rate for power problems. They replace the entire board and calibrate

 

[pvision]

There was a single KH120 on eBay recently for £325. That might be a better option as I'd end up with spare drivers as well

Nick Froome

 

[5v333]

I would check for fuse resistors that has fused.

 

[pvision]

Mosfet went in easily and blew the fuse even easier. Mosfet is now shorted. I think it's above my pay grade now

Nick Froome

 

[beatnik]

Usually the pwm driver ic blows very easily and if that is dead, putting in a new mosfet it's useless because it would short.

Man how much I hate smps. They are a bitch to repair

In this case not having schematics and labeling on the pcb makes things even harder.

But they are very nice little monitors

 

[pvision]

Which is the PWM driver? I have one more Mosfet left. Should have ordered more...

Nick Froome

 

[beatnik]

I had another one of these with the same symptoms, blowing the mains fuse.

This time I was successful in repairing it and actually it might have been the same story as the order of faults was identical as the previous failed repair attempt.

- blown mains fuse
- shorted switching mosfet
- dead switching controller ic (not confirmed but very likely)
- shorted diode on transformer secondary (on the +/- UB unregulated rails that power the amplifier ics)
- dead power amplifier ic tda7239 (the one for the tweeter was bad but I ended up replacing both)

I think last time I fixed the psu but one of the amplifier ics was pulling down the voltage and preventing the smps to start properly.

Hope this might help someone in the same situation.
If you need advice feel free to ask on this thread I can also point to reliable sources for some of the spare parts that are now obsolete and unavailable from mouser etc.

 

[Khron]

I kinda wish people sold their dead ones... But then again, i AM quite the scavenger... x)

 

[Tubetec]

Its an all too common fault with these units and many other powered monitors , they last a few years and thats it , one day they go up in a puff of acrid smoke . I was considering the KH120 until I saw the reports come back . For all the work you put into effecting a repair and the chance that in a couple of years even with a factory repair/new board the same thing will happen again , its not worth it . Maybe converting the boxes to passive might be one possible option , a external active crossover and bi-amplification would be another possible route , probably going to be a tough job to match the crossovers as good as the originals , Maybe a Frankestein job where you resurect the original Sennheiser active crossovers and run HF and LF line outs to proper power amps . I might keep an eye out for damaged units on the cheap for experimentation , At least the metal cabinet/baffle/waveguides and drivers would be a good base for a project .

Ive already got 20 years out of my Behringer truth monitors and they still do what there supposed to do , everyone who bought the Alesis M series has long since binned them , I was just as sceptical of SMPS back then as I am now and I know there are people here who swear by them but in close proximity to a power amp in a speaker these things fail with alarming consistancy . Ill bet theres many an old Klein & Hummel techs crying in their beer now over the mess the new parent company has made on the once glorious marque , it simply would not have been allowed to happen in the old days .

 

[Tubetec]

AHAHA Khron , my sentiments exactly !

 

[Khron]

AHAHA Khron , my sentiments exactly !

Well, that's how i got my pair of Tannoy Precision 6D's for ~200e, over 5 years ago. No SMPS issues there, only "overly optimistic" thermal management (or lack thereof), and some undersized power resistors. 5e worth of parts later, and they've been going strong ever since.

Downside is the config switches on the back get dicky, as do the tweeter level trimpots... One of these days i'll finally get around to the DSP + class-D upgrade i've been planning for them.

But i ended up with those due to another forum and the resourceful people there - Tannoy Reveal 6D - Badcaps Forums