Neumann KH120 studio monitor power supply repair

[beatnik]

Not sure how easy it would be to convert them to passive... surely a fun project but it's gonna be hard to find a dead stereo pair, and for cheap.

It would probably be worth to build an external power supply, all you need is a pair of unregulated rails about +/- 40V and a pair of regulated +/- 15V

At least the psu won't go up in smoke every time there is a fault in the amplifier

 

[Tubetec]

All to often you find the KH120 being sold as single orphaned working speaker ,although the seller wont tend to tell you the other blew up .

There is the K&H P110 the passive predecessor of the KH120 , might be a reasonable starting point for a passive crossover network design as the drive units are similar .

 

[MattLuie]

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.

Hi
I have a same problem as you said but I have a big problem....I don't have the service manual and I don't know what is the components value for exam: diodes before tda16846 Vcc and etc. Please if as possible help me...this thing is driving me mad really.

 

[beatnik]

There are no service manuals available unfortunately.

 

[Khron]

Hi
I have a same problem as you said but I have a big problem....I don't have the service manual and I don't know what is the components value for exam: diodes before tda16846 Vcc and etc. Please if as possible help me...this thing is driving me mad really.

Well-lit, well-focused photos of the board(s) might help.

 

[MattLuie]

Well-lit, well-focused photos of the board(s) might help.

It's a photo of Tda16846 and her kid's around her
On my left speaker board which is ok, on tda Vcc pin I can read 10 V
But in this case which has shorted mosfet I read unstable voltage on TDA's Vcc

 

[MattLuie]

There are no service manuals available unfortunately.

Yeah unfortunately I couldn't find it...
I contact the Neumann but they said take it to Neumann distributor in your country...
I took it but distributor gave me a hilarious price for repair something like 500 $

 

[Khron]

But in this case which has shorted mosfet I read unstable voltage on TDA's Vcc

Well, with a shorted MOSFET how and why would you expect the TDA to have a stable supply? It uses a winding from the main transformer (which is driven by the MOSFET that's now shorted) to supply itself***. If anything, you should count yourself lucky that the chip survived (assuming it did indeed survive undamaged).

***
https://techweb.rohm.com/product/power-ic/acdc/acdc-design/1400/

 

[MattLuie]

Well-lit, well-focused photos of the board(s) might help.

Another Pic would be helpful

 

[beatnik]

With a shorted mosfet I doubt the psu would power up at all, most likely it would just be blowing mains fuse.

If it's powering up but have an unstable voltage I suggest removing the amplifier ICs and see if the voltages stabilize

 

[MattLuie]

Well, with a shorted MOSFET how and why would you expect the TDA to have a stable supply? It uses a winding from the main transformer (which is driven by the MOSFET that's now shorted) to supply itself***. If anything, you should count yourself lucky that the chip survived (assuming it did indeed survive undamaged).

***
https://techweb.rohm.com/product/power-ic/acdc/acdc-design/1400/

I replaced a MOSFET
But it shorted in first 8 second
Original MOSFET (08N80C3) new one (10N80C)
And my guess was the chip drives MOSFET because the output of tda connected to MOSFET gate pin

 

[Khron]

Another Pic would be helpful

I'd start by replacing those two little "Jwco" capacitors with similar-value low-ESR ones from one of the good (Japanese) brands - Panasonic, Rubycon, Nichico, United Chemi-Con, Elna.

I replaced a MOSFET
But it shorted in first 8 second
Original MOSFET (08N80C3) new one (10N80C)
And my guess was the chip drives MOSFET because the output of tda connected to MOSFET gate pin

Sounds like it might be time to replace the chip too, then.

TDA16846 datasheet:
https://eu.mouser.com/datasheet/2/196/PWM-QRICTDA1684616847-73021.pdf
Assuming there's no great big hunk of steel (PFC inductor) to be found there, it might be safe to assume the schematic looks roughly like the one on page 25.

 

[MattLuie]

With a shorted mosfet I doubt the psu would power up at all, most likely it would just be blowing mains fuse.

If it's powering up but have an unstable voltage I suggest removing the amplifier ICs and see if the voltages stabilize

I always use test protector lamp for repair because of that fuse is ok
I'll try removing amp ICs
TNX

 

[MattLuie]

I'd start by replacing those two little "Jwco" capacitors with similar-value low-ESR ones from one of the good (Japanese) brands - Panasonic, Rubycon, Nichico, United Chemi-Con, Elna.



Sounds like it might be time to replace the chip too, then.

TDA16846 datasheet:
https://eu.mouser.com/datasheet/2/196/PWM-QRICTDA1684616847-73021.pdf
Assuming there's no great big hunk of steel (PFC inductor) to be found there, it might be safe to assume the schematic looks roughly like the one on page 25.

 

[MattLuie]

I'd start by replacing those two little "Jwco" capacitors with similar-value low-ESR ones from one of the good (Japanese) brands - Panasonic, Rubycon, Nichico, United Chemi-Con, Elna.



Sounds like it might be time to replace the chip too, then.

TDA16846 datasheet:
https://eu.mouser.com/datasheet/2/196/PWM-QRICTDA1684616847-73021.pdf
Assuming there's no great big hunk of steel (PFC inductor) to be found there, it might be safe to assume the schematic looks roughly like the one on page 25.

Yeah it's very similar to page 25 of datasheet
I ordered original 08n80c3 I'm waiting for this

 

[MattLuie]

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.

Can I ask you please last time that you got successful to repair your speaker which components did you replaced ?
And can you give me the components source's?

 

[beatnik]

If the mosfet was shorted I would definitely replace the controller IC as well. I got the controller IC from ebay

https://www.ebay.it/itm/16414614050...948dV8fLuaeUvFowyj0wF3ww==|tkp:Bk9SR8jx-v2yYg

 

[MattLuie]

If the mosfet was shorted I would definitely replace the controller IC as well. I got the controller IC from ebay

https://www.ebay.it/itm/16414614050...948dV8fLuaeUvFowyj0wF3ww==|tkp:Bk9SR8jx-v2yYg

Thanks for your reply

 

[francoislchatel]

I repaired many power supply in the KH120 the problem was the four 220uF25Vdc capacitor around the two regulator in the output, psu, if the primary can start you can try that, I worked for Sennheiser-Neuman for a long time, good luck

 

[francoislchatel]

By the way the output amp is TDA7293