MFB 545 - Bass driver oscillatie

Alles over onze geliefde MFB boxen in deze categorie.

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markkelsall
Berichten: 29
Lid geworden op: 05 apr 2017, 18:08

06 apr 2017, 11:13

Hallo

Hoop dat dit vertaalt OK in het Engels!

Ik overweeg de aanschaf van een paar 545's, maar er is een probleem met hen wil zeggen dat de bass driver oscilleert wanneer er geen muziek wordt afgespeeld

Denkt u dat dit een probleem in de terugkoppeling of de multifunctionele knop sensor ??

Erg bedankt
Mark

Hello

Hope this translates OK from English!

I am considering the purchase of a pair of 545's but there is an issue with them i.e. the bass driver oscillates when no music is playing

Do you think this is an issue in the feedback circuit or the mfb sensor??

Many thanks
Mark
esgigt
Berichten: 3790
Lid geworden op: 22 feb 2013, 15:13
Locatie: ergens in Zuid-holland

06 apr 2017, 13:44

Hi Mark,

If operating correctly the 545 is a great speaker... the oscillation-problem usually ought to be solvable.

Being a feedback-system MFB can have several sources of oscillation.
The system has two main feedbackloops. One in the power-amplifier and one for the MFB correction, fed by the sensor.

First thing we'd like to know is the frequency at which the system oscillates.. Is it a low, medium or high frequency? Can you provide some indication, please?

Also nice to know is which equipment do you have that can help you determine the culprit? And do you have any experience in tracking faults in electronic equipment?

Greets,
Edwin
CD880, FT880, RH544, PM2519, BX510A, 22RH720, GA312 (Aristona) ;)
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markkelsall
Berichten: 29
Lid geworden op: 05 apr 2017, 18:08

06 apr 2017, 20:19

Hi Edwin

Bedankt om mij te contacteren.

Ik heb niet gekocht van de 545's toch zo weet alleen wat de verkoper mij heeft verteld. Ik heb een vriend die is een ingenieur dus misschien met uw hulp kunnen we het probleem op te lossen, als ik ze te kopen

Een vraag echter

Zijn de componenten in de sensor direct beschikbaar als dit is de schuld?

Beste wensen
Mark

Hi Edwin

Thank you for contacting me.

I have not purchased the 545's yet so only know what the seller has told me. I do have a friend who is an engineer so perhaps with your help we can resolve the problem if I purchase them!

One question however

Are the components in the sensor readily available if this is the fault?

Best wishes
Mark
esgigt
Berichten: 3790
Lid geworden op: 22 feb 2013, 15:13
Locatie: ergens in Zuid-holland

06 apr 2017, 20:55

markkelsall schreef:One question however

Are the components in the sensor readily available if this is the fault?
The BF245 (not nock-offs) might not be too easy to obtain.. but replacement types might be available..
CD880, FT880, RH544, PM2519, BX510A, 22RH720, GA312 (Aristona) ;)
OgWok
Berichten: 53
Lid geworden op: 13 feb 2012, 16:26

07 apr 2017, 07:31

The FET BF245 is available in the webshop for € 1,50 :)
Salvador.
Berichten: 142
Lid geworden op: 24 jan 2017, 17:42

07 apr 2017, 17:25

OgWok schreef:The FET BF245 is available in the webshop for € 1,50 :)
Don't use the BF245B for repairing the MFB PCB, when the original BFW11 FET is defective, replace it with a new BFW11. Replacing the BFW11 with a BF245B is a lazy non professional way of repairing. Specially for the 545, which produces very low freqeuncies, it is not recommandable to put a BF245* with glue on the pcb... crappy solution.
esgigt
Berichten: 3790
Lid geworden op: 22 feb 2013, 15:13
Locatie: ergens in Zuid-holland

09 apr 2017, 12:40

Salvador. schreef:Don't use the BF245B for repairing the MFB PCB, when the original BFW11 FET is defective, replace it with a new BFW11.
Since nowhere in the schematics is indicated that a BFW11 is used in the sensor, I was under the impression that the BF would do also...

Thanks for filling this gap in my knowledge.
CD880, FT880, RH544, PM2519, BX510A, 22RH720, GA312 (Aristona) ;)
Salvador.
Berichten: 142
Lid geworden op: 24 jan 2017, 17:42

09 apr 2017, 21:16

esgigt schreef:
Salvador. schreef:Don't use the BF245B for repairing the MFB PCB, when the original BFW11 FET is defective, replace it with a new BFW11.
Since nowhere in the schematics is indicated that a BFW11 is used in the sensor, I was under the impression that the BF would do also...

Thanks for filling this gap in my knowledge.
The BFW11 can be swapped with a BF245B theoretically for the MFB PCB, but the housings are not the same, so mountment can not be done on a properly way. Glueing the BF245B is not a proper and long life solution of mountment on the MFB PCB. Nowadays the BFW11 is good available in a few eBay stores, in a few years it will be a bigger problem to find them, but that's then and not now :wink: :D

Of course, there a some people who did not already the disassembly of the MFB PCB, but if they do it carefully, it will succeed. I did it about 15 times. In these crucial applications, it's always better (if possible) to take original parts.

The BFW11 is technically in these applications almost the same (it is also be used in the 545 On-Off automatic circuit) in comparing to the BF245B, the biggest difference is that the BF245* has three pins and the BFW11 four pins, an extra pin for ground.
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Thomas
Site Admin
Berichten: 11839
Lid geworden op: 16 jun 2005, 14:55
Locatie: Lochem, Gelderland
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10 apr 2017, 13:49

Salvador. schreef:Don't use the BF245B for repairing the MFB PCB, when the original BFW11 FET is defective, replace it with a new BFW11. Replacing the BFW11 with a BF245B is a lazy non professional way of repairing. Specially for the 545, which produces very low freqeuncies, it is not recommandable to put a BF245* with glue on the pcb... crappy solution.
I would like to put a little nuance to this story. Although original replacement parts are always recommended for optimal performance, there is no harm in using a BF245 instead of a BFW11. It is technically compatible and will work in exact the same way. That is the technical part.

As for mounting the FET, removing the sensor PCB from the woofer is very tricky and the risk of damage is present. I think your mentioning of a "lazy non professional way of repairing" is quite exaggorated. When playing safe and working carefully, the BF245 mounted on the PC board the way I do it will be a good alternative. Putting a bit of glue on the FET to lock it to the PC board will be sufficient, even for the big 545 woofer.

We all know here that your standards are very high Salvador, but I must insist on respecting alternative repairing methods as well. I believe we have talked about this in the past as well.



As for the original question that Mark is asking, I think it is very important to see and or hear what exactly is happening in the oscillation that is described. It may be caused by the Motional Feedback circuit but there may be other factors in play as well. I have had 545's oscillating in the woofer amplifier whereas the cause was in fact that the mid range speaker was dead! This caused a powe build-up in the output capacitor which - in some way - reflected on the other elektronics.

That being said, Mark, I think it is wise to review the purchase price of the boxes. If they appear to be very cheap to purchase, I would say don't hesitate and buy them. Repairing must be possible with all knowledge that is available here in the Forum.
Thomas Baur
webmaster http://www.mfbfreaks.com
De Enige Echte Philips Motional Feedback Website!
Salvador.
Berichten: 142
Lid geworden op: 24 jan 2017, 17:42

10 apr 2017, 17:19

Thomas schreef:
Salvador. schreef:Don't use the BF245B for repairing the MFB PCB, when the original BFW11 FET is defective, replace it with a new BFW11. Replacing the BFW11 with a BF245B is a lazy non professional way of repairing. Specially for the 545, which produces very low freqeuncies, it is not recommandable to put a BF245* with glue on the pcb... crappy solution.
I would like to put a little nuance to this story. Although original replacement parts are always recommended for optimal performance, there is no harm in using a BF245 instead of a BFW11. It is technically compatible and will work in exact the same way. That is the technical part.

As for mounting the FET, removing the sensor PCB from the woofer is very tricky and the risk of damage is present. I think your mentioning of a "lazy non professional way of repairing" is quite exaggorated. When playing safe and working carefully, the BF245 mounted on the PC board the way I do it will be a good alternative. Putting a bit of glue on the FET to lock it to the PC board will be sufficient, even for the big 545 woofer.

We all know here that your standards are very high Salvador, but I must insist on respecting alternative repairing methods as well. I believe we have talked about this in the past as well.
Hi Thomas,

Yes, as said, the BF245B can be technically used as an alternative for the BFW11. It works but it is not a really nice solution.

Removing the MFB PCB is not that tricky if the repairman is carefully and have experience in repairing small technology. The alternative you requested, is much more dangerous than removing the PCB carefully. Removing the BFW11 from above, without dismantling the MFB PCB will cause the risk of damaging the copper PCB tracks on the PCB.
OgWok
Berichten: 53
Lid geworden op: 13 feb 2012, 16:26

10 apr 2017, 18:29

Salvador. schreef:
Hi Thomas,

Yes, as said, the BF245B can be technically used as an alternative for the BFW11. It works but it is not a really nice solution.
........
Hello Salvadore,

Why is using a BF245B, instead of a BFW11, "not a really nice solution"?
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Thomas
Site Admin
Berichten: 11839
Lid geworden op: 16 jun 2005, 14:55
Locatie: Lochem, Gelderland
Contacteer:

10 apr 2017, 19:03

Maybe we should first focus on finding the cause of the oscillation. Before someone starts cutting open woofers and replacing fets that maybe aren't the cause of the problem.
Thomas Baur
webmaster http://www.mfbfreaks.com
De Enige Echte Philips Motional Feedback Website!
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motoindo
Berichten: 3096
Lid geworden op: 01 jul 2010, 11:04
Locatie: Diemen
Contacteer:

11 apr 2017, 09:22

Thomas schreef:Maybe we should first focus on finding the cause of the oscillation. Before someone starts cutting open woofers and replacing fets that maybe aren't the cause of the problem.
Correct.
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markkelsall
Berichten: 29
Lid geworden op: 05 apr 2017, 18:08

11 apr 2017, 21:31

This is all very helpful, keep your thoughts coming

I will try to obtain a video if possible. That is if I can upload one to the forum!?

Mark
esgigt
Berichten: 3790
Lid geworden op: 22 feb 2013, 15:13
Locatie: ergens in Zuid-holland

11 apr 2017, 21:46

markkelsall schreef:This is all very helpful, keep your thoughts coming

I will try to obtain a video if possible. That is if I can upload one to the forum!?

Mark
The forum does not allow big uploads... You can however post a link to a location on Youtube, Drive, dropbox .... name it..

But don't forget to set the rights so others can view..
CD880, FT880, RH544, PM2519, BX510A, 22RH720, GA312 (Aristona) ;)
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