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Please Help with MFB 587 🙁


freakmfb
(@freakmfb)
Active Member
Deelgenomen: 8 jaar geleden
Berichten: 8
Topic starter  

Hi,

First of all I'm sorry if I write in english

I'm Andrea and I'm very happy that I discovered this beautiful website and forum about MFB speakers. I bought a pair of Philips 545 and I can't wait to test them, and a pair of 587 which I have already here in my studio.

I've got a problem with the 587.

FIRST SPEAKER:

1 - One speaker has the Midrange which sounds weak. I opened the speaker and tested the Midrange unit itself and it works perfectly. So I hit several time the speaker with my hand on the top of it and the midrange comes and goes but anyway sounds weak.
I read some other posts here on the forums and seems that the responsible of the intermittent sound could be the relay. Could be...Ok but my question is: could the relay be responsible also for the weak sound?

2 - How can I clean the Relay? Should I unsolder it, open it and clean inside? how?
3 - Is it possible to buy a new brand relay for my 587?

SECOND SPEAKER:

The second speaker both Woofer, mid and Tw, sound very weak. The sound between them is balanced, if I turn the volume knob all the way up and down the speaker sounds ok, just weak. Very Weak. Doesn't sound like a problem with the amplifier but not sure. Could be also the relay on this? By the way the relay switch on and off if I hit the speaker on the top.

Well, I hope that someone here have the time to answer me detailed because I can't find any useful informations on internet about this problem.

Thank you and have a nice weekend. 🙂

Andrea


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Remi
 Remi
(@remi)
Noble Member
Deelgenomen: 16 jaar geleden
Berichten: 2162
 

Hi Andrea and welcome to this forum!

The relay is for shure a problem, often when you would knock on the housing the sound changes.
This is caused by the contact which aren't clean anymore and give some contactresistant.
You have already opened the backplane, now to get access to the relay you would have to separate the amp board from the backplane.
For a clean job you better remove the white contact paste and apply some new when reinstalling the board later.
For easier work you may also remove the metal bracket on the relay side.

The relay cover can be taken of, it might be glued somewhat but you can pry it open.
To clean the contact you have to be somewhat carefull, you better not use sandingpaper but just plain paper instead.
If you cut a small strip and apply some contactspray to it and move this up and down the contacts while you keep them closed by hand you will solve the problem nicely.

The weak Mid is probably caused by a faulty elco, i would advise you to replace all the smaller ones with a blue or brown color. the 'silver' ones can stay.
I would do this for both boards. It's up to you to replace the biggest Elco's also. If you want to play at a loud volume this might be a good idea.
For all the Elco's you have to make sure that you place them right regarding the "+" and "-". the "-" is always indicated. They will blow if place them wrong.

You would also visualy check the (little) transistors, it is possible that some Japanese type are present. They happened to build an oxidelayer on the legs which will lead eventualy to failure.
These ones can be replaced by BC5xxB or C types, they are indicated in the servicemanual , the equivalent Japanese types are never mentioned.
This might also solve your problem with box 2 which holds back in volume.

While you're at it you might also dismount the potentiometer from the board to clean it with contactspray (i use Brunox) and or IPA alcohol.

If it al functions again it would be a good idea to set all the adjustments to the factory's specifications.
That is if you are able to use a oscilloscoop and sound generator (PC is great)

Be aware that the last setting, the MFB value is to be done with the box closed and airtight. All the other settings can be done 'on the table'

good luck!


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