544 Fault ,Sounds like Thunder

allant · 28 jan 2023, 15:34

Hello, thank you for letting me join the group & sorry for my lack of the Dutch language.
I have owned my MFB 544s for over 25 years now, but just recently one of them has started to sound like 'thunder' when it gets a signal, it continues when you cut the signal & only stops when you cut the power to the speaker.
If i leave the power off for 24 hours it seems to work fine for a few days, what is the most likely component that is failing.
Thank you.
Allan.

Remi · 28 jan 2023, 17:16

Hello Allant, welcome here!
it might be the FET in the woofer's pickup, quick way to know is change the backplane of both speakers with each other.
Is the effect now in the other box is the cause to be found in the amplifier, if it stay's, it is most likely the FET.
Or it might be a broken wire from the terminals to the woofers' coil or pickup.

Is the problem in the amp, i would first cut the feedback line to see if it stops, it could be a bad transitor, or zener diode or perhaps also a bad elco.
The BF245 directly behind the input might also cause some problems

allant · 28 jan 2023, 17:24

Thank you Remi, i will swap the back plates over & see if it changes anything first, i am an electrician but not an electronics expert at all.
Regards,
Allan.

Remi · 28 jan 2023, 17:29

if you need a manual, it's free to download with thanks to Wil Manshande:
http://freeservicemanuals.info/nl/servi ... s/22RH544/

you might have a different /xx number 544 but this will get you started

allant · 28 jan 2023, 18:12

Thank you .

allant · 30 jan 2023, 14:51

Hello Remi, right I have swapped the amps round and established that it is the amp at fault, it all looks good and I have sprayed it all with contact cleaner, all back together now and crackled when I first turned it on but seems great now, one thing though, the Red led has stopped working

. I will keep you posted. The faulty unit seems to have had a home made circuit installed, but it is disconnected, no idea what that was for, 2 extra sockets on rear.

Remi · 30 jan 2023, 22:15

ok! maybe this amp needs some more investigation.
is a wire for the LED on the connector lose now?

allant · 31 jan 2023, 08:49

I am hoping it's just that, has them on for 5 hours yesterday without any noises, will have another look today.

allant · 31 jan 2023, 15:59

Well still crackles today, all the connections seem good to the LED, what DC voltage can i apply to the terminals 2 & 4 to see if it light up?

Thomas · 31 jan 2023, 21:47

Question. Is the crackling coming only from the woofer or also from the midrange and tweeter? If only woofer, then it can be the FET inside the woofer. This can be repaired. If all speakers crackle, it could be the fet on the small input pcb. This can also be repaired.

allant · 01 feb 2023, 09:38

Hello Thomas, I have swapped the amps round on the speakers and it didn't change the problem, so the amp is the problem, so it could be what you suggest on the small pcb input board, but not being an electronics expert I don't know what part the fet is. Regards, Allan.

Remi · 01 feb 2023, 19:16

this one, the BF245

Thomas · 01 feb 2023, 22:15

In that case, if you have a technician nearby, ask him to replace the part that Remi highlighted in the picture above. Otherwise, you could send it to Remi or to me for repairs.

allant · 01 feb 2023, 22:42

Thank you Thomas, I have found the BF 245 on the board, looks like a black transistor to me, I will check for dry joints on it, but have no idea how to test it. Is it a common part that can easily be obtained. Regards, Allan.

Steerpike_jhb · 04 feb 2023, 19:16

That FET circled in red above is part of the automatic power on system. No audible signals pass through it.

So many failing components can produce rumbling sounds.
One diagnostic tool that may work is a can of freezing spray. Failing components often get better or worse if they are rapidly cooled. Spray suspect parts in turn and listen for changes.