MFB Speaker issue - unusual oscillation

[audiogymp]

Hi AK

I just landed a pair of Philips RH544 (AKA 22R544) MFB speakers (with built in amps). I might need to go to a different forum but there has been some discussion of speakers of the like on AK.

I needed to make up some power cords for the unusual mains input sockets. The service manual didn't make it clear which line needed to be connected to the active mains line and which to the return but it didn't look like it would make a difference because they were both only going to the power transformer.

I hooked them up through a dim bulb just to be on the safe side. Both speakers did the same curious thing. Didn't seem to make a difference which way I connected the mains.

There was an oscillation (about 1 Hz) in which the dim bulb would get brighter/dimmer and there would be a mid/high pitch beep at a point in the cycle. The speakers played input source (from an iPod) and it sounded OK but I didn't continue it for long for obvious reasons. The protector relay didn't seem to be tripping - I could hear it click after power down.

Could it be that the dim bulb is causing the problem? The speakers have buttons to turn on automatically when there is a signal - perhaps this is responding to the current limit. I'm reluctant to try them without the bulb.

Any other thoughts as to what might be the cause?

Thanks, Jon

 

[audiogymp]

BTW - the service manual can be found on this link.

http://www.mfbfreaks.nl/pdf/mfb/544/service544-50r.pdf

 

[audiogymp]

Thanks for the reply. Could well have been PSU-related. But it was curious that both speakers were doing the same thing.

I put a posting on mfbfreeks.com (main hub for this kind of gear) and got a response suggesting that it was the dim bulb causing the problem. I tried with progressively higher wattages and then no bulb and the problem went away. I haven't seen this before.

Interesting speakers/amps.

 

[tubed]

Hello, I have a set of 544's.
The power cord seems to be a simple extension cord with a unique plug shape.
I've had these for quite some time and at one point I lost the cords but Philips America was able to still replace them.
In the meantime while waiting for delivery, I fashioned a plug out of a simple plastic plug that I bisected and made to "fit".
It worked fine, perhaps I lucked out

Indeed, "interesting speakers/amps" and they sound extraordinarily good.

 

[MJB1959]

I have the 532 mfb's, and they are in need of a significant overhaul, they are so complicated looking inside that I am afraid to touch them, they are great little speakers that really could deliver, got them from my Dad after he passed, and I am determined to get them back to top condition.

I do fortunately have the proper cords, and have had them hooked up, source to left speaker(in), left(out) to right speaker(in), and ac to power input, it's a bit of a odd way to wire some speakers, but it works, I imagine that they are all similar in that regard. So the mains input is the power source for both speakers, as you relay the power from the mains in to the mains out, and utilize the channel button for channel designation.