ARCHIVED FORUM -- March 2012 to February 2022READ ONLY FORUM
This is the second Archived Forum which was active between 1st March 2012 and 23rd February 2022
Hello!
I recently decided to get into vinyl, and acquired myself a Beomaster 1000 set with matching speakers and a Beogram 1700 record player.
I'm new to audio systems, so I don't know the difference between good and bad sound, but after listening to a few records on this thing, I'm very impressed. Everything sounds great!
But alas, the following day I have a certain someone over for a glass of wine. Of course I have to put on a record to set the mood.
Things are going well, BUT suddenly the system starts making this loud thumping noise. BUMP BUMP BUMP BUMP. You can still barely hear the music, distorted, between the bumps. I jump out of my seat to stop it, and try to figure out what is going on. (I can only imagine what my downstairs neighbour, thought I was up to).
Red faced, I reassure my guest, that this has certainly never happened before. After wiggling a little with the wires, it's playing well again, but not 5 minutes later, the same thing happens! BUMP BUMP BUMP BUMP BUMP!
I give up, and turn it off, too embarrassed to wiggle any more wires, we spend the rest of the evening in silence.
The following day, I set out to figure this thing out! I have, over the last few days, tried my best to diagnose the problem, but I have not succeeded, I have searched the world wide web, and many a forum FAQ. I come to you, humble and desperate.
Here is what I gathered:
The bumping sound can be increased and decreased in volume with the volume knob.
The bumps are a deep bass sound almost like a kick drum.
The bumping can vary in speed (or frequency if you will), from 1Hz to a continuous hum.
It will only make the sound when it's getting a signal through input ll, which is the one with the built in preamp, as far as I can gather.
It doesn't matter which output I use for the speakers, or whether I set it to mono.
The bumping will usually start if I tap the Beomaster 1000 with my finger, or when I walk past it (shaking the floor). It will also do it by itself, when it's playing. I assume this is because of the vibrations, it produces itself.
The bumping will usually stop If I tap the Amplifier a few times OR (get this) turn the tuner knob.
If I play music from my phone through the other inputs (one and three?), or use the radio, there is no problem. I can't play music through input ll with my phone. I don't get why, but again, I am completely new to this.
To me, it seems like there is some loose wiring in the pre amp connected to input ll.
I have searched the forums for anyone with a similar problem, but have not been successful.
I really like this set, and would love to get it working. I'm fairly confident with a soldering iron, and automotive wiring, but I really have no idea where to start with this thing.
Sorry for the wall of text!
If anyone could point me in the right direction I would be eternally grateful!
Help me save myself from future embarrassment. I really need it.
Thanks,
Ian.
Welcome to Beoworld !
From your description it sounds like a phenomenon called "motorboating" (obviously from the sound of it).There can be many reasons for this, but it usually comes from bad decoupling in amplifier and/or power supply, causingthe receiver/amplifier to go into self-oscillation at a low frequency.Lack of decoupling/filtering in the power supply makes it go to its knees when loaded, this causes the supply voltage to dropand the amplifier to fade. When the amp fades it draws less current from the power supply, which in turn allows its output voltage tocome up again quite instantly, typically causing a blob in the speakers (from preamplifier outwards).This blob takes a lot of energy to amplify = it takes a lot of power from the power supply which then again goes down andwe have oscillation, that can only be stopped by turning down the volume.Replacing old and dried out capacitors would be the first step.
The larger capacitors can occasionally become slightly microphonic, because when the amount of liquid inside is no longer adequate,what is left can be moved around ever so slightly from vibrations. Actually, fitting new caps is almost a geenral rule by now for all things electronic more than 25-30 years old.
Martin