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
Hi all,
When using headphones on my Beomaster 901, I am hearing low level 50Hz hum on all sources even with the volume at zero. It does not increase as I turn up the volume.
But there is no audible hum on the speakers.
I'm just wondering if this could be either because the position of the headphone socket at the back close to the transformer causes induction into the cable, or whether the socket itself could be oxidised/faulty in some way.
Grateful for advice, as always.
What is the impedance of your headphones?
Martin
Hi Martin,
32 ohms.
Simon
Did you read the printing next to the headphones socket?
Er, no I didn't, as it's on the back !!
Can I use 32 ohm h/phones on, say BM 3000-2?
Thanks,
Beomaster 3000-2 has a series resistor for the headphones output, so yes.Does the hum go away if you unplug the speakers while using the headphones?
I unplugged the speakers from the BM901, plugged in the h/phones, pressed FM and was rewarded with a loud buzz and smoke so I've obviously blown one, if not both channels.
Does the 901 see 32 ohm headphones as a short?
To me it sounds like your headphones are defective.
32 Ohms is not seen as a short in itself, but in parallel with speakers the resulting impedance may be seen as close to a short.
Thanks, Martin,
Will have to do some poking around to try to figure out what the damage is.
Speakers were in any case disconnected, and as far as I can tell, the h/phones are ok, though I used a 6.35mm jack adaptor.
This may have been an unconnected fault just waiting to happen though I suspect my 32 ohms may have something to do with it.
Read the Small Print Lesson learned.
I have seen some (cheap) adapters that actually short the socket connections momentarily as you plug in/out.- Or for a longer time if the adapter hasn't been fully inserted/pulled out.These WILL cause problems in amplifiers without series resistors.
Had a look inside and it's clear the emitter resistors on the left channel are toast, R141 having lifted one leg. Frustrating as it was working so well after my earlier repair.
Not sure about the functionality of the right channel though it looks ok.
As Martin says, insertion of the jack causes a momentary short - on the right channel in this case. (So I'm not sure why the left went up in smoke).
The adapter looks to be deficient in terms of its isolating rings.
If it shorts on the way in and out, it's no good.
Just a quick postscript to this sorry episode.
I replaced pretty much everything in the l.h. output stage and idle circuit. If you use Fairchild TIP120 and 125 output transistors as I did, be prepared to dremel out the mounting hole as it's too small; thin little mounting plate as well, but they seem to work.
BC547B made a suitable replacement for BC171B.
Unit now sounds as good as it did before; a testament to the repairability of the BM901 even for the inexperienced.
(Just a thought, a sticky with transistor equivalents would be a useful resource).