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 folks.
i am new on here, having just registered.
i have just bought a Beocenter 7007 which is in great condition but it has a problem with the turntable.
the radio and tape deck work perfectly, with both sets of speaker inputs working like a dream. Really nice quality.
the issue I am having is with the tt part. It plays through the right Chanel but not the left. These are the symptoms:-
equal input to both speakers through the mmc4 cartridge (visually inspected under a strong microscope and in perfect condition) when the tt Is not spinning ( gentle drag of finger confirms) even if the arm is moved across the platter. As soon as the PH button is pressed, the left Chanel disappears. There is no earthing hum when I touch the headshell with the platter not spinning, however once spinning I have an earth hum through the left channel.
no other issues.
so, it is not the cart or the tonearm wiring, I am thinking some earthing issue.
i have opened up the unit and have discovered an earth wire that is not connected to anything ( short one by the tape deck) can't see where this goes, however to rule out that inadvertently causing a bad earth I have put insulation tape on it. Same issues. Where should this connect to?
i will attach a picture
does anyone have any ideas what it could be and what to check for? It appears to be either an earthing issue or a power issue but none of the capacitors appear damaged or leaky.
grateful of any advice.
thanks
john
Any internal photos of a 7007 would be greatly appreciated to see if i can trace the earth connection.
Any ideas?
If you are absolutely sure that the cartridge and tonearm wiring is fine, the problem will have to be somewhere in the preamp stage.Most likely a capacitor but cracked solder joints at connectors etc. has also been seen in these lovely machines.
The ground lead goes to the nearby black screw on the tapedeck sections metal chassis "ear".
Martin
Thanks for the reply. I presume you mean the metal tang that secures the tape deck to the chassis ( extreme right of my photo). I have just connected this so will retry.
the reason that I am certain the arm and cartridge are fine is that the output to both speakers is the same until the tt is started moving.,if I play and then stop the tt, wi the arm retuning to the rest point, a drag of the stylus reveals equal ( to my ears). As soon as I start it again, a drag accross the styles shows a much diminished output to the left speaker ( very noticeable). Where would I need to be looking for a bad capacitor? There is a lot crammed in under there.
No change with the earth lead attached. Everything else works perfectly
I am not sure if I understand you correctly, but if you have good signal volume touching the stylus while the deck is not running, then themuting is not working.A muting problem could also be the cause for the low/missing channel.
Ok. So with the tt not spinning, I shouldn't have any sound thought the stylus? If that is the case any idea what it would be in the muting circuit?
I have given all the circuitboards a good blast with compressed air to get rid of any dust. No change.
Any ideas what to test in the muting circuit?
Regards
John
Can anyone identify which board the muting circuit is on?
I have downloaded the wiring diagrams and I am trying to pick my way through them.
diagram b shows the left channel output amplifier with a muting input.
diagram f shows the pickup arm wiring, including the earth.
Does anyone have any suggestions on this. Really need some diagnosis help her, I am struggling.
Folks.
someone must have experience with one of these. If no one on here knows I don't know wherever to turn.
grateful of any assistance.
regards
The muting relay on the amp board is for muting the whole system with the mute button, the turntable has got its own separate muting relay. Sometimes you can get them to work properly by playing a record and cueing the arm up and down several times, this can clean any oxidisation on the relay.
Thanks for the advice. Do you know where the relay is?. If i can find it i can use compressed air to blow any dust out
I have a download of the schematics but not the full service manual. Anyone have a pdf of the full manual or can point me in the right direction.
If you follow the wires that come from the tone arm the first circuit they hit contains the muting circuit.
Download the 7700 manual - very similar and a better manual!
Peter
Had a look at the 7700 manual as well. Thanks for the pointer.
I have cleaned the contacts on the connectors that the tonearm wires are soldered to. No difference.
I am assuming that the mute circuit is the one below (rectangular with one corner missing)
This is the contact part that i cleaned.
I have visually inspected the circuit boards and nothing appears to bedamaged. No obvious leaking caps.
Where do i go from here. I have a multimeter but dont have any experience with these machines. I am content that thecart and arm are fine.
Any help gratefully recieved.
Hi. I am 99% certain the cart is ok. Cart upside down, problem identical. It is only when the tt spins that the left ch annel mutes.
Correct, which indicates that the cart is electrically fine.
Thanks.
Dont suppose you know which one is the muting relay?
The muting switch is not a relay.It's the sliding switch operated by the lift/lower arm that goes from the cam wheel to the tonearm base.The thin wires from the tonearm are soldered to it.
Right. Desoldered the tonearm wires and removed the switch. Removed the sliding contact bar (carefully), cleaned all contact points with very fine emery, put the sliding bar back in, re soldered the wires and crossed fingers.
Exactly the same. No difference whatsoever.
Any other ideas?
The muting switch doesn't actually carry the sound - it stays out of the way when the music is supposed to be playing, and short circuits the pickup signals when it is supposed to be silent. So a bad contact in it is not going to mute a channel, but exactly the opposite.
Check for some mechanical problem that keeps the switch from operating properly. In a pinch you could just disable it completely to see if you have proper sound without it. You'd then have to live with the pop sounds when the arm is lowering or rising - just like the owners of most other brand turntables do anyway.
--mika
Swap left/right signal leads from the tonearm to tell if the problem is with the cartridge/tonearm leads or somewhere in the following circuitry.