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 B&O enthousiasts !
This is my first post, I will share my first B&O restoration project and maybe ask you some support as I still face some some issues...
Some days ago, I have decided to get my tools and equipment out of the garage where they were sleeping for a while, and get back to some electronics activities.
But working on what ? It didn't take me more than 30 seconds to realize I will try to restore a B&O equipment, one of those I was dreaming but couldn't afford when I was young.
The good thing with this plan is that we can find many B&O systems for repair for cheap, all Service Manuals are available, I love the electronic components from the 80's... and there is a very nice B&O community on BeoWorld !
So, I bought a Beocenter 2800 "not working, for spare parts"...
Philippe
First I checked what was wrong: the front panel selectors were completely stuck, no way to move them. Nothing happens.
The Frequency index was moving loose, probably disconnected from the wire.
So I opened the box...
It looks like the old grease has turned into solid glue ! After reading some posts in the forum, I discovered that it is a very common issue.
I removed all the pushbutons, cleaned them and the metallic rail as well, put some silicon grease: all is working fine now !
As expected, the Frequency slider has one pin broken, the wire cannot stay in place anymore. The Volume slider is OK.
That is not good news, as it seems quite impossible to find this kind of spare part. So, I decided to go for my own solution: THE SARDINE CAN
I could make a pin out of a piece of metal, rolled and glued with Loctite. The glue is working ok on the plastic, here is the result.
The slider backside certainly is available: https://beoparts.com/2018/06/15/beomaster-beocenter-slider-backside-3015079/
Actually, it should only have two pins, with the string secured to it by a blob of glue, but your solution will work fine.
Martin
Then, I connected the unit to the mains, a loudspeaker, a FM antenna , pushed the FM button: no sound, impossible to get the green LED when moving the frequency....
It is time to study the wiring diagrams. Let's print them and try to understand how the thing works. This is part of the quest for restoring this Beocenter. I like this SM from the 80's TCA 750, MC 1310, TCA 420...
Some mistakes anyway (diodes upside down...)
PhG: Some mistakes anyway (diodes upside down...)
Where?
Hi Martin,
Thank you for the info about the spare parts.
I think the LED D4 is upside down in the drawing, am I wrong? It lits when AM tuner is selected (MW/LW).
I also found a diode upside down on a PCB drawing, can't remember where...
I checked all the DC Powers etc... OK.
But no radio, AM or FM. I used my pocket signal generator to inject a 1kHz signal on the Tape DIN connector: wonderfull, both Left and Right channels are working, so the Amp section is ok.
After checking the Tuner board, I found that the ground connection coming from the Preamp board was broken. After soldering and restoring the ground connection between the two PCBs, I have now tuning LEDs lit, as well as AM LED when LW or MW are selected.
At that stage, I have the system working with audio out on the 2 loudspeakers, AM radio, but no FM yet.
The phono section will be checked later, once I can reassemble the box. The good thing is that the unit came with a MMC 20 S cartridge, with a "small" problem anyway... we will see that later.
Back to the FM issue. I found many wrong voltage values on the FM section. I checked the +15 VDC feedind the TCA 420 (IC2) and TCA 750 (IC3): OK.
Humm... let's check again carrefully : TCA 420 pin 11 is 0V ! Although the nearest pin where I checked before was at +15V DC !
Finally, using a magnifier, I found a crack on the circuit board (blue line on the picture), pin 11 couldn't receive this +VDC. That was quite impossible to see without a magnifier.
So, I repaired this part of the board. Good, I have FM in stereo now !
So, everything is fine, I can reassemble and close the box… NOP !
The Right channel output is silent…. no FM, no AM, no 1 kHz Tape. It looks like the 22KB balance potentiometer is broken somewhere. I desoldered it, checked: one end of the right track is disconnnected. The track itself is good (let say acceptable) all the way, but the connection to one pin is cut (crimp).I opened the thing, and try to solder a small point of tin there, obviously it doesn't work… I cannot find this kind of slider from usual suppliers, that made me feel I am getting into troubles.But there may be a solution to be tested : silver conductive paint, used to restore tracks of heating system of car's rear windows. I order this one from internet, several others can be found.I painted carrefully 8 or 10 layers of this paint at the junction of the track and the output pin. You need to wait 10 mn between each layer to allow drying. The result was positive: I could measure 22K again between both ends of the slider. Sorry, no picture of this "rescue".
I have completed all the works inside the box, so I reassemble everything, connect the power string to the sliding index, the frequency string to the sliding Freq index. These 2 tasks turned me almost crazy, but that's it !
Now powering On for a final check: Argh !! No HP outputs ! The two 4A fuses blow immediatly !
After visually checking everything, grounds, HP outputs etc... nothing special. I checked the wiring diagram to understand.
I see that a Triac may short the transfo output in case of an issue detected by a protection circuit TR4 TR5 TR6 connected to both Amp channels outputs.
I checked the Amps (transistors, etc...) but noticed nothing. I also checked the diode bridge (D7) after the triac, looks good... Transfo output is 2 x 28VAC.
Anybody could help ? This protection circuit is often used in B&O amps as I could see in watching other equipment's SM, so there is maybe a well known problem there...
hi there,
I Got one Yesterday for free. How did you get the front off the unit ? I have the service manual but it is not informative.
best regards,
Per
Hi there,
how did you get the panel off ? How did you strip it down. best regards,