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
After having acquired a BM 1200 with 2 beovox 2700 boxes that produced miserable sound quality I replaced the big caps and refurbished the speakers, resulting in very pleasant results: no humming nor other disturbing things except for the fact that the 2 ( MW/LW ) lamps are very weak, even after replacing them by new 12v/35mA. If the enclosure of the picture of the pertaining part of the schematic made it to this post I wonder if somebody familiar with this problem can indicate where to check for the malfunctioning. Voltage measured : 12V. The ON lamp is OK as well as the FM, although it is located parallel to the MW and LW lamps. Could there be some unwanted resistance over the switches?
Measure the voltage "under" the lamps. Most likely they will not be the same. Indicating that there is a resistance located in the switches.
Caused by bad contacts in the switches itself.
The light output is to some extent indicating the received signal strength.Try with a better antenna.
Martin
So, by bridging/shorting C-E from TR12 (while switching between FM/MW/LW), there shouldn't be a difference in light output.
If there is, then the swithes are probably oxidated.
Thanks for the usefull response.
I cleaned the switches (TUNER 600 contact cleaner)), checked TR12 's C-E, seemed OK, reassembled everything (nasty little bolts), pressed the ON button: a faint smell and there was silence! No lamps, no sound.I will have to recover from this disappoinment and therefore go on vacation till the end of July.
Thanks again
Aad Jansse: Thanks for the usefull response. I cleaned the switches (TUNER 600 contact cleaner)), checked TR12 's C-E, seemed OK, reassembled everything (nasty little bolts), pressed the ON button: a faint smell and there was silence! No lamps, no sound.I will have to recover from this disappoinment and therefore go on vacation till the end of July. Thanks again
Most of us had that with one or another item, its just vintage and it do like that once in while, take a break and go back to it again, your long vacation comes in good
Collecting Vintage B&O is not a hobby, its a lifestyle.
Could not keep myself from having a quick look inside my beauty before getting into my caravan:
I had to improvise with mounting the smaller sized 3000u/25V cap; by closing the bottomplate the cap''s + and - touched: consequently invoking ashort-circuit; What is the damage done, where to look first? There is 0 V on board ; probably the rectifier?
Apart from the fuse, that's the only thing that could be cooked. And check the circuitboard traces.
Check the AC and DC voltage on the rectifier. Broken traces could also the culprit.
Are you sure about the 25V rating for the 3000uF cap ?I remember it to be much higher, 63V or so.
Also make sure to get the polarity on the cap right.
Martin, you are right,
I actually put in a 3000uF/63V; 25V was a mistyping in my post.
Aad
As Martin already mentioned, did you connect the cap with the right polarity?
Normaly these caps have a stripe on the side, marking the negative pole. If your marking on the leads are right, can it be that the positive lead is connected with the marked/striped side of the cap?
"Broken traces could also the culprit"
It seems so easy to suggest it, but one has to be much wiser/experienced than the undersigned : thanks for your wise advise.For those who do not share that wisdom : see picture.
With the recollection of the revived BM 1200 I can now go easily on vacation .
Again thanks to all of you.
I am familiar enough with the principle of the cap's polarity; although I tried to carefully solder the leads to the new (smaller) cap there was some solder contact between the positive and negative pole, resulting in burning damage to circuitboard, which a easily could bridge.
Thank you too
That will be a holiday without thinking about the BM1200's faillure
It feels probably much better with the thought that coming home after the holiday, you can switch on the BM1200 and have a break (again).
Just sit and relax.
Hi Martin,
I did and now I have nicely shining lamps.
Thanks.
Hi,
I replaced the lamps for the BM 1200.
The FM, LW, MW indicators started working after cleaning thei trmmer.
The stereo light worked for a short while but the trimmer needs to be replaced.
Can anyone tell which size it is? I didn't take out yet but I don't it's mentioned on the trimmer, also the schematics are not clear tot me.
Thanks
Kristof
Hey Kristof ,
Do you have the complete schematics?
If not I can send you a PM with copie of the relevant parts of the schematics and then we can see it from there if I can be of more help, although I am not an expert on these matters.
Hi Aad, I have a copy which is available on this site bit it's incomplete, there're also no numbers printed on the pcb which makes it a bit difficult to locate the pot trimmer on the schematics.
hello Aad, the circuit diagram extract only shows the transistor that turns on the lamps. The trimmer potentiometer at the base of the transistor controls the brightness of the lamps. The transistor requires a good to very good antenna signal to switch on the lamps !
wilfried
Hello Kristof,
IMHO there is no Trimmer Poti for the stereo lamp.
The Signal to switch on the stereo lamp comes form the 19kHz Detektor circuit and is depended on a very good FM antenna signal.
Witch Poti you want change?
To locate the Poti , can you sent a photo or it in your BM 1200? Then we can locate it in the circuit diagram an find out the value
Yes, I noticed that the pot controls the brightness of the stereo lamp, but if it's broken than the stereo lamp will not work. I will send a picture this evening
Hi Wilfried, it's not possible to send a picture via beoworld right now, I get an abend.
can this board plan help?