What I am doing now

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Christian Christensen Posted: Mon, May 19 2014 11:07 PM

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As I got this set of white BM1001, white BV1001, white BG1200 and white BG reel 1200

I though I should do a bit more usual

BG1200 is ready and refurbed.

This BM1001

Holy smokes, I have many many evenings with it.
When I opened it , it was a fat layer of , from most llikely kitchen steam, mixes up with animal hair that had created a smelly , sticky mess, ALL OVER. 
I spent most likely two evening clening it, where one evening where scale thread and the radio mechanics.

The scale mover, was really badly reparied with glue sticking out everywhere.
It looked like some repairman had reparied it with time stress. ( have another BM 1200 that has been reparied just as bad )
spent 3 evenings trying to get old glue of the plexi, and make the metal holder fit better from old lousy repair
A enormous time consuming work that needs a lot of patience and better eyes that I have.
Glued - took apart the scale mover 3 times, was not satisfied.
Now the scale mover looks way way  better, at least as good as my skills is.

Re-capped as long as my storage of caps is, around 12 caps exchanged.
( very frustrated of not haveing enough caps at home )

lamps exchanged. - one lampholder broke - bought a new . and of course it didnt fit. special fopr B&O
Had to pick one from a a BM1200 I have with a completly damage wooden frame.

Stereo searation fine tuned, FM indicator lamp strength adjusted

For now - its finished and I am very happy with my work

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Christian Christensen replied on Sat, Jul 19 2014 12:47 AM

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Calibration of BC1200

First done calibration of VU meters, due to service manual .- easy

Now running calibration tape for playback frequence.

Tricky. Service manual is not giving any volt reference in this prodedure, only telling the tech to plot in the result in a dB diagram.
Decibel is a value of reference, so witout mentioned reference, I have to guess.

It might be that the service manual is written in the higher technical school of technical documentation ethics that you leave out information that is to be taken for granted or can be read out be other information in order to lower risk for misinterpretation.
But this ethics has I never found in this kind of service manuals before, they have been step by step written for simpel manouvers....guess I need to process this a bit...read abit more theory before I do any conclusions on how to interpret the service manual.

Running the tape reference level shows about minus 1dB wrong on the VU meter, which is way enough good, if I am using correct calibration tape, or do I have to convert ? Serveice manual doesnt tell me which calibration tape to use.

And all the test tones from 63Hz to 20Khz gave 500 mV top to top value  ( not RMS )on the osciloscope, which  indicates  very straight frequence kurve, indicates also  that tape head is most likely in good shape, it also shows that reference level is close to 0dB on the Vu meter.
But questions are still un answered for me. 0dB to what calibration standard ? in what correltation to what the volt messure.
Service manual doesnt tell me.



Execption was 250 Hz that gave 600mV and 32 Hz that was abit lower. 

If I am lucky everything is just fine....but I would like to understand EXACTLY what I am doing.

Maybe the service manual is just badly written....Maybe I need to gain knowledge....