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Beocord 7000 - All good on 400Hz, new caps, issues with freq. response on 3kHz/10kHz

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alien2108
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alien2108 posted on Wed, Jan 6 2021 9:41 PM

Hey all!

I am restoring a Beocord 7000 and after Dolby Calibration, Record lever Calibration, PPM display level calibration I still have issues with right channel with higher frequencies.

I then changed all electrolytic capacitors (yes, all - Power, Signal and Control board) and issue is still there. 

I also confirmed that if I inject 10kHz AUX signal it comes out without an issue on right channel too! Which points me to the tape head itself or circuit from head->amps? Amps must be good if AUX signal works....

SIgnal 1 - Yellow - Left

Signal 2 - Blue - Right

Azimuth and signals on Dolby level 400Hz (test point P15) are perfection (400Hz ANT Dolby level tape):

Now if I switch to 3kHz signal from tape this happens:

And on 10kHz from tape:

What would be cause of such thing? Bad head, some other caps (non elco)?

Thank you for any tips!

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alien2108
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UPDATE:

Cleaning head - Since IPA did not do the trick I used Acetone which did help! A bit of Azimuth adjustment again and things are much much better!

Dillen
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Dillen replied on Mon, Jan 11 2021 9:57 AM

Well done!

But I wouldn't recommed using acetone on tape heads.
Not all heads will survive it, because acetone attacks certain types of plastics, used for insulating and casting in the coils.

Martin 

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