Improving on a Revox B77 frequency curve

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#42
Indeed, and I don't want to compromise too much on bias.
EQ should normally flatten the curve.
Anyway, I found what looks like a good compromise, I'll do some more testing with real music
and if I like it I'll call it a day.
I'm very grateful for all the help and more ideas are always welcome.
 
#46
The best you can do is trust Revox engineers at fixed passive component selection, i.e. don't change any component. Apart from this, calibrate at -20 dB rec/playback level the following way:
Record a 315 Hz or 400 Hz sine, note the playback level. Then record 10 kHz, and adjust the HF bias so that you get the same playback level. I think this is the best you can do.
Unless you have an oscilloscope, then you can adjust the playback head azimuth with Lissajous pattern.
 
#52
You mean like this? Or the same on the record head?

Repro.jpg
 
#53
THE FULL PROCEDURE:

1. You play a 'certified' playback test tape of your IEC or NAB format & zero reference level. ( I always used MRL test tapes )
2. If the adjustments are available > You adjust: Height, Wrap, Zenith and Azimuth for optimal performance.
If playback heads are worn to any significant degree, you DON'T touch any of the above adjustments.
3. You adjust your playback Zero reference output & metering.
4. You adjust your playback frequency response using the 'spot frequencies' of your test tape.
* If HF spec. can't be achieved in one channel, you can add small capacitance across the head AND/OR playback level pot.
5. Re-check playback Zero reference.
6. Adjust your "bias traps" at test points for maximum NULL.
7. With your desired tape type in record mode, adjust for "peak bias" @ 1Khz.
8. Adjust HF record EQ to achieve flattest response.
9. If either channel is not flat, or out of spec. use UNDER or OVER biasing to correct.
*Use this 'interplay' of bias & HF eq to achieve the flattest frequency response.
10. Allways double-check your Rec/Play zero reference.

PS.
Using pink noise IS A WONDERFUL WAY to adjust things by ear 🙂
 
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#54
That's basically how I did it. I used Heinz Bluthard tapes 1kHz NAB 250nWb/m 7.5 & 19cm/s
I didn't touch the heads except for a very slight adjustment of Azimuth. +/- 1/10th of a millimetre.
All other adjustments were done according to the service manual.
Final fine tuning with pink noise at -20dB and NAK T-100. (Better than my ears 🙂 )
Overall, I'm happy with the results.
Some more could be done, and I've ordered new ball bearings for the tape guides.
Blasting the capstan is another option if I can find someone to do it without having to send it to Nagra.
https://nagravox.com/products/capstan-shaft-reconditioning-service?variant=9074976751660
 
#56
Both methods will go: two heads shorted or both amps to one head. The idea is to look for the locus of the fault: amp or head.

Another idea may be to separate head from amp and feed amp with a source of white noise while watching the screen of your analizer. Ore some kind of anti-NARBT generator.
 
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