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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 20th Nov 2022, 3:03 pm   #1
sixtimesseven
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Default Revox A77 Crosstalk Problems

Hi

I got my A77 mostly working after recapping and replacement of the oscillator NPN's.

But I noticed that I can hear the music coming from the AUX faintly even if I set the output to NAB. What amount of crosstalk would be normal for such a vintage machine?

Another interesting problem is that I get a lot of noise if I set the input to MIC Low and to a lesser extend, on MIC HI.
From the overall diagram the signals should take the same path after the switch
Could it be just the rotary selector switch?
Why is there even a MIC LO / HI input? Does it just select front or back input?

Thx
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Old 20th Nov 2022, 6:51 pm   #2
Analogue man
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Default Re: Revox A77 Crosstalk Problems

The Mic Lo/Hi settings are to do with the impedance of the microphone being used.
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Old 20th Nov 2022, 8:15 pm   #3
TIMTAPE
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Default Re: Revox A77 Crosstalk Problems

What you describe may or may not indicate problems with the A77.With the "crosstalk" is it while actually making a recording and monitoring off tape? Higher noise at Mic Hi and even more at Mic Lo is normal. The preampifiers are running at much higher amplification to compensate for the expected much weaker input signal voltages. The noise should diminish when a suitable mic is connected to the mic input. Generally, various factors seem these days to have created in many people unrealistic expectations of "no" background noise or signals. Its not like that in real life. The manuals speak in relative terms of "signal to noise ratio" and quote a number, rather than absolute signal purity with no noise or crosstalk. Often unwanted signal is helpfully masked by another noise such as tape noise. We live in a world of noise. Managing rather than totally eliminating it is usually the watchword.

Last edited by TIMTAPE; 20th Nov 2022 at 8:39 pm.
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Old 21st Nov 2022, 8:55 am   #4
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Default Re: Revox A77 Crosstalk Problems

In addition to the last post:

The schematic shows that the input impedance in the MIC LOW position of the switch is 1M. In the MIC HI position, a 2K2 resistor is switched parallel to the 1M resistor.
 
Old 21st Nov 2022, 11:37 am   #5
DMcMahon
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Default Re: Revox A77 Crosstalk Problems

Technical details/Specification from various Revox A77 documentation for the microphone inputs:-

Low Impedance (Input Impedance 2.2kOhm) suitable for 50 - 600 Ohms, sensitivity 0.15 - 15mV

High Impedance (Input Impedance 220kOhm) suitable up to 100kOhms, sensitivity 2.5 - 250mV

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Old 21st Nov 2022, 11:51 am   #6
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Default Re: Revox A77 Crosstalk Problems

I overlooked the 220K resistor at both mic inputs.

I would think that the specified 220K for the high impedance input is not exactly right. There is the 1M resistor in parallel to the 220K resistor, resulting in a total impedance of about 180K. No big deal of course.
 
Old 21st Nov 2022, 3:53 pm   #7
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Default Re: Revox A77 Crosstalk Problems

Quote:
Originally Posted by sixtimesseven View Post

But I noticed that I can hear the music coming from the AUX faintly even if I set the output to NAB.
Similar to Post 3 it is not clear to me what you are doing, are you feeding in a source signal into one of the Auxiliary inputs then hearing signal crosstalk on the other channel during monitoring of the record signal ?

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Old 24th Nov 2022, 9:58 pm   #8
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Default Re: Revox A77 Crosstalk Problems

I have encountered a small level of “ crosstalk/breakthrough “ such as you describe on an A77. I just assumed it was normal.
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