Revox A77 Crosstalk Problems - UK Vintage Radio Repair and Restoration Discussion Forum

sixtimesseven · 20th Nov 2022, 3:03 pm

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

Analogue man · 20th Nov 2022, 6:51 pm

The Mic Lo/Hi settings are to do with the impedance of the microphone being used.

TIMTAPE · 20th Nov 2022, 8:15 pm

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.

Robert Gribnau · 21st Nov 2022, 8:55 am

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.

DMcMahon · 21st Nov 2022, 11:37 am

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

David

Robert Gribnau · 21st Nov 2022, 11:51 am

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.

DMcMahon · 21st Nov 2022, 3:53 pm

Quote:

Originally Posted by sixtimesseven

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 ?

David

Tim · 24th Nov 2022, 9:58 pm

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

20th Nov 2022, 3:03 pm	#1
sixtimesseven Diode Join Date: Nov 2022 Location: Zurich, Switzerland Posts: 1	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

20th Nov 2022, 6:51 pm	#2
Analogue man Hexode Join Date: Dec 2020 Location: Raunds, Northamptonshire, UK. Posts: 353	Re: Revox A77 Crosstalk Problems The Mic Lo/Hi settings are to do with the impedance of the microphone being used. __________________ Graham

20th Nov 2022, 8:15 pm	#3
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	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.

21st Nov 2022, 8:55 am	#4
Robert Gribnau Heptode Join Date: May 2017 Location: Konongo, Ghana Posts: 514	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. __________________ Robert

21st Nov 2022, 11:37 am	#5
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,603	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 David

21st Nov 2022, 11:51 am	#6
Robert Gribnau Heptode Join Date: May 2017 Location: Konongo, Ghana Posts: 514	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. __________________ Robert

24th Nov 2022, 9:58 pm	#8
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	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. __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly."