Revox A700 Mic inputs - UK Vintage Radio Repair and Restoration Discussion Forum

MikeCross · 1st Apr 2019, 4:39 pm

I have just purchased a Revox A700 Recorder and wish to use the Mic inputs.
These inputs fail to work with known good microphones.
The previous owner said he never used these so not sure if they ever worked.
I don’t have a user handbook but the circuit diagram indicates two mic transformer Interface boards.
Were these boards an option or where they are located within the machine.

Thanks for any help available

TIMTAPE · 1st Apr 2019, 10:35 pm

I think they were standard in the A700 but they are also "balanced" inputs as with XLR 3 pin connectors. The mic's output is connected to "tip" and "ring" on a stereo headphone jack plug. A mic with a normal tip and sleeve jack plug probably wont work. There may be nothing wrong with the mic input sockets and preamps.

To test with a standard unbalanced mic you could try temporarily bridging its sleeve and ring connections in the jack plug, or by making up an adaptor lead.

Radio Wrangler · 1st Apr 2019, 10:45 pm

Don't unbalanced mikes on mono jack plugs short the ring contact to ground, providing continuity for an unbalanced signal on the tip?

David

TIMTAPE · 1st Apr 2019, 10:48 pm

Good point David, they do. My apologies.

Michael Maurice · 1st Apr 2019, 11:08 pm

The amplification system for both Mic and Line is the same but with different gain set by feedback.

Radio Wrangler · 2nd Apr 2019, 12:36 am

Could be just dirty sockets.

David

MikeCross · 2nd Apr 2019, 12:18 pm

I was using RTS jacks connected to 600 ohm AKG mics.
Cleaning sockets has now given me 3 out of 4 sockets working with possible intermittent master record level slider.
4th jack appears to be loose, perhaps joints on pcb fault.
However still looking for user manual if available.

Thanks for your help.

ms660 · 2nd Apr 2019, 12:24 pm

Revox A700 user manual is available on the vintageshifi site.

Lawrence.

1st Apr 2019, 4:39 pm	#1
MikeCross Tetrode Join Date: Sep 2016 Location: Tring, Hertfordshire, UK. Posts: 87	Revox A700 Mic inputs I have just purchased a Revox A700 Recorder and wish to use the Mic inputs. These inputs fail to work with known good microphones. The previous owner said he never used these so not sure if they ever worked. I don’t have a user handbook but the circuit diagram indicates two mic transformer Interface boards. Were these boards an option or where they are located within the machine. Thanks for any help available

1st Apr 2019, 10:35 pm	#2
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Revox A700 Mic inputs I think they were standard in the A700 but they are also "balanced" inputs as with XLR 3 pin connectors. The mic's output is connected to "tip" and "ring" on a stereo headphone jack plug. A mic with a normal tip and sleeve jack plug probably wont work. There may be nothing wrong with the mic input sockets and preamps. To test with a standard unbalanced mic you could try temporarily bridging its sleeve and ring connections in the jack plug, or by making up an adaptor lead. Last edited by TIMTAPE; 1st Apr 2019 at 10:44 pm.

1st Apr 2019, 10:45 pm	#3
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,866	Re: Revox A700 Mic inputs Don't unbalanced mikes on mono jack plugs short the ring contact to ground, providing continuity for an unbalanced signal on the tip? David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

1st Apr 2019, 10:48 pm	#4
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,969	Re: Revox A700 Mic inputs Good point David, they do. My apologies.

1st Apr 2019, 11:08 pm	#5
Michael Maurice Moderator Join Date: Feb 2006 Location: Wembley, Middlesex Posts: 7,223	Re: Revox A700 Mic inputs The amplification system for both Mic and Line is the same but with different gain set by feedback. __________________ Forum Moderator http://www.michaelmauricerepairs.co.uk/

2nd Apr 2019, 12:36 am	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,866	Re: Revox A700 Mic inputs Could be just dirty sockets. David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

2nd Apr 2019, 12:18 pm	#7
MikeCross Tetrode Join Date: Sep 2016 Location: Tring, Hertfordshire, UK. Posts: 87	Re: Revox A700 Mic inputs I was using RTS jacks connected to 600 ohm AKG mics. Cleaning sockets has now given me 3 out of 4 sockets working with possible intermittent master record level slider. 4th jack appears to be loose, perhaps joints on pcb fault. However still looking for user manual if available. Thanks for your help.

2nd Apr 2019, 12:24 pm	#8
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Revox A700 Mic inputs Revox A700 user manual is available on the vintageshifi site. Lawrence.