The No.1 Website for Pro Audio
77DX Output Wiring
Old 14th August 2020
  #1
Gear Addict
 
Kroc's Avatar
77DX Output Wiring

I’m finally getting around to replacing the disintegrating half-century old cable on my 77DX.

The previous cable was wired up for 150 Ohm output, but I'm looking to wire it for 250 Ohms, and the service manual diagram is a little convoluted. As I understand it, the ground and red wires will be connected to the same places, but I'm unsure as to whether the black XLR wire goes to the same terminal tab as the red wire (bottom right in attached photo, with light green wire attached), or to the bottom left terminal with the yellow wire attached.
(Edit: Seems like the photo was rotates anti-clockwise during upload. I’m referring to the terminal tabs with yellow and green wires on the right)

And it seems like the braided metal ground wire in the XLR (not the green wire) just gets trimmed off?

Also, I’m replacing it with a 14 foot NOS cable. Is there any harm in leaving that much cable length in terms of signal degradation?

Thanks
Dan
Attached Thumbnails
77DX Output Wiring-0c7abe1b-3eb9-403d-b95a-46de29eb00f2.jpg  
Old 14th August 2020
  #2
This should answer most of your questions https://www.google.com/url?sa=t&rct=...jsSdjU9s1axOhP

Best of luck!
Old 14th August 2020
  #3
Lives for gear
 
kludgeaudio's Avatar
 

Quote:
Originally Posted by Kroc View Post
I’m finally getting around to replacing the disintegrating half-century old cable on my 77DX.

The previous cable was wired up for 150 Ohm output, but I'm looking to wire it for 250 Ohms, and the service manual diagram is a little convoluted. As I understand it, the ground and red wires will be connected to the same places, but I'm unsure as to whether the black XLR wire goes to the same terminal tab as the red wire (bottom right in attached photo, with light green wire attached), or to the bottom left terminal with the yellow wire attached.
The service manual diagram says that the two pins on the top are used for signal and the ground remains on
the lower pin for 250 ohm output. That is, the ones with the black and green wires from the transformer. The big diagram on the upper left on the service manual shows it strapped for 250 ohms.



Quote:
Originally Posted by Kroc View Post
And it seems like the braided metal ground wire in the XLR (not the green wire) just gets trimmed off?

Also, I’m replacing it with a 14 foot NOS cable. Is there any harm in leaving that much cable length in terms of signal degradation?
The original cable that you see in the manual has three twisted wires. They are used for two signal wires and the ground, and the shield is lifted (disconnected) at the microphone end. This is a very very smart thing to do when you are concerned about RF noise and have high impedance connections. Most people today do not do it, and instead would connect the cable shield in place of the green wire so the only ground is through the shield.

Because this microphone really can't drive a lot of current and it's probably being used into high impedance preamps, you really want to make sure that the cable capacitance is low. If possible, use a short cable. If you can't use a short cable, you can't, so don't put yourself out too hard for that. Use cable with the lowest capacitance you can find (and that will not be star quad cable).

Gotham GAC-3 is an old-style three-conductor microphone cable that will allow you to keep the wiring original. It is not expensive, and it is designed for this sort of thing. However, I have to admit that all of my RCA mikes are wired with Belden AES/EBU cable or old Apple Localtalk cable as cheap routes to low capacitance.
--scott
Old 14th August 2020
  #4
Gear Addict
 
Kroc's Avatar
Quote:
Originally Posted by Thomas W. Bethe View Post
This should answer most of your questions https://www.google.com/url?sa=t&rct=...jsSdjU9s1axOhP

Best of luck!
That's actually the exact service manual with the confusing diagram that I was referring to in my original post.
But thanks.

Quote:
Originally Posted by kludgeaudio View Post
The service manual diagram says that the two pins on the top are used for signal and the ground remains on
the lower pin for 250 ohm output. That is, the ones with the black and green wires from the transformer. The big diagram on the upper left on the service manual shows it strapped for 250 ohms.

The original cable that you see in the manual has three twisted wires. They are used for two signal wires and the ground, and the shield is lifted (disconnected) at the microphone end. This is a very very smart thing to do when you are concerned about RF noise and have high impedance connections. Most people today do not do it, and instead would connect the cable shield in place of the green wire so the only ground is through the shield.

Because this microphone really can't drive a lot of current and it's probably being used into high impedance preamps, you really want to make sure that the cable capacitance is low. If possible, use a short cable. If you can't use a short cable, you can't, so don't put yourself out too hard for that. Use cable with the lowest capacitance you can find (and that will not be star quad cable).

Gotham GAC-3 is an old-style three-conductor microphone cable that will allow you to keep the wiring original. It is not expensive, and it is designed for this sort of thing. However, I have to admit that all of my RCA mikes are wired with Belden AES/EBU cable or old Apple Localtalk cable as cheap routes to low capacitance.
--scott
Thanks for your response.
I'm looking again at the manual and the Coutant article. Now I'm unsure if my microphone was actually wired for 150 or 250 Ohms? (The attached photo above). If it was already wired for 250, then it's all super easy.

The new cable I'm putting on is actually a new old stock 14 foot brown cable from the '50s, which looks identical to the original.
Old 15th August 2020
  #5
Gear Addict
 
Kroc's Avatar
Never mind! My bad - I was running on four hours of sleep yesterday, and for some reason I was convinced the pictured configuration was 150 Ohms. Which would have made the connection diagrams incongruous with the visual terminal layout. Things made a lot more sense today!

But I wired it back up the same way and all is well. Thanks for the help anyway.
📝 Reply
Topic:
Post Reply

Welcome to the Gearslutz Pro Audio Community!

Registration benefits include:
  • The ability to reply to and create new discussions
  • Access to members-only giveaways & competitions
  • Interact with VIP industry experts in our guest Q&As
  • Access to members-only sub forum discussions
  • Access to members-only Chat Room
  • Get INSTANT ACCESS to the world's best private pro audio Classifieds for only USD $20/year
  • Promote your eBay auctions and Reverb.com listings for free
  • Remove this message!
You need an account to post a reply. Create a username and password below and an account will be created and your post entered.


 
 
Slide to join now Processing…
🖨️ Show Printable Version
✉️ Email this Page
🔍 Search thread
🎙️ View mentioned gear
Forum Jump
Forum Jump