triple screen wire

I am restoring two Scully 284 eight track tape machines and one of them needs the wiring from the magnetic heads to the electronics completely replaced (some idiot who de commissioned the decks had the wiring severed)

One of the heads uses a coaxial wire with three separate layers of shielding / insulation. The shields terminate at different points on the connectors, so I don't think it can be substituted with a wire that has less layers of shielding

I am having trouble sourcing a suitable replacement, was wondering if someone has any suggestion where to find this type of wire ?

I have attached a picture of a stripped end to show a section of the wire. The overall diameter measures around 1/4"

Best source of audio related cables that I have seen would be Redco.

But I'm a little skeptical that you need triple shielding. Maybe there's a high frequency bias signal over it? Or maybe shielding of the original cable wasn't that great?

The wire is used for connecting the tape heads to the electronics, so it is carrying a high frequency bias signal. I guess that's actually the reason for the extra shielding

The screen on the original cable is spiral type, so it probably gives less shielding than a braided screen

I have been suggested triaxial wire that is normally used in video applications. This has only two separate layers of shielding, but the screen is braided, so it probably offers a better performance, but I am not really sure it would be enough

It will be quite a lot of work to make all the cabling from scratch and I would like to make sure I am using good materials

Post deleted - I couldn't see the third screen due to screen / photo size until I scrolled.

Could be for a ‘driven shield ‘ to reduce cable capacitance. You could go through the Belden catalog. My guess is you will have to construct the cable from coax and braided shield.

The Scully 280 reproduce section does indeed employ a "driven shield".

You are fortunate that there is an cable to copy but try to find some original cables, perhaps from parted out 2 or 4 track machines.

The value of the resistor coupling the internal shield to the first transistor emitter changed over the years,
especially when the germanium part was replaced with a  2N4250.

beatnik said:

I am restoring two Scully 284 eight track tape machines and one of them needs the wiring from the magnetic heads to the electronics completely replaced (some idiot who de commissioned the decks had the wiring severed)

One of the heads uses a coaxial wire with three separate layers of shielding / insulation. The shields terminate at different points on the connectors, so I don't think it can be substituted with a wire that has less layers of shielding

I am having trouble sourcing a suitable replacement, was wondering if someone has any suggestion where to find this type of wire ?

I have attached a picture of a stripped end to show a section of the wire. The overall diameter measures around 1/4"

Click to expand...

"Driven shield" is quite common in instrumentation, when a conductor must be shielded but the capacitance of teh shield is too high. You may try vendors specializing in lab equipment.
This is interesting, too.
https://www.copperbraid.co.uk/tubular-shielding-braids/?gclid=Cj0KCQiA_JTUBRD4ARIsAL7_VeWoftXnZx-ANqbiYmTSSfTWX2MBMUZPz7nmspPkAdZIGFVUwPq4fJkaArTYEALw_wcB

abbey road d enfer said:

"Driven shield" is quite common in instrumentation, when a conductor must be shielded but the capacitance of teh shield is too high.

Click to expand...

Yes but you need "only" Triaxial cable for a driven shield with one screen driven by a low impedance 'copy' of the signal and the outer screen at 0V for screening.
The OP's cable has an extra shield - just extra screening or something more complicated ?
btw I suggest people brace themselves if they end up looking at the cost of triaxial connectors 

Newmarket said:

The OP's cable has an extra shield - just extra screening or something more complicated ?

Click to expand...

A glance at the schemo would tell.

I actually have another cabling assembly from a second machine that is complete, so I have taken it out to make some comparisons

This cable actually has two shields only

The internal screen is the return signal of the playback head. The outer shields are all joined together on the heads side, but left unterminated, and on the other end they connect to chassis.

This is different than the severed cable, and from the original schematic, which indeed shows three shields.

In the schematic the internal shield is actually connected to pin 19 on the electronics connector which is connected via some resistors to the first transistor in the playback amplifier

This configuration is rather strange, what would be the purpose ?

I have attached a schematic of the electronics unit. It's small enough for the permitted size so not the best quality but should be usable

On the severed cable  the internal shield (pin 19 on the connector) is left unterminated at the heads end

I don't know where that shields goes on the other end, because i don't have the connector side anymore, but looking at the schematic it seems the internal screen is actually acting as a shield, but isn't referenced to ground, it goes to the playback amplifier first transistor emitter

Since the complete cable has a different configuration, I am wondering wether it has been retrofitted or the factory has changed wiring over the year

If the two shield configuration is fit for this purpose, I might just copy that. It seems impossible to find a cable with three shields anyway

Talk to Steve at Sonicraft. I would not be surprised if he has an original for you. He's also very generous with Scully expertise.

http://www.sonicraft.com/Analog_To_Digital_Transfer_Gear.html

beatnik said:

In the schematic the internal shield is actually connected to pin 19 on the electronics connector which is connected via some resistors to the first transistor in the playback amplifier

This configuration is rather strange, what would be the purpose ?

Click to expand...

That's the principle of active guard or driven shield. The emitter signal follows almost 100% the base signal, so there is no differential voltage between the active conductor and the active shield, which cancels the capacitive load.

so its purpose is to "counteract" the capacitance of the cable ?

would be possible that with a lower capacitance and better shielding cable, later machines were made with just dual shield wire ?

I cannot explain why the two machines have different wiring, and one doesn't actually follow the schematic

I guess I should just try the dual shield wiring I have and see how it works , but it would be good to know what's really the best way

beatnik said:

so its purpose is to "counteract" the capacitance of the cable ?

Click to expand...

Yes; it's also known as "bootstrapping".

would be possible that with a lower capacitance and better shielding cable, later machines were made with just dual shield wire ?

Click to expand...

Maybe, but in teh absence of teh original schemo, it's very hard to tell.

I cannot explain why the two machines have different wiring, and one doesn't actually follow the schematic

Click to expand...

It is very likely the heads were changed from High -impedance to low-impedance. At the beginning, tape recorder manufacturers used the same heads as their tube counterparts, which caused a lot of problems. With low-Z heads, most of the capacitive issues are solved.

I guess I should just try the dual shield wiring I have and see how it works , but it would be good to know what's really the best way

Click to expand...

Actually, I can't really figure out why the need for triple-shield, so I can't predict the consequences of using double-shield only; my crystal ball is under repair...