fazer

United Audio 1108 Raysistor control
« on: June 24, 2020, 05:09:42 PM »
I have  two 1108 UA preamps using Raysistor by Raytheon CK 2037's for control of level.  It says in Raytheon's old brochure to use not more than 4 volts and not more than 55 ma. of current to control these elements.   So thats doable but these things use an incandescent bulb to change resistance of the sealed  Cadmium cell.   Seems like something ready to die at any moment. 

What are other people doing to  modify these things for service.

The TO 5 can is the Raysistor to control .   


Re: United Audio 1108 Raysistor control
« Reply #1 on: June 24, 2020, 05:20:39 PM »
A lot that I've seen have the part just bypassed.  I don't know if this was because it had died or just because?
I seem to remember on the units I played with @ UA that the amp is quieter without it as well.

If you kept the leads short, I suppose you could wire in a pot instead.  The original idea behind doing it as they did was to control level from a greater distance - ie. the amps were in the back of the desk, the control was on the desk surface type of thing.
Jeffrey Toobin: "This is the most embarrassing week ever."

Rudy Giuliani: "Hold my pants..."

fazer

Re: United Audio 1108 Raysistor control
« Reply #2 on: June 24, 2020, 07:42:40 PM »
The cell is 1K to 5 meg adjustable -  between the the transformer and base of Q1.   So seems like a small value resistor or just jump it and build a pad on input for adjustment. 

I'm waiting on a pair of 10 pin card edge connectors to power up and record.   

Thanks for help .

mjrippe

Re: United Audio 1108 Raysistor control
« Reply #3 on: June 27, 2020, 06:15:36 PM »
If you don't want a gain control you yank the LDR and insert the jumper wire that connects to the junction of R1 & R2 into pin one of the LDR socket.  Then connect pins F and 6 to pin 4 (B-) on the card edge connector.

If you want a gain control without the LDR, jumper pin 1 to pin 2 on the LDR socket and pin 3 to pin 4.  Then run a shielded 2 conductor wire from pins F and 6 with the shield connected to ground on the connector end only.  The far end goes to a 1M log taper pot.  Pin 6 goes to the wiper, Pin F goes to the outside lug of the pot (zero ohms to the wiper when pot is full CCW).

I have used this on racked modules with no issues.  If you are not inside a rack enclosure, keep the leads to the pot as short as possible.

Mike

fazer

Re: United Audio 1108 Raysistor control
« Reply #4 on: June 28, 2020, 09:47:10 AM »
If you don't want a gain control you yank the LDR and insert the jumper wire that connects to the junction of R1 & R2 into pin one of the LDR socket.  Then connect pins F and 6 to pin 4 (B-) on the card edge connector.

If you want a gain control without the LDR, jumper pin 1 to pin 2 on the LDR socket and pin 3 to pin 4.  Then run a shielded 2 conductor wire from pins F and 6 with the shield connected to ground on the connector end only.  The far end goes to a 1M log taper pot.  Pin 6 goes to the wiper, Pin F goes to the outside lug of the pot (zero ohms to the wiper when pot is full CCW).

I have used this on racked modules with no issues.  If you are not inside a rack enclosure, keep the leads to the pot as short as possible.

Mike

Comprehensive description.  Thanks Mike.

Re: United Audio 1108 Raysistor control
« Reply #5 on: June 28, 2020, 05:18:53 PM »
mjrippe, is the gain control without LDR outlined here controlling negative feedback, or something else ?

I'm wondering if it changes the sound and how well it works. I have a group of these that I am planning to turn into to a sidecar, and was planning on just using an input pad and output attenuator.  But maybe I should include this too.

thanks

Re: United Audio 1108 Raysistor control
« Reply #6 on: June 28, 2020, 06:24:28 PM »
mjrippe, is the gain control without LDR outlined here controlling negative feedback, or something else ?

I'm wondering if it changes the sound and how well it works. I have a group of these that I am planning to turn into to a sidecar, and was planning on just using an input pad and output attenuator.  But maybe I should include this too.

thanks

It isn't changing negative feedback, it's just a level control after the input transformer.

I've seen a couple of original desks (Bushnell designed?) that utilised the 1108 as well as the more recently built "Copper" desk that Neil Young put together for live stuff.
In those desks neither the LDR or a pot were used as far as I recall  but, rather, a multi input pad.   I remember there being a noise and frequency response penalty with the LDR/pot scheme myself but  Mike says he's done it that way so I could be imagining it.
 Try both and see.

P.S.  Thanks Mike for the rundown  :)
Jeffrey Toobin: "This is the most embarrassing week ever."

Rudy Giuliani: "Hold my pants..."

Re: United Audio 1108 Raysistor control
« Reply #7 on: June 29, 2020, 07:43:03 PM »
thanks Winston.  I've seen pictures of two desks which were designed by the UA Studio Electronics division that was doing custom gear in the mid/late 60s,  which also just used pads before the 1108s.


mjrippe

Re: United Audio 1108 Raysistor control
« Reply #8 on: June 30, 2020, 04:31:06 PM »
Hey folks, I think we have talked about the possible penalties of the post-transformer attenuator elsewhere recently so I won't go into all that.  A multi-position pad is certainly a practical alternative.  And yes, for those who need the fine gain trim, it could be added at the output.  Again, all this has been talked to death if you do some searching of the forum.  I was just posting what I have done and know to work.  Whether you like the method or the sound is up to you.  ;)

Mike

EmRR

Re: United Audio 1108 Raysistor control
« Reply #9 on: June 30, 2020, 04:36:13 PM »
Do we know input transformer ratio? 
Best,

Doug Williams
Electromagnetic Radiation Recorders

"I think this can be better. Some kind of control that's intuitive, not complicated like a single knob" - Crusty

"Back when everything sounde


mjrippe

Re: United Audio 1108 Raysistor control
« Reply #10 on: June 30, 2020, 09:18:15 PM »
And just in case anyone wants to refer to the UA docs - https://groupdiy.com/index.php?topic=45039.msg592060#msg592060


 

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