Custom 600-Ohm T-PAD for 1176

anonyfous · Feb 16, 2023

Hi,

I came across lot of old NOS dual pots, lot of different values and tapers (Linear, logaritmic, exponential), they are easily disassembled, so I can make a custom one, that led me to the idea that I could replicate the T-PAD attenuator that they sell here: New UREI 600 ohm T-Bridge Input Pot for UREI and UA 1176 & 1176LN Rev A-F. UM. I only need to know what values and type of taper should I combine. The original schematics for the input section are here.

and here are the pots:

any suggestinons?

Thanks,

Lukas.

beatnik · Feb 17, 2023

I am not sure about the dual deck, what I have been using for the 1176 is a triple section pot 500ohm lin - 1k log - 500ohm lin this is sold by several diy shops.

anonyfous · Feb 17, 2023

Yes. hairball sells bourns triple deck custom pot that si 2x 500 ohm lin and 1x 1K log, but I have found out that in the original UREI unit there was a dual deck pot and two 620ohm resistiors, but I would like to know the values and types.. It is pretty easy to customize the dual deck pot, you just swap the tapers. Not so easy to do the triple deck, not impossible, but lot more work mechanically..

emrr · Feb 17, 2023

3 is a T, 2 is a bridged T. The modern thing isn't really a T, it's an approximation that works mostly OK across some of the range. A real one goes close to 5K on the shunt before going open at '0'. Modern 3 deck minimum loading is 1K.

I'll try to remember and check if I have any 2 deck to measure.

13engrsapper · Feb 17, 2023

I have 500ohm bridged t clarostat units.

anonyfous · Feb 17, 2023

Thank you!

Ok, so we are talking about bridged T here. From the schematics I just need the value and type (log/lin) of R1A and R1B.

emrr · Feb 20, 2023

Haven't found a continuous bridged T yet.

3 deck continuous T in front of me is a 3K6 shunt that goes open at '0' and a pair of 600R.
The 600R's are about 450R at 50% rotation, the 3K6 is about 250R.

3K6 would be the same for a bridged T.

10K pot in parallel with 1240R could be a good guess, really depends on the taper, max value 1103R at fully 'off', where a real one should also go open leaving just the 1240.

I suspect the exact tapers aren't available, and that it also doesn't matter in an 1176 so long as it feels ok in action.

limo · Feb 22, 2023

I have the attenuator removed from the two 1176 revision "B".
The original attenuator has two legs each. R1A does not have pin 1 and R1B does not have pin 3.
Where there are no pins, they are floating.
The manufacturer is Allen-Bradley, and although it says 600ohm, it is not the value of this Pot.
I actually measured one R1A is 330K ohms and the other is 300K ohms. R1B are both 35K ohms.
The curve is unknown, but it is not a linear curve because R1A is about 5k5 to 6K5 ohms and R1B is about 80 ohms at the approximate center.　
I think it's close to the Log curve, but it could be more complicated.
My 1176 has a Bridge-T attenuator made by ”Tokyo Ko-on Denpa”. Please check this out for https://ameblo.jp/junker-life/entry-12730884646.html