AW: [sdiy] Urei 1176

[jhaible at t-online.de]

> I have one of these. I have a few notes I made based on measurements 
I made
> when fixing/setting it up- not from a service manual but it works 
fine and
> the threshold was not changed from when I got it.
> It starts: 'Adjust Q1 bias preset (R81- the one whose wiper is 
connected to
> the release control) so wiper is minus 2.6V with no input signal'


Thanks - that's one more valuable bit of information.
It could mean that Q1 is selcted for 2.6V threshold, but it could
also mean a smaller threshold and some "reserve" before the FET
starts to conduct.


There is some threshold from the diodes in the envelope follower,
too (are these Si or Germanium, btw ?), and a variable offset to
this that is switched with the compression ratio buttons.

All this works together; so even with adjustment of the bias
trimmer to fit a varying FET threshold, I expect different
behaviour from the interplay of these factors. 
Simple circuits are the most difficult ones, sometimes ...


> Did an answer come out for this? I always kinda wondered what would 
make
> a good replacement part....


I made first tests with a BF245A (threshold smaller than 2.6V).
It does compress, and compression varies with the knob settings
(ahem ... alligator clips at the resistor dividers), but so far
I have only looked at it on the scope with tone bursts from the
Synthi. Hope I can make some listening tests soon.
I think I'll select one close to 2.6V.

BTW, there is a slight treble boost designed into the signal
path. (Look at the feedback loop of the line driver amp!).
I don't know if this is to compensate the frequency response
of an output transformer, or to give the signal some extra
brilliance that might be lost from compression.
This is all very new to me ...

JH.