[time-nuts] Resoration of 5065A - continued...

Bob kb8tq kb8tq at n1k.org
Mon Sep 11 12:53:34 EDT 2017


It’s been a *long* time since the 709 or 741 were “best” in any regard. I would not go with 
socketed parts. I’ve seen more problems from sockets than from the parts that go into them.
Semiconductor quality / reliability has come a *long* way since the 1960’s.

How to redesign the board mostly involves figuring out what it does and how things like offset
and drift impact the overall performance. The choice in general is between very low leakage 
(= high impedance) parts with high(er) offsets or very low offset parts (= low impedance) with
high(er) bias currents. Yes, there are a bunch of other things, but that’s the first branch in the 
path for DC control stuff. Noise is obviously an issue, but the 709 / 741 are pretty noisy by 
today’s standards. 


> On Sep 11, 2017, at 5:56 AM, Ulf Kylenfall via time-nuts <time-nuts at febo.com> wrote:
> Continuing my efforts to fix the 5065A...
> There was something wrong in the A7 (AC Amplifier assembly).
> DC balance of the input stage was completely off-scale
> and one of the 1854-0023 transistors had an open junction.Luckily I had a small supply of them but I also discovered that
> the OP-Amps were old "seven of nines" (709).
> There are several different manuals floating around on the internet,
> and the hard copy that I have shows two different versions
> of A7. One with 709 and one using 741 that is also different
> in terms of circuit design and layout.
> I am considering redesigning A7 using modern CAD and also
> purchasing boards from a professional vendor, but first I would like
> to know if anyone has tried both versions and if there are any
> improvements. The modern 741-version uses DIP-socketed
> OP-Amps so that version could be tried with even better
> types.
> BR
> Ulf Kylenfall   SM6GXV
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