[time-nuts] "The Penultimate HP5065 A15"

[Bob kb8tq]

Hi

> On Aug 4, 2020, at 3:07 PM, Poul-Henning Kamp <phk at phk.freebsd.dk> wrote:
> 
> --------
> Bob kb8tq writes:
> 
>> If a more extensive rebuild is in the works …..
> 
> Yes, that's where we're headed eventually, but I prefer to do it
> incrementally.
> 
>> +/-20V is (as has been observed) not an ideal voltage for “modern” electronics. 
> Right, and being able to drop that down later is in the design.
> 
> However, from a noise point of view, there is a lot to be said for
> having local LDO's on individual subassemblies.
> 
>> A11 Unless you want to redo the heater windings on A12, you are
>> stuck with +20 to +30V. Rest of the board sort of begs for a modern
>> op-amp approach.
> 
> I have not given much thought to A11 yet, and I do find a certain
> elegance in the wien-bridge approach.  It is worth noting that the
> actual heaters run of the unregulated supply, when I stabilized
> that, A11 worked less hard.

The AC bridge approach certainly worked well “back in the day”. The 
thermistors that are in there are well adapted to that approach and not
quite the value that one might want for a more modern controller. One 
could deal with that.

The biggest issue I see with the design is that there are a few points of 
failure on the board that could take out the whole device. One could 
improve the MTBF of the control circuit and upgrade the ratings on the 
pass transistors at the same time. 

You also would get rid of the residual audio flying around in the heater
winding. There may not be much, but there likely is some (espically if the
caps have started to die off ….).  Watching how various examples of 
the 5065 warm up (and ring), I suspect the A11 boards may not be in
ideal shape ….


> 
> Unless you plan to run the HP5065 at the freezing point, I expect
> the heaters could run of a lower voltage, though +15V may be too low.

The real question is - how long do you want to wait for it to warm up at
normal temperatures? I suspect it would take a pretty long time to get
going in your frozen garage in the middle of the winter, even with full voltage
on the windings. 

If you really want to go nutty: Do an AC controller with an MCU in the 
middle of it …..

Bob

> 
>> A14 If the upstream boards get changed, this likely does as well.
>> Sort of begs for a $1 MCU and a handful of resistors as a replacement. 
> 
> I was actually pondering doing that on A17.
> 
> -- 
> Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
> phk at FreeBSD.ORG         | TCP/IP since RFC 956
> FreeBSD committer       | BSD since 4.3-tahoe    
> Never attribute to malice what can adequately be explained by incompetence.