Hmmm, love the story so far. Sitting on the edge my chair re the
900Hz. I am trying to get back to a project that needs approx.
200W power at 1600 - 1800 Hz [A Rebecca IV]. I have made a couple of
tiny 400Hz supplies for selsyns and looked at the variable frequency
supplies often on ebay [for spindle motors]. One cheap supply spec
said 0 - 2500Hz. Turns out that is period not frequency :-( It
is 0 - 400Hz.
I have enough 'junque' to try to make a FET 'vibrator' supply or a
high power audio amp. Have even bought a couple of transformer design
books.
Looking forward to the next instalment.....
John K
----- Original Message -----
From: neonixie-l@googlegroups.com
To:
Cc:
Sent:Mon, 23 Jun 2014 04:14:21 -0700 (PDT)
Subject:[neonixie-l] Saga of the Sperry part 2
Hi all,
Part 1 described the innards of the Sperry radar indicator that I
wanted to use for a clock and it's difficult internal structure. Now
I'll get started on the design.
The first step was to work out what frequencies and voltages I would
need. Experience has shown that CRT clocks need to run at a scan
frequency that is related to the mains frequency in order to prevent
stray electric and magnetic fields in the environment from wobbling
the display. This was not likely to be a big issue here as the neck of
the CRT was shielded and by the time the beam got out of the shield it
would be accelerating fast enough to not be seriously affected.
Nevertheless I decided on 900 Hz as it is a multiple of both 60 and 50
and was in the design range of the power transformers. Looking at the
ratings of those transformers I worked out that I needed about 50
watts, possibly less if I could derive the +/- 12 volt and the shift
coil supplies from the mains. I also thought it would be pretty nifty
if I could drive the scan coils from a rotary resolver driven by a
stepper which in turn would be controlled by an AVR and GPS receiver.
Cool huh?
The only snag was that I didn't have, and would almost certainly
never have, the proper resolver! I did have a couple of surplus 50 and
400 Hz synchros though, so why not try?
The 100 volt 900 Hz power supply had to be a sine wave as that was
expected by the area balance circuit. This meant that the power supply
would be a linear amplifier driven by a sine wave source. Inefficient
I know, but making a switch mode sine wave inverter was just too hard
given the difficulty in finding suitable cores to work at 900 Hz. I
designed up a 900 Hz exciter using a 4046 PLL phase locked to the
mains followed by an active low pass filter to knock off enough
harmonics to make it look like a sine wave. I included a voltage
controlled attenuator to allow feedback control of the output and
built it up on a little PCB - it worked very well and supplied several
other outputs that I thought I would need.
For the power amplifier I chose a LM3886 which was cheap and capable
of generating lots more power than I needed providing it was heat sunk
properly and supplied with enough volts. It would need a transformer
to shift the output voltage to 100 and I raided both my and several
friends junk boxes to find what I though might work. I had a suitable
80 VA mains power transformer already - it would develop +/- 25 volts
and had taps to also get +/- 17 volts for a pair of 12 volt 3 terminal
regulators, and 6 volts for the CRT heater thus allowing me to remove
one of the 900 Hz transformers. The junk box yielded a suitable very
large heatsink so away I went.
I got to work removing all the unnecessary stuff from the indicator,
Dismantling is definitely a lot of fun :-) Out went the heater
transformer, IF strip, the power relays, fuses and the multi-pin
connectors. All the associated wiring came out of the looms too. A few
charred power resistors and all the electrolytics were replaced as
were some messy replacement rectifiers that sat on blackened areas on
the rear panel(!). The variable video time constant parts and
switching went, as did the heading indicator and signal strength
meter. After doing that there was enough room to mount the power
transformer and the big heatsink for the LM 3886. I connected it all
up, attached my 400 Hz synchro and reached for the power switch - a
scary proposition for something that hadn't seen moving electrons for
along time.
Well, the good news was that the CRT lit up OK (whew!) and the
waveform generators all worked but that's where it ended. It was
pretty obvious that my little synchros would never develop enough
secondary current to adequately deflect the beam. In addition the
LM3886 was getting very hot and working far too hard to force enough
power through my junk box transformers into the unit, even with a
power factor correction capacitor - now that's a phrase I never
thought I'd ever use again! On top of all that, the 900 Hz
transformers howled like banshees. It was obvious a radical rethink
was going to be needed!
That's where I'll leave it for now, so stay tuned for the next
exciting installment folks!
Morris
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