"Bart Rowlett" wrote in message
news
On Tue, 27 Dec 2005 02:02:15 +0000, James F. Mayer wrote:

I need to generate 6v DC and 90v DC from a 12v DC automotive
electrical
system to power an RT-70A/GRC surplus military radio. I need about 250
mA
at +6 volts and about 75 mA at +90 volts. I was thinking about using
the
guts from an old battery back up but it would be a bit of a kluge. Are
there any 90 volt regulators in the 78xx series? How do I get the
voltage
up to where I can get something that I can get the 90 volts from.

Probably the easiest is to obtain one of the original power supplies.

Most installations using the RT-70 were in conjunction with an RT-66/RT-67
or RT-68 and an audio distribution amplifier. A separate vibrator power
supply was supplied those cases where the RT-70 was used 'stand alone'.
One approach to homebrewing a vehicular power supply is to mimic the
original vibrator supply. Provide the heater supply through dropping
resistors and emulate the vibrator supply by building an inverter using a
pair of switching transistors and a junk 60 Hz power transformer.
Regulation isn't necessary if you get the right transformer. The only
thing the least bit tricky with this approach is properly snubbing the
primary leakage inductance to prevent destroying the switching
transistors.

Probably the most compact approach is to build a flyback supply with both
a six and 90 volt output. Regulate the 6 volt output and let the 90 volt
output track. The advantage of the flyback supply is that it can be
designed to operate over a wide range of input voltages while maintaining
a regulated output. In addition, the overall size will be much smaller due
to the high switching frequencies (300 kHz typical) used by modern flyback
controller chips.

Getting
the 6 volts doesn't seem to be a problem. A 7806 off the battery should
work for that unless any of you can see a problem doing that. Maybe the
common common would be a problem. Right now I'm running it off of an
HP6299A
and an HP6236B with commons jumpered. I'd like to be able to go portable
with it.


Here's a reference to 'Silicon Chip' magazine, who ran some articles about
these devices.
http://www.siliconchip.com.au/cms/A_30674/article.html

I recall seeing a commercial unit which was a semiconductor equivalent of
the vibrator, two AC132 transistors (IIRC) as a multivibrator running into a
transformer. The article above gives enough hints to be useful though.

Here's some other useful links:
http://www.radios.freeserve.co.uk/vibrators/vib.htm

Woops, here we go, found the one I was after:
http://www.vmarsmanuals.co.uk/newsle...s/vibrator.pdf
Circuit for the solid-state vibrator.

Cheers.

Ken

On Wed, 28 Dec 2005 16:29:14 -0600, clifto
wrote:

Rich Grise, but drunk wrote:
"... Accurate imitation of the Hammond sound with simple electronic
circuitry was difficult, because the subtly-changing phase relationships
between tonewheels could not be easily replicated...."

OK, fair enough. :-)

I guess a piano is even harder - they don't even reproduce well from a
live recording! :-)

I heard a decently accurate electronic piano sound long before I heard
anything remotely resembling the timber of a B3.

---
Oak, was it, or maple?


--
John Fields
Professional Circuit Designer

John Fields wrote:
On Wed, 28 Dec 2005 16:29:14 -0600, clifto
wrote:
Rich Grise, but drunk wrote:
"... Accurate imitation of the Hammond sound with simple electronic
circuitry was difficult, because the subtly-changing phase relationships
between tonewheels could not be easily replicated...."

OK, fair enough. :-)

I guess a piano is even harder - they don't even reproduce well from a
live recording! :-)

I heard a decently accurate electronic piano sound long before I heard
anything remotely resembling the timber of a B3.

---
Oak, was it, or maple?

Not that I would be one to pass up a good straight line, but just in case...

Main Entry: tim·bre
Variant: also tim·ber /'tam-b&r, 'tim-; 'tam(br&)/
Function: noun
: the quality given to a sound by its overtones: as
a : the resonance by which the ear recognizes and identifies a voiced
speech sound
b : the quality of tone distinctive of a particular singing voice or
musical instrument
?tim·bral /'tam-br&l, 'tim-/ adjective

Source: Merriam-Webster's Medical Dictionary, © 2002 Merriam-Webster, Inc.

--
If John McCain gets the 2008 Republican Presidential nomination,
my vote for President will be a write-in for Jiang Zemin.

I assume that you do not have the companion AM-65 audio amplifier?
Shame, because internal to that unit is a power supply (don't remember the
numbers now) that allowed you to operate the AM-65/RT-70 combo from 6, 12 or
24 volts depending on the power supply used. The usually supplied unit used
24 volts (Korean War American military standard elec system). If you can't
get the 12 volt version, modification instructions can be found in a 73
Magazine article called
"New Orders for the R-108" which was a companion receiver for the big
brother of the RT-70, the RT-68. R-108 used the same series of plug in
internal power supplies that the AM-65 did.
There was also a magazine article in the now defunct as well Ham Radio
mag titled "Get on Six Meters-The Easy Way". It dealt with a manpack 6
channel radio of Canadian ancestry that also needed 90 Volts. I have the
entire run of that magazine from 1969 to 1990 on cd. If I can figure out how
to extract the stuff off the disk, I'll email it to you. Now, the fee for
such information.....
You will regale me with tales of operation and modification of
aforementioned Olive Drab garbed electronic equipment!!! I'll email you
later from my own account so that you will get my real email address, or you
can use the arrl reflector. 73

Chris



"James F. Mayer" wrote in message
nk.net...
I need to generate 6v DC and 90v DC from a 12v DC automotive
electrical system to power an RT-70A/GRC surplus military radio. I need
about 250 mA at +6 volts and about 75 mA at +90 volts. I was thinking
about using the guts from an old battery back up but it would be a bit of
a kluge. Are there any 90 volt regulators in the 78xx series? How do I
get the voltage up to where I can get something that I can get the 90
volts from. Getting the 6 volts doesn't seem to be a problem. A 7806 off
the battery should work for that unless any of you can see a problem doing
that. Maybe the common common would be a problem. Right now I'm running
it off of an HP6299A and an HP6236B with commons jumpered. I'd like to be
able to go portable with it.