Garey Barrell <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Gerry -
One other factor. Electrolytic capacitors of that era were rated in DC
Working Volts (DCWV), with a peak voltage of some 30-40% higher. So in
the terminology of the day, a 300 DCWV capacitor would be a reasonable
choice for the filter of a 250V supply.
Take a look at the bottom chassis photo of an early AC-3 at
<http://www.wb4hfn.com/DRAKE/DrakeEquipmentPictures/Equip_EarlyAC3_Bottom.htm>
and you'll note that the HV filter capacitors are clearly marked 450 WVDC.
I think the only problem with the AC-3 was that the _necessary_
"minimum load" resistor was undersized, even considering that it was
returned to the +250V supply rather than ground. Kinda makes you wonder
if Drake got a good buy on those resistors! Or perhaps that was the
largest one available in that "single-ended" style. The choke supply IS
more tightly regulated than a capacitor only version, _IF_ the minimum
load is maintained by an intact bleeder resistor.
73, Garey - K4OAH
Atlanta
Drake 2-B, 4-B & C-Line Service Supplement CDs
<www.k4oah.com>
Gerry wrote:
"Gerry" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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What you describe is typical of choke input power supplies. Text books tell
us that choke input supplies are better at regulation but that is simply not
true if you have a wide ranging dynamic load. Choke input power supplies
require a minimum load. There are formulas in the old handbooks to calculate
the value of resistance to draw a proper load current. Without a minimum
load it would be as if no choke existed (except for its DC resistance) and
the filter capacitors will charge to 1.414 times the value of AC rms. In
other words, if you have 750VAC secondary the capacitors will charge to over
1,060 volts with no load! With a properly chosen resistor this would be 0.9
times 750 volts or 675 volts. Please note the HV filter capacitors are only
rated to 350 volts for a total of only 700 volts. So as you can see, the
10k, 20w load resistor on the output of the HV side of the supply is
crucial. That resistor dissipates around 16 watts. Notice that the low end
is referenced to +250V and not ground. It should be rated for twice the
actual power dissipation or 32 watts. Put it another way, without the load
resistor, key up voltage would be around 1kV while key down would drop to
about 675 volts. I should mention that the choke has DC resistance which may
be a significant contributor to voltage drop. If you draw 450 mils and your
DC choke has 50 Ohms of DC resistance, you will drop an additional 22.5
volts across the choke. Also look at the +250 volt side and see that the
filter caps are rated to only 300 volts. This is much too close for comfort
and is really a very slim margin of safety. Typically the voltage rating of
electrolytics is related to the maximum voltage it can withstand for a given
numbers of hours at some temperature. Component manufacturers usually try to
embellish the specifications and say their caps will take 1000 hours of use
at room temperature. But in actual operation ambient temperature will be
higher than 25 degrees C and they typically don't say too much about self
heating due to ripple current which brings up temperature even more. This is
why it is always best to over rate voltage in electrolytics when selecting
components. I see that Drake realized this was not a good design and soon
changed to a capacitive input supply which is better suited to SSB service,
and less expensive. As far as I'm concerned I would avoid the original AC-3
in daily use. You wisely chose to use it on the test bench instead.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of Donnie Garrett
Sent: Tuesday, December 12, 2006 5:27 AM
To: Tom Taylor
Cc: drakelist@www.zerobeat.net
Subject: Re: [drakelist] AC-4R upgrades
"Donnie Garrett" <[EMAIL PROTECTED]> made an utterance to the drakelist gang
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Tom:
I have one of the early AC3 supplies that used a HV choke. Garey
K4OAH and I had a lot of discussions about its design some time back
after I discovered the ceramic 25W load resistor was open. You better
check yours to see if its OK. It will still operate but the no load
HV will float very high with an open load resistor. I was never able
to find an exact replacement at the time I needed it so I installed a
25W wire-wound resistor in a vertical fashion with a small bolt
running through it. Somewhat similar to the original setup. I choose a
25K 25W instead of a 15K to reduce the heat dissipation produced by
the resistor. The 25K still loaded down the HV and was near the same
as was prior with the original 15K 25W resistor which seems to
regularly fail due to running it at or near it's maxium wattage
dissipation.
Also one last thing, If I recall the HV winding on this early choke
version AC3 was higher than the later AC3 and AC4's that didn't use
the HV choke. Seems I temporally jumped across this choke just to see
what effect it had on the HV and if I recall correctly the HV jumped
up in voltage considerably, and was then out of speck and was near the
max voltage ratings of the 2 HV caps. (No Good) I would appreciate
your findings on the HV secondary windings if you don't mind.
Too put it quite frankly I don't care for the early model AC3 choke
type supply due to these issues. The only nice thing about this model
is that all the caps mount on the underside of its chassis. This AC3
supply now serves as my test bench supply and is not used in my
station.
Hope this helps, Don / WA9TGT
On 12/11/06, Tom Taylor <[EMAIL PROTECTED]> wrote:
I have a number of Drake A, B, and C twin combos. I decided to upgrade all
the power supplies with AC-4R boards. Since most of the caps are now 30 to
40 years old, I don't want to risk a bias supply going out and ruining the
finals or some short ruining the transformers. I order a bunch of blank
boards since I already had some of the parts. The rest of the parts I
ordered from DigiKey. I assembled all the boards at the same time and then
started upgrading the AC-4s, one by one.
So far, all of the power supplies are different in one way or another and
all of them have been modified by previous owners. Here are some examples:
#1) A previous owner replace the mid-voltage supplies' capacitors with a
bundle of three caps mounted to the upper end the supply.
#2) A previous owner replaced the sand resistor below the chassis with two
higher wattage resistors mounted to the upper end of the supply.
#3) A previous owner replaced one of the diodes in the HV supply. The
transformer in this supply has transformer bolts inserted the wrong
direction to mount the AC-4R board. Unfortunately, it's not as simple as
removing the bolts and inserting them in the other direction because the
bolts are just long enough to reach, but not long enough to add the board
spacers.
#4) This is an AC-3, serial #101. I've never seen a Drake power supply
like this. There's a large choke mounted adjacent to the transformer. All of
the other parts, including the capacitors, are radial lead caps that are
packed underneath the very low chassis. Has anyone else seen an early AC-3
like this? I'm planning to remove the choke and rebuilding the supply just
like all of the others since all of the circuitry is now on the AC-4R board.
Thanks,
Tom N7TM
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