Carl Strode <[EMAIL PROTECTED]> made an utterence to the drakelist gang
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What I may do is to install a NTC (Negative Temperature Coefficient) high current thermistor in series with the incoming AC line. These are really cool devices. At room temperature (25C/ 77F) they exhibit a high resistance, as they self heat, the resistance drops. The one I will try starts out at 47 ohms, then slowly drops to less than one ohm in less than a second. Steady state current rating is 4 amps.
Perfect for a surge limiter.
Gerry wrote:
Unfortunately, inserting series resistors in the B+ leads will, depending on the value, adversely affect regulation. It may work fine for static loads such as the +250 supply but is not advisable for HV supplies with dynamic loads. Years ago I tried a home brew power supply constructed with mil surplus components. What I found is that choke input supplies need a critical value resistive load to maintain the voltage below allowable limits. With no load on the +650 volts, it went almost to the rating of the filter caps which I believe was 800 volts. Trying another approach with a capacitive input filter and a choke meant I had to find another transformer. When I did, the 50 Ohms resistance of the choke became somewhat of a problem which all made sense later. At 450 mils, that would be a 22.5 volt drop on top of whatever the drop was in the first place. The thump comes from the transformer and is loudest when power is switched on at the peak of the primary AC cycle. Sometimes you can hit it just right. When I built my Heathkit SB-1000 linear, there was an alignment procedure which called for removing the screws and sliding the case back about an inch or so. This was done per the assembly instructions to align the input coils. When I turned on the amp for the first time, there was a loud KABAM! After changing my underwear, it became obvious what happened: there was a surge at turn on and the steel cover acted like a sounding board. Nothing bad happened to the amp, it was just one of those things that makes you tremble whenever you reach for the ON switch. This brings up another point about the need for slow-start on such devices. I don’t use the linear often but it is on my to-do list.
-----Original Message-----
*From:* [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] *On Behalf Of [EMAIL PROTECTED]
*Sent:* Thursday, March 03, 2005 2:54 AM
*To:* [email protected]
*Subject:* Re: [drakelist] AC-4 Cap values
Carl,
Now might be a good time to review some slow-start feature design reviews,
looking forward to incorporating one into the turn-on function of the power
supply. When you have a "thump" from turning on a power supply, it
generally is indicative of too great of a surge current going into the filter
capacitors and stressing them unnecessarily. It doesn't help the rectifiers,
either. Also, I'm not sure that it is a "leaky" capacitor that causes the
"thump" you heard at power supply turn-on....I've always been led to
believe that the "thump" was from the large flow of current into an
uncharged capacitor, although I've never had it proven to me that this
was the actual cause.
You could, however, benefit the power supply considerably, by inserting a
resistor of the proper resistance and wattage, into the 650 volt output,
between D2 and the connection to R1. This would also serve to bring your
650 volt HV (now approx. 700 volts, as you indicated) back down nearer to
the correct 650 volt value.
Your could also do the similar action with a resistor added between D5 and
the connection to C3, and also reduce the 290 volts down to the original
design value of 250 volts.
Possibly, the original design of this power was slighted somewhat, by not
including a choke in each of the two points I noted above. That would
have produced a significant reduction in the capacitor surge current at
turn-on. Chokes would also have been more effective in reducing the
ripple you originally saw on the outputs.
I'm naturally assuming, of course, that you do intend to reduce power
supply outputs down to their specified voltage levels.
I haven't intended to sound critical of your modification efforts here, Carl.
Moreso, I am critical that Drake didn't put choke filtered supplies in this
equipment. I would sure have liked my T-4XB/R-4B's AC-4 to have been
choke filtered. (Guess they figured the cost of the choke, a place to put it,
and also a slight change in the secondary output winding in the power
transformer secondaries would cost too much to add into the equipment
price.
Anyhow, and Meanwhile, back at the Ranch (as "they" say, somewhere)
keep up the good work, and keep us posted.
Bill Flowers K5VKL
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On Wed, 02 Mar 2005 20:37:21 -0800 Carl Strode <[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>> writes:an utterence to the
Carl Strode <[EMAIL PROTECTED] <mailto:[EMAIL PROTECTED]>> madesave alldrakelist gang ---------------------------------------------------------------------- I have completed re-capping the AC-4.
After looking at the actual caps installed, and comparing it to my supply of modern electrolytics, I have replaced the 40 mfd caps (shown as 40 mfd on most schematics) with 33 mfd.
The HV filter caps were replaced with 150 mfd 450 volt units. ( I had a bunch of these laying around).
Since my original late model AC-4 had a 100 mfd cap installed at C3
and C4a, and an 80 mfd cap installed at C4b, I decided to keep
everything simple and replace these with 150 mfd caps as well. I'llof my 82 and 100 mfd caps for some other project.
The 150 mfd caps I have are circuit board mount types, with both leads on one end. This made installation above decks a snap. I removed the old caps, and then reamed out one of the holes used for the old cap ____________ __________________ ________________
When I was finished, there was essentially no ripple at all! onlyabout a tenth of a volt on the 650 and the 250 volt, and about0.2volts on<mailto:[EMAIL PROTECTED]>>the bias supply. Oh yes, as usual nowadays, the 650 is really closer to 700, and the 250 is about 290. When the power supply is switched on, it thunks a bit harder than it used to, due to the lack of leaky caps and the increased cap values. The supply is much stiffer than before. Very little sag and absolutely no detectable hum.
I guess this demonstrates that with brute force, we don't need no
steenkin regulators!
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