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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.
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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]> writes: When I was finished, there was essentially no
ripple at all! only
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- RE: [drakelist] AC-4 Cap values Gerry
- Re: [drakelist] AC-4 Cap values Carl Strode
- Re: [drakelist] AC-4 Cap values Jim Shorney
- Re: [drakelist] AC-4 Cap values Ron
