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Author Topic: Lifetime of 500 WV 100 ufd Power Amp Capacitors  (Read 1297 times)
VE7ALQ
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« on: March 24, 2005, 12:01:53 PM »

Hi!  My Yaesu FL2100z power amplifier runs six 500WV 100 ufd electrolytic capacitors in series, to have an effective capacitance of 17 ufd at 3000 volts.  The plate  voltage on the twin 572B tubes is 2400 volts no load, 2200 volts idling (90 milliamperes bias) and 1900 volts full load (450 milliamperes)  My amplifier was made sometime around 1980 so it is about 25 years old.  I would like to know if I am reaching the end of useful lifetime of the six 500WV 100ufd capacitors in my Yaesu FL2100b linear amplifier, as evidenced by the voltage drop.  Another cause could be the 117 VAC that I am forced to use to energize the amp.  There is a 12 amp 250 volt slow blow fuse in the amp. which has yet to die.

Another motive for possibly replacing the filter capacitors in my Yaesu FL2100z could come from reports of hum on my signal.  As of yet, nobody has complained...
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W5JI
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« Reply #1 on: March 26, 2005, 08:58:36 AM »

I doubt if the supply voltage drop is a sign the caps are too old. It is most likely due to your use of 117VAC rather than 230VAC, as you surmised. If you start getting hum reports, it will most likely signal a cap problem. As for the lifetime of your caps....my experience is that caps tend to go bad more quickly when used little. A sure way to kill electrolytic caps is to let them set idle for a few years. The lifetime is also dependent upon manufacturing processes, electrolyte used and chemical reactions. I have had caps go bad in five years or less and then I have some in my boatanchor gear which are over 25 years old with no problems. On the other hand, if you intend to keep the amp long term, you might want to replace the caps while 500WV, 100 uF caps are still relatively available (at least you might want to buy some now for later use)......Jim
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VE7ALQ
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« Reply #2 on: March 26, 2005, 11:45:33 AM »

Regarding w5JI's response

(1) I agree that the 117 VAC circuit is probably to blame
(2) Thanks for pointing out future Capacitor shortages.
(3) I have had no reports of hum on my CW signal.
(4) My Amplifier is powered up at least once a day.
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W9GB
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« Reply #3 on: March 26, 2005, 03:00:31 PM »

Your more likely "off the shelf" replacement capacitors will be 450 VDC units of 100 or even 140 mfd.  Snap in or screw moutn are the most common.

Mouser
http://www.mouser.com/catalog/621/484.pdf

My Alpha uses six 210 mfd at 450 vdc Mallory caps.

w9gb
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K0IZ
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« Reply #4 on: March 26, 2005, 03:23:12 PM »

I agree with above posts that most of your voltage drop is likely due to 120v operation.  Also recommend replacing the caps precautionary (when a cap goes, it really stinks and might expel a corrosive paste inside your amp).

Go to a good source like Mouser.  You will find that the newer caps are much smaller for a given capacity compared with old stuff.  Buy the PC (radial) style with both pins coming out one end.  Place two caps pin to pin (ie lenghtwise) and solder (+ to +), then tape together.  This will make a capacitor about same size as original and much more capacity (probably double).  Connect a couple of hookup leads to the soldered pins.  This will help stiffen your power supply voltage.
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VE7ALQ
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« Reply #5 on: March 27, 2005, 05:39:00 AM »

Thanks!  I found some reasonably priced "Snap-In" caps at http://www.mouser.com with 270 ufd at 450 working volts.  The original caps in the linear amplifier are 100 ufd at 500 working volts, six of them in series.  The main question I have is whether 450 working volts * six capacitors will work at 2400 volts with an adequate safety margin?

If the cause of my voltage droop is insufficient capacitance, then going from 100 ufd to 270 ufd ought to stiffen the output voltage up.

I'm sorry (pudding between the ears) but isn't modulating the B+ with audio and applying this signal to the plates of an RF amplifier the way we used to get AM?  Surely supplying poorly filtered AC to the plates of the linear amplifier will result in an AC modulated RF envelope?  I may be wrong.

Curiously in the wee hours of the morning the AC line voltage attains 117 VAC and the linear DOES show 2400 volts, no load and 2000 volts idle.  Current when dipped for maximum output on 40 meters is 0.45 amps.
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W9GB
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« Reply #6 on: March 27, 2005, 10:49:07 AM »

Ya'akov -

The 450 VDC versions will likely be fine (450 VDC x 6 = 2700 VDC) ...

500 VDC versions would raise this to 3000 VDC

Read the manufacturer and part number as you remove these HV capacitors --
exact repalcements may still be available (current mfg).  Here are CDE current models (with spec sheets)
http://www.cornell-dubilier.com/alumframe.htm

CDE now sells specific capacitors "on-line".  This is a great resource for that occasional replacement -- and allows you to select some of the "top of the line" 105 degree (and 500 volt) units - not normally available in small quantities to the end-user.

Switching power supplies -- with their higher internal voltages -- have revived this part of the electronics industry -- many new products are actually better in tolerance.

w9gb
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VE7ALQ
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« Reply #7 on: March 28, 2005, 08:53:54 PM »

I see http://www.mouser.com has some nifty 470 ufd aluminum snap-in electrolytics at 450 working volts DC.  They cost slightly over USD$8 each, so buying a full deck of six of these capacitors should reduce my voltage sag, if it is due to lack of capacitance.  Before I start fiddling around and possibly drilling holes to get balky capacitors to fit in the boards, I would like assurance that the 2400 volts key_up and 2000 volts key_down at 450 milliamperes droop is not caused mainly by the IR loss in the 117VAC circuit breakered at 15 amperes.  The linear has a 12 amp slow blow fuse which should be expected to blow first in case of overload.
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W9GB
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« Reply #8 on: March 29, 2005, 03:16:13 PM »

I agree with W5JI - "I doubt if the supply voltage drop is a sign the caps are too old. It is MOST LIKELY due to your use of 117 VAC rather than 230 VAC."

You should address this problem or consider 230/240 VAC operation.  This may be problematic of an older home electrical system (old 60 amp service upgraded to 100 amp in the 1970s).

Changing capacitors are a good idea for older amplifiers (> 20 years), since electroltyic capcitors has a specific life (CDE/Mallory have pages you can read on life versus temperatures, etc.).

I would also CAUTION you not to install capacitors above 50% of original values.  You may have to change the equalization and/or bleeder resistor values.

w9gb
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VE7ALQ
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« Reply #9 on: March 29, 2005, 03:43:23 PM »

Unfortunately upgrading the power wiring is out of the question as I live in a rented apartment.  Similarly I could not gain access to 234 VAC although the stove has it for the oven.  I would prefer to interact with the landlord as little as possible - it is enough that he allow me to put up a vertical on the roof and drill holes to lead in the Belden 213 coax to my apartment which is on the ground floor - 125 feet of coax away from the antenna.  The amplifier seems capable of 2000 volts at 450 milliamperes in the wee hours of the morning, but later on during the day the B+ weeps to 1900 volts and I can only obtain 420 milliamperes plate current.  This must be due to other people's putting a load on the AC mains in the apartment building, if not an issue with the electric power utility.

I could always rewire the amplifier to 110 VAC instead of 117 VAC, but hesitate to do so mainly due to the implications this might have on tube and incandescent filiament life.  If I remember correctly, the life of an incandescent filiament varies inversely with the twelfth power of the voltage applied.

There are two main issues I see with upgrading the power capacitors.  The first issue is, will the new snap-in capacitors physically fit?  The second issue is whether the other components could handle the higher inrush current of the new capacitors, should I decide to increase the capacitance from the existing 100 ufd to a potential 470 ufd as www.mouser.com has 470 ufd on sale for USD$8 ?  I have no problem with replacing the bleeder resistors if that becomes necessary.  As pointed out, hum is not an issue, because, being linear, the output of the amplifier should remain independant of any ripple from the B+ supply.
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WA9SVD
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« Reply #10 on: March 31, 2005, 09:21:59 AM »

Be careful if you change the capacitors.  A higher capacitance value than original will result in a higher in-rush current, and might cause fuse/breaker problems, especially at 117 VAC.
    While you might be able to get away with 450 WV caps vs. the original 500 Volt caps, it would probably be prudent to ALSO replace the equalizing resistors with new units.  Old resistors can and often do change value, and what might work for the original caps could leave little tolerance with new, slightly different voltage, capacitors.  Just be sure the power rating of the new resistors is equal to or greater than that of the original.  (Yes, I HAVE seen someone try to use a 1/2 watt resistor in place of a 10 W.  They paid a heavy price for that lesson, if they really learned it in the first place.  "They shoulda knowed better.")
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WA1RNE
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« Reply #11 on: April 02, 2005, 06:27:54 PM »


 First off, how much output power are you seeing into a dummy load? If the output is normal, the resistors used in the meter circuit HV divider have probably changed value. That's a distinct possibility with age.

 Second, I wouldn't assume that low line voltage is the problem.  Get a good RMS voltmeter and MEASURE the line voltage at the power receptacle.

 Start with a no load reading (amplifier idling), then key down in CW mode.

 If you see a correlation between loading the amplifier and low line voltage, try running the amplifier on a dedicated 117 volt line or one that is not used by many other devices.

 73,  Chris

 
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VE7ALQ
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« Reply #12 on: April 02, 2005, 07:48:06 PM »

I see that someone is advertising in RfAnpsCorner Yahoo Group large power supply of 2500 ufd 450 volt Capacitors for $15 each.  The capacitors are made by Cornell-Dublier and are part of their 400C series.  Physically they are 4 inches tall and 2 inches in diameter.

If I replaced the six elderly 100 ufd 500 volt capacitors in my Yaesu fl2100z amplifier by six 2500 ufd 450 volts pacitors, would my fl2100z linear amplifier work better, and/or lose the droop in plate voltage from 2400 Volts no load/2200 Volts idling at 90 milliamps to 2000 Volts at 450 milliamperes.
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WA1RNE
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« Reply #13 on: April 03, 2005, 09:44:49 AM »


 Some absorb what they read, others do not.......
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WA9SVD
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« Reply #14 on: April 03, 2005, 12:34:42 PM »

Replacing 100 uF caps with 2400 uF caps, (assuming there is enough tolerance to allow 450 V vs. 500 V. of the originals without failure of any of the caps due to over-voltage; that damage might not be immediate, but could show up at any time, such as with a power surge, etc.) would almost assuredly blow the breaker/fuses; and quite possibly destroy the rectifiers and possibly damage the plate transformer due to the much higher inrush current.  Your use of the "slow-blow" fuses will in fact exacerbate the problem; allowing the inrush current to last longer before the fuses blow.
    There's a reason the original supply used the rectifier, transformer, and especially capacitor values (capacitance AND voltage)it did; it was designed to take into consideration the inrush current, load/no load voltage values, steady state current demand, etc.  If you wish to second guess the designer(s,) that certainly is your option, but be prepared for some unexpected, unpleasant, and possibly quite expensive, surprises if you do so.
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