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Author Topic: Rejuvenating Dormant Filter Capasitors  (Read 16937 times)
WA9AFM
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« on: June 18, 2016, 12:03:19 PM »

Have what appears to be a perfectly good Crown solid-state PA amp which has sat unused for about 3 years.  Don't want to apply operating voltage as the filter caps might blow unless properly rejuvenated.  If I understand the correct procedure, plug the amp into a Variactor and bring the voltage up slowly.  Two parts; is this a valid procedure and how fast should the voltage be increased?
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KG6YV
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« Reply #1 on: June 18, 2016, 03:21:29 PM »

3 years should not be a problem but you can start at around 20 volts and increase 10 volts per hour until you are at 120 volts.
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WB9TEN
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« Reply #2 on: June 19, 2016, 09:14:12 AM »

Idle curiosity, what model? I worked at Crown from 1973-1999. Left just before the Harmon buy out.
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WA9AFM
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« Reply #3 on: June 20, 2016, 10:26:01 AM »

I have found out there are four Crown amps in the rack:

two D-75 (ran the monitor speakers)

one D-150A

one 460CSL(ran the main center cluster)

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KB4QAA
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« Reply #4 on: June 20, 2016, 10:50:59 AM »

http://www.w7ekb.com/glowbugs/Military/PDF%20files/ReformingElectrolytics.pdf

Here is the MIL-HDBK-1131A  handbook for reforming capacitors. 
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KA0HCP, ex-KB4QAA Relocated to Ks. April 2019.
AC5UP
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« Reply #5 on: June 20, 2016, 04:19:53 PM »

1+

Excellent reading in the link immediately above.  Note the following:

4.5  Disposal period
.  MIL-C-62 and non-military grade aluminum electrolytic fixed capacitors more than 12 years old should be disposed of.  MIL-PRF-39018 aluminum electrolytic fixed capacitors more than 15 years old should be disposed of.


While Unca' Sam often works to a higher standard of reliability due to the combat thing, reforming an electrolytic can be an exercise in postponing the inevitable...  Especially when you read further to find the process involves controlling both the VOLTAGE AND CURRENT delivered to the part being evaluated.  It's common eHam lore that only the voltage need be increased over time, but the example given includes an incandescent lamp as a current limiting device with the condenser(s) being reformed out of circuit.  A Variac won't give you the same level of control.

BTW:  There is something to be said for allowing weak parts to fail as part of a restoration project.  I'd rather replace ten parts in one bench session than two parts each in five bench sessions...   Wink
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G8HQP
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« Reply #6 on: June 21, 2016, 02:15:05 AM »

Reforming an electrolytic is far more about current than voltage, so just using a variac is likely to only successfully 'reform' those caps which don't really need reforming. It is the current which reforms the cap via electrolytic action. The voltage merely creates the current, but a small rise in voltage can cause a big rise in current for an unreformed cap.

The best way to do it is to connect to an appropriate DC high voltage supply via an appropriate resistor, and then monitor the cap voltage. The resistor should be chosen to limit the current to a few mA at most. The DC should be about the same as the rated cap voltage. If the voltage seems to get stuck then switch off the supply and allow the cap to discharge via its own leakage. Switch on again and see if the voltage now rises smoothly to about the place where it got stuck and then slowly rises from there. If so, continue. If not, the cap has had it and needs to be thrown out. As the cap voltage rises you can switch to a lower value resistor.
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WB9TEN
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« Reply #7 on: June 21, 2016, 10:12:37 AM »

The D-75 and D-150 are DC coupled so the don't have any coupling caps to short so no problem there. That leaves the big electrolytics and some smaller caps in the protection circuits. The only way the protection caps charge is when there is an actual signal so reforming those won't happen with a Variac or other method. The big filter caps, the ones bolted to the chassis, will most likely blow the line fuse if they short and not do other damage. You won't pull much current from these amps unless you drive them under load so reforming with a Variac may not do much. Both of these amps are likely to be at least 20 years old.

The D150A was my favorite amp. Back in the day I would run a pair with active crossovers for the input and then drive the mids and highs from one channel and the woofer from the other.


I'm not familiar with the 460 CSL.

That's my free advice/observation and you know what they say about free advice.
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WA8UEG
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« Reply #8 on: June 21, 2016, 11:45:26 AM »

I agree with AC5UP. I have reformatted many capacitors and most last a while but sooner rather than later fail. I gave up on reformatting them several years ago.

In the long run much easier to just change them out while it's open and on the bench.
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G8HQP
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Posts: 969




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« Reply #9 on: June 21, 2016, 01:03:10 PM »

Quote from: WB9TEN
The big filter caps, the ones bolted to the chassis, will most likely blow the line fuse if they short and not do other damage.
Unreformed electrolytics don't necessarily short; they may explode instead. Hopefully the vent will blow as it is supposed to, so you will only have some electrolyte mess to clear up.

Quote
You won't pull much current from these amps unless you drive them under load so reforming with a Variac may not do much.
I'm not sure I understand this sentence. Reforming has nothing to do with the amplifier drawing a DC current from the PSU.
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KD0REQ
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« Reply #10 on: June 21, 2016, 02:57:29 PM »

a lot of the old-time capacitor testers have a neon lamp on the front, along with that voltage control.  I have an Aerovox with the feature, Heathkits also did.  while you're dialling up the voltage, stop and back off if the neon lamp glows.  that's your current indicator, and it's sensitive enough.

I'm personally a fan of shotgunning old caps, but if a tool can be hand, I must have it....
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AA4HA
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« Reply #11 on: July 04, 2016, 09:10:06 AM »

Just using a current regulated power supply to reform capacitors may give you the ability for that capacitor to not blow up when exposed to circuit voltages. It does not make it in to a "good" capacitor.

As capacitors age;
Their voltage ratings decrease
Their capacitance values decrease
Their ESR (equivalent series resistance) goes up

You can have a capacitor that does not blow up and may even have a somewhat decent capacitance value but it is a terrible filter cap because under AC voltages it has a very high internal resistance. This is a parameter that usually does not recover with reforming as there are electrolytic changes happening in the capacitor as the electrolyte dries out over time.

High ESR capacitors run hotter so they are on "the highway to hell" as the heat makes things even worse on an old cap. It is like asking an 87 year old to run a marathon; some may make it but how many do not wake up the next morning because they "kicked-off" in the middle of the night.
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Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama
G8HQP
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Posts: 969




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« Reply #12 on: July 05, 2016, 12:58:55 PM »

Quote from: AA4HA
You can have a capacitor that does not blow up and may even have a somewhat decent capacitance value but it is a terrible filter cap because under AC voltages it has a very high internal resistance. This is a parameter that usually does not recover with reforming as there are electrolytic changes happening in the capacitor as the electrolyte dries out over time.
Drying out and oxide degradation are two separate ageing mechanisms. One can be present without the other. A dried out electrolytic will not recover with reforming because reforming is not intended to re-insert the missing water. A cap which has not dried out can recover with reforming. Drying out is largely a matter of cap build quality. Oxide degradation merely needs time.

If a reformed cap has significantly high ESR then it needs to be discarded.
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N8FVJ
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« Reply #13 on: July 05, 2016, 03:32:27 PM »

I replace old electrolytic capacitors. Not worth the risk.
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G3RZP
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« Reply #14 on: July 05, 2016, 03:54:08 PM »

I have a number of pieces of gear with 50 and 60 year old electrolytics which have been successfully recycled, It appears that they need to be in circuits which aren't stressed in terms of ripple current or voltage. Now it doesn't work every time, but I have gear with old capacitors that are working fine. I also have equally old gear where the caps could not be salvaged......YMMV.
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