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Old 1st June 2012, 05:26 PM   #1
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Default About to Replace Filter Capacitors with Different Type- Help Needed!

Since my Crown Comtech 400 is still using its original filter capacitors from 1996, I feel that it is a good idea to replace them as a preventative measure. The originals are also probably fairly dry now. The problem is that new 6300uf 160v capacitors from Crown cost more than $70 a piece (one needed for each channel). These are large computer-grade screw terminal type capacitors.

I checked on Mouser and it is much more cost effective to purchase multiple smaller capacitors and to wire them up in parallel. I also spoke with DJK and he mentioned that I can get by with 75 volt capacitors if I stick to 4/8 ohm mode on this amplifier. Theoretically, the 160 volt filter capacitors are only needed for 70 volt mode.

With this knowledge, I went to Mouser and purchased four of these 100v snap-in capacitors:

SLP332M100E4P3 Cornell Dubilier Aluminum Electrolytic Capacitors - Snap In

Will I be OK if I wire up two in parallel for each channel? Also, will these perform better than the original 16 year old filter capacitors?

Please let me know if I should not do this! I'm planning to install them over the weekend so hopefully I won't kill my amplifier!

Thanks

Last edited by techbiker; 1st June 2012 at 06:00 PM.
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Old 2nd June 2012, 02:46 AM   #2
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At about 20 years I always replace e-capacitors, but if the amp has some other problems or is lacking rated power (VAC into a known load resistance) you might replace the main caps. Certain people here always recommend measuring capacitance and ESR before replacement, but a meter that does that pre-calibrated is about $150 so wth. I don't feel such a reading is valid anyway unless made at stable running ambient (120 deg F?) and cold start ambient (50 deg f?) since you're talking about a water system with a rubber seal that might or might not be cracked.
I've made subassemblies of 3300 uf caps to replace $40 10000 uf caps by mounting the snap in caps on a board I make out of sheet 1/16" polycarbonate (imitation lexan) with holes drilled for the snap in terminals, and holes for mounting screws. I bought the polycarbonate sheet at the lumberyard for a broken window, and used the leftover bits. It saws with a sabre saw, 18 tpi blade or so. I put 18 ga bus wire along the outside of the board, and bend the snap in terminals over them for a good solder joint. Then I plug the end of the buss wires in the holes on the original amp PCB where the single caps used to go. I put another piece of lexan over the top of the caps, sandwich the whole assembly with a 2" 6-32 screw, and screw it with the same screw and fender washers to the original PCB so it will take rough PA type handling. I glue the edges of the caps to the lexan with silicon seal or wallboard adhesive. My PA amp has a fan so I'm not to worried about cooling. If you don't have a fan you might leave more space between the individual caps than I did.
Do one channel at a time and see if you improved anything. A radio as source that can drive to full volume, and a 20 VAC scale on your meter will help you to determine output power before clipping (honky sound). If you measure the speaker impedance, you have some sort of power output measurement. Warning, most DVM's with a minimum 200 VAC scale are insensitive to music frequencies, as is my Sears DVM. They only give useful results at power line frequencies. DVM's rated for RMS voltage might be more likely to be okay, although I've never owned anything that expensive. I use a Simpson 266 100kohm/volt analog VOM for output music frequency voltages, but you can't buy those anymore, the market has bankrupted them for "obsolescence".
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Last edited by indianajo; 2nd June 2012 at 02:51 AM.
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Old 2nd June 2012, 08:33 AM   #3
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Indianajo,

Thanks for the information! I actually have a digital ESR meter and a oscilloscope that is currently undergoing maintenance. As far as you know though, would using these smaller capacitors instead of regular replacement ones not result in a catastrophic failure? If not, I will go ahead and securely install them, then test one channel at a time.

Please see page two of the schematic for the capacitors in question. The filter capacitors are labeled C126 and C226. Also I apologize, but my original assumption was incorrect. These are 200v capacitors, however only 60 volts are entering the circuit in 8/4 ohm mode.

Thanks again!

http://www.crownaudio.com/pdf/legacy/ct400_main.pdf
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Old 2nd June 2012, 10:36 AM   #4
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Since you do have an ESR meter, by all means check the old electrolytics. (I would expect <100 mOhm, somewhere in the two-digit mOhm range maybe. If in doubt, check the datasheet for nominal values.) It would seem more than silly to misimprove the unit needlessly.
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Old 2nd June 2012, 10:57 AM   #5
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I would not put in lower volt caps than original without removing the wire from the switch that allows the 70v mode. However, I think 3 caps bussed together with 18 ga wire or fatter should perform as well as one giant cap. I can't prove it yet though, my amp has driver board issues that turn on both pnp and npn output transistors at the same time on one channel. After I figure that out, I'll do a full voltage power check into 5 ohm 450 w resistors.
As far as replacing goes, do you start out on a 1000 mile road trip on 14 year old tires? Even if they have plenty of tread and the air pressure is correct, doesn't mean the seals aren't going to blow. Like I say, 20 years is a sure shot. 14 years, they might still have good seals, but not a lot of years left unless they were epoxy sealed caps. Not likely in a PA grade product like a Crown. Some shop owners have said they always replaced all e-caps in out of warrantee units that come into their shops with problems. Do you have problems? Measure your output voltage and speaker resistance and calculate p=V^2/R
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Last edited by indianajo; 2nd June 2012 at 11:01 AM.
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Old 2nd June 2012, 09:45 PM   #6
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Quick update:

I just replaced the filter capacitor in the side of my amplifier that drives my right speaker. So far everything looks good and although it might just be my imagination, my right speaker is a bit louder than the left now. Bass does appear to be tighter and more defined. I'm getting more of a solid thump when the low notes come up rather than a hollow thud that I am used to.

The old capacitor was reading .08-.1 ohms of ESR by the way.

One last question though. Do you think installing another 3300uf capacitor (for 9900uf per side) on each channel would push this amplifier too hard? After reading several power supply guides, my amplifier should use about 10,000 microfarads of capacitance per side for the filter capacitors. I believe that some of the Macrotechs of similar size use this much.

Thanks again.

Last edited by techbiker; 2nd June 2012 at 09:48 PM.
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Old 3rd June 2012, 12:53 AM   #7
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The only drawback to using more capacitance is inrush current at switch on.
add as much as you dare.... even rewire the power rails with the heaviest wire you can manage.
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Bass does appear to be tighter and more defined
when it comes out and kick you in the *** then you know its good.!
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Old 3rd June 2012, 05:18 AM   #8
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9900F should be fine.
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Old 7th June 2012, 08:52 AM   #9
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Hey! Thanks for all of your replies. I ordered in a couple more 3300uf capacitors and installed them so that I have 9900uf of capacitance per channel. Bass is now MUCH improved. I can even feel some of the lower frequencies vibrating through my desk.

I'll double check the amp's bias and then reassemble everything. I'm just curious though, would I be able to increase the quiescent current at all without causing damage? Stock, bias is set to 300 ma however I've been wondering if I could boost it to 400 ma or so. This might be a dumb question, however modifying/ building amplifiers is fun.

Thanks again.
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Old 7th June 2012, 11:02 AM   #10
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Glad your bass improved. Power out is the best measure of rail capacitance, IMHO. I use ZZ Top Afterburner for bass test when buying or repairing anything. Those bass drum hits are radical, and very like a classic full size drum.
Output transistor bias current capability depends on the output transistors and the heat sinks. Since the ear is a logarithmic device, I don't feel going from 300 to 400 ma would be very audible, but a temporary experiment shouldn't hurt. If you're going to make it permanent you have to do some calculation on SOA for the output transistors, and maybe a temperature measurement on the heat sink. Datasheets for transistors are on datasheetcatalog.com, although most manufacturers have house brand numbers on the transistors so you "have" to buy replacements from them. If you search on here, someone might have posted a cheat sheet for conversion of Crown numbers. I found one here for a conversion sheet for Peavey, that I own.
Frankly, ensuring a minimum 40 ma bias current for the one output transistor pair on my ST120 amp (from OEM 0 cold) seriously cut the distortion on the sound. I use solo piano sources as a distortion ear test, as I own a wood Steinway for comparison of sounds. I would think 40 ma X the number of output transistor pairs you have should be adequate. For a 400 W/ch amp, probably 3 or 4 output pairs, or 120 to 160 ma.
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Last edited by indianajo; 7th June 2012 at 11:19 AM.
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