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Topic: Cap Job on a Vibrasonic Custom |
Neil Harms
From:
Ionia, Missouri, USA
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Posted 15 Oct 2007 7:17 am |
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It's time to change the caps on my Vibrasonic Custom. It's the 95/96 model w/PC boards. Anyone know what values this amp uses? Also any recommendations in terms of brand/type of caps to use. It's a good sounding amp but I've been losing headroom. Starting to distort earlier and hums more than it should when it's idling. Don't know if that's clear or not.... Any help much appreciated! N.
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Buckaroo ShowPro, Quilter Aviator, (plus all that other stuff....) |
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Jim Phelps
From:
Mexico City, Mexico
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Posted 15 Oct 2007 4:07 pm
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I couldn't find a schematic diagram or anything online about it... but if you take the chassis out of the cabinet the caps (I assume you mean the power supply filter caps?) are easy to find and you should be able to read the values on them. If you do remove the chassis, watch your fingers... most newer electronic devices have bleeder resisters to drain the charge in the capacitors but just in case, I always short the positive lead of the power supply caps to chassis with a screwdriver to make sure there's no charge in them.
Sprague Atoms are generally a good choice, maybe not the most "boutique" out there... Actually, It's hard to believe that your amp should need them, only being about 10 years old, but those do sound like the typical symptoms... I just changed the originals on my '68 Bandmaster Reverb and all but one were still good... |
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Jim Sliff
From:
Lawndale California, USA
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Posted 16 Oct 2007 4:19 am
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| I assume you mean the power supply filter caps? |
Often missed is the bias supply cap - it's critical as wll and usually needs an upgrade to 100uf/100V for safety. A normal cap job consists of the filters and bias cap - - bypass and tone caps last for decades and are replaced only when leaking or tonal changes are desired.
The filters are easy to identify and almost always well-marked. Just replace them with similar uf values (they do NOT have to be exact - 20uf can sub for 22 or 16, 100 for 70, etc.) but make sure the voltage rating is at least as high or higher.
A cap job at 10 years is exactly right according to most tech''s recommendations. They DO last longer, but after 10 years you are taking chances.
Last - you DO know how to discharge filter caps, right? If not, do NOT do this yourself. In fact, if you do not know how to discharge caps you should not even be looking at an amp chassis - it's a good way to die, even with the amp unplugged.
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No chops, but great tone
1930's/40's Rickenbacher/Rickenbacker 6&8 string lap steels
1921 Weissenborn Style 2; Hilo&Schireson hollownecks
Appalachian, Regal & Dobro squarenecks
1959 Fender 400 9+2 B6;1960's Fender 800 3+3+2; 1948 Fender Dual-8 Professional |
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Neil Harms
From:
Ionia, Missouri, USA
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Posted 16 Oct 2007 5:51 am
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Thanks for your comments. I've done one cap job so far and survived. Not that tough really. I do know how to discharge the caps. Safety first! Good advice on the bias supply and I was planning on catching that since I had the amp open. Should be a fun little project. Hopefully this weekend I'll get to dive in. I've got the DVD and book from Kendrick and found it to be very helpful in starting to learn how to do this stuff. Thanks again! N.
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Buckaroo ShowPro, Quilter Aviator, (plus all that other stuff....) |
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J Fletcher
From:
London,Ont,Canada
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Posted 16 Oct 2007 7:36 am
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| I know you've done this, but it should be mentioned that the power tubes should be replaced with known good ones, before you go changing the filters. The symptoms you are experiencing could be caused by bad output tubes. Or even a bad preamp tube. |
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Neil Harms
From:
Ionia, Missouri, USA
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Posted 16 Oct 2007 9:01 am
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| Did forget to mention that I had swapped out the power tubes and tried different pre-amp tubes as well. Symptoms always the same. Maybe there will be something obvious (visually) when I pull the amp out. |
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 16 Oct 2007 1:35 pm
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If you're going to discharge big electrolytic caps, it's advisable not to short them directly to ground, but to make up an insulated jumper with a bleeder resistor that lets the cap discharge slowly. Shorting caps directly can punch holes in the dielectric, and cause premature failure. There's ample info on the web about bleeder-jumpers, so I won't go into them here.
Also, make doubly sure you have the polarity right when you replace an electrolytic, as they tend to blow up when you apply power - if you install them backwards.  |
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Jim Phelps
From:
Mexico City, Mexico
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Posted 16 Oct 2007 4:15 pm
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Yes Donny, which is why I wrote, <i>"most newer electronic devices have <b>bleeder resisters</b> to drain the charge in the capacitors but <b>just in case</b>..."</i>.
If you look up that info on the web, you'll see that most of it says to use a discharge tool like Donny mentions (and you can see how to make one) and then to CONFIRM after bleeding with something like a SCREWDRIVER. You'll also see advice that bleeder resisters can fail, and should not be relied upon without confirming with a discharge tool or screwdriver to ground, and some capacitors can even still have a charge after sitting with bleeder resisters.
And you surely wouldn't want to use a screwdriver on extremely high voltage circuits as it could shatter the screwdriver right in your face, but guitar amps do not contain that high of voltage...but high enough to kill you.
To tell you how long a good cap can hold a charge, I just took some out of a dead computer monitor that has been in the storage shed for over a year... a small cap with a 160 volt rating made a pretty good spark, after all that time. 
And of course Jim Sliff is right too about including the bias cap... I changed mine with the rest of the electrolytics but forgot to mention it... and also true about a bad tube being the cause, which I guess you've already ruled out.... good luck with it, let us know how it turns out after the re-capping.
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