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Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: Jason G on December 11, 2007, 12:01:57 PM

Title: WV rating on a filter cap.
Post by: Jason G on December 11, 2007, 12:01:57 PM
I have a Yamaha Bass amp that needs a new PS filter cap. The cap has a 100 WV rating. My guess is that a 100 V cap can be subbed in but, I have never seen a WV rating on a cap so I am not sure. Why would they rate a cap like that?
Title: Re: WV rating on a filter cap.
Post by: teemuk on December 11, 2007, 12:06:07 PM
WV = "Working Voltage"

Edit: Yeah, forgot to mention, the capacitor should be operated at about 75% of its WV rating. This prevents the electrolyte from drying but allows some marginal for higher voltage peaks.
Title: Re: WV rating on a filter cap.
Post by: Jason G on December 11, 2007, 01:13:05 PM
Thanks
I measured 54 V on the transformer under load. (54X1.41)-1=75.14 Does that sound right ?
Title: Re: WV rating on a filter cap.
Post by: teemuk on December 11, 2007, 01:30:03 PM
Yes.
Title: Re: WV rating on a filter cap.
Post by: Jason G on December 12, 2007, 11:36:50 PM
Ok , so I put in the new cap. Fired it up and it sounded great.Till today... The main problem was that when it started up it would slowly fade in with white noise then be fine. Today rather than starting up like it should it faded up  but without white noise. I tested the bridge rectafier (on AC volts) by placing the Dmm probes on oppisite ends. I got a ( more or less) 53 on one set of corners and a 152 volts on the other set of corners.
I think that its off. My guess is one of the diodes is bad. Am I in the right neighborhood or should I look elsewhere?
Thanks
Title: Re: WV rating on a filter cap.
Post by: joecool85 on December 13, 2007, 06:05:59 AM
Quote from: Jason G on December 12, 2007, 11:36:50 PM
Ok , so I put in the new cap. Fired it up and it sounded great.Till today... The main problem was that when it started up it would slowly fade in with white noise then be fine. Today rather than starting up like it should it faded up  but without white noise. I tested the bridge rectafier (on AC volts) by placing the Dmm probes on oppisite ends. I got a ( more or less) 53 on one set of corners and a 152 volts on the other set of corners.
I think that its off. My guess is one of the diodes is bad. Am I in the right neighborhood or should I look elsewhere?
Thanks

I'm going to say you are probably right with a bad diode, but I'd wait for Teemuk before you go replacing it.
Title: Re: WV rating on a filter cap.
Post by: teemuk on December 13, 2007, 07:06:40 AM
AC testing is ok before the rectifier but I believe it will give false results for the following DC part.
Title: Re: WV rating on a filter cap.
Post by: Jason G on December 13, 2007, 09:33:30 AM
Ok, tested the BR with dc and it was good. So its back to the drawing board.
Thanks for the help!
Title: Re: WV rating on a filter cap.
Post by: Jason G on December 14, 2007, 01:53:36 PM
So It looks like the power amp and pre amp are fighting for current. The power supply puts out 75v the pre amp is taking 31 and the power amp is taking 50. So my guess is that the pre amp zener is has gone bad and that the excess load on the PS filter cap is what destroyed the last cap.
There is an x with 4 dots around it on the pc board. I know I have seen it some where but don't remember where. Can anyone help me with that one. I have not been able to find it online.
Next thing on the list is to put a 25v zener in.  If I knew what those x 's mean I would be more sure about moving ahead.
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