Fener BXR300C hum

lowell · 11-05-2009, 11:05 PM

This amp is humming. The positive supply is only 46v whereas schem says 70+. The negative supply is fine, rectifier good, filter cap good, 20vAC with PA disconnected from supply. With the R106 disconnected the positive supply goes up to 64vDC, but not 70vDC. ?? Currently there is NO load on the power supply and it's still low. What to check next? I'm suspecting CR33 cause it's reading odd, but currently it's not even connected to the supply so maybe I'm missing something. Oh and the + supply with no load has 5vAC on it and the - supply 1.5vAC with a load. Guess it's possible the main + filter is leaking... but don't they usually totally fail?

Enzo · 11-06-2009, 02:34 AM

How did you determine the filter cap to be good? You are reading higher ripple on the lower voltage. SOunds like a cap going soft to me.

DO caps usually totally fail? No, not in my experience. I do find shorted caps, but I replace a lot more leaky ones.

lowell · 11-06-2009, 10:13 AM

I ohmed it and it tested ok. Is there a way to determine a leaky cap other than abnormal ac voltage readings or replacing it to see?

Enzo · 11-06-2009, 11:41 AM

Ohmed it? AN ohm meter will tell you if the cap is shorted, but it won't tell you how leaky it might be at 40v, or 400v.

The primary way to see if a cap is working is to see if it is doing its job. Low DC voltage and excessive amounts of ripple are usually signs of a bad or missing cap. Be aware that excessive current draw from a power supply will also drag the voltage down and increase the ripple, but I don;t get the impression that is your problem.

ANother test? OK, remove the cap from its circuit, use clip wires and charge it up to something close to its rated voltage. Now unhook the clip wires and see how long it will hold its charge. The larger the cap value the longer it should take to discharge. Try this with a variety of caps on your bench to get a feel.

And what is wrong with replacing it? I have a few different caps in the shop with clip wires soldered to them. I can at any time clip one of them into a circuit. If I think a cap is dried out, I can clip another in parallel and see if it makes a difference.

lowell · 11-06-2009, 08:34 PM

perfect. thanks for the info. I wasn't sure if the "cap charging" and metering the DC was a reliable method. I have read about that in G Weber's books.

lowell · 11-09-2009, 09:01 PM

through the closest value cap in there that I have (2200uf 100v) and the + supply looks good. thanks enzo. gonna order the correct value.

Jazz P Bass · 11-09-2009, 09:57 PM

Way to go guys.
Now both of you, on the count of three, take a bow.

11-06-2009, 02:34 AM	#2
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,266	How did you determine the filter cap to be good? You are reading higher ripple on the lower voltage. SOunds like a cap going soft to me. DO caps usually totally fail? No, not in my experience. I do find shorted caps, but I replace a lot more leaky ones. __________________ Education is what you're left with after you have forgotten what you have learned.

11-06-2009, 11:41 AM	#4
Enzo Senior Member Join Date: May 2006 Location: Lansing, Michigan, USA Posts: 9,266	Ohmed it? AN ohm meter will tell you if the cap is shorted, but it won't tell you how leaky it might be at 40v, or 400v. The primary way to see if a cap is working is to see if it is doing its job. Low DC voltage and excessive amounts of ripple are usually signs of a bad or missing cap. Be aware that excessive current draw from a power supply will also drag the voltage down and increase the ripple, but I don;t get the impression that is your problem. ANother test? OK, remove the cap from its circuit, use clip wires and charge it up to something close to its rated voltage. Now unhook the clip wires and see how long it will hold its charge. The larger the cap value the longer it should take to discharge. Try this with a variety of caps on your bench to get a feel. And what is wrong with replacing it? I have a few different caps in the shop with clip wires soldered to them. I can at any time clip one of them into a circuit. If I think a cap is dried out, I can clip another in parallel and see if it makes a difference. __________________ Education is what you're left with after you have forgotten what you have learned.

11-09-2009, 09:57 PM	#7
Jazz P Bass Senior Member Join Date: Dec 2008 Location: Wernersville, PA Posts: 128	DC smoothing caps Way to go guys. Now both of you, on the count of three, take a bow.

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11-06-2009, 10:13 AM	#3
lowell Senior Member Join Date: Jun 2007 Posts: 941	I ohmed it and it tested ok. Is there a way to determine a leaky cap other than abnormal ac voltage readings or replacing it to see?

11-06-2009, 08:34 PM	#5
lowell Senior Member Join Date: Jun 2007 Posts: 941	perfect. thanks for the info. I wasn't sure if the "cap charging" and metering the DC was a reliable method. I have read about that in G Weber's books.

11-09-2009, 09:01 PM	#6
lowell Senior Member Join Date: Jun 2007 Posts: 941	through the closest value cap in there that I have (2200uf 100v) and the + supply looks good. thanks enzo. gonna order the correct value.

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