Fener BXR300C hum

[lowell]

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]

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]

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]

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]

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]

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]

DC smoothing caps

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

[lowell]

Surprisingly I still have this amp. I pulled it out of the junk pile and am trying to wrap it up. I replaced that bad filter and the supplies are good. I reconnected everything and replaced some burnt traces with wire. It almost works.

I have it rigged with a bulb limiter and speaker is connected. There IS sound when I play through it. However, when I turn the volume up to a certain point, and only at that point (about half way) there is a pulsating current surge. It is about 3-4 seconds apart. There is a low thump when it happens and the bulb glows bright at each thump. If I disconnect the speaker cable from the amp it does not happen. If I leave the speaker connected and plug my guitar into the PA-in it does not happen. I'm missing something. The power amp must be fine if going into the PA in works perfectly. Also if I disconnect the speaker and use the preamp-out the amp is fine.

So the amp does this thump thing only when there is a load AND J4 has nothing plugged in. I don't see any protection circuitry related J4 or any mute on the inputs or speaker jacks.

[Enzo]

If the amp is not longer putting DC on the speaker, and the amp is not drawing excess current at idle, get rid of the bulb. Now how does the amp work on actual mains power?

[lowell]

Good call... it works. I guess the supplies couldn't keep the circuit stable with the bulb limiter in there.

[Enzo]

Every time a peak comes through the amp, it draws more current from the mains to produce it into the load. Of course that extra mains draw causes a larger voltage drop across the bulb, meaning the amp is reaching out for more current and the thing dials down the voltage to the amp. WHat then can happen, as it did for you, is the amp reacting to the result of it reacting to the result of it reacting... and so on.