Fender BXR60 - Hi pitched buz

[dndemattia]

Good Morning,

This is my first post! having an issue with Fender BXR60 Bass amp. There is a hi-pitched "tone" (low in volume, more like a non directional vibration) when there is nothing connected to amp and volume is at zero. When I connect my scope to the speaker, I get a very high frequency waveform that is about 10millivolts @ 0 volume and .4 volts @ full volume. I traced back in previous stages and do not see it, I think the problem is in the last stage. Can anybody point into a troubleshooting direction? I have replaced the power supply filter caps. Thank you very much in advance and I have posted the schematics. David

[J M Fahey]

Please re-check at TP17/15/13/7 and then plug a shorted plug at J4 , Power amp <In.
Still oscillating or it disappears?

[dndemattia]

Here is what I found:

TP17 - Oscillating
TP15 - Oscillating
TP13 - None
TP7 - None

Oscillating stops when J4 shorted. Also, there is a sawtooth pattern on the anode side of CR14 and the cathode side of CR15. What do you all think it is? Can someone explain to me what would cause this type of oscillation?

[dndemattia]

Good morning all! I'm in the shop and ready to work on the Fender. Can someone point me in a direction? Am I providing enough info? I'm a little rusty with my analog electronics having been in the digital world for 20+ years...any help would be appreciated. Thank you again.

David

[joecool85]

I'm a little rusty with my analog electronics having been in the digital world for 20+ years...

David, unfortunately I can't help you on this one (although I'm sure others can).  But I AM curious what you do in the "digital world", care to enlighten me?

[Toner]

C32 would be suspect, also C33&34. It seems U7 has somehow become unstable in the high frequency range.

[dndemattia]

Toner, Thanks! Just checked the cap. and it tested good. Pulled it anyway and had a 10micoF 35V cap in my parts bin. Still oscillating - will check R55 as well and the other caps you suggested. R55 & C32 look like a "snubber" circuit used to protect from high frequency noise that I've seen in industrial controls. I will start again in the morning!

Joecool85 - After the electronics basics in school I ventured into computer control, automation and PLC stuff. Worked in industrial environments with digital process controllers. Troubleshooting a digital circuit is much different than working on a analog one. But because of my electronics background, it seems that my friends are always dropping things off for me to look at. The last few years I've hooked up with a local music shop to help him out with guitar electronics & amp repair. Just finished repair on an old Kustom Tuck & Roll. The amp repair has caused me to dust off my old oscilloscope & signal generator -  just need a little push sometimes to help figure out some things. Thanks for the Forum!

[joecool85]

Joecool85 - After the electronics basics in school I ventured into computer control, automation and PLC stuff. Worked in industrial environments with digital process controllers. Troubleshooting a digital circuit is much different than working on a analog one. But because of my electronics background, it seems that my friends are always dropping things off for me to look at. The last few years I've hooked up with a local music shop to help him out with guitar electronics & amp repair. Just finished repair on an old Kustom Tuck & Roll. The amp repair has caused me to dust off my old oscilloscope & signal generator -  just need a little push sometimes to help figure out some things. Thanks for the Forum!

Neat background.  And your welcome for the forum :-)  It's always good to hear when people appreciate it.

[dndemattia]

Here is the status update for this uncooperative amp. I replaced all the capacitors in the output section and it still makes the high pitched whine. There was something not right about this whine, it sounded like it was coming from a physical place on the board. So I disconnected the transformer secondary from the board (just having the transformer power up) and discovered that the whine was coming from the transformer! So is it a bad transformer? I've been looking online for a direct replacement and am coming up short - any ideas on an aftermarket replacement or has anyone run into this problem?

[J M Fahey]

Then it´s nothing .
Probably some lamination sheet vibrating and buzzing.
Talk about chasing ghosts!
Carefully, keeping away from 1110/220V line connections, press the laminations , the coil, push it sideways, etc., until you damp (or increase) the buzz, trying to find it.
Then drop there a couple drops of glue or somewhat thick paint/varnish (*not* water based, please).
You'll probably damp it.
Transformers are dipped in a special varnish at the factory just for that purpose.
You might even re-dip the transformer yourself, but you´ll have to fully remove it from the amp, it´s messy and smelly, if you use classic "synthetic/oil" household paint or varnish, plus you´ll have to wait 3 or 4 days before remounting.

[dndemattia]

You are correct about chasing ghosts! I tighten up the acorn nuts and the buzz/whine went away - and so did the apparent "oscillation". Analog circuits, go figure! Thank you all for your help and suggestions.

[joecool85]

You are correct about chasing ghosts! I tighten up the acorn nuts and the buzz/whine went away - and so did the apparent "oscillation". Analog circuits, go figure! Thank you all for your help and suggestions.

Great to have an easy fix, even if it did take a while to figure out what it was.  Also thanks for posting back what was wrong, it may help someone else later on down the road.

[dndemattia]

An end of the day status update: Tightening nuts did not totally fix problem. When I removed the transformer from the chassis the whine went away totally - the transformer was REALLY reacting to the amp chassis. After pouring black enamel model paint into the transformer and moving it around every so often to disperse the paint - the problem is now fixed! Thanks J M Fahey for the tip!  I bolted the transformer back in to the chassis and no whine! I'm going to plug my bass in and do a little testing. Hopefully this is it. Thank you all again and I really appreciate the forum. I'll hang around and hopefully I can help somebody some time. One another note, if you want to see some of my animatronic characters - check out : www.sleepingdragonstudios.com. Later!

[J M Fahey]

Hey !!
INCREDIBLY cool !!!
Don't miss watching dndemattia`s Animatronics projects!!
Just for breafast:
http://www.youtube.com/watch?v=0qR31Ek39cg
http://www.sleepingdragonstudios.com/PProjects/Grant/GrantProject.htm
FUN !!!!!