No sound at all: missing B+ ???

[slidincharlie (Carlo P)]

A friend of mine brought me his bf Vibrolux. This amp is dead silent: not even a faint hiss comes out of the speakers. I checked the tension and it is 0 (zero) volts at both the standby switch lugs and at the OT center tap.
There is normal VAC on the power switch.
What should I check? Should I simply read voltages at every node starting from the power switch until I find the dead point?

[Don Symes]

Normal VAC at the power switch -

What about the fuse? (check it with an ohmmeter)
ANY secondary voltages at all?
Do you have 5VAC at the rectifier?
Do you have 6.3VAC at the heaters?
While we're at it, what about the bias string (AC, recto, filter)?

[Amp Kat]

I would first check the secondary VAC HV right at the rectifier diode. If it's not there it's not going to be DC'ed. Then I would disconnect it from the diodes to verify something isn't dragging it down. If there is HV VAC at the rectifiers check the other side of the rectifier and see if it's there. It's also possible a resistor on top inside the can opened up so pop the top and inspect for burnt resistors and caps. RG's site at Geofex.com has some good transformer troubleshooting tips too. http://www.geofex.com/ampdbug/nosound.htm

[TD_Madden]

just a few stray thoughts....

what is the status of the rectifier...does it (and the tubes, of course) light up?

Speaker plugged into the correct speaker-jack? (although being in the "wrong" one should still result in some sound...)

[Enzo]

Heck, is there mains voltage on the power tranny primary wires?

ANother way to test the primary is to hold the mains plug in your hand, turn the main power switch ON, then measure resistance between the main prongs of the power plug. You should see the fairly low resistance of the PT primary winding. If it measures open, then either the mains cord, the fuse, the power switch, the internal wiring, or the PT primary are open.

There should be normal mains voltage between the pins of the power switch ONLY WHEN IT IS OFF. When it is on, there should be zero volts between the pins. Think about it.

[slidincharlie (Carlo P)]

Fixed!

I can't believe it!
It was just the wire going from pin 8 of the rectifier to the main board (i.e. the first filter cap I suppose): it was disconnected from pin 8!
I resoldered it.

Thanks,

[Don Symes]

Do you think it was a poor solder joint?

You might want to go an retouch the iffy-looking ones so you don't wind up with another mysterious (or intermittent) failure.

[slidincharlie (Carlo P)]

It was a poor solder joint, since the wire was intact.
And yes, I should do a big retouch (the next time at least... the amp is back to his owner's home now, because the guy needed it urgently).
This amp isn't in optimal conditions: lots of rust on the chassis, non-original PT and OT, power tubes mismatched by 10 mA, low B+ (around 380 V when I select the correct AC, that is 230V for my place)... sure it needs some work...

[mykey]

A friend of mine brought me his bf Vibrolux. This amp is dead silent: not even a faint hiss comes out of the speakers. I checked the tension and it is 0 (zero) volts at both the standby switch lugs and at the OT center tap.
There is normal VAC on the power switch.
What should I check? Should I simply read voltages at every node starting from the power switch until I find the dead point?

burned out winding on power transformer or
open circuit rectifier tube
did you buy it on e bay?
caution high voltage.

[Enzo]

Hi MyKey, welcome to the group. You might read through the whole thread, he didn't buy the amp, it was a friend's, and then he found a loose connection.

[slidincharlie (Carlo P)]

Thank you Enzo for clarifying things!

And welcome to the forum, MyKey.

[mykey]

Hi MyKey, welcome to the group. You might read through the whole thread, he didn't buy the amp, it was a friend's, and then he found a loose connection.

should have offered him $20 for it and used it to learn with.
greetings, ampheads- from grover beach.