VH-140C little or no signal without jumper in effects loop

[toddler68]

Just bought an Ampeg VH140C head with a known issue - the amp only works when there is a jumper in the effects loop. Pull the jumper and there is little or no sound. With the jumper in place, the think rocks like nobody's business. Any ideas? I re-flowed a loose solder joint on one of the jacks, which didn't help. I also sprayed/cleaned both inputs and effects send/returns. How else should I (an electronics newb) go about troubleshooting this? Thanks in advance!

[52 Bill]

If cleaning didn't help, you will probably need to replace the return jack.

I've gone as far as to burnish the contacts clean with a tiny strip of 1200 grit sandpaper. I place the strip between the contacts and let the spring action pressure of the jack hold it in place while I pull the paper out.

[kenda47]

Common problem on most amps with effects loops. Save yourself the trouble replace both send and return jacks. Either one or both are sprung and remain open circuit when you unplug the jumper lead. Do you use the loop? If not save yourself the cost of the jacks and stick with the jumper. Far more reliable.