Function Generator / Oscope

[Gibsonman63]

A few wayward amps are starting to find thier way to my workbench. I have a friends HRD on my bench now that has a problem somewhere in the preamp section. I can plug a guitar into the effect return (power amp in) and get a nice powerful sound, but jumpering the effects loop with a cable yeilds no sound at all. Plugging a guitar into the input also produces no sound. It doesn't seem to have the common Hot Rod Deluxe ailments. The switching works fine and the two cement resistors don't look like they cooked the board. I have already reflowed the solder joints and chopsticked with no joy. I have also tried a different "known-good" set of preamp tubes.

I need a cost effective way to inject a sine wave and trace it through the preamp section. Suggestions for reasonably priced equipment to do this?

[Phil_S]

Do you have an old Walkman-type CD player around? You can burn a CD with several 5 minute tracks at specific frequencies. (You can download a free tone generator to your computer to provide the source.) All you need after that is to jury-rig an output cord into a 1/4" phone jack, or alternatively to a pair of gator clips for working on an open chassis. The frequency range of a guitar is roughly 82Hz to around 5,000Hz. In addition to the standard 1K test signal, you should have plenty of room on the CD to record several other frequencies. It's a bit of work to set it up, but practically cheap as dirt. Heck, you even get a volume control on the CD player.

[Gibsonman63]

Great suggestion. The schematic calls for a 4v, 1kHZ sine wave test signal. That should be easy enough to measure with my trusty VOM to set the level.

[surfsup]

freesounds.org has a 1khz WAV file you can download for free. FYI

[TUBEDUDE]

A scope and generator are essential tools to be sure. In this case however, did you clean or reform the contacts on the effect in/out jacks? Or jumper around with clips? Sorry if I mention the obvious but i couldn't tell if you cabled around them or plugged the cable into them.

[Leviman]

Get someone to strum on a guitar while you trace around!

[Gibsonman63]

A scope and generator are essential tools to be sure. In this case however, did you clean or reform the contacts on the effect in/out jacks? Or jumper around with clips? Sorry if I mention the obvious but i couldn't tell if you cabled around them or plugged the cable into them.

Tubedude, I plugged the cable into them. Good point. I should be able to plug a cable into the send and meter it back to the board through the jack. The return jack definitely works. cableing through and jiggling the send side doesn't even produce static.

[TUBEDUDE]

Depending on the failure mode, (if the jacks are bad), you might not be getting signal fron the out jack to cable it anywhere. try jumpering it with clip leads from the resistors/capacitors involved, then insert naked phono jacks to take them out of curcuit completely.