Harmony H304

[mattvon]

Hi all, first post...I've got a bit of a mystery on my hands and I'm hoping you all could help me with it.

I recently picked up a Harmony H304 for a reasonable price. Playing a bit of guitar through it, it sounded pretty cool--although the speaker (8" Jensen) is pretty tired. It still had the 2-prong power cord on it, so I opened it up and discovered it appeared to have the original filter caps as well! Transformers are stamped 1960, so it was high time to replace those electrolytics.

I replaced the 2-prong cord with a proper 3-prong grounded one and removed the "death cap"...next, I looked at the electrolytics and found a 20/20/20uf 450v Illinois can cap, a 20uf 300v cap, and two 25uf 25v caps on V1 and V2.

I had Sprague caps on hand to replace the 20uf and the two 25uf caps, so I did that, planning on ordering a multi-cap can for the 20/20/20uf. I brought the power up on my variac and let the caps form up, then plugged it in and...nothing. No sound. Power was good, all tubes and the pilot light lit, so I figured I must have missed something so I double-checked the work. it all seemed okay.

Here's where it gets weird: I plugged my signal generator into an input and turned it on, hoping to poke around with a chopstick and see if I had any loose/bad solders. I was expecting to hear nothing--except I DID hear the signal generator! Both volume and tone work, both inputs work with the signal generator. I can adjust the frequency range and hear it sweep.

BUT, if I take the same cable from my signal generator and plug it into my guitar...nothing. I don't even get a shorting sound when touching the guitar cable tip and sleeve. I've tried two guitars with the same result--and checked to make sure the guitars worked on a different amp.

What the heck?!? I'm still a bit of a newbie at all this, but I can't think of what would make the signal generator work but a guitar not? I've swept across a wide range of frequencies, wondering if somehow I was blocking certain ranges...no problem with the generator, nothing at all on the guitar.

Any thoughts? I'm pretty well stumped, so I really appreciate any help anyone might have for me!

Here's the schematic for the Harmony H304 for reference.

[g1]

I'll take a wild guess and say maybe you are missing a ground at the input jack or somewhere?
Does the signal generator have 3 prong? Maybe you are getting a ground through the 3prong for the generator, but the guitar has no ground reference.

[mattvon]

Thanks for the reply...I was thinking ground issue myself (especially since it makes NO sound with a guitar cable plugged in, bridging the tip and sleeve.

I do have continuity from the sleeve of a known-good guitar cable to chassis ground. I tested a number of other ground point throughout the amp to make sure I have continuity and it seems good...

[madkatb]

Have you checked continuity from the tip of the jack to ground? If you're getting no sound at all then maybe your signal is going to ground.

[g1]

Also, you did not mention what the signal level was from the generator. Did you set it for a level similar to what a guitar puts out, or did you just turn it up till you heard something?

[mattvon]

Thanks for responding! I do not have continuity from my jack tip to ground. I have my signal generator set at .775VAC, but it's audible at far lower voltages--just requiring more volume on the amp...

I did notice something I hadn't before...after letting the amp warm up for about 15 minutes, I got an intermittent crackling or sputtering from the amp, even with no signal applied to the input. In my limited experience, I've heard this in amps which needed recapped, but I've replaced all the electrolytics save the multi-cap can.

Thoughts? Should I replace the large can and see if it's the source of my woes?