Eden WT800 gone silent

[Sowhat]

I got a perfectly good working David Eden WT800 in the shop that had a busted input jack, it still worked, it just had the nut and part of the housing broken off. The customer was fine with me swapping the busted jack with the headphone jack because he never used the headphones.
Both jacks are identical stereo design (3 circuit) cliff jacks. I told him that I would order a new jack and he could drop it by sometime in the future and I would restore the headphone jack, for right now he needed the amp for some upcoming shows this weekend. I pulled the preamp board, swapped the jacks and stuffed it back together again. When I power up tested it I got not sound out of either channel... damn!!! why me and why now!

I figured I screwed something up so it came apart again but everything looks fine. I shot a signal into the input and I could see it plain as day at the preamp tube (12AX7) grids but this is where things get weird, there's no signal coming out of the tube or at least almost nothing. Before I continue here's a schematic of the input stages...



The signal path comes off the cathodes of VT1 but I get nothing from there on. I checked the supplies and I get -78.5 volts on the cathode and somewhere around +64 volts on the anode... this should give me some output signal, right? well, it don't. I tried another 12AX7 and even an ECC83 but I get the same results. The tube has 12.6 volts on the heater and the tube lights up and gets warm, it's got to be O.K.

I fiddled around with this thing for a while but have not injected signal past this point to see if the rest of the preamp and power amps work but I'm pretty confident that they will, If I'm not getting signal through the first stage then the rest is just academic at this point.

Any ideas?

[olddawg]

Are you sure the jacks are exactly the same internally? I would swap them back before you do anything else.

[Enzo]

Don't assume. How long does it take to inject some sort of signal into the signal path after the tube? 10 seconds? A quick look and I see the tube is a cathode follower, so they are not looking for any gain. So we ought to be able to lose the tube. A cap tacked from pin 2 to pin 3 ought to do it. Got sound now?

Does the DI work?

Did you leave the HP jack spot vacant? The schematic doesn;t show any, but any chance the closed shunt contact is missing and cannot unmute the amp?

And the best advice I can provide: don;t try to figure out what you DID, just find out what it NEEDS. Once you know that, you can figure the other. It is probably a result of your work, happens to us all, but it might not be, it might just be a coincidence, so if we waste 10 hours looking for a mistake we made, the real problem just sits there. regardless of how it got there, we have the problem, so just look to solve it as if you were handed the amp as it sits.

[Sowhat]

I had that thought for a moment but cliffs have external contacts... you see what you get, no hidden internal circuitry, aside from that, the jack is working just as it should... signal in and signal out. Signal get's all the way to the VT1 grids and then that's where this adventure begins. If you look at the schematic you see that only the tip circuit is part of the signal path, the second signal circuit is completely open and unused and the 3rd circuit is signal return ground. When a plug is not inserted it shorts the signal path to ground as you would expect... kind of standard stuff.

I just might do that but I hate resoldering and resoldering fragile circuit board traces. They have not lifted so far and that's a big plus. One thing I'm considering is that I had to disconnect a lot of connectors to get that preamp up and out of the chassis, I'm checking over all of them (and there's a lot) just in case I zigged when I should have zagged. Nothing has burst into flames yet so that's a good thing.

[Sowhat]

Did you leave the HP jack spot vacant? The schematic doesn;t show any, but any chance the closed shunt contact is missing and cannot unmute the amp?

Sage advise indeed! That broken jack I put in the HP location is a bit more broken than I realized... sure enough some screwdriver pressure on the contacts brought the amp right back to life. I'm going to toss that one out and put some jumpers in it's place for right now. My repair shop is also my recording studio which I named Busted Junk Studios... how appropriate.

Once again... Thanks Enzo!

[Enzo]

Maybe you'd like the Palindrominator from Funk Logic for your studio.

Funk Logic | Palindrometer Rack Panel



Oh lord in heaven, Sweetwater even has it on sale currently.