Dumble Clone issues

[dankayaker]

Hi everyone,

I bought a really nice clone made by an experienced builder from the original owner. It's a #183 clone . . . . sounds very nice but the PAB switch and the FET input don't work.I pulled the chassis to have a peak and nothing is obviously broken. I adjusted the FET trimmer all the way up which made the amp hiss a bit more but still nothing from the input. I have some closeup pictures if any one thinks they may be able to help.


thanks,
Dan

[sonicmojo]

For the FET symptom, check your jacks, they work together. The input from the NOR jack is shunted when nothing is plugged in. Maybe you have a bent shunt not making contact when nothing is plugged in to NOR.

[dankayaker]

For the FET symptom, check your jacks, they work together. The input from the NOR jack is shunted when nothing is plugged in. Maybe you have a bent shunt not making contact when nothing is plugged in to NOR.

I was wondering about that . . . . . both jacks have a piece of metal that is supposed to make contact with the bent piece (that touches the tip of cable) ?

Sorry about the pedestrian lingo but I'm not a builder . . . just a player.

Here is a picture . . maybe you'll be able to tell me what to look for:
http://filebox.vt.edu/users/dmirolli/suhr/IMG_4519.JPG
http://filebox.vt.edu/users/dmirolli/suhr/IMG_4523.JPG


THANKS !!

[CHIP]

A little hard to see because of the red striped wire, but it does look as though the switch (metal tab) is not making contact with the tip connector on your normal jack.

[dankayaker]

So the normal jack shunt should be touching the main tip portion when nothing is plugged in ? Can I safely bend the shunt without electrocuting my self ?

[sonicmojo]

A little hard to see because of the red striped wire, but it does look as though the switch (metal tab) is not making contact with the tip connector on your normal jack.

Yep,

Looks to be that is exactly the problem on that bottom (NOR) jack. Here's a trick: plug a cord into that bottom jack and with a screwdriver pry the shunt switch (inner straight metal tab) out while you have more space. You don't want to bend the tip of the jack (the outer long, bent one) at the same time. Hopefully when you then unplug, it will connect. It should not connect when you are plugged in, but it should connect to the tip of the jack when you are unplugged.

Oh, and unlplug the amp and make sure you drain your caps to be sure before you poke around.

[dcribbs1412]

Oh, and unlplug the amp and make sure you drain your caps to be sure before you poke around.

+1

[dankayaker]

Oh, and unlplug the amp and make sure you drain your caps to be sure before you poke around.[/quote]


I don't know how to do that.

Just so I know what you're talking about . . .I need the bend the little shunt so it rests against the longer bent piece when nothing is plugged in . . .like this picture:
http://filebox.vt.edu/users/dmirolli/su ... AA300_.jpg

thanks

[sonicmojo]

Also, the unshielded resistor wraps around pretty close to the NOR jack's tip. I would for sure put some heat shrink on the leads of it just in case. Jacks are shifty little animals.

[sonicmojo]

Oh, and unlplug the amp and make sure you drain your caps to be sure before you poke around.

I don't know how to do that.

Just so I know what you're talking about . . .I need the bend the little shunt so it rests against the longer bent piece when nothing is plugged in . . .like this picture:
http://filebox.vt.edu/users/dmirolli/su ... AA300_.jpg

thanks[/quote]


Yes, like that picture.

I highly suggest you read up on things a little before poking around or have someone with some electrical knowledge help you.

One way I drain things is to turn the amp off but leave standby on operate for a while, while it is still plugged in. Then after a few minutes, I unlpug and ALWAYS measure the DC voltage on the larger capacitors in the amp before I put a hand in it. The reason I always measure is because I can forget to unplug it, or something stupid. Don't risk it.

[ToneMerc]

These things work so much easier when you use the most conducive jacks. Nevertheless, does anyone else see an issue with the way it's wired?
I can't see in front of that volume pot, but it looks like to me that there are two wires (wht/brown) terminating on the ring terminal (ground) of the FET jack that come around the right side of the volume pot, another termination on the tip lug and the switched lug is unterminated.
One of those "two" leads appears as though the opposite end terminates at the NOR jack's switched terminal along with the FET output. Since the FET jack isn't isolated from the chassis, wouldn't this shunt the FET output signal to ground?

Correct me if I'm wrong here.

TM

[sonicmojo]

Yes, this area gets quite confusing so there may be multiple things going on here. On my last 2 recent amps I used Switchcraft jacks, with the FET using a 3 conductor, not like this amp being discussed which looks like it uses just a 2 conductor for both. I never isolated the chassis nor had an issue with that. If I were trying to debug it, I'd probably just replace the top one with a 3 conductor and wire like the 183 or 124 layout in the files section, but check to make sure things made sense with a meter. Here are the Mouser descriptions and part numbers if you go that route:

Switchcraft Phone Connectors (3 CONDUCTOR 1/4") 502-14B
Switchcraft Phone Connectors (2 CONDUCTOR 1/4") 502-12A

[CHIP]

I think you should contact the original builder and discuss these issues with him, before you go messin around inside that amp.

[ToneMerc]

Yes, this area gets quite confusing so there may be multiple things going on here. On my last 2 recent amps I used Switchcraft jacks, with the FET using a 3 conductor, not like this amp being discussed which looks like it uses just a 2 conductor for both. I never isolated the chassis nor had an issue with that. If I were trying to debug it, I'd probably just replace the top one with a 3 conductor and wire like the 183 or 124 layout in the files section, but check to make sure things made sense with a meter. Here are the Mouser descriptions and part numbers if you go that route:

Switchcraft Phone Connectors (3 CONDUCTOR 1/4") 502-14B
Switchcraft Phone Connectors (2 CONDUCTOR 1/4") 502-12A

Yes, the preferable FET jacks if one is two use Switchcrafts are the 13E and 14B. However, two 12A's can be used as well, you just have no way to shunt the FET output signal to ground when no jack inserted, which minimizes hiss.

I've never had an issue when wired just as those actual amps (124, 183) were, using the Kobinconn jack and better yet, it only costs a few pennies more than a Switchcraft 12A.

TM

[dankayaker]

One way I drain things is to turn the amp off but leave standby on operate for a while, while it is still plugged in. Then after a few minutes, I unlpug and ALWAYS measure the DC voltage on the larger capacitors in the amp before I put a hand in it. The reason I always measure is because I can forget to unplug it, or something stupid. Don't risk it.[/quote]


I left the amp plugged in with power off and standby in operate just as you recommended. If I'm careful not to slip can I safely just bend the shunt back on the input jack without getting close to the dangerous stuff . . .or is it all dangerous ? I watched a few youtube video's on draining caps.
thanks