AB763 help please.

[Stratz]

Does anybody happen to have a picture of the bottom/underside of a AB763 main board?
I searched the net for an hour and could not find one.

I want to install my Deluxe Reverb build board in my chassis but I want to make sure I have the bottom connections wired properly first.

Thanks.
Rob.

[Tom Phillips]

I don't have a picture but do you realize that the connections on the bottom of the board are shown in the layout diagram? The view is from the top and the connections under the board are shown as dashed lines. (I.e. x-ray view)
Layout drawing attached.
Deluxe-Reverb-AB763-layout.pdf

[Stratz]

Hi Tom. Thanks for the reply.
I do know that the Fender layout shows the underside connections but I have THIS .pdf file from Mojotone.
If you look at "Photo 5 - Fiberboard Assembly" it shows the underside of the board. What has me stumped and worried about installing my main board is the long green jumper wire in pic 5. I dont have anything like that on my board.

I do have some of the longer runs (X and Y) running twisted along the top side of the board as you may be able to see in the attached pic of my main board but that green wire in photo 5 of the Mojo .pdf has me scratching my head.

Thanks again
Rob.

[Justin Thomas]

Hard to go by just the pics given, put I am willing to bet that that is a ground wire to tie all the preamp grounds together. If you look at the final pic, it shows the overhead view with the (at least two) green wires going from the board to the input jack grounds. Also, looking at the spacing, I feel my suspicion is confirmed.

I personally was under the impression there should be only ONE preamp ground wire, not one to each channel's Jack, but I imagine they have sold hundreds of these kits without problem or incident with ground loops.

The difference being, the original Fender grounds just went right to the chassis. The circuit is the same, just physically different.

Justin

[Stratz]

After much staring at layouts and my blackfaced '68 Bandmaster Reverb I see that the white wire is X (I have mine running above the board along with Y and twisted with two others at times) and the green wire is the common ground for all of the cathode bypass caps which I wont be wiring from underneath but rather from the top to a terminal strip.

Thanks very much to all for your help.
Perhaps I should have just started a thread about my build so you all could follow along from the beginning and help by keeping an eye on my work.
Rob.

[Tom Phillips]

After much staring at layouts and my blackfaced '68 Bandmaster Reverb I see that the white wire is X (I have mine running above the board along with Y and twisted with two others at times) and the green wire is the common ground for all of the cathode bypass caps which I wont be wiring from underneath but rather from the top to a terminal strip...

Good idea Rob. I was going to suggest routing that wire on top rather than hidden under the board. I'd just bend it to run along side the board. I don't think you need to add extra terminal strips.
Tom

[Stratz]

Thanks for the reply Tom. The terminal strips are already installed. One strip next to the PT for the AC ground wire, pilot ground and bias board ground.
All other grounds including pot grounds will go to the other strips on the opposite side of the chassis near the normal channel pots.

I'm just hoping that I have all wires that go under the board in place because once the board is installed that's it.
Attached are a few a pics of the bottom of the main board. I'm pretty certain I have everything wired correctly. I just need to trim some component leads and re-flow a few eyelets.

The far right side of the board wasn't finished at the time I took these pics.
Thanks
Rob.

[Tom Phillips]

Rob,
I have a tip about the wires on the underside of the board. It is good practice to route the under-board wires around the back of any eyelets that are between the end connection points. You do not want the wire insulation touching the back of a solder eyelet because 1) The insulation could become damaged when an eyelet is re-heated to do repairs or mods in the future and 2) the wire could interfere with the proper initial flow and potential solder sucker removal of the solder in the future. In addition, you should make sure that the wire is not stretched tight between its connection points. This is because a tight wire can break free if the board warps over time or is flexed during installation or service. That creates a difficult problem to fix and a nightmare situation if it causes an intermittent contact. For otherwise straight runs you can just allow for a very slight "S" turn in the wire routing. It only takes a little tiny bit of slack to avoid the stress.
Cheers,
Tom

PS: Be it known that Fender soldered the board sub-assemblies from the back side during production. That gives easier access to the eyelets without touching adjacent wires or components with the hot soldering iron. It is also why the old Fender boards have that characteristic solder dome on the top side of original solder joints.

[Enzo]

And since components were pushed into the eyelets from the top side, they could solder and trim excess lead wire at the same time from underneath.

[Tom Phillips]

Rob,
I was typing while you were posting the pics. You have fewer long under board wires than the Mojo example. That's good.
See the attached commented photo.
HTH,
Tom

[Stratz]

Rob,
I was typing while you were posting the pics. You have fewer long under board wires than the Mojo example. That's good.
See the attached commented photo.
HTH,
Tom
[ATTACH=CONFIG]39859[/ATTACH]

Yup, that's just two component leads twisted together and soldered. I'll clip it off and replace it with a jumper wire top side.

I want to put the main board in and I'm about 95% certain that I have all of the jumpers installed but it's very hard to tell.


Thanks to all for the excellent advice guys. If I didn't mention it before or you haven't guessed by now, this is my first amp build.
I know I should have started with something easier but I have most all the amps that I really like.
Thanks again
Rob.

ps: Is there any way to get more midrange from this circuit? I would love a mid control to increase mids.
I'll make sure it fires up and works good stock before I do any mods though.

[Richard]

the 6.8K resistor on the bass control is replaced by a 10k pot on AB763 amps that have a mid control. Some modern Fenders have a 25K pot for the mid control. Increasing the value of this will give more mids. Decreasing the value of the .047 capacitor connected to the blue wire going to the left lug of the volume pot will raise the frequency of the bass shelving, more lower mids. A Super Reverb has a .022. You can download Duncan's Tone Stack Calculator to see how the components interact.

TSC

[loudthud]

I like to lay the board down component side up next to the layout and use an Ohm meter to check continuity between eyelets that are supposed to be connected by wires on the back side. It sure beats having to remove the board after installation. It's nice to have a full size printout of the layout even if you have to print it in two pieces and tape it together.

[Stratz]

Thanks for the great advice. I'm going to look the board over one more time and drop it in. If I missed anything I'll install it from the top.
I'll keep you all posted.
Last question, since I have all of my cathode bypass cap and pot grounds, and other preamp grounds going to a terminal strip should the ground wires be twisted?
Thanks again
Rob.

[Tom Phillips]

...since I have all of my cathode bypass cap and pot grounds, and other preamp grounds going to a terminal strip should the ground wires be twisted?
Rob.

There is no need to twist ground wires together. They could be twisted to make things look neater but that could cause a hassle if you need to alter or troubleshoot the grounding system. A word of caution about the grounding system. The original Fender grounding system was pretty random but it worked well enough for them. There is lots of room for improvement and I can see from the diagram that Mojo has made some changes. Your mention of multiple grounds "going to a terminal strip" indicates that you are making your own changes to the grounding scheme. You need to be aware that incremental changes can make things better or can make them worse so proceed with caution. If I was doing a from scratch build I would probably do the grounding differently than the classic Fender method but I would do all the changes in concert as a complete grounding system. If you do not understand the theory of grounding systems then I would stick with a published tried and true grounding layout. Either the one Mojo shows, the classic Fender scheme or another that someone has tested and published. The point is that it can go bad if you mix various grounding scheme features.

Cheers,
Tom