Deluxe AB763 new build :-) but LDR vibrato started thumping?

[rogb]

Hi, hope someone out there can help with this weird thing.
I finished this build on Monday, powered it up and was sounding good, if a little hissy. I used Ohmite carbon comps throughout. I decided to change a bunch of them for Dale RN65 and Xicon carbon films that I had around.


The LDR was working fine but a had a slight tick so I added the Fender cap trick to the 10M and ground legs. The tick was gone.

Added the new resistors and some RG174 shielded to the inputs and effects loop.

Powered up again and it was sounding much better, almost no hiss but the LDR started thumping like a bass drum. The neon is still flashing and responsive to the Speed pot.

I have taken all the vibrato components off the board, cleaned the board and the eyelets and re soldered, the thump is still there.

Bias on the JJ6V6s is fine, 24mV with the GZ34 and 27mV with the diode recto, all voltages seem good, I mean it sounds just great, even with no reverb and you can jumper the channels as there is no out of phase effect in the non reverbs. Got an NFB lift switch that adds more gain (and hiss) to proceedings.

Any ideas welcome.
Thanks
Rog

[jelle]

.....

[jelle]

Pics? Must be coupling....

[Firestorm]

Where did you ground the shield on the effects loop input. Maybe someplace noisy?

[rogb]

Thanks guys, I think coupling might be the clue. There is also scratchy noise on the Intensity pot, thinking that DC is getting through there.

I will look at the caps marked on the schematic for leakiness,

The grounding for the effects loop shields is the same as I use on D style builds, isolated jacks and a star ground near the jacks. It was fine before just suddenly appeared so I guess a cap might be leaky.

[martin manning]

The caps in the LFO are not likely to be involved. If the oscillator is working, they are ok, and the intensity pot is isolated from that part of the circuit by the opto-bug-thing. The 0.001 PI input cap could put DC on that pot if it were to be leaky, though.

[rogb]

The caps in the LFO are not likely to be involved. If the oscillator is working, they are ok.

Correct! I subbed all the caps I highlighted and still the same. I disconnected the effects loop and still thumping. I chopsticked all the wires on every tube and pot, still the same.

I'm sure I will find it but it's very annoying

[martin manning]

Oops, see my edit above... Add to that the 0.047 coupler out of the normal channel.

[rogb]

Oops, see my edit above... Add to that the 0.047 coupler out of the normal channel.

Yeah, I will investigate that area too, something is leaking DC onto the pot, I'm sure thanks

[jelle]

Can you measure dc on the pot? And if so, can you measure dc on the preamp board material? If so, lift the decoupled side of the cap up and hook the wire from the pot to that lifted leg of the cap. Does that solve it?

Btw, measure the sides of the circuitboard. I have frequently seen 50mV.....and yes that is bad. Good luck!

[rogb]

Nailed it in one you guys! Jelle, it was coupling. After taking the whole board out and measuring the coupling caps, I found a dead 0.1uF TAD "mustard" on the anode pin 6 of V2, that was where the DC leak was from, I found a 715P to replace it, its an 800v so SLIGHTLY big, but no matter, it works perfectly

THANKS A MILLION TAG Guys!

The slight tick was there so I added RG174 to pin 2 grid of V3, silent as you can get with these LDRs.

All I gotta do is get this damn fx loop running. Bypassed it is fine but no fx as yet. I followed the #102 layout for wiring but I may have connected it backwards as it is a kind of a mirror image.

On a downer note, I have now trashed and bumped most of the PVC wire that I had in this, there is plenty of Teflon, but I will swap the melting stuff out soon enough when I order some more.

Does it ever end?.......

[martin manning]

Great! Was it the 0.1 you circled in the schematic above?

[rogb]

Great! Was it the 0.1 you circled in the schematic above?

Why yes it was!

This is the first time I have strayed away from ODs caps, and now I have put some more back in.

Do they ever go down like these Mallory types?

I'm not sure if this vintage stuff is for me, I like the Dumble sound, and when I play I'm comparing it in my mind to my old HRM 50.

Guess I will build another someday, like Tom I like the clean and I like the dirt....

It's nice though the AB763, maybe I need to spend a bunch of time with it, tweaking and twiddling. Sounds good too with a 12AX7 in the PI. Sounds completely different with silicon recto, AT7 PI - like a little BF Super, really want to put some 6L6s in this too, to see how clean it will go.

[jelle]

Sweet, glad you found it. Jelle

[ampdoc1]

rogb,

Here's a project for you. Trainwreck Expresss preamp with switching EQ channels and a switching Dumble OD and power amp. I built this without the dual EQs into one of those shitty Bandmaster Reverb TFL heads a few years ago and it keeps the integrity of the original amps. SWEET!

a'doc1