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FDP Forum / Amp Mods, Repairs, and Projects / AA1164 Vibrato Not Working - New Build

JJuran
Contributing Member
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Boston Area

Feb 17th, 2017 07:41 PM   Edit   Profile   Print Topic   Search Topic

Hi all -

Brand new build. Just finished this evening. All voltages check out within +/- 10%. Everything sounds great. But no vibrato.

When I've got a shunted RCA plug in the Vibrato Pedal jack on the back panel, speed and intensity all the way up, nothing. Just a very slight reduction in signal volume - just barely noticable and quite small.

V4 voltages (without the RCA shunt inserted) on pin 1 = 245V. Pin 6 = 184V. When I insert the RCA shunt, pin 1 drops about 100v.

I've re-examined all the wiring and ran continuity checks. Everything checks out.

Before I replace the the .02, and both .01s, any hints or thoughts? I don't have a meter to test those smaller caps.

Thanks as always

JJuran
Contributing Member
******

Boston Area

Feb 18th, 2017 07:25 AM   Edit   Profile   Print Topic   Search Topic

forgot to mention that I've got brand new pre-amp tubes all around and have swapped them in and out of V4.

JJuran
Contributing Member
******

Boston Area

Mar 6th, 2017 07:58 PM   Edit   Profile   Print Topic   Search Topic

I've discovered a wiring error. Duh... I've got continuity where I should NOT have it. Something is shorting under the board. Maybe a jumper under the board where it should not be, maybe component leads are touching somewhere...

The 220K resistor that that feeds 400v to the plate on V4 pin 1 (point B on the schematic) is shorted with the junction of 1M resistor and the 2 caps (.02 uF to ground and .1 uF to the intensity control pot.

Time to lift the board.

Peegoo
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The spotlight

looks like a prison break
Mar 6th, 2017 08:18 PM   Edit   Profile   Print Topic   Search Topic

Did you install an insulating plate behind the board to keep it from contacting the metal chassis?

You're lucky the amp didn't let the Magic Smoke escape!

JJuran
Contributing Member
******

Boston Area

Mar 6th, 2017 08:44 PM   Edit   Profile   Print Topic   Search Topic

Sure enough ... extra bare lead length that jumpers the 400v to the reverb circuit was way long and touching the solder on the juntion of the 1M resistor and and the 2 caps (.02uF to ground and the .1 uF to the intensity control pot.

Now to re wire the leads to all the tubes and power supply ... I'll be extra careful here ...

JJuran
Contributing Member
******

Boston Area

Mar 6th, 2017 08:45 PM   Edit   Profile   Print Topic   Search Topic

Actually I'm using 3/8" standoffs to mount the G10 board so "hypothetically" plenty of room for bare wire ... but yes - you're right. I'm lucky here... could have caused some damage....

Peegoo
Contributing Member
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The spotlight

looks like a prison break
Mar 6th, 2017 09:34 PM   Edit   Profile   Print Topic   Search Topic

GOOD catch!

JJuran
Contributing Member
******

Boston Area

Mar 6th, 2017 10:06 PM   Edit   Profile   Print Topic   Search Topic

Ah ... luscious tremolo .. eh hem .. I mean vibrato ... I mean ... I'm psyched ... fixed. Nice amp. Time to put it into the 1x12 cab I got from Mojo...

Thanks for chiming in Peegoo. Was all alone because I didn't effectively examine ALL my work before posting. Triple checked the wiring and all was good. It's the continuity I DID have where it shouldn't be.

I love this addiction ... can't explain it. But I love it. Thanks for sharing it with me.

Hammond101
Contributing Member
**********

So. Cal. USA

Mar 13th, 2017 10:08 AM   Edit   Profile   Print Topic   Search Topic

Under the board, yes a good grab that you were able to find it with a DVOM.

Under the board is were I'm most careful and at least triple check my work. So far so good but we are only human, right?

Building amps is very satisfying. With the exception of 61 Concert and a 67 Dual Showman I have sold off all my vintage Fenders and only play my builds at home or on the road.

Congrats!

FDP Forum / Amp Mods, Repairs, and Projects / AA1164 Vibrato Not Working - New Build




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