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FDP Forum / Amp Mods, Repairs, and Projects / Custom Vibrolux Reverb problem

Yreva

United States

Nov 19th, 2016 03:15 PM   Edit   Profile   Print Topic   Search Topic

I'm stumped on this one. I have an intermittent problem where the amp won't power on when I flip the power switch. Fuse is good, tubes are new, no obviously bad connections after removing from cab and checking visually. Do the power switches go bad? Never had this problem in 40 years of playing Fender Twins, Deluxe Reverbs. Any ideas or suggestions greatly appreciated.

willie
Contributing Member
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Too Near Atlanta GA

Amp Tech Emeritus
Nov 19th, 2016 04:11 PM   Edit   Profile   Print Topic   Search Topic

Power toggle switches do indeed go bad. If you have access to an ohmmeter it is easy to check the switch to see if it is functional (be sure to disconnect from the mains power before you test the switch. :) )

w

Yreva

United States

Nov 19th, 2016 06:49 PM   Edit   Profile   Print Topic   Search Topic

Thanks Willie,
Are there any decent tutorials out there, books etc. that I can refer to for the exact testing procedure and replacement if necessary?
Thanks again,
Mark

Peegoo
Contributing Member
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Willie Nelson

Mandela Fitzgerald
Nov 19th, 2016 09:03 PM   Edit   Profile   Print Topic   Search Topic

Howdy Mark

If that were my amp, I'd

- unplug the amp
- pop the chassis out
- drain the filter caps

And use a DMM to test the switch for proper operation. Another way would be to

- unplug the amp
- pop the chassis out
- drain the filter caps
- solder a jumper wire across the power switch (to bypass it)
- reassemble the chassis in the amp
- plug it into the wall

If it works reliably that way, time to replace the switch.

WARNING: lethal voltages exist in an amp chassis--even when unplugged. If you don't know what you're doing, don't take a chance. People have died messing around with electrons.

Yreva

United States

Nov 20th, 2016 02:45 PM   Edit   Profile   Print Topic   Search Topic

Thanks for the tips. I thought I'd just change out the switch. Is it necessary to drain the filter caps to do that?
Thanks,
Mark

Peegoo
Contributing Member
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Notice the music

not the guitar brand
Nov 20th, 2016 06:34 PM   Edit   Profile   Print Topic   Search Topic

YES.

Every time you have the chassis open. No exceptions. The "one hand in the pocket" rule can still get you a bad shock if the caps are still energized.

Even if you don't touch anything and receive a shock, something as goofy as accidentally dropping tool in the chassis can create a short and fry something--making it necessary to do a repair that wasn't originally planned.

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

So. Cal. USA

Nov 21st, 2016 10:00 AM   Edit   Profile   Print Topic   Search Topic

If I remember correctly from the CVRs I owned, the power switch is connected with spade lugs and no soldering is required.

Unplug the switch from wiring and test for continuity after draining the filter caps. It should be a DPDT switch (hot and common are both switched)so check both sides.

You can also test the chassis hot looking for approx. 120 VAC at the input to the power transformer, usually two black wires. Be careful here and observe the one hand in pocket rule above. Best to connect with jumpers and the power off then power up for measurements.

Yreva

United States

Nov 22nd, 2016 07:03 AM   Edit   Profile   Print Topic   Search Topic

Thanks for the replies. Yes the switch is connected with spade lugs. Just to confirm draining the filter caps, which I haven't done: attach a lead to ground and then to pin 1 of the first preamp tube, then check volts with DMM. Check switch and replace if bad.
Thanks,
Mark

Yreva

United States

Nov 24th, 2016 02:44 PM   Edit   Profile   Print Topic   Search Topic

Switch replaced without incident. The amp continues to fail to power on intermittently, fuses, tubes intact. Where to look next?
Thanks,
Mark

amphead4
Contributing Member
********

Cincinnati, USA

Nov 25th, 2016 06:56 AM   Edit   Profile   Print Topic   Search Topic

If the problem is on the secondary side, you might see tubes light but no B+ voltage (or vise versa). If all secondary voltages are zero, it really points to something on the primary side.

To help determine if the problem is on the primary side of the PT, open the connections closest to the primary winding itself and check that line voltage is being consistently applied every time the switch is closed. Could be intermittent at the fuse.

The schematic I found online is not legible. It shows something (a thermal device?) in series with the PT primary. This would be suspect as well. Does anyone have a link to a better schematic?

There are very few components in that primary circuit - line cord, double pole switch, fuse holder, fuse, the blurry thing on the bad schematic image, connectors and the primary winding itself. Power off, filter caps drained, test all with ohm meter.

All this assumes your house AC outlet is good....

If my comments don't make sense to you, find a helper. I don't want you to get shocked or worse.


Roly
Contributing Member
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Whitehorse Canada

I don't get out much
Nov 25th, 2016 11:58 PM   Edit   Profile   Print Topic   Search Topic

Replace the fuse with a one amp slo blow. Pull the power tubes, bypass the switch...... Short it out with a clip cord. Turn the amp on.
Does the fuse hold?


Roly
Contributing Member
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Whitehorse Canada

I don't get out much
Nov 26th, 2016 12:12 AM   Edit   Profile   Print Topic   Search Topic

"Turn the amp on"
.....oops... that should read.....plug the power cord into the wall outlet.
sorry.

FDP Forum / Amp Mods, Repairs, and Projects / Custom Vibrolux Reverb problem




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