Barn find Black Face 1964 Vibroverb with a short circuit

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Barn find Black Face 1964 Vibroverb with a short circuit

Postby mballentine on Sat Apr 22, 2017 7:49 pm

I have a barn find 1964 Vibroverb. Blows fuse when switched on. No history on the amp.
Opened it up and looks like it had never been into. All caps and resistors looked original.

Problems found and addressed:

1. All caps in the dog house where bad (cases bulged and corrosion). All are replaced and are in the correct polarity.

2. Tremolo "roach" leads corroded and broken so roach was replaced (Caps for the roach kit have not been installed).

3. Treble and Bass pots on Normal channel corroded and could not be turned. Both replaced. All other pots tested fine and met specification.

4. Checked all resistors and all met the specification values and tolerance.

5. No broken wires or black spots.

5. All tubes tested good ( I have a working vintage Super Reverb AB763 as a test platform) but I replaced the rectifier tube (GZ34) because of age.

Used a light bulb current limiter to trouble shoot the amp.
With all tubes removed the amp still blows a fuse (the light stays bright) when switched to ON.
Ground switch and cap is not the problem.

Now that everyone has the background.
I don't want to blindly replace parts, solder and dis-solder other testing, so what is my next step?
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Re: Barn find Black Face 1964 Vibroverb with a short circuit

Postby jaywalker on Fri Apr 28, 2017 3:20 am

Did you replace the bias cap too?
Some close up pictures would help of the guts would help. Use a host like photobucket.com for storage then past the "img" link in your reply post. Hit preview and if the pic shows then post. Meanwhile check out the tubeamp debugging page at www.geofex.com.
Welcome to the palace!
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Re: Barn find Black Face 1964 Vibroverb with a short circuit

Postby Dean Jr. on Fri Apr 28, 2017 9:16 pm

Well, at this point I say you are going to have to desolder some connections. first, +1 one on checking the bias circuit. That cap should have been replaced if you did not replace it. Lift the secondaries, install a fuse, fire it up...does the fuse blow? IF so, the PT primary is shorted. IF the fuse holds, then you are left with the secondaries as suspects. You could check the continuity of each winding and also check for a short to the frame in each secondary. Each secondary should show no continuity with another secondary winding of course.
Nice amp to have when it is working correctly. My first one had to have a new PT.
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Re: Barn find Black Face 1964 Vibroverb with a short circuit

Postby stratele52 on Sat Apr 29, 2017 9:50 am

Disconnect all Power Transformer secondary wires,
Power amp ON .
Report

A short like this is easy to fix if you follow the right trouble shooting procedures.

Show us good pictures
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Re: Barn find Black Face 1964 Vibroverb with a short circuit

Postby mballentine on Sun Apr 30, 2017 1:17 pm

Thanks for everyone's help.
Here are a few pictures of the amp for your review.
http://i1243.photobucket.com/albums/gg5 ... wptf6m.jpg
http://i1243.photobucket.com/albums/gg5 ... 6bgnvm.jpg
http://i1243.photobucket.com/albums/gg5 ... 03bqfj.jpg
http://i1243.photobucket.com/albums/gg5 ... mjd5w9.jpg
http://i1243.photobucket.com/albums/gg5 ... jfcnfe.jpg
http://i1243.photobucket.com/albums/gg5 ... heee69.jpg
The resolution should be good enough to see everything.
If there is a place on the circuit that may need an additional photo, please let me know and I can try to get a better view.
Working this afternoon on looking at the power transformer primary and secondaries. If I disconnect and check impedance on the power transformer winding, any idea of what the primary should be in ohms as well as the secondaries?

Thanks again for your help.

MARK...
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Re: Barn find Black Face 1964 Vibroverb with a short circuit

Postby stratele52 on Sun Apr 30, 2017 1:28 pm

mballentine wrote: If I disconnect and check impedance on the power transformer winding, any idea of what the primary should be in ohms as well as the secondaries?

Thanks again for your help.

MARK...


You can't check impedance only DC resistance,

Don't waste you time, do the test I write if you want to fix your amp
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Re: Barn find Black Face 1964 Vibroverb with a short circuit

Postby mballentine on Sun Apr 30, 2017 7:37 pm

Disconnected the secondaries of the power transformer.
Short was still detected.
Checked the reverse polarity switch and cap for issues with none found.
Removed power transformer from the circuit and checked the "DC resistance" of primary and secondaries.
Power transformer shows shorted inputs (zero ohms) and only one secondary winding showing an ohm value.

Thanks for pointing out that impedance (Z) can not be measured with a DMM only R in ohms. Some how I thought I had covered it in the post.
I'll double check my wording before posting to keep from misleading anyone. Remember folks, I'm still new at this.

Based on what I have found I would think that the power transformer is an issue, so unless I missed something, what are your recommendations for replacement of the power transformer (125P5D)?

Thanks for your help,

MARK...
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Re: Barn find Black Face 1964 Vibroverb with a short circuit

Postby stratele52 on Mon May 01, 2017 8:46 am

Power transfomer DC resistance at primary ; 1.9 ohms with Fluke Meter.
Your P.T. look short.

If you live in North America ; I order parts from Antique Electronics.
The sell good Canadian made transformer made by Hammond, the one I'll put on your great amp
https://www.tubesandmore.com/products/t ... reverb-etc


Other manufacturer;
https://www.tubesandmore.com/products/t ... 5-v-200-ma
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Re: Barn find Black Face 1964 Vibroverb with a short circuit

Postby stratele52 on Mon May 01, 2017 9:10 am

Check the two 470 ohms resistors on 6L6 socket

Replace bias circuit filter caps by a F & T brand, 100 volts 100 mfd

Available at Antique Electronics
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Re: Barn find Black Face 1964 Vibroverb with a short circuit

Postby wilson32 on Thu May 04, 2017 7:29 pm

How do you find a 64 Vibroverb in a barn??? Post the details so that we all may have some hope.....
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