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FDP Forum / Amp Mods, Repairs, and Projects / 66 Bassman suspect bad power transformer

Mars Hall

USA NW Indiana

Telefunkin'
Nov 7th, 2018 03:34 AM   Edit   Profile  

Everything points to the PT going out. I want to confirm by checking the PT. AS I remember, I need to remove the leads, from the circuit, and check continuity on each pair?

SonicBlue

Sunbury-on-Thames

Nov 7th, 2018 06:53 AM   Edit   Profile  

Yep, remove the leads from the circuit and check each coil for continuity. Bear in mind the primary and secondary high voltage windings will show an appreciable resistance, what you don't want is 'infinity' ohms as that indicates an open circuit.

Also measure between windings (primary to secondary and to heater, secondary to heater), to ensure there are no short circuits going on. Do the same between each winding and laminations too.

(This message was last edited by SonicBlue at 08:54 AM, Nov 7th, 2018)

Peegoo
Contributing Member
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Well ya go to a bar

on a Friday evenin'...
Nov 7th, 2018 06:57 AM   Edit   Profile  

You can also compare AC voltages at the secondary pairs with those shown on the schematic.

BbendFender
Contributing Member
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American Patriot

About as ordinary as you can get.
Nov 7th, 2018 03:30 PM   Edit   Profile  

Be careful whatever you do.

Elmo66
Contributing Member
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Midwest USA

Nov 8th, 2018 04:25 PM   Edit   Profile  

A simple primary test is to connect your multimeter leads to the two ac plug prongs. (amp unplugged from wall) Put the meter to read ohms and throw the power/standby switches on. If you get a low reading, say under 10 ohms, the transformer primaries are good, the fuse is good and your switches are good.

BbendFender
Contributing Member
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American Patriot

About as ordinary as you can get.
Nov 8th, 2018 07:13 PM   Edit   Profile  

I have a question.
Another Bassman PT. I get readings of 292v on one secondary and 394 on the other. Total 687v.
Is this bad? Why the difference.

Elmo66
Contributing Member
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Midwest USA

Nov 8th, 2018 08:08 PM   Edit   Profile  

Each reading is one side of the center tap. Ideally these numbers would be equal, but in your case the center tap is not quite in the center of the winding.

Not a big deal if you're seeing about 450-475v total on your B+

pdf64

UK

Nov 13th, 2018 06:38 AM   Edit   Profile  

Are you sure that the PT HT CT and bias supply tap wires haven't been mixed up?
394V seems extremely high - was the amp in 'standby' or 'operate' mode at the time?
If 'operate' mode, I suggest to verify that the HT rectifier diodes are good.

rfrakes331K
Contributing Member
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IL USA

RonHalen Jokingly He Says
Nov 15th, 2018 10:18 AM   Edit   Profile  

In the older amps, the lamp socket rotates and eventually shorts out the PT. I had this problem in a 64 Bassman I bought out front of a bar.

BbendFender
Contributing Member
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American Patriot

About as ordinary as you can get.
Nov 23rd, 2018 09:07 AM   Edit   Profile  

PDF, this PT is completely out of the chassis. It is one of two that I salvaged from some smoke damaged '65 Bassman amps.
Everything else tests well within tolerance under powered up testing.

FDP Forum / Amp Mods, Repairs, and Projects / 66 Bassman suspect bad power transformer




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