Dynaco ST 70 bad PT or not?

[Randall]

OK. With all 12 secondary wires snipped free, I still get a bright limiter bulb, and 9.4 vac across the primary. Can we agree this PT is toast?

And, so how was it I got the neon to glow per my original post?

[Helmholtz]

I had thought he was still testing with the 10VAC on the primary and so TX must be ok. My bad.

Low reflected primary impedance means low voltage drop in series with the bulb. I have no doubt that there must be either an internal or external PT short.

[Helmholtz]

Can we agree this PT is toast?

I do. Must be an internal winding short.

And, so how was it I got the neon to glow per my original post?

Seems the step-up ratio of the PT of around 5 was high enough to just ignite the neon. I found considerable variation of neon ignition voltages. Some ignite well below 60V.

The neon test works most reliably if battery and neon are connected to the same winding. The neon should flash when the battery is disconnected as it's the flyback voltage which should strike the neon.

[Jazz P Bass]

https://www.ebay.com/itm/Power-trans...0AAOSw-0xYlP2L

[Randall]

"The neon test works most reliably if battery and neon are connected to the same winding. The neon should flash when the battery is disconnected as it's the flyback voltage which should strike the neon."

I have never heard of doing it this way. If they are both connected to the same winding, then you must mean in parallel? And does this method test for all windings at once, or must you test one winding at a time?

Jazz, that PT isn't a real one, these amps are furniture, so that ugly piece ain't gonna cut it. There are other correct options for the same money, fortunately. And as I read a little more, it seems failure with these is not uncommon. Some say they are a little under designed.

[Helmholtz]

And does this method test for all windings at once, or must you test one winding at a time?

Testing one winding is enough as all windings are coupled and if there is a short in any winding it will prevent high flyback voltage in all windings.

neontest.pdf

Other than described in the text the push button switch should be closed for a second or so to allow for ramp-up of inductive current. The neon should flash when the button is released (meaning sudden disruption of current).

[nosaj]

"The neon test works most reliably if battery and neon are connected to the same winding. The neon should flash when the battery is disconnected as it's the flyback voltage which should strike the neon."

I have never heard of doing it this way. If they are both connected to the same winding, then you must mean in parallel? And does this method test for all windings at once, or must you test one winding at a time?

Jazz, that PT isn't a real one, these amps are furniture, so that ugly piece ain't gonna cut it. There are other correct options for the same money, fortunately. And as I read a little more, it seems failure with these is not uncommon. Some say they are a little under designed.

Does your transfomer have bellcaps? They gussy up the most ugly of transformers.

nosaj