Home » Audio » Thermionic Emissions » Tranformer testing???
Tranformer testing??? [message #8179] Fri, 21 May 2004 07:45 Go to next message
jim denton is currently offline  jim denton
Messages: 162
Registered: May 2009
Master
My Sansui 500A receiver is suspect to having a bad transformer---I have evidence that it has leaked a thick waxy substance ---although it was working before a tube flared out---upon inspection under the chassis I found a "glob" (highly techincal term guys---hope I haven't gone over too many heads here!) of a waxy material -off white in color----can get another rec on e-bay for parts to replace this tranny---would like to see if it can be confirmed though as a way to start Jim

Re: Tranformer testing??? [message #8181 is a reply to message #8179] Sat, 22 May 2004 03:37 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18792
Registered: January 2001
Illuminati (33rd Degree)
You can test to see if any of the windings has opened up with an ohmmeter. The connectors to the primary winding(s) should show continuity and the connectors to the secondary windings should also show continuity, but the meter should show open between the primary and secondary. This won't tell you if windings have shorted, but it will tell you if they've opened up.

The most likely failure mode of a power supply transformer is for it to open, so if there's continuity, I'd feel pretty comfortable powering it up. A technician might use a variac to apply power slowly, measuring the voltage output on the secondaries after verifying that the windings weren't open. A drastically low voltage condition indicates that windings are open. With the input power at say 1/4 line voltage, you'd expect the secondaries to all be at 1/4th their voltage rating and if that weren't the case, the transformer might be shorted. But like I say, the most common failure mode is open windings.

Re: Transformer testing??? [message #8189 is a reply to message #8181] Mon, 24 May 2004 22:03 Go to previous messageGo to next message
metasonix is currently offline  metasonix
Messages: 103
Registered: May 2009
Viscount
>The most likely failure mode of a power supply transformer is for it to open

Not always. In my experience, it is more common for either
the primary to open, OR to develop a partial or full short.
And plate secondaries can go either way.

I just recently saw 2 quite different plate transformers
from different tube amps, develop dead shorts across their
high-voltage secondaries. (Both amps were Chinese made,
sorry to say...)

One way to help minimize this is simple: put a 2 ohm to 5 ohm
10W resistor in series with the primary. It limits the
huge inrush current at turn-on, helping prevent internal arcing.

Also a 100 ohm resistor in the plate winding, BEFORE THE RECTIFIER,
can help protect the secondary. Two 50 ohm, one on either side, if
it is a center-tapped winding with the CT grounded. Yes, even if
you have tube rectifiers.

These are simple and cheap things you can do, which mfrs.
usually don't--to save a few dollars.


Re: Transformer testing??? [message #8190 is a reply to message #8189] Tue, 25 May 2004 02:07 Go to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18792
Registered: January 2001
Illuminati (33rd Degree)
I can see how a power supply transformer used for step-up to generate high plate voltage might be vulnerable to short. Certainly flyback transformers are vulnerable to that condition, and take out the horizontal output when they do. But I do tend to find the most common failure mode as being opens especially with low voltage power supply transformers that step down to ten or twenty volts.

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