[SHIF]

Greetings fellow Cornet enthusiasts.

Right after a great two hour vinyl session last night I heard a funny whooshing sound in my speakers.  This turned out to be the 1 amp slow-blow in my Cornet puking.  Tried another fuse which popped right away so I figured something was seriously wrong. The tranny was warm to the touch as usual, but not hot.
 
Today I pulled out the 270BX to bench test it.  None of the secondary coils are shorted to each other or to the primary.  The primary coil measures very low resistance, a few ohms.  This unit is toast.

Should I buy another 270BX or will the 370BX be a better choice and less prone crap out?

If going with the 370BX, I shall wire the primary blue + black to Load and  brown + white to Neutral.  Of the secondary wires, would the center taps (yellow/black and green/yellow) be attached to the PCB too?  What to do with the violet wire?

Any opinions??

Thanks again,
STEVE

[hagtech]

Well, there have been at least a half-dozen 270BX failures.  Not sure if it was old stock (some look pretty old) or if any were new production.  I tested a few and found the primary to have a shorted turn.  Probably an insulation breakdown due to heat, possibly overvoltage.

The 270BX is not overstressed in this application, not for power or current.  However, the primary is rated for 115V, and we all are running 120V or higher now in the USA.  That is one big reason for the extra heat.  Nonetheless, transformers are made to run hot and typically can handle some extreme heat without incident.  This model apparently doesn't.  Or perhaps it is not properly being used.

I discovered the problem using a variac.  Transformer ran fine until about 60Vac, then it snapped (or arced) into failure mode.  All output voltages dropped by some large percentage.  At a few more volts the fuse would blow.  I have found no problems with the secondaries.

My recommendation is to switch to the 370BX. It is a larger tranny with a bigger core and windings specified for 120V.  It costs a little more, but I think it is worth it.  Should fit pretty closely to any holes already drilled.  For this same reason, I switch to the 370BX on the Cornet2 and Clarinet.

The purple wire can be left hanging.  Center taps on low voltage outputs not needed.

jh  

[SHIF]

Cornet is sounding better than ever now with it’s new 370BX transformer.

Seems like a quieter background, much reduced hum, very acceptable.  And the thing does run somewhat cooler then the 270BX.  Fit is good enough, need to tap two more mounting holes in my case.  Gonna add a Molex Mini-Fit Jr. connector on the secondary leads to facilitate PCB removal for future component mods.

Turn up the music,
-Shif

[GRD]

I found this description by another DIY of problems with his transformer primary shorting.  Don't know if this applies to the 270BX problems, but interesting reading.  To quote part of the article:

"see, unless your trans is a split bobbin, the primary winding is most probably the innermost layer, and the various secondaries will be on top of this layer, thus forming the outer layer. After winding, the trans is supposed to be soaked in lacquer so that when it dries and harden, it will "coat" the windings and ensure zero (or at least minimal) movement of the windings. Now the problem is that some trans winder, after winding the trans, they don't soak the trans long enough. As a result, the lacquer doesn't get to seep into the primary windings. Now this seems okay but after some usage, due to the primary coil "vibrating" at mains frequency, sooner or later, the insulation of the magnet wires will start to wear out, you know, rubbing against each other. Now when this insulation is completely kaput, you have a short! "

http://www.diyparadise.com/taleof2trans.html

So far my 270BX is still up and running with no problems.