[Amps] Trio TL-922 Filament Transformer...

[R.Measures]

On Nov 19, 2004, at 7:58 PM, r391 wrote:

> Tonight I decided to pull some PM on my Japanese Trio TL-922...
>
> Filament voltage 4.8 volts... at the tube pins... thought hmmm... 
> better a little low than a little high?
>
> Felt the filament transformer and it felt quite warm after only 5 
> minutes or so of operation...  this is the first time I've done this 
> so I don't know how warm the transformer should feel after 5 minutes 
> so...

The 922's fil.  transformer should Not be running this hot.
>
> Out of curiosity I decided to check mains input voltage to the 
> filament transformer and it was 122 volts on one leg and... 102 volts 
> on the other so that the two primaries input voltages are unbalanced 
> to each other by 20 volts...
>
> Thinking "that can't be right" as the schematic shows "0" to "0" and 
> "120" to "120" between the two transformers so that  their both shown 
> to be phased the same way and that my 122 volt line voltage should 
> appear on all of the primary windings equally... I decided to trace 
> the wiring.
>
> Found the installed wiring different from the schematic...  this 
> difference is either from the factory or by previous owner design but 
> is such that one of the 120 volt lines from the filament transformer 
> is connected to the 100 volt connection on the HV transformer thus 
> feeding one of the filament transformer primaries 100 volts instead of 
> 120 volts while the other primary was connected to the 120 volt 
> connection on the HV transformer and receiving 122 volts...
>
> Was wondering if this is a factory wiring error or a deliberate 
> attempt by the factory or a previous owner to adjust the filament 
> voltage... anyone seen this before?

No.  With the  primary connected for 220v (100v-pri. seriesed w. 
120v-pri.) and 220v input, the filament voltage at the sockets should 
be c. 5.3v (loaded), not 4.8v.  This, combined with the unusual temp 
rise in T2 suggests that there could be a shorted turn in one of T2's 
primaries.  Since T2 is unfused, if one of the tubes grid-filament 
shorts, the +110vdc cutoff-bias supply that is powered by the 80Vrms 
winding on T2 sees a short -- so the transformer can overheat and 
sustain damage unless the power is quickly switched off.  //  One way 
to test T2 for a shorted turn is to pull the tubes, put the cover on to 
switch off the interlock, switch the 922 on for 10 minutes or so, pull 
the mains plug, and see if the transformer is cooking.  If it gets hot 
with no load, it's history.  (note -- the Heath SB-220 has the same 
design error)
>
> tnx, Ott
>
>
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org