[Racal] RA6790/GM Help

[2002tii]

Lee wrote:

>I need some help with my RA6790/GM I need to replace the audio 
>output transformer
>looking at the circuit it looks like 8ohm primary to 600ohm ct 
>secondary am I correct?
>The manual only gives a racal part number 08535 which really does 
>not help me find a replacement.

Are you talking about the line output transformer, T1 on the Main 
IF/AF assembly, A4?  My schematic (unfortunately, quite indistinct 
because it was scanned at way too low a resolution) indicates that 
there is a 140 ohm resistor (R 142) in series with the primary, and 
the coupling capacitor
(C 107, I think it says) is way too small for an 8 ohm load.  I'd be 
inclined to think that T1 has a primary impedance of at least 150 
ohms, probably more like 600.  The secondary is probably 600 ohms 
nominal, center-tapped.  I'd think a 600:600CT transformer of 
suitable ratings would work fine.  A UTC A-20 (or even an A-21) would 
be way overkillI.  Something electrically resembling a UTC A-30 is 
probably what's in there.  In a pinch, a small 1:1 mains isolating 
transformer could be pressed into service.

Alternatively, you may not really need transformer-isolated, balanced 
audio, so you may be able to just jumper the (vacant) transformer 
position.  You would have to chase down where the J7, pins 7 and 8 
signals ("Main Line Output") go and verify this.  (I tried, but my 
schematic is just too jaggy to work with.)  Perhaps you can jumper 
former T1 pin 5 to either pin 3 or pin 1, earthing the other of these.

Best regards,

Charles