Rotel RA-935BX Transformer Replacement

[J1mbo]

Hi All

Rotel RA-935BX amplifier (c.1993) in need of new toroidal PSU. No Rotel spares available; transformer is shorted on the primary winding.

Rating plate power is 300W so I'm assuming 300VA transformer?

Fitted unit (TT-279GF) is 2x 28.2V centre-tapped 110mm x 51mm. There is no space for a bigger transformer. I can't find any 300VA units that small.

The amp is only 40W RMS into 8 ohm (x2), various calculators seem to suggest even 160VA should be enough.

I will be replacing the electrolytics whilst I'm there, with:

• PSU 8200uF - nichicon BK
• Smaller Signal Caps - ELNA SILMIC II

Any and all input very much appreciated.

 

[J1mbo]

Based on the size and weight (2kg) the transformer is 225VA at most, fortunately 225VA 2x 30V are readily available here. Schematic calls for 29.2V so hopefully close enough.

I'm very interested in any opinions on Nichicon LS series vs Gold Tune for PSU smoothing caps. At the given can size (35x50mm), LS is 10,000uF/80V/6.6A ripple, whereas Gold Tune is 8,200uF/50V/4.4A ripple. The dissipation factor is slightly lower on the LS too.

Also I read that changing the rectifier, a KBPC802, for fast-recovery diodes like MUR420 should help. Any thoughts on that greatly appreciated too!

 

[Hyperion]

If you are assuming 300VA why cheap out on a 225VA?

what about this http://www.maplin.co.uk/p/2x30v-500va-toroidal-transformer-a60qj

Or this (not in stock) http://www.maplin.co.uk/p/2x30v-300va-toroidal-transformer-a58qj

The rest of what you are suggesting seems to indicate you want to do a good job with this...

"The power transformer is the heart of a good sounding audio amplifier".

 

[J1mbo]

Thanks, but unfortunately the absolute maximum size is 110x51mm, there is no way the OEM unit was 300VA so I'm left a bit puzzled by the wattage rating plate!