A few months ago I bought an SRD-4 with a pair of SR-30 and SR-40 headphones off eBay.  The seller said that one side had a lower volume than the other.  He thought it was the headphones so he bought the SR-30 headphones and they had exactly the same problem so the problem was probably the SRD-4.  He was right.  One of the step-up transformers in the SRD-4 was bad.  I disassembled the transformer to find out what the problem was.  It turned out that there was some arcing and half of the secondary was shorted.

While unwinding the transformer I made notes on the number of turns and the stackup so I could rebuild it.

The inner most winding of the transformer starts off with the Red wire which is one end of the center tapped secondary.

Red (secondary) 2395 turns, 40 AWG
Green (secondary CT)

Black (Primary) 90 turns, 28 AWG
Yellow (primary)

Green (secondary CT) - continuation of the Red/Green winding above
Blue (secondary) 2395 turns, 40 AWG
 

If you have a SRD-4 that has a bad transformer or two you can either rewind it yourself or buy a replacement from the Canadian distributor for Stax.  Don't bother calling the US distributor, they don't have any.

Morishita & Associates Limited
(416) 244-6200
(416) 246-0829 fax
I think the price was something like $50 USD each.  They're not cheap.  I don't know if it's because they know there isn't anywhere else to go or what.

I thought about rewinding mine at work since we have a transformer winder.  But decided not too as it would take a while.  There are nearly 5000 turns of really fine, 40 AWG wire.  You don't want to go too fast and risk breaking the wire.