Reviving this thread as I just finished working on a green board SRM-Xh to replace caps, and enable the normal bias output by populating two missing components on the board. I replaced the pro bias socket with a normal bias one from an SRD-6 (I have too many of those) which is a perfect fit.
I still plan to replace or bypass the volume pot, but listening impressions so far are not bad: soundstage is much smaller than my SRM-313 but that's the main difference in sound on a quick listen. This amp will also be very easy to power from a USB PD power bank with a USB PD 12V cable.
This is the board with the missing capacitor and resistor that need to be filled in for normal bias, and the LO hole for the normal bias wire to the socket
What I used for cap replacement
- for the 6 22uf 250V I used 33uF 250V (DigiKey pn 493-16294-ND)
- for the 1uf 50V I used 2.2UF 50V (DigiKey pn 493-15488-ND)
- I left the 33uf 16V capacitor alone as it's tiny and very close to other components
- to enable normal bias I added a 2.2M 1/2W resistor, and a generic non polarized 0.1uf 630V capacitor
I would not mind adding a switch to flip from normal to pro bias, but am having trouble finding one with the right specs. Any suggestions appreciated.
Here is a pic of the patient undergoing operation