Starting a thread about modifications on smaller Stax amps (SRM-X, X pro, Xh, 212, 252, etc).
*** warning: these amps have high voltage supplies inside. Before doing any work on the circuit board, always unplug the power and confirm HV rail voltage is at 0 (usually drops to 0 within a minute) .
First up, some mods on a SRM-Xh (to drive a pair of 303, for work use)
The Xh board looks pretty simple, a lot less components comparing to the later models such as 212 and 252.
a) power supply section:
-- polarity protection: since this is intended for work use, there is a possibility of somebody, for whatever reason, plugging a wrong power plug into the amp. The Xh has an on-board voltage regulator (12v) to prevent over-voltage, it only needs an in-line diode for reverse polarity protection. I added the diode right at the power switch. (desolder the out-most white wire and solder the diode to the switch terminal, then the white wire to the other end of the diode).
-- a 35v electrolytic is installed right after the diode as buffer. The one in the photo is a nichicon muse 2200uf/35v.
-- Extra heatsink fins are added to the existing heatsink to help the voltage regulator stay cool.
-- C35 is at the output pin of voltage regulator, it is changed to a larger value one. Voltage rating needs to be at least 16v. I used a 330uf.
-- C38 is a buffer cap, after D31. Not sure what the intended purpose of D31 is, maybe to prevent damage to the step-up supply section in the case of a reversed polarity? Since I already have the in-line diode at the power switch, reverse polarity is no longer a concern. I replaced c38 with a muse 2200uf (not direct replacement, had to add extra wire). D31 is short across, that increased the HV rail voltage from around +/-195v to +/-205v.
-- speaking of HV rails, C39/40 are HV reservoir caps, stock value is 10uf/250v. The Xh is a very old model so it is prudent to replace these high voltage caps with fresh ones, and while you are at it might as well increase the capacitance value and use better parts. The size constrain here is mainly that the height has to be 1" or less. A quick search on digikey and mouser returned many suitable options for direct replacement, such as these low impedance nichicon and kemet.
(The ones shown in the photo below are blackgate VK 22uf/350v, they were the only right voltage caps I got, at the time of the mod, that would possibly fit into this case.The VK seems to be working quite ok so I will probably just leave them in there.)
***!!! the exposed aluminum top might be "hot" depending on the cap's design (the VK's bare top measures 200v....and guess how I figure that out...."ZZZZZZZ! ouch! " ). I'd recommend adding insulation tape to cover it.
-- finally, c02 and c03 were replaced with larger value film caps (stock value is only 0.01uf).
*** warning: these amps have high voltage supplies inside. Before doing any work on the circuit board, always unplug the power and confirm HV rail voltage is at 0 (usually drops to 0 within a minute) .
First up, some mods on a SRM-Xh (to drive a pair of 303, for work use)
The Xh board looks pretty simple, a lot less components comparing to the later models such as 212 and 252.
a) power supply section:
-- polarity protection: since this is intended for work use, there is a possibility of somebody, for whatever reason, plugging a wrong power plug into the amp. The Xh has an on-board voltage regulator (12v) to prevent over-voltage, it only needs an in-line diode for reverse polarity protection. I added the diode right at the power switch. (desolder the out-most white wire and solder the diode to the switch terminal, then the white wire to the other end of the diode).
-- a 35v electrolytic is installed right after the diode as buffer. The one in the photo is a nichicon muse 2200uf/35v.
-- Extra heatsink fins are added to the existing heatsink to help the voltage regulator stay cool.
-- C35 is at the output pin of voltage regulator, it is changed to a larger value one. Voltage rating needs to be at least 16v. I used a 330uf.
-- C38 is a buffer cap, after D31. Not sure what the intended purpose of D31 is, maybe to prevent damage to the step-up supply section in the case of a reversed polarity? Since I already have the in-line diode at the power switch, reverse polarity is no longer a concern. I replaced c38 with a muse 2200uf (not direct replacement, had to add extra wire). D31 is short across, that increased the HV rail voltage from around +/-195v to +/-205v.
-- speaking of HV rails, C39/40 are HV reservoir caps, stock value is 10uf/250v. The Xh is a very old model so it is prudent to replace these high voltage caps with fresh ones, and while you are at it might as well increase the capacitance value and use better parts. The size constrain here is mainly that the height has to be 1" or less. A quick search on digikey and mouser returned many suitable options for direct replacement, such as these low impedance nichicon and kemet.
(The ones shown in the photo below are blackgate VK 22uf/350v, they were the only right voltage caps I got, at the time of the mod, that would possibly fit into this case.The VK seems to be working quite ok so I will probably just leave them in there.)
***!!! the exposed aluminum top might be "hot" depending on the cap's design (the VK's bare top measures 200v....and guess how I figure that out...."ZZZZZZZ! ouch! " ). I'd recommend adding insulation tape to cover it.
-- finally, c02 and c03 were replaced with larger value film caps (stock value is only 0.01uf).