Mean well RS-15-5 SMPS modification

[jans]

Hi all, I'm trying to increase the voltage of a 5 volts Mean Well supply to 6.3 volts to power the heaters in my ampex 350 tubes. I saw this video on Youtube about a bigger one with through hole components:
https://www.youtube.com/watch?v=RHVPpO3XG0g

In the video, one resistor is replaced with a pot, so that the voltage can be varied over a larger range. However, the smaller ones have the SMD resistors. Does anyone know which one to replace? Here's a picture:

 

[jans]

In the youtube video it seems to be one of the resistors close to the variable resistor.  So would it be R124, or R106 or R123?

 

[gridcurrent]

Hi all, I'm trying to increase the voltage of a 5 volts Mean Well supply to 6.3 volts to power the heaters in my ampex 350 tubes.

low voltage power requirements for an Ampex 350 are:
6.3 VAC @  2.1 A
12.6 VDC @  0.75 A

 

[jans]

Thanks! The Mean Well is rated at 3 amps for 5 volts, so that would be enough probably.

 

[Ricardus]

Surely they must make an inexpensive supply that will put out 6.3 volts?

 

[Bo Deadly]

Thanks! The Mean Well is rated at 3 amps for 5 volts, so that would be enough probably.

Yeah but I think what gridcurrent is saying is that you could get an RS-15-12 and then it's adjustable range is well within 12.6. That would be a lot cleaner.

Otherwise, to figure out which resistor, momentarily jumper each resistor with a 100k in parallel and monitor the voltage. There's no high voltage there so I think it would be safe to just press the leads of a typical resistor across the smt pads. Maybe touch them with a little flux and the iron just to get the oxidation off. Or use some meter probes and allegator clips to make a nice rig. If you see the voltage tick up slightly, try 50k and adjust your jumper resistor repeatedly until you zero in on 6.3 with the trimmer in the middle. Then just clip the leads down and tack it on.

 

[zamproject]

Hello

I see a ZD52 label at bottom silkscreen, 99.9% chance it's a 5.2V zener, which is probably there as over voltage protection or limit...
Just a guess but you probably need more trick than just a resistor change.

Best
Zam

 

[JohnRoberts]

See if you can find a part number for the switching regulator IC. Then search for a data sheet, maybe even an app note.

If you can tweak it up to 6V it will only make 5/6th the current all else equal.

If you haven't purchased it yet, maybe find something closer.

JR

 

[jans]

Thanks, i will try this. I think i have a 12 volts one lying around also.

Yeah but I think what gridcurrent is saying is that you could get an RS-15-12 and then it's adjustable range is well within 12.6. That would be a lot cleaner.

Otherwise, to figure out which resistor, momentarily jumper each resistor with a 100k in parallel and monitor the voltage. There's no high voltage there so I think it would be safe to just press the leads of a typical resistor across the smt pads. Maybe touch them with a little flux and the iron just to get the oxidation off. Or use some meter probes and allegator clips to make a nice rig. If you see the voltage tick up slightly, try 50k and adjust your jumper resistor repeatedly until you zero in on 6.3 with the trimmer in the middle. Then just clip the leads down and tack it on.

 

[moamps]

Ratio of R123/(R124+Rtr) defines output voltage. Look into attached picture for a reference.
Additional good article worth reading:
https://audioxpress.com/article/repairing-switching-mode-power-supplies