How to: recapping the Mega STe power supply

[exxos]

I recently got a MSTE and on inspection found, that in the PSU the R23 resistor ist cut out (the wires of it are still soldered) - it looks like on the picture from the first page of this thread (while the unit shown on exxos last update PSU page does contain the resistor).

Looks like a feedback resistor based on the 12V.. maybe it was cut as the 12V was too high and was causing the 5V rail to be too low in voltage.. measure 5V and 12V, and put the resistor in see what happens...

EDIT:

Wasn't there some resistors cut to set the 5V correctly on those PSUs ? "poor mans 5V adjust preset" ...

[czietz]

R23 and R24 being present / absent is part of the factory alignment procedure of the PSM-5341 to the set correct voltage levels.

[pixelpusher]

I recently got a MSTE and on inspection found, that in the PSU the R23 resistor ist cut out (the wires of it are still soldered) - it looks like on the picture from the first page of this thread (while the unit shown on exxos last update PSU page does contain the resistor).

Looks like a feedback resistor based on the 12V.. maybe it was cut as the 12V was to high and was causing the 5V rail to be to low in voltage.. measure 5V and 12V, and put the resistor in see what happens...

EDIT:

Wasn't there some resistors cut to set the 5V correctly on those PSUs ? "poor mans 5V adjust preset" ...

Thx exxos & czietz for the info! Will measure with (power) resistor load before and after the recap...

[pixelpusher]

R23 and R24 being present / absent is part of the factory alignment procedure of the PSM-5341 to the set correct voltage levels.

A question regarding the opto: The opto used is the cny 17-3Z (from QTC). Does anyone of you know if what the Z stands for at QTC (couldn't find a datasheet)? Guess it's probably not the floating base of the F type of other vendors (as recommend in this thread), or is it?

Best,

[beeka]

I recently got a kit from Exxos but it doesn't quite match up with the photo... I think the 22uF 25V, which the photo says to replace with a 10uF 50V, is meant to be populated by the 22uF 50V in the kit... but this is only by a process of elimination, so wanted to check. Everything else seems to match up.

Once I get the caps replaced and convince myself I haven't made things worse, the other parts of this discussion are confusing me. For the record, the PSU is from a MegaSTE:

C302074-001 (on lid)
S/N S12903813A6 (on lid); 9127 on PCB
REV-A1 (on PCB)
5 screws, not insulated
no cuts to PCB
CNY17-3Z
R23 has been cut out

With that in mind... do I need to replace the opto? And if so, is a generic PC111 from random ebay stores going to be worse, or is the CNY17 F the chip of the moment?

Do I need to cut the PCB / isolate the post? I notice that in icky's photo there is a resistor on the back, which I don't have either.

Thanks,

Steve.

PS: I measured the ripple of the PSU before I started by using the AC mode of my multimeter... on the basis that some figure is better than none. Has anyone compared the how well / badly this method matches up to an oscilloscope (where actual noise could be visualised).

[exxos]

I recently got a kit from Exxos but it doesn't quite match up with the photo... I think the 22uF 25V, which the photo says to replace with a 10uF 50V, is meant to be populated by the 22uF 50V in the kit... but this is only by a process of elimination, so wanted to check. Everything else seems to match up.

C23 ? I originally change that to 10uF as the power supply I had at the time took 3 to 4 seconds to power up otherwise.. But then later I found other power supplies were powering up way too fast.. So I actually put back the original value of C23 which is 22uF.

With that in mind... do I need to replace the opto? And if so, is a generic PC111 from random ebay stores going to be worse, or is the CNY17 F the chip of the moment?

I don't recall offhand, but it is mentioned somewhere in the thread I think it was the one you mentioned. I have not personally tried or tested this Opto, but apparently it gives better performance.

PS: I measured the ripple of the PSU before I started by using the AC mode of my multimeter... on the basis that some figure is better than none. Has anyone compared the how well / badly this method matches up to an oscilloscope (where actual noise could be visualised).

It is unlikely multimeters will measure the frequency of the power supply on AC..

The only scope waveforms I have are here..

https://www.exxosforum.co.uk/atari/last/ ... tm#PSM5341

To my knowledge nobody else has tested a before and after recap, or before and after replacing the Opto.

[czietz]

To my knowledge nobody else has tested a before and after recap, or before and after replacing the Opto.

I tested my MegaSTE supply before and after recapping and found no discernible difference. I guess the original caps were still good.

The output ripple I saw was entirely caused by the optocoupler. I also measured the 5 V rail with different optocouplers: CNY17 (that was originally in my PSU), PC111 (that should have been used originally) and CNY17F (recommended replacement):

megaste-psu-measurements2.png

As you can see, the 50 Hz ripple is reduced by about 30 dB (= to about 1/31 of the original amplitude) by the PC111 and CNY17F.

[exxos]

Thanks @czietz Very interesting !