Ultra Xfinity 600w 5vsb mod

[ratdude747]

I have the power supply from this thread:

https://www.badcaps.net/forum/showthread.php?t=12683

it appears to have the dreaded two transistor 5vsb... and I would like to fix that.

How does one perform the modification? I have never done this before so some guidance would be nice.

[everell]

Re: Ultra Xfinity 600w 5vsb mod

The first step is to build the add on pwm board. There are several good chips out there to use and experiment with. Most of my experience is with the DM311, so that is the one I will show you. For attachments, here is the schematic of the DM311 add on circuit. Then two photos showing each side of the add on pc board, including a heat sink made from the tin lid of a peanut can. There are five wires coming out from this board, and I have colour coded them as shown in the schematic.

If you decide to make a heat sink as shown in the picture, make sure the tin can material will take solder. Some tin cans have special things added to the metal and they simply will not take solder. The right material to use will take solder very quickly, and it will become obvious what is acceptable and what is not.

The small board is cut from a larger piece of "perfboard" readily available from Radio Shack.

These are the drawing and pictures I made when modifying a Bestec, and the add on board is shown mounted to the main switching transistor heat sink. I have seen this same circuit in Fortron power supplies.

Any questions so far?

[ratdude747]

Re: Ultra Xfinity 600w 5vsb mod

I will probably bolt my board to the secondary heatsink, which already has tapped holes for use:



I have yet to buy and order parts, once I have them I will begin.

[mockingbird]

Re: Ultra Xfinity 600w 5vsb mod

The Antec I refered to earlier is a SP-450... Which Bestec was it? The 12E or the 12Z? I read here that the 12E is a motherboard killer and that it requires lots of modification...

I don't want to bother you now with questions because I can't commit to this project right now. I have like 8 PSUs I have to order caps for and reference the old capacitors. Is it ok if I bug you about this later?

[ratdude747]

Re: Ultra Xfinity 600w 5vsb mod

the 12z doesn't need the 5vsb mod, the 12e does

that reminds me, I have a kind rusty 12z sitting (humid basement) that could use a recap... its a spare unit right now.

[everell]

Re: Ultra Xfinity 600w 5vsb mod

Here are a couple of diagrams of the two transistor 5vsb circuit. Look it over and get familiar with it.....see how many of the components you can identify in your 5vsb.

[everell]

Re: Ultra Xfinity 600w 5vsb mod

One of the first components you need to identify in the 5vsb circuit is the 5vsb transformer. On the half bridge power supplies, the largest transformer is the main switching transformer. The middle transformer is the driver transformer for the main switching transistors. The remaining transformer is the 5vsb transformer.

The 5vsb transformer has three windings:
the switching coil driven by a transistor (or FET)
the feedback coil
the 5vsb coil

The attached drawing shows two basic types of 5vsb transformer used in the half bridge power supply. The switching coil and the feedback coil are the same, but the 5vsb winding is sometimes a tapped coil which drives 5vsb and other circuits.

[everell]

Re: Ultra Xfinity 600w 5vsb mod

To identify the 5vsb winding, look at the drawing below. First, locate the violet wire inside the power supply. Follow it back and you should see the load resistor R1, then the filter capacitor C2, then coil L1, then filter capacitor C1, then diode D1, then the 5vsb winding. Sometimes the diode is located on the other side of the coil.

During the conversion process you will not do any changes to this winding and its circuitry. This description is only to help you locate which winding is the 5vsb winding.

[everell]

Re: Ultra Xfinity 600w 5vsb mod

Next step is to identify the switcher winding. First, locate the two large "mains" capacitors. One pin on each capacitor is tied together. The remaining pin on the minus side of one capacitor is common (NOT GROUND, do not confuse the two). The remaining pin on the positive side of the other capacitor charges to about 320 volts.

One side of one of the transformer windings goes to the 320 volts. The other side of this winding goes to the center pin of a power transistor usually attached to a heat sink. I have labeled this transistor Q2. The other transistor in this circuit (labeled Q1) is usually a 1815 or 945, and its collector pin (the middle pin) connects directly to the base of Q2.

Try to locate these two transistors and the switcher winding.

[everell]

Re: Ultra Xfinity 600w 5vsb mod

Once you have located the switcher winding and the two transistors, we can begin the conversion process. Refer to the drawing below, and locate the snubber circuit consisting of capacitor C5, resistor R5, and diode D5. This snubber circuit is connected directly across the switcher winding. It is used to remove negative going peaks and oscillations. Do NOT remove these components.

Locate the startup resistor (labeled R2) connected between 320 volts and the base of Q2 (or the collector of Q1). Remove this resistor to prevent any power going to the old circuit.

Remove power transistor Q2. The orange wire from the new DM311 board will connect in the middle hole where Q2 was.

[everell]

Re: Ultra Xfinity 600w 5vsb mod

The feedback winding is the winding that is left. Study this circuit carefully.

There should be a diode (labeled D4) between one side of the winding and common. Remove the diode and insert a piece of jumper wire where the diode was.

Using an Xacto knife, cut the circuit trace between the other side of the winding and pin 4 of the optoisolator chip. Make the cut such that the circuit trace path between optoisolator pin 4 and capacitor C4 and resistor R4 is left intact.

[everell]

Re: Ultra Xfinity 600w 5vsb mod

Next, connect the yellow wire from the DM311 board to the 5vsb feedback winding where you made the trace cut. See drawing below.

[everell]

Re: Ultra Xfinity 600w 5vsb mod

LAST STEPS!

Next step is to correct the optoisolator chip wiring. Remove the resistor (labeled R3) which is connected to pin 3 of the optoisolator chip. Insert a jumper wire between Optoisolator pin 3 (using one of the holes resistor R3 was in) and common.

Remove capacitor C4 and Resistor R4 which are connected to pin 4 of the optoisolator. Connect the brown wire from the DM311 board to the optoisolator pin 4 (using hole where C4 or R4 was).

Connect the green wire from the DM311 board to common using any unused "common" hole on the pc board.

Connect the red wire from the DM311 board to any unused +320 volt hole on the pc board.

If you can't find an unused hole for the common or +320 volt connection, I use a Dremel drill and a #60 drill bit to make whatever holes are necessary.

[ratdude747]

Re: Ultra Xfinity 600w 5vsb mod

i forgot about this thread

in another thread, a new idea was posted to fix the issue. is there any way a high esr poly cap could be used to fix the issue without all the work required by this method? or would that introduce new issues?

[ratdude747]

Re: Ultra Xfinity 600w 5vsb mod

perhaps, like this?

http://www.mouser.com/ProductDetail/...F6GdgW2mG7g%3d

[everell]

Re: Ultra Xfinity 600w 5vsb mod

I suggest that you try your idea, then report back to us on your results. Let us know if it stops the squealing.

[ratdude747]

Re: Ultra Xfinity 600w 5vsb mod

so have I ID'd the correct cap that needs fixed? the 50v 10uf? (highest voltage I can get is 25v, will that be high enough?)

[mockingbird]

Re: Ultra Xfinity 600w 5vsb mod

The only flaw with this idea I can think of is that I heard that polymer caps do not handle ripple well (But they do handle heat well - that's why PSUs with electrolytic caps fare better in ripple tests than polymer PSUs). But like you said, this cap is not there to suppress ripple, it's there to take an unreasonable amount of heat.

[ratdude747]

Re: Ultra Xfinity 600w 5vsb mod

another question: for the other caps in the psu, is nichicon HE ok? I am having a hard time sourcing the two 6800 uf 16V caps in the unit; the only two brands with the size in a "low esr" format are Cornell Dubilier and lelon

[c_hegge]

Re: Ultra Xfinity 600w 5vsb mod

I'm sure Nichicon HE would be fine. I use them all the time in power supplies, and they work every time.

EDIT: as for the 5vsb cap, I think 25v should be OK. some PSUs have 47uF@25v in that position. If in doubt you can check it with a DMM.