VOGONS


VRM 5v 3.3v

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First post, by narcotic

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Hi I have an IBM 330-450DX2 with a dx2 processor at 50mhz. I have a dx4 at 100mhz running at 3.3v. On the board there is a vrm pin which is simply a jumper that lets the 5v through. I have been looking for a 5v to 3.3v vrm for this board and can't find it anywhere. Does anyone know where I can find it? Thanks.

Reply 1 of 6, by majestyk

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I doubt it´s just a jumper with 2 pins. Any VRM will need a ground connection also. For lower resistance at higher currents there are multiple pins for input and output in many cases.
Do you have a picture available?

Reply 2 of 6, by narcotic

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majestyk wrote on 2021-05-01, 10:55:

I doubt it´s just a jumper with 2 pins. Any VRM will need a ground connection also. For lower resistance at higher currents there are multiple pins for input and output in many cases.
Do you have a picture available?
yes.

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Reply 3 of 6, by majestyk

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Well that´s 10 jumpers = a jumper block. Most of them are connecting 5v to the CPU socket some are ground connectors.
Finding the original (proprietary) VRM will be quite a challange.
A DIY VRM is an option if you can handle a soldering iron well.
The easiest way would probably be using an interposer socket that has the 3,3V voltage regulator onboard. These are offered on Ebay frequently.

Reply 5 of 6, by majestyk

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You can use this schematic, but you will have to adapt the pinout.
First you must measure the voltages of your 20-pin connector.

1. Remove the jumper block and the CPU
2. turn on the machine
3. measure with a voltmeter between ground and the pins to determine which ones are the 5V input pins
4. measure between one of the +5V rails of the PSU connector and the pins to determine which pin(s) are ground potential (you will measure full 5V there)
5. If you´re lucky and measure with an ohmmeter (machine now turned off) between the remaining pins (that are NOT 5V and NOT ground) they are all connected = 0 ohms. Then these pins are 3.3V output pins.

I just made some similar VRMs here:
What´s wrong with this FIC 486 PAK-3 ??