Tech: Robotron power supply question - How to adjust +5? Mine is only at 4.1V.

jehuie

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Tech: Robotron power supply question - How to adjust +5? Mine is only at 4.1V.

So my Robotron voltage is low. Do I need to do a full rebuild on the power supply or is there some way to adjust the voltage up? I just read in the manual something about adding or removing a resistor to adjust the voltage....should I go that route or does Bob sell a rebuild kit that I should look into and rebuild the whole darn thing?

Thanks!

John
 
There are a couple of things that will cause that.

1) Check the voltage at the test points 5 & 6. They should be approx. +13V and -13V. If not check the fuses F4 & F5. Make sure they do not have corrosion and that the clips are tight.

2) If the voltages at TP 5 & 6 are OK, replace the voltage regulator (LM723). Put it in a socket while you are at it.

3) Test all of the resistors. You will need to unsolder one end because you cannot get accurate readings in-circuit.

The resistor mod (removing R10) is just to boost the voltage about 0.1 to 0.2 V. You have bigger problems with that PS board.

The quickest way to isolate what is going wrong is to use a voltmeter and check the voltages listed in the schematics. That will let you know if it is before the voltage regulator or after it. Or if it is the voltage regulator.

ken
 
If you've never rebuilt your power supply, I'd do a full rebuild on it. Bob Roberts has a very nice kit that provides you with all the parts you'll need. It comes with the 723s and sockets to put them in as well as the caps that usually go bad and many other parts that fail over the years. once rebuilt, I've never had to mess with the resistors so far. Once its rebuilt they are usually right on spec. Don't forget to resolder all the pins too.

The part about the fuse holders is also very true. I've replaced more than a few fuse holders over the years.
 
If you've never rebuilt your power supply, I'd do a full rebuild on it. Bob Roberts has a very nice kit that provides you with all the parts you'll need. It comes with the 723s and sockets to put them in as well as the caps that usually go bad and many other parts that fail over the years. once rebuilt, I've never had to mess with the resistors so far. Once its rebuilt they are usually right on spec. Don't forget to resolder all the pins too.

The part about the fuse holders is also very true. I've replaced more than a few fuse holders over the years.
Did it work? I am in the process of going back to a linear supply from the switching one that was in it and I get .2-.3 v drop for each board that is connected to the harness (its a stargate, but same 2 board powersupply)
 
As others mentioned definitely do the rebuild before any mods. A properly rebuilt linear should last years.

If you want to do the pot mod, change R2 to a 5k pot. Then WITHOUT J2 plugged in, adjust the output voltage to about 5.1V and put a dab of hot glue on the pot to prevent drifting or an accidental bump. Setting to 5.1V should account for the harness voltage drop and get you about 5V on the CPU board when connected.
 
ok, I've got 2 psu's with different issues so I'll just rebuild them. the other one has 2 volts on +5 with nothing attached
 
Bob Roberts also sells a kit to replace all the connectors in the game. Those connectors were great for production but sucked in the long haul.

Bad connectors will cause lower voltage on the boards even if you have god voltage out of the power supply. Worse yet, it will draw more current and burn things up if the connectors are really bad and not making good contact.

Like mentioned earlier in this thead, be sure to reflow the header pins.
 
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