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| Keithley 2306 Dual Battery Simulator Repair (maybe) |
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| Smokey:
Plot thickens.... The +5VD rail is powered by an LM2940CT: https://www.ti.com/lit/ds/symlink/lm2940-n.pdf The input to that regulator is +6.5VD, which measures about 6.40V at the input to the LM2940CT. As I said in the last post, the +5VD rail is measuring about 4.95V. Normally I would say this is pretty damn good. Test = power the 5V rail from an external supply: So far, when I power the whole system +5VD rail from an external supply, and confirm with measurement that the voltage of the +5VD rail at the display connector is the full 5.00V, the supply powers up consistently every time as confirmed by the external display. When installed in the system, the internal VFD display only powers up every time consistently when I boost the +5VD rail to 5.13V. With the whole system running, the +5VD rail pulls about 575mA. It just seems insane that having the +5VD rail be only 50mV low would make the whole system unreliable. Any ideas? ::NOTE:: I should add that I have looked at the 5V rail on the scope and it is really solid. No excessive ripple or noise or anything nasty. |
| Smokey:
I'm thinking I can replace the LM2940CT with something that has a variable output and just bump it up to about 5.1V, but that seems like a band aid for some underlying issues. This looks like it might be viable: https://www.digikey.com/en/products/detail/microchip-technology/MIC29302WT/771593 I can probably hack this in the same footprint without needing a carrier board since they are both through hole. I can solder the new part into the IN,OUT,GND pads of the existing TO-220 footprint, and just air wire the adjust and enable pins. |
| Smokey:
Looking at the LM2940CT 5V part datasheet, the recommended input voltage range is 6.25V to 26V. This is surprisingly close to the 6.4V that is actually on the input to the regulator (even through the dropout plot does say it's rated at 0.5V dropout at 1A). The output voltage range, depending on the current level, can be anywhere from 4.75V to 5.25V. So the normal operating range extends way into the non-functional zone at the low end of the regulator output tolerance. There has to be something else going on. |
| AVGresponding:
I take it you have checked for dry caps on the output side of the regulator, and the display board, which could easily cause such symptoms? What is the ripple voltage? |
| Smokey:
The LM2940CT has maybe 50mv of high frequency ripple at about 200kHz, which I would think is coming from the square wave drive into the transformer primaries for the output voltages. I don't think it looks that bad at all really. When I power the unit from the bench 5V supply, the character of the voltage rail doesn't change at all, it just increases the voltage a little. That's what so strange to me. The voltage rail looks solid at 4.95V. It's not like its dipping whenever there is a current demand. I actually don't think there are any electrolytic caps on the 5V rail... but there are a ton of tantalums. With that said, the in circuit resistance across the 5V rail is about 750ohms, but that's in circuit. The total current on that rail is under 600mA, which is well within range for the regulator. It's not even getting very hot. While the display board and the system board are both showing what looks like essentially the same symptom (extreme sensitivity to under voltage on the 5V rail), the main board has the problem even when the display board is completely disconnected. |
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