Products > Test Equipment
Keithley 2306 Dual Battery Simulator Repair (maybe)
<< < (5/10) > >>
mnementh:
Have you tried isolating the input to the LM2940CT and powering it with a slightly higher voltage; like 8-12V? What you're describing sounds to me like it may be right on the verge and dropping out.

mnem
 :-/O
Smokey:

--- Quote from: mnementh on July 21, 2022, 01:00:26 pm ---Have you tried isolating the input to the LM2940CT and powering it with a slightly higher voltage; like 8-12V? What you're describing sounds to me like it may be right on the verge and dropping out.

mnem
 :-/O

--- End quote ---

That's a good test.  I'll set that up. 

One thing I don't understand is that the LM2940CT datasheet lists this part as 0.5V dropout, and the dropout plots clearly show 0.5V at 1A as the dropout.....
But the electrical characteristics for the 5V version clearly show the input voltage as:
Input voltage : 5 mA ≤ IOUT ≤ 1 A: Min=6.25, Max=26
https://www.ti.com/lit/ds/symlink/lm2940-n.pdf (page5)

I don't necessarily just want to start replacing parts that aren't showing failure symptoms, but the LM2940CT is probably going to be the first to go.
AVGresponding:

--- Quote from: Smokey on July 22, 2022, 03:55:47 am ---
--- Quote from: mnementh on July 21, 2022, 01:00:26 pm ---Have you tried isolating the input to the LM2940CT and powering it with a slightly higher voltage; like 8-12V? What you're describing sounds to me like it may be right on the verge and dropping out.

mnem
 :-/O

--- End quote ---

That's a good test.  I'll set that up. 

One thing I don't understand is that the LM2940CT datasheet lists this part as 0.5V dropout, and the dropout plots clearly show 0.5V at 1A as the dropout.....
But the electrical characteristics for the 5V version clearly show the input voltage as:
Input voltage : 5 mA ≤ IOUT ≤ 1 A: Min=6.25, Max=26
https://www.ti.com/lit/ds/symlink/lm2940-n.pdf (page5)

I don't necessarily just want to start replacing parts that aren't showing failure symptoms, but the LM2940CT is probably going to be the first to go.

--- End quote ---

Have you checked the input ripple voltage? If it's reading close on DC, it might be falling under the dropout voltage at the bottom of ripple voltage. Though the output you measured looks good to me; I'd be surprised for anything designed to work using a standard 5V linear regulator like this to stop working at 4.94V if it's not faulty in some way.
Smokey:
I've been out all day.  I'll check the input soon.

But I agree 4.95v is usually pretty stellar for an ldo.

I had this idea that there may be some adc or comparator somewhere that is confirming rail voltages before enabling the system and a voltage divider resistor is out of spec causing the thing to never turn on even though the actual voltage is fine.
But there is no schematic....
Smokey:
Considering that both the +25V and -9V test points are expected to have unloaded voltages much higher than what is listed on the silk screen....

...I wonder if the +6.4VD testpoint that feeds the 5V regulator is actually supposed to have much higher voltage than that as well. 
::Update:: ... its' not.

I know you guys probably already put your supplies back together, but if you could measure one more voltage, it would be interesting to know what a good unit measures at the +6.4VD testpoint on the board right next to the fan.
::Update:: No need for this.  It's just the output of a switching regulator is that is programmed for 6.4V.  It's doing what its supposed to.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod