Products > Test Equipment
Keithley 2306 Dual Battery Simulator Repair (maybe)
Smokey:
--- Quote from: AVGresponding on July 22, 2022, 05:41:49 am ---...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.
--- End quote ---
About 15mv ripple on the +6.5VD rail. There is some +/- 50mV high frequency stuff coming from the switching regulator frequency that's generating the voltage, but I wouldn't say it's excessive.
U112 is the switching regulator that's driving the +6.5VD rail. It's listed as "ADJ VOLTAGE REG STEP DOWN" in the component list on the data sheet. Thanks Keithley.
The part is actually a LM2675:
https://www.ti.com/product/LM2675
https://www.ti.com/lit/ds/symlink/lm2675.pdf
The output from the regulator is essentially a 260kHz 0-12V square wave into the inductor/cap.
R124 = 2.55K
R125 = 11K
So assuming R124=R1 and R125=R2, The output voltage is set to:
(((R2/R1) + 1) * 1.21) = 6.429V, which is pretty much right on.
NOTE: Since this looks good, no need to get any other measurements on this. It is what it is.
Smokey:
As a test, I bumped R124 to 2.4k.
...Which increased the measured input voltage of the LM2940CT to 6.68V.
The measured output of the LM2940CT is still super solid at 4.95V, and the 2308 supply still won't consistently power up with just the internal regulators.
So... It's not looking like it's an issue with the input to the LDO powering the +5VD rail.
AVGresponding:
Time to start looking at the control logic then... IC decoupling caps, reset signals etc...
Smokey:
Filling in some part numbers for "Model 2306 digital board":
U132 = "INTEGRATED CIRCUIT " = LM2937 = "500-mA, 26-V, high-PSRR, low-dropout voltage regulator"
https://www.ti.com/lit/ds/symlink/lm2937.pdf
U135,U136,U235,U236 = "INTEGRATED CIRCUIT" = LM7121 = "235-MHz Tiny Low Power Voltage Feedback Amplifier"
https://www.ti.com/lit/ds/symlink/lm7121.pdf
On the "Model 2306 display board parts list"
U601 = "UNDERVOLTAGE SENSE CIRCUIT" = MC34064
https://www.onsemi.com/pdf/datasheet/mc34064-d.pdf
Smokey:
Checking some processor lines:
This is all in the "failed to power up" state.
{
Checking (not)RESET, (not)HALT, and (not)BERR line on the MC68331CPV16 Main processor...
Reset line goes high about 525ms after 5V rail comes up. Stays high.
HALT line goes high at the same time as 5V rail. Stays high.
BERR (bus error) is also high at power up. Stays high.
Clock out looks good at 25MHz
Freeze line stays low.
}
Nothing strange here.
Powering the system with an external 5V (successful power up)
{
Same behavior. No changes to the above signals
}
That all looks good.
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