[volt-nuts] Fluke 5205a Repair
d.garrido at me.com
Tue Sep 29 14:23:47 EDT 2015
I am making a little more headway on this issue of the OVERLOAD & FAULT led’s being triggered and shutting down the system.
In paragraph 3-34 on page 3-4 of the manual it states:
Once U2-3 has been pulled low and a 3-9ms delay, the Operate FF J Input, U41-4 is supposed to go high. And it does, BUT the next thing in the line is a short delay caused by R121 & C43 and that signal showing up at U41-1 should be low. It is NOT and neither is the Q Output (U41-14). So I thought ok GREAT, only pin 4 should be high and both of the others must be low. Easy fix, plop in another SN74106N and I should be GTG.
Not so much………..still the same issue after warm-up, the OVERLOAD led comes on, followed very shortly by the FAULT led. Now I checked U3-9, it is low and U3-8 high. Next was U4-9 and it was high and pin 8 was low. On to U40-9, it is low and pin 8 is high which is what was pushing U41-1 high.
So I went and checked R121, she measured 389 ohms and C43, a 4.7uf TA, measured 5.1 with 1.2 ESR. That all seems good.
So I decided to try putting the amp into the OPERATE mode and see if that changed the input to U41-1as it is supposed to follow the low from U3-8, it did. Yes, U3-8 is now 100mV along with U3-9. U4-9 is low and U4-8 is high. U40-9 is high and 8 is low. Now the signal at U41-4 is high as before, The signal at U41-1 is low as expected, BUT………..the signal at U41-14 did not go low following the low at U41-1. Hmmmmmmmmmmm
Scratching my head………..
In summary in the standby mode:
In summary in the OPERATE mode:
I could use a little more brain power as this too seems to be a bad U41-FF, but I changed that. So unless my other U41 FF was bad in the very same way……………what am I missing????
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