Q1 and Q2 are switched out for voltage measurements, so even if they failed short it should not affect voltage functions. About the only scenario I can come up with is that the PTC opened up permanently. Or, given that the Hz function feeds the ADP input almost directly, the DMM IC is fried.
Hi ModemHead,
Thanks for giving a play by play on the schematic and input protection. Interpreting schematics is one area I'm weak in and appreciate the full detailed explanation.
I had Martin pull and test all the 3 pin transistors and they all tested fine.
The PTCs seem to test okay? Martin reported
PTC1: 1K
PTC2: 567 ohm...out of circuit
PTC3: 589 ohm...out of circuit
I have no idea what a good PTC should measure other than relying on the FS9721 datasheet values (that is 1K). So PTC2 and PTC3 could be bad or they could be used together in series (giving it a 1K value)?
For my own learning, I decided to measure the DC voltage on pins 26 (TENM) and 24 (ONEM). I found two points on my UEI board that fed into the above pins making it easier to measure.
Since I don't have a variable DC power supply, I got 4 semi-dead 9V batteries and noted the following:
9V battery voltage TENM ONEM
8.08v 80.7mv 0.0
4.90v 52.6mv 0.0
3.76v 37.5mv 0.0
2.466v 228mv 228mv
Now my UEI display is stuck on "all segments" test, but what I'm trying to show with the above testing is that the input voltage divider is working by measuring pins TENM and ONEM. My UEI has two PTCs.
So if Martin does this same test, he can verify that the input section is working up to the FSP9721 (implying that the PTC works)?
Martin, to test this (probably 15 to 30 minutes of your time)
1) Use your variable power supply to supply DC voltage to the input jacks of the UT90C.
2) Use a working multimeter to measure the DC voltage at pins TENM and ONEM.
3) The black probe on the working multimeter goes to the UT90C COM jack for ground.
4) The red probe goes on TENM and ONEM (or some physically bigger component that feeds these pins to make probing easier - looking at your photo, I can clearly see traces leaving pins 26 and 24).