[sdiy] MOTM 310/300 debugging discoveries & dual FET question

[Aaron Lanterman]

Short version: My MOTM 310 and 300 VCOs each use a dual LS3854 FET.  The 
oscillator cores of both circuits appear identical. With the power off, in 
situ, the source-drain resistances of both FETS in the 310 measure 600 
ohms. But, in situ, one of the source-drain resistances measures 600 ohms, 
but the other measures 25K. Does this mean the LS3854 in the 300 is 
buggered?

(i.e. I don't think it was an organic flux bridge problem as I thought 
earlier)

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Long version with debugging audit trail details:

Earlier I posted about trying to get an MOTM 300 to work. I get noise, and 
when I press down on the FET, I sometimes pick up radio stations. I hence 
suspected the FET. Also, measuring the resistance between two of the FET 
legs with the power off, I got 600 ohms, making me think I had a solder 
bridge, so I started scraping away.

I think I started scraping prematurely.

I just got my MOTM 310 working, which is basically a stripped-down version 
of the same circuit.

The FET that buffers the integrating cap is a LS3854 dual FET.

When I measure the resistances between the FET legs (with power off of 
course) of my _working_ 310, I get:

(pins 7 6 5)
gate 2 - drain 2: 1.9K (pin 7 - pin 6)
drain 2 - source 2: 520 ohms (pin 6 - pin 5)
source 2 - gate 2: 1.4K (pin 5 - pin 7)

pins (3 2 1)
drain 1 - gate 1: open circuit (pin 2 - pin 3)
source 1 - drain 1: 520 ohms (pin 1 - pin 2)
source 1 - gate 1: 3.5M (pin 1 - pin 3)

pin 5 - pin 7 makes since, since there's an 1.5K resistor on the schemtiac
connecting them.

pin 1 - pin 3 and pin 2 - pin 3 make sense, since the FET is supposed to 
not be drawing current in at its gate

Now that I think about it, the source-drain low resistances make sense, 
since FETs are naturally open until you close them (IIRC)

pin 7 - pin 6 measured 1.9K... hmmm, I'm not sure how to interpret that 
one. But hey, it works.

So, let me now go to my MOTM 300 and take the same measurements.

(pins 7 6 5)
gate 2 - drain 2: 2K (pin 7 - pin 6) (same as 310)
drain 2 - source 2: 600 ohms (pin 6 - pin 5) (same as 310)
source 2 - gate 2: 1.5K (pin 5 - pin 7) (same as 310)

pins (3 2 1)
drain 1 - gate 1: open circuit (pin 2 - pin 3) (same as MOTM 310)
source 1 - drain 1: 25K (pin 1 - pin 2) (that's suspicious)
source 1 - gate 1: open circuit (pin 1 - pin 3) (sort of same as MOTM 310)

Huh. The only difference is that the source-drain resistance of FET 1 in 
the MOTM 310 measures 600 ohms, but it measures 25K in the MOTM 300.

Does this mean that one half of my LS3854 be buggered in the MOTM 300 is 
buggered?

Checking the schematic, pins 5-6-7, the ones that measure the same as in 
the MOTM 310 and 300, is the current source for the buffer. Pins 1-2-3, 
for which the MOTM 300 and 310 resistance readings differ, is the actual 
buffer. Perhaps the FET isn't conducting the way it should. I measure the 
same resistances if I measure on the top side of the board directly 
touching the leg FETs just after they come out of the FET body, so I don't 
think it's my solder joints...

- Aaron

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Dr. Aaron Lanterman, Asst. Prof.
and Demetrius T. Paris Junior Prof.    Voice:  404-385-2548
School of Electrical and Comp. Eng.    Fax:    404-894-8363
Georgia Institute of Technology        E-mail: lanterma at ece.gatech.edu
Mail Code 0250                         Web:    users.ece.gatech.edu/~lanterma
Atlanta, GA 30332                      Office: Centergy 5212