Hi Patrick,
I have some good news: my SG503 is fixed and running. With your and David's help, I identified Q180 (2N3906) as having failed . C180 was also damaged (open). R180 and R216 had signs of cracked epoxy but their values were good. I replaced all of these components and the unit is running fine.
The scenario that seems plausible to me is that the original culprit was Q180, failing intermittently and causing current surges. These surges blew C172 (1mF tantalum) a couple of times. C180 was good initially because I had replaced with a new 1mF/50V tantalum along with C172. Eventually when I applied external power, Q180 failed completely taking C180 along with it.
For future reference, here are some measurements for Q180 and Q190.
Bad Q180 Good Q180
V(B-E)=0.288 V, R(B-E)=350 Ohms V(B-E)=1.481 V, R(B-E)=6.0 KOhms
V(E-B)=0.273 V, V(B-E)=0.673 V
V(B-C)=0.289 V, R(B-C)=352 Ohms V(B-C)=0.750 V, R(B-C)=4.92 KOhms
V(C-B)=0.275 V, V(C-E)=0.643 V
V(C-E)=0.000 V, R(C-E)=6.5 Ohms V(C-E)=0.666 V, R(C-E)=1.63 KOhms
V(E-C)=0.000 V, V(E-C)=1.305 V
Q190
V(B-E1)=0.701 V, V(E1-B)=1.339 V
V(B-E2)=0.735 V, V(E2-B)=1.340 V
V(C-E1)=0.719 V, V(E1-C)=0.609 V
V(B-C)=0.694 V, V(C-B)=1.087 V
V(C-E2)=0.748 V, V(E2-C)=0.614 V
Thanks again,
-Achilles
toggle quoted message
Show quoted text
--- In TekScopes@..., "Patrick Wong" <patwong3@...> wrote:
Hi Achilles,
I'm wondering whether you've made further progress on your SG-503? Would like to hear the outcome when you have a chance.
Thanks,
Patrick Wong AK6C
