I'm tuning a CP5176 and am unable to reach the required 13.5 VAC output. The problem appears to be in the SK25 DOA.

With a 1 kHz signal and JMP1 on pins 2–3:

At 100 mVAC input, TP1 = 65 mVAC, TP2 = 1.336 VAC, and L1/SK25 output = ~0.97 VAC.
TP1 and TP2 appear to behave normally as input is increased. At maximum input I get TP1 = 1.194 VAC and TP2 = 7.54 VAC.
However, at about 300 mVAC input, the SK25 output starts behaving abnormally. L1 stops increasing and begins decreasing as input is raised.
At maximum input, L1 falls to about 0.4 VAC and develops approximately -10.5 VDC.
With the signal removed, L1 returns to 0 VDC over roughly 25 seconds.
TP1 and TP2 remain essentially 0 VDC during the fault, so the large DC shift appears to originate in/around the SK25.
TP6 remains stable at -5.5 VDC at both low and high signal levels.
SK25 V+ remains stable at ~+16.0 VDC under load.

Some internal SK25 measurements also follow the negative shift. For example:

R9 bottom: +0.12 VDC low -> -11.4 VDC high
D5 anode: -0.13 VDC -> -10.7 VDC
D5 cathode: +0.15 VDC -> -11.1 VDC
D6 anode: +0.23 VDC -> -11.4 VDC
D6 cathode: ~0 VDC -> -12.1 VDC

I've checked Q2–Q10 and D5–D9 with the meter's diode test and haven't found an obvious failed/shorted semiconductor. I also checked the assembly-guide warning about R1 contacting the D5 solder joint; R1 is definitely clear. D5/D6 are thermally coupled, and the heatsink-board solder connections look good.

The key symptom seems to be: the SK25 works at low signal, but at ~300 mVAC input it begins developing a large negative DC shift and its AC output collapses instead of continuing to increase.

Any suggestions for where to look next on the SK25 would be greatly appreciated.


Steve