HH Build Problem

[WesP]

I'm putting together a HH rev D and things looked good up to the point of checking the voltage on TP4 while adjusting R18. The instructions state that the voltage should vary from -1.6v to +1.6v. I get +.42v to +.14v. All the other voltages match with the build instructions. Any ideas on what may be wrong or what to check. I'm also wondering about R7, R8 and R15. The resistors are listed on the parts list but the instructions state they may be shorted. I have them shorted.

Class? Class? Anyone?


Thanks

[Carl-NC]

Shorting those R's is fine. Try tacking another 1M in parallel with R16, see if the output offset will go negative. You might have an opamp with a higher-than-normal input bias current.

- Carl

[WesP]

Thanks Carl I'll give it a try.

[WesP]

Carl,
No luck with the additional resistor. It did lower the voltage but even with a short across that resistor it would get to just barely negative then slowly go to zero. Any other thoughts?

Thanks

[Rob in KS]

I had the problem that R18 wouldn't adjust the voltage. I had substituted 270 ohm for R9, and it was leaving the MOSFET partially turned on. Not exactly the same problem, but something to watch for.

[WesP]

Carl,
I found my problem. I had two 1M resistors instead of two 100K resistors. Now it measures out OK. Thanks for the help.