how I determine my capital account if already depreciated all the assets and I still owe for one, can I have capital account negative after substracting accumulated dep

If you have fully depreciated all your assets and have no cash in the bank, your total assets would be zero. It's possible you have "loans from shareholders" in the asset portion of the balance sheet due to the balance due on one of the assets. If this is the case, then your capital (equity) balance would be zero. Capital accounts cannot be negative.

Example of Schedule L:

Assets

   $2,000 Capital Assets

   ($2,000) Depreciation on Assets

   $2,000 Loans from Shareholders

Total Assets = $2,000

Liabilities

   $2,000 Short term loan for capital asset purchase

Members Equity

   $0

Total Liabilities and Members Equity = $2,000