Hi, Im new to this forum and I on my back day I do DB rows, Chins on the chin up machine (not strong enough yet to do them without any support) and close grip pulldowns. I feel as though my arms are getting more out of the exercise than my back when I do chins because when I lower myself I do it until my arms are locked out before lifting myself up again, and when I do the close grip pulldowns when I let my arms go up, I let them go up until they lock out. How far should I go down with the chins and how far should I let my arms go up with the close grip pulldowns?
Don't ever lock your elbows ... keep tension on the lats and flex them throughout the entire ROM. If you feel too much bicep in your back exercises, try a false grip, it's helped me a great deal.
You could try fatiguing the lats first with an isolation exercise so that they fail before the biceps. You could do stiff arm pulldowns or pullovers (if you have access to a pullover machine). Do a set of these and then go straight into you chins and/or pulldowns.
By directly working the lats first and going directly into a compound exercise they will get as much or more stimulation than the biceps.