When does Willy talk about earning $170 in 1928 in Death of a Salesman?

The mention of making $170 in 1928 can be found in act 2, scene 1 of Death of a Salesman. 

In this scene, Willy is talking to Howard Wagner, his boss. He is trying to get Howard to grant him a full-pay job in New York so that Willy does not have to travel anymore. As we know by now, Willy is in his 60s, loses his sense of direction easily, and is quite tired. Moreover, he really needs money and has never had a job in the company where he has made an actual, steady paycheck; he works on commissions only.

Unfortunately for Willy, Wagner is too busy playing with a new gadget that he is very excited about: a tape recorder where he has taped the voices of his children. Rather than listening to Willy, Howard continuously deflects the conversation, focusing mainly on the new contraption. 

On the other hand, Willy is very intent in showing Howard that the late Mr. Wagner had "made promises" to Willy based on the latter's excellent commissions, which supposedly averaged at $170 per week, back in 1928. When Howard Wagner finally says to Willy that he cannot give him an office job with set pay, Willy loses his temper, especially when Howard doubted Willy and responded that Willy has never averaged that amount of money. 

Death of a Salesman

I averaged a hundred and seventy dollars a week in the year of 1928! And your father came to me — or rather, I was in the office here — it was right over this desk — and he put his hand on my shoulder.

We can make the assumption that Howard is correct; that Willy was never good enough a salesman to average so much money. However, we can only feel sad for Willy because, regardless of his performance, he had loyally served that company for almost his entire life. Now tired, old, and still underachieving, Willy sees the harsh reality of having to retire from a job that he never gained anything from to the fullest.