Adam Triple Beam Balance (TBB-610S)

I've now ordered 4 of these balances. Of the first two (ordered a couple years ago), one had a calibration knob that never worked. I worked around the uneven calibration by using a was of clay near the pointer. The other was okay and works well. They each arrived with auxillary weights, as well. However, on both balances the larger riders can slide past the zero mark which consistently causes confusion for my 8th graders. Despite these flaws, for economic reasons I chose to order two more this year. I thought that it'd be unlikely to have such problems again. Well, this time, two balances arrived lacking the auxiliary weights, and in each case, the mass under the pan was either too high or too low to zero out the balances, no matter how far out or in the calibration knob went. I did read the directions and unscrewed the pan to access the dead weight, thinking it would be fairly straightforward if the directions told me to do so. Ha. I took out the tiny screw, unscrewed the pan...lo and behold, the space below was filled with what look like chipped off tips of screws, coated in a mystery dust (hope it wasn't lead), that tumbled out when opened! I'm guessing that the pan's dead weight area was filled with the whole kit upside down. I managed to put it back together without some of its pelletized masses (I improvised) but when I discovered the second balance was also unbalanced, I opted not to do that again. I just taped masses to the underside of the tray in order to zero it out. In sum, 3 out of 4 balances had problems. Yes I can rig changes, but the kids undo them in their inquisitive ways, which just makes more work for me and confuses them. I'm wishing now that I had gone for the pricier balances.