1802 problems

Does anyone really know the RCA 1802 processor hardware? I am still debugging the microwriter, and have not got very far. I have an RCA user manual on the processor which seems quite comprehensive, I am asusming it's accurate. Now, according to that manual, the processor can be in one of 4 states (fetch, execute, interrupt responze, DMA), and which state it's in is output on 2 lines called SC0 and SC1. The INT/ line is high, both DMA IN/ and DMA OUT/ are also high. I can detect no gltiches on any of these signals. The clock is running, the bus looks active, etc. The problem is that I am getting active-high pulses on SC1. This, according to the data sheet means it's doing a DMA or interrupt response cycle. But why the heck should it? I thought it was a defective 1802 chip, but another one pulled from an old board shows identical behaviour (right down to locking up after typing 8 characters on the microwriter keyboard). I can't believe 2 have failed in exactly the same way. Unless of course this is a well-known problem. Any thoughts? -tony