Jack; That's useful to know. I don't have the assembly manual -- all I got with my ETA-3400 was the operation manual, 595-2271, with a copyright date of 1979. I did get the schematic, though. Walter Moore posted the following modifications he traced out on the Heathkit mail list: ----start of quote I guess I'm missing that important document also! I'm also missing the schematic for the ETA-3400, so I cannot guarantee what I'm about to tell you is 100% accurate. It would be nice to see if the wires in 3 & 4 are correct. I compared an unmodified ET-3400 (no 'A', no 'Series') with one that was modified and found the following differences: 1) A 40 pin connector added (OK, I had to say that :) ) 2) D0-D7 connected on said 40 pin connector. If you examine the back of the board, you will find 8 solder pads for D0-D7 below the 40 pin connector. Each of these was soldered to the corresponding via (is that what those holes are called???) next to IC9 and IC10 (to the side on an ET-3400, below on a 3400 "Series"). 3) Pins 6 & 35 of the 40 pin connector (NC on the schematic) connected to IC10 pin 1, soldered at the first via as the trace moves towards the top of the board. 4) Pins 15 & 20 (also NC on the schematic) connected to ?2 near IC4. Looking at the X-Ray layout picture, this was confusing to me. It wire was connected to the right pin of the 4x2 breadboard socket. From the front, this looks like it is labeled "?2", from the X-Ray it looks like "?2" is the 3rd pin). 5) The RAMs were removed. This system was purchased on eBay, and at least powers up. I've never hooked up a terminal or tape deck to try anything else out. Do you have the Software Reference Manual (595-2271) for the ETA-3400? ..walt ----end of quote I'm thinking the clock was also changed to raise it to 1 MHz. Can you look at your ET-3400 and see what values are installed for the resistor and capacitor on the clock generator? I'm going to start a bibliography for all these manuals and put it on my web page. There seem to be three versions of the ET-3400. Later, Paul Pennington Augusta, Georgia