Hi, I'm currently building an X-Y display out of a DG7-32 CRT. I've got the power supply and deflection amplifiers pretty much sorted - no problems there. Before I continue, the schematic is online at . OK, what I want to do is use a voltage between 0V and 5V to vary the brightness of the spot on the screen. Note that the brightness control allows V(G1) to vary between -600 and about -500V (just above the cathode voltage). -600 is full blank, -500 is full brightness. I want the external control to work with the brightness control. If the brightness control is set so that the beam is just off, then 0V will leave the beam in that state, and 5V will make the beam visible. In other words, the 0-5V variation is added to the brightness setting. The problem I'm having is with the -600V offset. I'm trying to take a 0V-5V signal and use that to tweak G1, but the transistor amplifiers I've tried all stick on when I connect their base to the input signal. Floating the base or wiring it to -600 will turn the transistor off fine. Does anyone know of a way I could do this? If I've done something really badly wrong in the PSU design, please feel free to criticise it... I know it's probably not the best (ha, not even close), but it seems to work fine... for the most part... BTW, I know the bias divider circuit in the schematic is crap. For instance, pulling the G1 or cathode around tends to throw the focus out of whack. I'm working on separating the dividers out, but I'm still looking for the notes I made on the design of this divider, so I can get the bias voltages right... Thanks. -- Phil. | Acorn RiscPC600 SA220 64MB+6GB 100baseT philpem@philpem.me.uk | Athlon64 3200+ A8VDeluxe R2 512MB+100GB http://www.philpem.me.uk/ | Panasonic CF-25 Mk.2 Toughbook ... --T-A+G-L-I+N-E--+M-E-A+S-U-R+I-N-G+--G-A+U-G-E-- -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist