HP 2420d - quite an old b/w laser.
It has recently started printing "echoes" and, more markedly, lines across the page every 40mm or so.
My suspicion is the new printer cartridge which I installed shortly before I started noticing this problem. Sadly I had already thrown the old one out, so I can't pop it back in to compare. That's pretty annoying as it was quite OK, but I had been getting a message to "Replace Toner" for quite a while and I imagined that prints were getting less intense (so I changed it as a precaution).
Does anyone have details on exactly what and how to clean the printer (if this is what's needed) ?