If you experience an Error 52 on early HP LaserJet printers such as the LaserJet II or III (Canon SX engine) it is usually caused by a scanner malfunction. This can also be caused by a problem with the DC controller. On a LaserJet IIp or IIIp (Canon LX engine) this is usually caused by a scanner motor malfunction which was a very common issue that resulted in an extended warranty. To it’s credit, even after the extended warranty expired, Canon continued to sell the part for about $3 while HP charged a LOT more.
Since we never did board level repairs back in the day I don’t know what part needs replaced but I would start with the checks in the service manual below. After that I would look at caps and other parts in the scanner assembly first, followed by the DC controller.
