class) Completed as Riga-class frigates. Slightly more heavily armed than the Soviet Riga class (Project 50) it is based on in that the two twin 25 mm gun mounts on original Riga class are replaced by a second pair of twin 37 mm gun mounts in Chengdu class. In addition, the original RBU-2500 ASW launchers on the original Riga class is replaced by RBU-1200 on Type 6601. All units were converted to Type 01 in the early 1970s. Type 01 (Chengdu class) By the early 1970s, Type 6601 class went through mid-life upgrade with their torpedo tubes replaced by a twin launcher for SY-1 anti-ship missiles. Although redesignated as Type 01, these ships were still called Chengdu class. Retired in the 1980s. Type 065 (NATO codename Jiangnan) Based on the Type 6601/01. design first started in Dec 1962 by the 701st Institute at Wuhan, and the construction begun in Aug 1964, with the first ship entering service on Aug 1, 1966. Powered by modified civilian diesel engine rather than military-grade steam turbines. Main guns were mounted one forward and two aft, instead of two forward and one aft on the Riga class. Completely withdrawn from active duty in the 1980s, but remained as training, museum, and public relations ships. The ships remain on the PLAN's roster, and their upkeep at museums is provided by the PLAN. Type 053K (NATO codename Jiangdong) Air-defense frigate armed with two twin-armed HQ-61 surface-to-air missile (SAM) launchers. Only two built, and retired from active service in the early 1990s. Yingtan (531) is docked at a museum in Qingdao; the PLAN retains ownership and provides upkeep. Type 053H (NATO codename Jianghu-I) "Mass production" surface warfare frigate whose design and equipment were hopelessly outdated before the first ship even completed. The only nod to modernity was the four anti-ship SY-1s in two twin-box launchers. Remaining armament consisted to two single 100mm dual-purpose hand-loaded guns with fire control by a very simple stereoscopic rangefinder, limiting the guns to effective fire against surface targets in daylight/clear weather only. The six twin 37mm short-range anti-aircraft guns were all locally controlled, severely limiting their effectiveness. These ships are equipped with Chinese SJD-3 sonar, which is modification of Soviet Tamir-11 (MG-11, with NATO reporting name Stag Hoof) hull mounted sonar: instead of being fixed to the hull, SJD-3 has a telescoping arm, so when not in use, the sonar is stored in