These are very nice instruments. Just finishing up the resurrection of one that had been written off by a previous owner due to the battery leakage issue. Somebody had unfortunately powered it up with the damaged A80 board, and blew the line fuse. Further troubleshooting found the 5V pass regulator transistor (Q3) to be defective (open emitter--internal bond wire melted), and the big zener diode (CR20) from the emitter to ground shorted. 5V supply must have "shot for the moon", causing the zener to conduct heavily and overheat. Transistor is easy enough to get (I found an OEM HP part on eBay for a few bucks, but NTE makes an equivalent NTE245 for about $5. It is a 10A NPN darlington. The zener diode was the painful one. Anyone bought a 6.2V 10W studmount zener recently? Better part of $30 for the NTE equivalent (NTE5180A), and that was about the only source I could find. The HP part (1902-1217) used to be available from Sphere for cheap, but he sold them out. There is an obscure note in the manual concerning this part. Apparently early production units used a 5.6V zener, but a production change shifted to 6.2V due to excessive current draw causing objectionable ripple on the 5V rail. The mounting of the diode is also not optimal. They put a toothed solder lug for a ground wire between the diode body and the heatsink, and bolted the diode down with no heatsink grease. I moved the ground lug to one of the screws that secures the thermal switch, and put some heatsink grease between the grounded diode case and the heatsink to improve cooling.
The other internal carnage that needed to be dealt with was the edge connector for the A80 board. It was heavily corroded by the electrolyte leakage. Fortunately, the damage didn't get through it to the motherboard itself. Unfortunately, replacing that connector is kind of an ordeal. You are dealing with a heavy multilayer motherboard, and a lot of heavy high current traces. Best option for connector removal is to break the plastic away in bits, and pull the pins individually, and then open the holes as needed with vacuum or solderwick. To get good access to the connector, the motherboard needs to come out from the main chassis. This involves removal of front and rear panels, ALL the coax cabling and pulling all the plug in modules from the unit. Then after all the module screws, you get to remove dozens of 5mm hex head screws that hold the motherboard to the diecast aluminum RF shield box.
If anyone has a badly damaged A80 board in their unit, I am offering "aftermarket" replacement boards for for these meters. Same circuit as original, but replaces NiCd battery with a small lithium cell. Drop-in replacement for the original card electrically and physically. PCB is high quality with gold plated edge contacts and dark purple soldermask. Asking $125 fully assembled and tested. Will sell blank boards for $45, if you prefer to source your own parts and stuff your own board. PM me for details or to order one.