I was PM'd about repairing an 8060A. I thought the information might prove useful to other owners of 8060s.
Bob in Port Coquitlam BC wrote:
"I recently pulled an 8060A out of non-function land back to life. Thanks to Mr.Modem-head, a 7660, and a bunch of electrolytics its now working again ... except the ohms range. A little troubleshooting revealed a broken RT1. A quick 1k resistor in place of it proved the ohms is working. I have sourced out the 446849 Fluke part but the greedy people on Ebay and other places want outrageous amounts to ship a few $ part across the 49th parallel. Is there any source you know of (Digikey, Mouser etc) that would have a suitable 1000v PTC thermistor substitute? Or am I stuck looking for an affordable Fluke part?
Also, should I worry about a ratio check that starts at 10025 and settles to 10015? More cleaning maybe?"
DRTaylor reply:
Regarding the PTC. The original part was made by a company no longer in existence. There are many candidates that would do the job. The 8060A was designed before the international safety rules went into effect, and the ohms protection wouldn't pass todays tests. That being said, the ohms protection is pretty simple in the 8060A. It consists of two selected 2N3904 transistors in BC to E diode mode. The resulting zener diodes are connected back to back. This yields a very low leakage Zener Diode clamp (leakage typically <1pA) that will clamp at around ±6.4V. Having a low leakage clamp is critical to making accurate measurements in the higher ohm and nS ranges. The current rating of these zeners is around 100mA. So the trick to this protection circuit is to have a PTC that reduces the current well below 100mA in time to keep the clamps from burning up. The initial resistance of the PTC is important in that it is in series with the reference resistor. But only the 200 and 2k ohm ranges have any worries about the series resistance. The max current through the PTC will top out at around 1.6mA.
BTW, the overload protection in the ohms range is rated to 300V, not 1000V. This is due to the breakdown rating of the PTC. So a cheap alternative that will not compromise performance is the Epcos B59885C0120A070. This is a PTC rated at 500V. These are in stock at DigiKey. However, it is bigger than the original part and you'll have to find a place to tuck it in. This is insulation coated which helps when you have to kinda fold it in. PTCs are noisy devices, but in this case it is just in series, and no precision measurements include the voltage across the PTC.
As far the ratio test failing, the most likely reason is board contamination, probably from spewwed (sic) electrolyte from the old Electrolytic caps. This requires a thorough cleaning. You should immerse the entire board (but not the switches) in IPA. Let it soak, then give it a good brushing with a stiff acid brush. Ratio error is can also be caused by DA in the integrate cap or by leakage current. It is probable you have a contamination under one or more of the critical A/D converter caps, or in the MAC chip socket. Usually the film caps do not degrade with time, so I would put that at a distant possibility. The ratio test shows that you probably have turnover error. Try putting 1V in and then reverse the leads. If all is well it should read exactly the same.