Lots of vintage Flukes on display here. I'll toss in my recently repaired 8010A.
I always wanted an 8010A. Because...well, just because. After all, this is the TEA. I don't have to explain why. A few months ago found this beauty on flea bay that appeared to be in good physical shape, had a bright LCD, and was less than $40 USD. But it had the typical “powers up but not tested” ad line. I figured for the price I'll take a chance.
The DMM arrived as advertised. In good physical shape, clean, powered up with bright LCD. But that's about all it did. DC volts 20 range and above was grossly out of spec. 10 volt input sometimes yielded as high as 14 volts. Ohms was way off too. AC volts totally non-functional and showed permanent over range across all ranges. Clearly this Fluke had a hard life. I wasn't very confident that it was going to be worth fixing.
First thing I did was get a service manual. Artek Manuals had it in PDF format for a reasonable fee. If you've never used them I highly recommend them. Started basic trouble shooting with the power supply first. Measured all the supply voltages and as insurance re-capped it. Found no issues here which eliminated the power supply as the source for the issues observed.
Next I went after the DCV trouble. Started at the front end and isolated the varistors (VR1, VR2, VR3) which made no difference in the observed readings so I assumed they were OK. I was getting some wacky and inconsistent resistance readings around precision resistor network U1. Decided to remove U1 and replace it with selected 0.1% discrete resistors. Since this is a high impedance area I positioned the resistors above the main board as much as possible. The repair looks ugly and a total bodge but after fixing additional issues it's working fine. Those resistors can be seen in the photo just to the right of the selector switches.
After replacing U1 DC volts improved but still had an offset of approx 100mV – 200mV. Figured this was due to board contamination so scrubbed it with IPA then distilled water and a bake at approx 150 degrees F for 2 hours. No real improvement. Next on the hit list were Q1, Q2, Q3, and Q4. I figured one or more of them must have some high leakage. I lack a transistor tester so I changed them all. They are common 2N3904 (3) and 2N3906 (1) so it was a no brainer. Progress! After changing them the DC offset dropped to under 20mV.
I figured the remaining off set was board contamination due to all my handling. So some more scrubbing with IPA and distilled water and a bake cycle. Success...at least at first. Once everything fully cooled the off set came back. I let it sit for a few days then scrubbed it and baked it again. It's fixed! The DC volts calibrated just fine across all ranges and is stable. Ohms is now OK too. It's been 2 months since this repair and it's still rock stable.
And finally to the AC volt issue. For some reason the RMS to DC Converter (U8) was outputting a high level DC voltage all the time to the A/D Converter causing the over range. That turned out to be a leaky 10uf tantalum cap (C33) which can be seen in the photo just to the left of U8 and above the 8 pin DIP. Decided to also change the other tantalum caps in that area as well as the IC AR-1 (LF351N op amp). The AC volts is now fixed and calibrated.
So here it is in a prominent location on my bench ready for work. It even has it's own LED book light to better illuminate the LCD display. I must have TEA bad because there were a few instances where I considered junking the Fluke. But I was determined I was gonna fix it!