Old Fluke 8600A Bench Multimeter Gets Put Back Into Service
1. Introduction
I was recently given this old multimeter by a local amateur who had found it while tidying up under his house. He said that it had been there for many years.
It was marked as ‘not working’. Of course I said “yes, please” when asked whether I wanted it; there was no way I was going to pass up a Fluke bench multimeter without at least having a go at fixing it.
Radio Museum lists it as a 4½ digit autoranging multimeter, first manufactured in 1974.
DC Voltage: 200 mV - 1200 V; AC Voltage: 200 mV - 1200 V; DC Current: 200 µA - 2000 mA; AC Current: 200 µA - 2000 mA; Resistance: 200 Ohm - 20 MOhm;
I suspect that such a meter would have been a very high end piece of test equipment in 1974. The main chip in my unit is marked 02-85 which presumably dates it to 1985.
2. Battery Power Supply
The unit is marked as having the ‘battery supply’ option and as being for 120V operation. The batteries consist of four nicad D-cells which, from what I can figure, are constantly trickle charged via the mains circuitry, and regardless of whether the unit is switched on or off. The zener diode current and possibly the charging current is limited by the capacitor in series with the transformer. S13F increases the current while the unit is switched on, I think.
The ‘5V’ that powers the electronics comes from the nicads. It appears that the unit will not work unless the batteries are fitted.
The ±15V that powers the analogue circuitry comes from the DC-DC converter daughter board that is marked as ‘battery power supply PCB’.
I applied 5V DC-DC converter PCB, pushed the power switch and was happy to see the unit come to life. It appeared to work just fine. I measured the current to be about 440mA.
3. Replacement Power Supply
The mains comes in via the IEC connector, on to the main PCB and through the fuse (shown under the white capacitor), then onto the vertically mounted DC-DC daughter board and through the capacitor, then back onto the main PCB and off to the front-panel power switch through the yellow, red, and purple wires show to the left of the fuse, then back onto the main PCB and finally to the transformer. Quite a torturous path if you ask me.
The mains input wires to the transformer have been desoldered from the board and sheathed in heat-shrink.
A chap on EEVBlog forum here decided to replace the batteries with a 5v mains power supply in the form of the PCB removed from a discarded phone charger.
I thought about doing the same but figured that, in my situation, it would be better to use a DC-DC converter to generate the 5V and therefore allow for operation from DC, 12V to 24V or so.
These little LM2596 step-down modules are all over eBay etc, so I decided to use one of them.
The 10-turn trimpot was set to give 5V output, then desoldered and removed. It measured 3.3kΩ and was replaced with a fixed resitor. This should prevent over voltage that might have occurred if the trimpot was accidently turned.
Pin 5 of the LM2596 is an E̅N̅A̅B̅L̅E̅ input. When connected to ground the device operates normally, when above 1.3V the device is shut down and draws only about 80μA.
Pin 5 was desoldered and lifted from the board and connected to Vin via a 10kΩ resistor.
Using the mains power switch (S13F on circuit diagram) to connect pin 5 to ground allows for the power supply to be effectively turned off.
Coloured wires previous carrying mains to the front-panel switch were desoldered from their positions on the board that are marked A,B,C.
Purple is now connected to ground and red to SHTDWN|E̅N̅A̅B̅L̅E̅ pin on the new power supply board.
Without the need for the four nicad D-cells, there is plenty of room for the new PSU board shown fitted to the left of the transformer.
Unfortunately I broke the mains input connector while drilling the hole for the 12V power cable, I also managed to snap off the ground pin. These, along with the nicad holders, I stored next to the transformer so that they are available for anyone in future who might want to attempt to restore it to original condition.
Back in service on the workbench and measuring my 2.5V voltage reference. Okay, it might need some calibration, but nonetheless it’s a nice addition to the bench.