Defctive filter

Defective filter unit
(Click images to enlarge)

The mains filters of old Solartron/Schlumberger model 7150 and 7150plus multimeters have been known to deteriorate over the years and eventually fail with a bang and a whiff of smoke.

Spares for the filter/fuse/voltage switch block are available but expensive. However, the filter part can simply be cut off and replaced with a new standard part (a short metal-can model), keeping the voltage selector/fuse holder unit in place.

(For all I know, the problem and suggested solution may also apply to the 7151 model, but I do not have one of these myself.)

KO4BB makes the Service Manual for the 7150+ available for download.

(Here's another repair description from the Time-Nuts mailing list.)

CAUTION: These instructions are for experts ONLY. The filter is connected to mains voltage, which can kill if handled improperly. Incorrect wiring may also create a fire hazard! It is of course self-evident that mains voltage and all inputs must be disconnected completely before working on the instrument!

Preparation: Open the enclosure, flip the analog (top) circuit board upward, remove the two screws holding the mains transformer, then take the digital (bottom) circuit board out of the lower half of the enclosure together with the display board and back panel. Disconnect the power cable from the analog board, the ground connector from the shield between the boards, and the two interconnecting cables from the digital board. Unplug the display board from the digital board and separate the digital board (analog and display remain connected). Push in the retainer clips on both sides of the mains filter block and slide it out of the back panel. (Chances are you'll have to clean up some smelly goo left behind by the failed filter.)

Cut off the old filter

Cut off the old filter

Using a metal saw, cut between the filter unit and the voltage selector switch ...

Use a sharp knife to trim remaining plastic protrusions on the voltage selector switch ...

Filter part cut off

End result after cutting and trimming

Covering a trace on the circuit board

Cover circuit board trace

Just under the filter block there is a trace on the top of the circuit board running to a voltage regulator mounted to the back panel. This should be covered with insulation tape to make sure the new filter's metal case does not short out the trace.

The new filter needs to be fastened to the back panel with small self-threading metal screws (2.5mm). Nice that there's just enough space for these ...

Screw holding new filter

Screw holding new filter

If the new filter has a metal case rim extending above the plastic, you may have to file or grind that down a bit to make the filter fit next to the voltage selector.

Just underneath tne new filter you will have to mill or cut the lower part of the enclosure down a bit so that the back panel can be fully inserted again.

Cut-out in lower half of enclosure

Cut-out in lower half of enclosure

Before final re-assembly, attach the new filter to the plastic case of the voltage selector / fuse holder using thick double-sided foam tape. You may have to use more than one layer to make sure the combined unit fits with enough pressure so that the plastic retainers hold the left side of the voltage selector case properly to the back panel .

End result

End result

While you are at it, you may want to replace the dim old yellow LEDs used to "illumniate" the display with modern bright-white LEDs — doing so will significantly enhance the display's readability!

But that is a story to tell another day ...

New display illumination

New display illumination

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