VTVM HP410B restored.

LW1DSE

Super Member
I owned this legendary instrument from a guy, at the internet, some km far from me. It was on service, but with some issues:the AC probe was adulterated and didn't perform. It was complete: tubes, cables and probes, DC test lead has breacked inside so measurements was unstable.
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As for safety, the main electrolytic capacitor was replaced with a new foil unit, from 4uF to 6.8uF @ 400V. Also a line filter is added at the primary of the power transformer.

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The meter itself has some humidity inside, when it warms, some condensation appears at the glass, but trying to open it was unsuccessful, with risk of damaging it irreversibily.
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It measures properly in both AC and DC voltages, and in resistance scales. However, a still unidentified fault makes the DC measurement to drop to zero while warm up, but is OK both at cold and warm states. I cleaned the switch contacts, measured resistances, and all look prety fine. Perhaps a faulty tube? It uses two 12AU7 units, heater voltage reduced with a barreter in series and a pot in parallel. The AC probe was broken: the internal capacitor was lost, I used 4 small SMD units 0.001uF @1KV and seems to be performing OK, although the original tip probe was missing too. Someone made one of his own, larger than the original. Surely the owner break it.

Tha case was repainted using alumunium based one, looks not too identical to the original, I suppose dust painted and ovened.

The leather handle is OK as it came to me, and I don'tn reworked it.

Both DC and AC probe cables are new, the AC lacks the connection to diode's heater and intermitent short circuit between both heater wires. And the DC probe showed faulty too, I used a piece of 75R coaxial cable with both inner and outer conductors, copper made.
 
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