Got curious about the cause of this recently. It's a first generation 77. Bought it new in 1983. Worked fine up until last year. Perhaps some techs here have seen/heard of this? A 'net search didn't really turn up anything.
Powers on normally. Passes self-test. On Vdc, display reads -2.246 V then slowly begins to fall. With leads shorted, it reads 0.00Vdc. Unshorted, it reads -2.165 for a bit then slowly falls. I don't leave it shorted for long, just to see what happens
On 300mV, display reads "- OL"
On Ohm setting, reads -2.884M Ohm. I short the leads it reads -0.1 Ohms. Unshort back to earlier reading.
On diode check, display reads OL.
On Aac reads 0.02 amps with last digit ticking slowly. Looks normal for Aac.
On Adc, reads 0.00 amps. Looks normal.
Given the above, something's FUBAR on Vdc, 300mV dc/ac, Ohms and diode test settings. Ampere settings are unaffected.
This negative DC offset sounds like something is shorted. Capacitors block DC, but not a shorted one. Thus DC would get in somewhere it's not supposed to. On 300mV, this -2.3V is clearly over-range, so the display reads OL. On the Ohms settings, the -2.xx MOhms reading indicates a voltage being fed into the ckt, which isn't right.
May also be a shorted transistor. I opened the case. Inside is primarily film caps, a few tant. and one or maybe two AEC's under the display. The AEC's are suspect. Film caps don't usually go bad on their own. Tant. though....not sure if these go wobbly after decades like AEC's.
BJT's can go wobbly though. I'll have to dig up a service manual + schematic.
You bench guys ever seen this before? Dang odd. Just suddenly happened one day.
Another reason to heat up the iron.......
Powers on normally. Passes self-test. On Vdc, display reads -2.246 V then slowly begins to fall. With leads shorted, it reads 0.00Vdc. Unshorted, it reads -2.165 for a bit then slowly falls. I don't leave it shorted for long, just to see what happens
On 300mV, display reads "- OL"
On Ohm setting, reads -2.884M Ohm. I short the leads it reads -0.1 Ohms. Unshort back to earlier reading.
On diode check, display reads OL.
On Aac reads 0.02 amps with last digit ticking slowly. Looks normal for Aac.
On Adc, reads 0.00 amps. Looks normal.
Given the above, something's FUBAR on Vdc, 300mV dc/ac, Ohms and diode test settings. Ampere settings are unaffected.
This negative DC offset sounds like something is shorted. Capacitors block DC, but not a shorted one. Thus DC would get in somewhere it's not supposed to. On 300mV, this -2.3V is clearly over-range, so the display reads OL. On the Ohms settings, the -2.xx MOhms reading indicates a voltage being fed into the ckt, which isn't right.
May also be a shorted transistor. I opened the case. Inside is primarily film caps, a few tant. and one or maybe two AEC's under the display. The AEC's are suspect. Film caps don't usually go bad on their own. Tant. though....not sure if these go wobbly after decades like AEC's.
BJT's can go wobbly though. I'll have to dig up a service manual + schematic.
You bench guys ever seen this before? Dang odd. Just suddenly happened one day.
Another reason to heat up the iron.......