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This is a multiplexed display.? There's a switch/transistor applying power to each of the displays in turn.? That same switch changes the input of a single decoder to the various counters, likely through a multiplexer.
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There's a counter that selects a digit and the appropriate input.? If that counter doesn't run, you get whatever input there is to the decoder and the single digit much brighter than normal. Scanning averages out the peak current (selected) to the average current desired, hence the digits would be dimmer. FIrst there needs to be a scanning clock.? Secondly, the counter and decoding circuit that selects the digits needs to work. If it's a 555, they're easy to come by.? RadioShack/Tandy sold them, as did/does Jameco, etc. If you had a signal generator that put out TTL pulses, I'd suggest pulling the 555 and patching a 1 Khz (roughly) output into the former output of the 555 timer and see how it goes.? What their exact frequency is will be in the manual. Depending on what state the decoding circuitry comes up in, you could be looking at any one of the counter outputs. Analyze the circuit starting at the oscillator and go from there. Harvey On 7/16/2021 7:36 PM, Richard Peterson wrote:
Sorry for hijacking this discussion, but i'm working on a DM502 as well and wonder if you all could help. The unit powers on and has +5V and +/- 12V (step 1 of the check/adjustment procedure) but I don't have a clock signal - step 2. I'm guessing the NE555 timer IC is fried, but before i start randomly replacing parts, I checked out as much as i could. Voltages at various other points i tested seem OK and the resistors, capacitors and transistors within the display drive circuit all test OK as well. When the unit is powered up, i get a '0' on the display no mater what setting the switch is on with the decimal point showing up on a couple of the switch settings. Sometimes the display will go dark after being on for a while but not always. The sn7447 and LD110 ICs both feel very warm but not too hot to touch - maybe OK? |
