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Hi Rich,
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Not a hijack at all -- you properly changed the subject heading, so you're good. :) If the 555 is getting the proper supply voltage (measure that at the chip), then there are very few things that could cause a no-clock condition. Either the frequency-determing RC is bad (watch out for bad or dirty trimmers), or the output is being loaded down. The latter is easy to check -- just disconnect the output from the rest of the circuit. If you start seeing clock, you know where to look next. Check the resistances in the RC network. The capacitor is almost certainly not bad. Don't bother to try checking it (other than, say, to verify that it's not a short) unless you exclude all other possibilities. Good luck! --Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 7/16/2021 16:36, 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? |
