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Hi Rich,

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

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?

I tried Roger's suggestion to pull, clean and reseat the ICs but that didn't change anything. So . . . should i try just changing the NE555 to see if it comes back on line. Replace all the ICs? Are there other tests I can try to isolate the root cause of the problem? possibly a dirty switch?

Mouser has the NE555, the SN7447 and the SN7416N but the LD110 and LD111 appear to be approaching extinction although there is one seller on the fleabag selling sets.

Thanks for your help.

Rich

I have a pair of DM501As that I use as my every day go-to DMMs on my bench. They work well. I have noticed, though, that with an open input on the 200mV range, one reads about +11mV, and the other about +3mV. Goes to zero when the input is shorted. On the 11mV unit, this corresponds to about 1.1nA on board leakage into the A/D input node and the associated 10Mohm input resistance. When I isolate the A/D input and ground it through 10Meg, the reading is around 0.01mV. So it is not the A/D chip itself.

The schematic shows a chain of switches in the S1 assembly, between the input attenuator assembly and the A/D. By watching the display as I switch ranges and short the input, it appears that the leakage injection is into that chain of switches. I isolated the relatively long pc track from the last switch (S1-N) to the A/D (R1613) and it looks like the injection is somewhere in the S1 assembly itself. I sprayed PC cleaner in, around and through the switches liberally, but after drying it - same result. But mere dirty switches aren't enough - there has to be a source of +V somewhere close by. To isolate possible switch leakage from other parts of the circuit, I have grounded, in turn, all of the feeds from the ohms, AC and temperature circuits at the respective switch terminals, with no difference to the display. I can't readily see any other sources.

And now I am out of ideas.
So I am appealing to the wealth and breadth of knowledge in this forum for any suggestions. What else can I do to identify the leakage source?
Is switch removal REALLY as simple as the manual suggests "...by carefully unsoldering the connections and pulling....". Dozens of pins have to be completely cleanly unsoldered for this to work... I really do not want to go there just yet.

Yes, I realise that this parameter is not specified in the manual, and this effect does not detract from the usability of the instrument. It's because I can :-)!

Roman

I'm trying to calibrate my 2465 but don't have the Tektronix calibration equipment. For the CAL 01 Horizontal adjustments, I used square waves from my Hantek oscilloscope/multimeter/waveform generator, which only goes up to 10MHz square waves. I think this was not great but was adequate for these calibrations. However, when I progressed to the vertical calibrations, I'm not even sure what was required. When it says to connect a 0.5 V, standard-amplitude signal from the Calibration Generator, what does that mean? The service manual says that the Calibration Generator is "Fast-rise, low-abberation amplitudes: to 1V. Rise time: 1ns or less. Repetition rate: 1 kHz to 100 kHz. Precision amplitudes: 0.01 V to 50 V +/-0.25%." Okay, so when they say a 0.5V standard-amplitude, what frequency do they want? Is that 0.5V peak-to-peak? Is it AC (goes positive and negative) or DC (all positive)? Do they want a square wave, or do they want a constant DC voltage?

I'm trying to calibrate my 2465 but don't have the Tektronix calibration equipment. For the CAL 01 Horizontal adjustments, I used square waves from my Hantek oscilloscope/multimeter/waveform generator, which only goes up to 10MHz square waves. I think this was not great but was adequate for these calibrations. However, when I progressed to the vertical calibrations, I'm not even sure what was required. When it says to connect a 0.5 V, standard-amplitude signal from the Calibration Generator, what does that mean? The service manual says that the Calibration Generator is "Fast-rise, low-abberation amplitudes: to 1V. Rise time: 1ns or less. Repetition rate: 1 kHz to 100 kHz. Precision amplitudes: 0.01 V to 50 V +/-0.25%." Okay, so when they say a 0.5V standard-amplitude, what frequency do they want? Is that 0.5V peak-to-peak? Is it AC (goes positive and negative) or DC (all positive)? Do they want a square wave, or do they want a constant DC voltage?