| Electronics > Repair |
| Tektronix DM501A repair |
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| David Hess:
--- Quote from: Rax on May 30, 2024, 02:51:18 pm ---Most units seem to experience a systematic issue - even if they measure very well in the (DCV) 200mV and 2V range, they have large offsets in the rest of the ranges. This tells me that the Caddock precision resistors arrays are busted, probably due to abuse from the user. --- End quote --- There is nothing wrong with the Caddock precision decade dividers. High offset on ranges which rely on the decade divider is caused by excessive input bias current from the multiplexer, or damage to the input protection diodes. The multiplexer is a known failure point in these designs, but it can be replaced with an external multiplexer and low input bias current precision operational amplifier. I have the parts but have not quite gotten around to fixing mine. Later versions of the DM501A used the alternate chipset which supported an external operational amplifier. |
| Rax:
David - thank you for this. --- Quote from: David Hess on May 30, 2024, 05:19:04 pm --- --- Quote from: Rax on May 30, 2024, 02:51:18 pm ---Most units seem to experience a systematic issue - even if they measure very well in the (DCV) 200mV and 2V range, they have large offsets in the rest of the ranges. This tells me that the Caddock precision resistors arrays are busted, probably due to abuse from the user. --- End quote --- There is nothing wrong with the Caddock precision decade dividers. High offset on ranges which rely on the decade divider is caused by excessive input bias current from the multiplexer, or damage to the input protection diodes. --- End quote --- By the "multiplexer," I assume you mean the ADC? (U1601/LD120) And if yes, you'd recommend replacing it? --- Quote from: David Hess on May 30, 2024, 05:19:04 pm ---The multiplexer is a known failure point in these designs, but it can be replaced with an external multiplexer and low input bias current precision operational amplifier. I have the parts but have not quite gotten around to fixing mine. Later versions of the DM501A used the alternate chipset which supported an external operational amplifier. --- End quote --- I seem to have two out of five having this revision applied. But they still exhibit issues similar with the rest of them - namely, that the 200mV and 2V ranges work OK, but the rest of them don't. There may be one difference in the way these two groups of meters are defective in the 20V through 1000V ranges - more specifically that the one having the revision mentioned above is completely dead on those higher ranges, and the DCA is completely dead too. The "non-revised" ones are defective in a different way, where the 20V through 1000V ranges are responsive but wrong, and the DCA works OK. I may be switching here to a more invested approach - remove the smaller board attached to the switches (desolder) to gain good access to the switches for cleaning and conditioning (which is what the manual requires) - and am not sure how to resolve the input bias issues. For starters, as I explain above, I currently assume the fault to be located in U1601/LD120. |
| David Hess:
--- Quote from: Rax on May 30, 2024, 06:53:01 pm ---By the "multiplexer," I assume you mean the ADC? (U1601/LD120) And if yes, you'd recommend replacing it? --- End quote --- The problem is with half of the LD120. The input goes through a multiplexer, then into a voltage follower, and then comes out before going back in on a different pin, which is why it can be repaired. The multiplexer gets damaged because it is directly exposed to the input. You could find another LD120, but they are not common. iMo posted a diagram showing what is required here. It is not complicated. I bought some OPA140 operational amplifiers and DG419 multiplexers to fix my DM501A, but have not gotten around to it yet. --- Quote ---I seem to have two out of five having this revision applied. But they still exhibit issues similar with the rest of them - namely, that the 200mV and 2V ranges work OK, but the rest of them don't. There may be one difference in the way these two groups of meters are defective in the 20V through 1000V ranges - more specifically that the one having the revision mentioned above is completely dead on those higher ranges, and the DCA is completely dead too. The "non-revised" ones are defective in a different way, where the 20V through 1000V ranges are responsive but wrong, and the DCA works OK. --- End quote --- The modification was to fix a different problem. The built in CMOS voltage follower lacks enough common mode rejection to support 4-1/2 digits. The modification replaces the LD120 with the LD122, which lacks the built in voltage follower, but keeps the multiplexer, so an external precision voltage follower can be used instead making the ADC more accurate. |
| Rax:
Thank you, David, this is very helpful. I'm new to these meters, and the background issues they have, so thanks for catching me up with what needs to be known and what needs to be done. --- Quote from: David Hess on May 30, 2024, 10:06:12 pm ---The problem is with half of the LD120. The input goes through a multiplexer, then into a voltage follower, and then comes out before going back in on a different pin, which is why it can be repaired. The multiplexer gets damaged because it is directly exposed to the input. You could find another LD120, but they are not common. iMo posted a diagram showing what is required here. It is not complicated. I bought some OPA140 operational amplifiers and DG419 multiplexers to fix my DM501A, but have not gotten around to it yet. --- End quote --- I'm flirting with the idea of doing the mod (I have some OPA140s and OPA189s handy, alongside other parts I'd need...). But it's more invested than what I was planning for these. May be fun though! Any reason for choosing an "11 ohm through resistance" part? By which I mean the DG419. The 417 has 6 ohms, I'd be inclined to think less resistance is better here?... --- Quote from: David Hess on May 30, 2024, 10:06:12 pm ---The modification was to fix a different problem. The build in CMOS voltage follower lacks enough common mode rejection to support 4-1/2 digits. The modification replaces the LD120 with the LD122, which lacks the built in voltage follower, but keeps the multiplexer, so an external precision voltage follower can be used instead making the ADC more accurate. --- End quote --- Makes sense from the standpoint of what you helped clarifying for me. Thanks again. |
| Rax:
One unfortunate, and a fortunate thing happened today. The first one is that I dropped the best performing DM 501A in the batch (the only thing with zero reaction to input excitations was the DCA) on the floor (from my lap, so not a tall fall), and the second one is... I dropped etc. etc. Initially, the plug seemed pretty shaken with zero readings on most inputs. After reading the manual (nth time) I had a hunch and reseated the display module and things came back to normal. By normal, I mean actually, everything works 95% now (all modes and ranges!...). I regard this outcome being the result of: * This is one that I both washed with IPA, and conditioned with David's recommended contact conditioner. * The fall must've helped with whatever mechanical issue the DCA was inflicted with. * This unit is one of two with the revision David mentioned being applied to the later units (and earlier ones that probably visited the Tek shop and had it applied, and I'm pretty sure the other one I have that has the revision is also my oldest unit). Now, I'm saying 95% because there's a bit of "wander" of the indication of a measured value. It doesn't happen all the time, and when it does, I have to re-actuate the range switches (up-down-up...) to restore the correct indication of the meter. I'm tempted to blame this for some leftover flakiness of the switches. But I have a nagging gut feeling there's some flakiness of the input circuitry - maybe the multiplexer? - which creates or just adds to this wandering. |
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