Siglent SDM3045X Input Damaged - Page 1
Forum section: electronics / repair

author: Max Gain
author_rank: Newbie
timestamp: April 30, 2025, 03:00:35 pm
content: I have plenty of safe ways to test an unknown transformer but without thinking I connected the secondary to my SDM3045X and powered it up. I know now that the secondary was 830Vrms. To make matters worse I had not switched from the ohms function to the AC volts function. I can’t believe I was so careless with my precious DMM but here I am.Sending it in for repair will cost almost as much as a new one and I was told the schematic is proprietary so no help there.When I opened it up I did not see anything obviously damaged. It appears to have gas discharge tubes and MOVs to protect it from idiots like me. Those devices seem OK and they all test open circuit. Here is how it behaves at this point:1. DC Volts function: Manual range – reads voltage on 60V range and higher. Sometimes also on 6V range. Auto Range - stays on 600mv range and reads 0 volts2. AC Volts Function: Manual range - reads millivolts regardless of input voltage. Auto Range – pretty much the same.3. DC Current – Normal operation Auto and Manual Range4. AC Current – Normal operation Auto and Manual Range5. Ohms – Basically reads zero under all conditions (2wire and 4wire)6. Freq and Capacitance – not functional7. Continuity/Diode – Reads 0.1ohm/beeps continuously8. Temperature – reads about 19C with no temp probe connectedWith a schematic in hand this thing could probably be saved. I’m hoping maybe someone has reverse engineered a schematic of the input circuits or has experience that could help. Thanks in advance.

author: Martin72
author_rank: Super Contributor
timestamp: April 30, 2025, 03:39:47 pm
content: Hello and welcome to the forum.The resistance measurement range will also be protected by the input fuse circuit.This has definitely suffered a defect, which then also affects the other areas in terms of measurements.Please take a clear photo of the input area of the meter; you may be able to see what is defective just by looking at it.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 30, 2025, 03:48:41 pm
content: AFAIK there is no public available reverse engeniered schematics. So it would be your turn to take good pictures and get at least a partial schematics for the input part. To a large part it should follow the SDM3055, but with lower grade parts in some places. From this the 6 V range is a bit odd: up to some 2 or 2.5 V it follows the low voltage path and for the higher voltage it goes though the high voltage divider like the 60 V and higher). There is a hidden automatic switching between the 2 hardware ranges.The sparK gap and MOVs are mainly to protect against ESD, so short bursts of even high voltage. The ohms range shoudl have 2 paths: one from the test current source (likely via the PTCs and THT diode). The 2nd path would be via the low voltage input path (the chain of 13 K resistors).One could check if one still gets the test current out when in ohms mode.It is a bit odd that the AC part is also broken. This input part is under the shield. There may be a common fusible (may well be THT and larger form factor, like R400 for the 3055) resistor for the ohms and AC ranges.

author: J-R
author_rank: Super Contributor
timestamp: May 01, 2025, 12:05:15 am
content: Lots of good teardown photos here:https://lygte-info.dk/review/DMMSiglent%20SDM3045X%20UK.htmlI'm no expert like the previous two posters, but I think I'd start by checking all those resistors which appear to be an input divider, and maybe the relays?

author: Max Gain
author_rank: Newbie
timestamp: May 01, 2025, 02:29:24 am
content: Thanks for all the support and info. It helps to know a little more about the circuit and how it works. As a result I have made some improvement. AC Volts is now working and working with auto and manual ranging. Also the frequency measurement function is working. After careful examination of the bottom side of the circuit board I saw what appeared to be a burned trace on an inner layer. The trace went from relay 1 to relay 2 and adding a jumper wire in place of the burned trace restored the AC Volts and Freq functions.DC Volts still does not work on auto range or ranges below 60V manual. I could almost live with that. The ohms function is still completely non-functional. There is no test current in ohms mode. Both PTC measure about 700 ohms in circuit and the two THT resistors under the shield read 499K. The THT diode D5 checks OK in circuit. The manual gives voltage readings at 8 points on the analog board. Those all measure within spec.The diode D302 is a little suspect. When checked with my Fluke DMM it reads 0.7V both directions. Also the THT transistor Q39 which is a MPS6560 NPN reads short circuit from base to collector. I suspect these components might be associated with the two ohms range paths Kleinstein mentioned so tomorrow I will probably remove them and test out of circuit.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 01, 2025, 06:48:01 am
content: The THT diode would likely be OK (should be a 1N4007 or similar rather high voltage diode), it was mentioned as an easy to identify part to give a hint where the current should flow. If D302 is a dual diode it could well have the 2 diodes back to back and thus reading the voltage in both directions.The shorted BJT is highly suspect - the transistor should be nothing that special and for a test a different small type (e.g. BC547) should be OK.The current source should be using U501,U502, likely U500. Damage to the ohms current source is a quite common issue, as here the protection is often not that good.One could use the THT diode as a point to check the ohms test current to ground (with some 1 K in series as protection).700 ohms for the PTCs is very reasonable.With a blown trace the fault current was quite high, so likely triggered the spark gap and thus has the current flow through the MOVs too.I still would not expect the MOVs to cause the defect - they should still read high impedance (there may well be some 1 M in parallel on the PCB).I would not expect a relay to fail and cause the fault for the low voltage ranges.

author: tautech
author_rank: Super Contributor
timestamp: May 01, 2025, 08:33:30 am
content: Drop a post in the main SDM3000 thread asking for help and pointing them back to this thread.https://www.eevblog.com/forum/testgear/siglent-new-bench-dmm-sdm3055/

author: Max Gain
author_rank: Newbie
timestamp: May 01, 2025, 01:36:16 pm
content: I had high hopes for the BJT Q39 but it tested OK when I removed it. While I had it out I could see that there is a trace on the top side between collector and base so I guess it is serving as a diode or maybe a temp sensor. SMT diode D302 probably is a back to back device as you mentioned. The side with two leads are connected together so that would make sense. The other side connects to the base of Q39. So I think all that is OK.With a 1K resistor from the cathode of THT diode D5 to my DMM to ground I measured 1ma on the 600 and 1K ohm ranges, 0.1ma on the 60K range and 0.01ma on the 600K range. My old DMM has 0.01ma resolution so I couldn't check the higher ranges. I assume these readings are about right. If so I guess that is good news for the current source U501, U502 and U500.One bit of information I had not mentioned previously - immediately after this happened I tried to measure DC volts from my 12V 1A power supply and it went into current limit. That issue went away shortly thereafter so I assume one of the MOVs or spark gaps was conducting. Those devices has since tested open circuit as they should.

author: Martin72
author_rank: Super Contributor
timestamp: May 01, 2025, 01:41:06 pm
content: Why no pictures?

author: Kleinstein
author_rank: Super Contributor
timestamp: May 01, 2025, 03:12:13 pm
content: With the actual current source still producing a sensible current it would only be the protection part between the current source and the ouput. So not that many more parts to check.Getting a short and high current already from 12 V is indicating an unusual path for the short. The spak gap and MOVs should not conduct at 12 V. The ohms source would likely have a PTC in series and thus not much current.For the lower voltage input the path should be though the chain of 13 K resistors. So one could check the path (e.g. relays) to the input and maybe if the ADC gets some voltage from the 13 K resistor chain. The very front part with the relays, MOVs, spark gaps including the connection to the ohms source should be relatively easy to reverse enginier.

author: Max Gain
author_rank: Newbie
timestamp: May 02, 2025, 01:23:59 pm
content: Relay K1 is at least part of the problem. I have a new one on order. The current source connects to N.O. contact pin 8. Pin 4 is the N.C. contact which connects to the big blue capacitor and pin 4 is the relay common for that half of the relay. The N.C. contact appears to be welded closed because it does not open and the N.O contact never shows continuity to the common. That is not too surprising because the current that burned up the thin mid layer trace also traveled through that relay contact.Thank you for all the excellent help I received here. If it weren't for your information and advice this DMM would be in the recycle bin. It may still have a few issues after the relay is replaced but I think it will be a useable instrument. I will report back after I have attempted the repair. Best Regards.

author: Max Gain
author_rank: Newbie
timestamp: May 19, 2025, 01:59:20 am
content: Just closing the loop. I received the replacement relay for K1 and replaced it yesterday. It was a success! The DMM is now functional in all modes and appears to be operating as per normal. So the issues were: bad K1 relay and burned mid layer trace (as described in previous posts). Thanks again to everyone for the assistance.