Lab microwave PSU blows fuses instantly, what to check? - Page 1
Forum section: electronics / repair

author: Hend
author_rank: Newbie
timestamp: July 16, 2025, 07:44:34 pm
content: Hi all,I’m troubleshooting a lab microwave setup and could really use your help.The system:Magnetron: Muegge MH1000S-212BB (2450 MHz, water-cooled)https://drive.google.com/file/d/1v1uziLP9O0MmE1ckK4w_dOTZykzcSHLS/view?usp=sharingMW Power Supply: Muegge MX1000C-112KLhttps://drive.google.com/file/d/1WIsPw9zhrVfn34AVbeSQuidKWtZLDviJ/view?usp=sharingPIC24-based interlock (customized to communicate with the system using PLC communication)It ran fine for ~15 years, powered either from direct mains (20A 125/250V sector) or a 230 V transformer.The problem:Suddenly stopped working.Found the old magnetron had the RF choke coils broken, so replaced it with a new magnetron.Still have the same issue.When we hit “Microwave ON”, the fuses blow instantly.Tried on both inputs:On Sector: first L fuse blew, replaced, then N blew.On transformer: both fuses blew together.I have 2 thoughts & questions:I noticed that the PIC24 interlock board has VCAP floating. Datasheet says it needs a cap to GND. Since it worked for years like that, is it critical to add now? Could it be related?I'm planning to test the PSU with the magnetron disconnected, but I can’t easily remove the HV cable, it looks soldered inside (photo attached below).Is it safe to remove the yellow insulation to detach the red HV cable, and run the PSU without the magnetron connected?Any other recommended checks or diagnostics before digging deeper inside the PSU (to look for shorted IGBTs or HV diodes)?

author: Harry_22
author_rank: Frequent Contributor
timestamp: July 16, 2025, 08:55:44 pm
content: Hi!Try to disconnect X1 connector from magnetron head (X5 at PS side).As I can see mains passes through PS to magnetron. It is necessary disconnect it and check the integrity of the fuses. Thus we simulate a break in filament circuit.Another possible fault point is HV converter.It is inside PS. Correct?Don't disconnect HV cable especially ground bolt.PSAnd of cause the good guts photos will help.

author: CaptDon
author_rank: Super Contributor
timestamp: July 17, 2025, 01:36:57 pm
content: It looks like the high voltage for the mag is generated by an inverter arrangement. I would suspect blown MOSFETS or BJT's. I see 8 with clip on heatsinks. One or more may be dead shorted.

author: Ian.M
author_rank: Super Contributor
timestamp: July 17, 2025, 02:02:17 pm
content: Domestic and commercial microwaves normally have door interlock switches that disconnect the supply to the magnetron and associated HV PSU when the door is opened. It is usual for the interlock switch to short its output when activated (or for a second interloc switch to do so), as a failsafe for its series contacts failing to open. When the interlock switches are in good condition and their mating part(s) on the door properly adjusted, the series contacts open before the shorting contacts close and visa versa on door closing. A common fault if the door interlock is worn or out of adjustment is for its shorting contacts to close before the series contacts open, crowbarring the mains supply and blowing the fuses.If your lab microwave has such interlock switches it should be the first thing to check.See more about interlocks and microwave oven repair here:https://www.repairfaq.org/sam/micfaq.htm

author: Harry_22
author_rank: Frequent Contributor
timestamp: July 17, 2025, 02:47:21 pm
content: Yes, it looks like 4x voltage multiplier supplied by inverter.I can't understand the mission of second transformer with yellow wires. Looks like serial connection of both transformers.X-Ray also have twin remote transformers to supply high voltage module positive and negative arms.