Keithley 182 Won't Turn On - Page 1
Forum section: electronics / repair

author: gamalot
author_rank: Super Contributor
timestamp: July 30, 2025, 01:17:00 pm
content: I bought a faulty Keithley 182, which the seller described as having no display. I connected it to my computer's GPIB port, but the device wasn't detected.I've repaired a 182 with a damaged high-voltage power supply module of the VFD, but this one seems more complex.--- Update ---I opened the case and took some measurements. The +8V power supply on the digital board was working fine, but the +5V output was only 0.8V. The regulator, an LM2940 (U514), was very hot.No shorts were detected at its output, and thermal imaging didn't show any other components getting hot. I highly suspect the LM2940 is damaged.The VFD's high-voltage module also had no output, but I decided to try repairing the +5V power supply first.

author: gamalot
author_rank: Super Contributor
timestamp: July 30, 2025, 02:36:31 pm
content: I removed the LM2940 from the board and used a benchtop power supply to provide +5V to the digital board, and I realized my mistake!Although my multimeter couldn't detect a short circuit, the power supply's output was pulling down to 1.5V, and the current was reaching my preset upper limit (2A). I took out the thermal camera to check again, and this time I could clearly see that the VFD power supply module was heating up. Its large size dissipates heat quickly, so it wasn't as hot as expected, only slightly warm to the touch.I couldn't understand why it wasn't connected directly to the +8V supply instead of the LM2940's output.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 30, 2025, 03:46:23 pm
content: A 5 V input is more common than 8 V for a ready made DCDC converter. The VFD power module may be (may be EOL however) an off the shelf product.

author: voltsandjolts
author_rank: Unknown
timestamp: July 30, 2025, 03:53:20 pm
content: E705-E905VFDC-AC/DC5VDC --> 5VAC / 60VDC

author: gamalot
author_rank: Super Contributor
timestamp: July 30, 2025, 03:56:29 pm
content: I think you're right, Keithley just used an off-the-shelf module.The last time I repaired a 182 VFD power module, it was only partially damaged. The filament supply was working properly, but the anode high voltage output was missing. This time, unfortunately, it not only doesn't work, but it also pulls down the +5V rail.I'm now considering whether to make a replacement VFD power module or replace the VFD with an OLED/TFT display.

author: gamalot
author_rank: Super Contributor
timestamp: July 31, 2025, 08:40:35 am
content: I removed the VFD power supply module from the board and soldered the LM2940 back in, and now it works fine without the display.

author: gamalot
author_rank: Super Contributor
timestamp: August 20, 2025, 05:08:24 pm
content: I built an AC power supply for the filament (about 5Vrms) and got the anode voltage from an SMU (55V). Now the VFD is lighting up.

author: gamalot
author_rank: Super Contributor
timestamp: September 11, 2025, 04:07:37 am
content: The new PCB arrived yestoday.A quick test showed that both the anode high-voltage supply and the filament supply were working properly.My only complaint was that I overlooked the fact that the +8V rail was actually 9.5V, causing about 2.7V voltage drop across the LDO (U2), and heat it up to nearly 60°C.I connected a 1N4007 diode in series to share the about 0.8V voltage drop and cool the LDO down a bit.