Fluke 199C  intermittent noise  on both channels - Page 1
Forum section: electronics / repair

author: hafrse
author_rank: Regular Contributor
timestamp: August 01, 2025, 08:04:51 pm
content: Hello,I have discovered intermittent noise on channel A and B (more on B ) , not sure where it comes from, noise is visible.I tested to ground the inputs, change DC/AC mode, etc... same problem.Higher noise on channel B ( Ch B is nearer the power supply management and flyback transformer)Could it be dried electrolytes in the internal power supply/inverter ? thanks for any help!Video 100mv setting on each channel:https://www.youtube.com/shorts/Z8wuLUo_1poattached picture shows 1KV setting on each channel

author: asis
author_rank: Frequent Contributor
timestamp: August 02, 2025, 02:18:44 pm
content: Hi,You can try to reduce the display contrast, and then look at the noise level of channel B.Or, use any ferrite choke, connecting it to channel A (increasing its sensitivity) and bring it closer to the CCFL pulse HV transformer.Also, at the M4210 test point, the drain of the MOSFET V4200 (MTSF2P03HD) can be indirectly assessed for the inverter operation.It is possible that the CCFL lamp itself is starting to degrade or C4214.-https://www.eevblog.com/forum/repair/fluke-199225c-repair-blown-part-id/msg3957929/#msg3957929https://www.eevblog.com/forum/repair/fluke-scopemeter-199-repeair/

author: hafrse
author_rank: Regular Contributor
timestamp: August 02, 2025, 09:42:47 pm
content: Thanks for the information, I tried to lower the contrast but still the same, I am thinking to shutoff the HV converter and apply light on the display and see if the trace noise disappears to rule out the converter circuit, is it ok to unsolder L4201 to shutoff the converter? thanks

author: asis
author_rank: Frequent Contributor
timestamp: August 03, 2025, 03:28:51 am
content: Hi,Don't rush to do this.The thing is that there is a current control circuit via CCFL V4204 (BC858C).There is an internal feedback loop on PWM N4200 (UC3872) via R4201 (100kOm) and C4200 (100nF).Also, via R4206 (10kOm) this feedback loop (TP4212) is connected to the D4300 multiplexer (pin 2), which analyzes all anomalies.It is possible that the scope will go into an error, which will be indicated.You can simply disconnect the CCFL from the X4200 connector (it is next to the flexible KBD cables) so as not to unsolder anything.But it is better to do these manipulations with the scope disconnected.-Read this post, perhaps it will be clearer.https://www.eevblog.com/forum/repair/fluke-199-scope-noise-at-faster-time-bases-and-meter-noise-(with-video!)/--There is another way to localize the fault:1. charge the battery.2. select the "Recorder" mode.3. set the recording mode for channels A & B with sufficient sensitivity.4. start recording.5. after completing all manipulations, start REPLAY.

author: hafrse
author_rank: Regular Contributor
timestamp: August 03, 2025, 07:54:24 am
content: @asis, Great!!! you are completely correct, the noise went away after disconnecting the X4200 ! now I will replace C4214

author: hafrse
author_rank: Regular Contributor
timestamp: August 03, 2025, 08:36:11 am
content: @asis replaced tantalum capacitor C4216, No noise!!! and the backlight is more brighter!!!! I want to thank you so much for your help!!!!

author: asis
author_rank: Frequent Contributor
timestamp: August 03, 2025, 08:54:55 am
content: Ok!I recommend you to check the network power adapter.It is quite heavy and if it falls on the floor, there is a chance that something will fall off.-Good luck!