Fluke 123 / 124 / 125 Scopemeter T-ASIC needed - Page 1
Forum section: electronics / repair

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 04, 2021, 02:52:32 pm
content: I'm working on a Fluke 123 that powers on with a single beep, backlight on, black screen, responds partly to RS-232.The PWM section of T-ASIC (OQ0257HP) appears whacked.Does anyone have one to sell or know of a source that sells OQ0257HP at a reasonable price?Thanks!

author: bob91343
author_rank: Super Contributor
timestamp: January 04, 2021, 06:09:52 pm
content: Have you contacted Fluke?

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 04, 2021, 06:49:30 pm
content: No, I guess I should give it a try...I've never had much luck when asking manufacturers for help.

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 05, 2021, 12:29:50 pm
content: I overlooked the SM a bit:Problematic T-ASIC isn't the PWM but the voltage references, some of which are needed for the PWM section of D-ASIC, one of the PWM outputs in turn is needed for LCD contrast control.All the voltage reference OP-AMP outputs on T-ASIC seem bad. (Attached screenshots)They seem to be floating around with very low output current, I'm considering overlapping them with an external quad OP-AMP as an attempt to fix. This may work if the analog MUX is still good.Another thought:I have no experience with this equipment hopefully there are more knowledgable people out here.The person I bought this 123 Scopemeter off had taken it for a repair estimate before selling it as is. The repair company returned it saying: "Digital PCB defect. Corrupt firmware. Parts unavailable."Of course this could be a load of B.S. but still brings this question:The Scopemeter has SN: 9444 001 23411, DM7840039.Battery cover says "Ni-Cd".PCB revision is 4022 245 0443.8 with Motorola "GARFLD" TQFP D-ASIC.When asking for ID over RS232 it responds as having FW v01.08, would this be the right firmware version for the hardware?Any chance that having an incorrect FW could cause the T-ASIC to malfunction, I'm thinking of something like power sequencing for Vref in T-ASIC that could be controlled over I²C...

author: shakalnokturn
author_rank: Super Contributor
timestamp: January 14, 2021, 03:06:00 am
content: Small update...Emailed Fluke waiting for an answer.Emailed "ScopemeterRepair" very responsive and has the part but out of my price range. (Unless I can avoid customs maybe...)Inquired on several "hard to find" I.C. sources, got mostly no replies and one having no availability.Piggybacked a quad Op-Amp on T-ASIC, this brought back the display and scope traces, it could have done the job if the missing reference voltages had been the only problem.Unfortunately the relay control outputs are also dead, I assume they share a common supply with the Vref Op-Amps which has fused in the IC.Due to the lack of relay control the Scopemeter is only usable in scope mode with AC coupling.Sniffing the I²C bus for the relay control commands and taking care of relays with an added microcontroller could be a workaround but at this point it's getting a little discouraging in terms of time investment.