Fluke 8050a Won't Read AC volts - Page 1
Forum section: electronics / repair

author: Who_Me?
author_rank: Newbie
timestamp: September 30, 2024, 03:39:17 pm
content: I have a Fluke 8050a that won't read AC volts. The screen will show "1" and "HV" and doesn't budge when the probes are connected to AC. It will read DC just fine as well as resistance and so forth. Any ideas? Thanks.This is an ebay purchase that I am trying to resurrect.

author: KungFuJosh
author_rank: Super Contributor
timestamp: September 30, 2024, 03:43:18 pm
content: Have you tried clicking through the ranges?

author: Who_Me?
author_rank: Newbie
timestamp: September 30, 2024, 04:01:39 pm
content: Quote from: KungFuJosh on September 30, 2024, 03:43:18 pmYes.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 30, 2024, 04:13:35 pm
content: These meters have a "Hybrid RMS converter", U32. When switched to AC, the converted DC signal is measured from the output of this converter. The input to the converter is ac-coupled - i.e. if there is a dc level being measured, regardless of the signal on the test leads, then it is probably coming from the RMS converter.Do you have another meter. There are test points provided in the 8050 - TP10 prior to the converter and TP11 after the convertor. With no signal present, there should be no dc level at TP11.

author: Who_Me?
author_rank: Newbie
timestamp: September 30, 2024, 04:21:46 pm
content: Quote from: Andy Watson on September 30, 2024, 04:13:35 pmWith no signal present, TP10 measures 7.78v dc, TP11 measures 12.29v dc.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 30, 2024, 04:31:16 pm
content: Quote from: Who_Me? on September 30, 2024, 04:21:46 pmTP10 is probably correct - can you verify that C15 is blocking that voltage from the input to the RMS converter.My money would be on a faulty RMS converter. Search this forum for "8050A RMS", much has been written about U32 !Edit: This thread may be useful:https://www.eevblog.com/forum/repair/fluke-8050a-true-rms-u32/msg623816/#msg623816

author: Who_Me?
author_rank: Newbie
timestamp: September 30, 2024, 04:50:57 pm
content: Quote from: Andy Watson on September 30, 2024, 04:31:16 pmHow do I do that? Measure voltage at input pin of RMS converter? Which pin is that?Please note that I'm a hobbiest here.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 30, 2024, 05:01:06 pm
content: Pin 3 of U32.Are you measuring with respect to the correct ground?

author: Who_Me?
author_rank: Newbie
timestamp: September 30, 2024, 05:23:43 pm
content: Quote from: Andy Watson on September 30, 2024, 05:01:06 pmI'm using one of the negative battery posts on the board as ground.Pin 3/U32 measures 7.7v dc.Oddly, I've noticed the voltage at TP11 isn't consistant. It seems to alternate between 7.7 and 12 volts all on it's own.

author: Andy Watson
author_rank: Super Contributor
timestamp: September 30, 2024, 05:58:36 pm
content: The battery voltage is offset from ground. The correct ground is the same as the "COM" input terminal.I've just measured the pins of a 8050A-U32 (which is being a bit intermittant - so treat these with a pinch of salt).All of the pins are within a few tens of mV of zero, with the exceeption of the power supply pins - 1 - 5.94V, pin 5 - 4.97V, pin 29 -5.2V, and pin 9 which is hovering around -1.0VPin1 5.942 03 04 05 -4.97V6 07 09 -1.010 014 015 17 mV16 11 mV19 020 021 11 mV22 023 -35 mV24 -67 mV25 -9 mV26 029 - 5.2 V

author: Who_Me?
author_rank: Newbie
timestamp: September 30, 2024, 06:43:17 pm
content: Using "COM" as ground:1 5.87v2 03 04 05 -4.95v6 07 09 -1.46v10 014 015 .7mv16 1.7mv19 020 0214.50v22 023 -2.0mv24 -3.9mv25 -1.8mv26 027 028 029 -5.13Pin #21 is also TP11...So.....something on the "Hybrid RMS" board is faulty? CA3046 or CA3140E as in the thread you referenced shows?

author: Who_Me?
author_rank: Newbie
timestamp: October 13, 2024, 09:51:20 pm
content: I removed the "Hybrid RMS" board (U32) and replaced CA3046 and CA3140E on it. The meter now reads AC voltage. Woohoo!