Fluke 8050a Won't Read AC volts

[Who_Me?]

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.

[KungFuJosh]

Have you tried clicking through the ranges?

[Who_Me?]

Have you tried clicking through the ranges?

Yes.

[Andy Watson]

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.

[Who_Me?]

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.

With no signal present, TP10 measures 7.78v dc,  TP11 measures 12.29v dc.

[Andy Watson]

With no signal present, TP10 measures 7.78v dc,  TP11 measures 12.29v dc.

TP10 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

[Who_Me?]

....can you verify that C15 is blocking that voltage from the input to the RMS converter.

How do I do that?  Measure voltage at input pin of RMS converter?  Which pin is that?

Please note that I'm a hobbiest here.

[Andy Watson]

Pin 3 of U32.

Are you measuring with respect to the correct ground?

[Who_Me?]

Pin 3 of U32.

Are you measuring with respect to the correct ground?

I'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.

[Andy Watson]

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 -1.0V

Pin
1  5.94
2  0
3  0
4  0
5  -4.97V
6  0
7  0

9  -1.0
10 0

14  0
15  17 mV
16  11 mV

19  0
20  0
21  11 mV
22  0
23  -35 mV
24  -67 mV
25  -9 mV
26   0
29 - 5.2 V

[Who_Me?]

Using "COM" as ground:

1  5.87v
2  0
3  0
4  0
5  -4.95v
6  0
7  0

9  -1.46v
10 0

14 0
15 .7mv
16 1.7mv

19 0
20 0
21 4.50v
22 0
23 -2.0mv
24 -3.9mv
25 -1.8mv
26 0
27 0
28 0
29 -5.13

Pin #21 is also TP11...
So.....something on the "Hybrid RMS" board is faulty?  CA3046 or CA3140E as in the thread you referenced shows?

[Who_Me?]

I removed the "Hybrid RMS" board (U32) and replaced CA3046 and CA3140E on it.  The meter now reads AC voltage.  Woohoo!