Electronics > Repair

Fluke ScopeMeter 199 no Channel A trace.

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Hi, I wonder if anyone can help with a problem I have on a Fluke ScopeMeter 199.  I think the 192 and 196 are basically the same.
The unit powers up fine but there is no trace. The alphanumeric information is displayed.  If I turn on channel B the trace comes on and seems to works as it should. The DVM part of the unit also works as normal.
There are two identical circuit sections for A channel and B channel so I was able to compare the two.
It looks as though both channels are fed an AC supply to a transformer and the output of this is then rectified to give +5,  +3  and -3 volt supplies and these seem to be the same on both channels. I checked these supplies to both main chips N1000 and N1200 and they looked the same.
Having tried numerous things I rearlised that the AC/DC relay for channel A was not switching when the button was pressed. You can hear it click when channel B is selected. This seemed something pretty basic so I follwed this path.
When pressing the AC/DC button with channel B selected I could see pulses on the S clock and S data  pins on IC N1200 and a pulse on pin 61 labeled ACDC and the relay clicked.  When doing the same on channel A I could see the same pulses on S clock and S data but no pulse on pin 61 so no relay operation.
My gut feeling is that the A channel ASIC ( application specific intergrated circuit )  N1000 has failed which means the unit is beyond repair but would welcome any comments or suggestions.
As always, many thanks in advance.

All control is carried out through the optocoupler isolation H1150 (HCPL4534 / 02 0) to the terminals 23-SCLK and 22-SDAT chips N1000 C-ASIC OQ0260 (Channel-A) and, accordingly, N1200 C-ASIC OQ0260 (Channel-B).
This data bus (I2C; SMB; UART) is indexed in the diagram as {CONTROL}.
Check if there are control pulses by comparing them on both channels. This is best done using a logic analyzer.
This part of the circuit has galvanic isolation from the rest of the circuit N2001 (S-ASIC) and D3500 (D-ASIC), so feel confident.


Thanks for the reply Vladimir.  As I said in my original post " with channel B selected I could see pulses on the S clock and S data  pins on IC N1200. When doing the same on channel A I could see the same pulses on S clock and S data "  These are on pins 22 and 23 of the chips. I only have a scope and not a logic analyzer but the pulses on both channels looked the same as far as I can tell.


OK Dave.
Now let's look at the presented schemes and collect a test scheme for stimulating both channels simultaneously (Pic 3).
This is a calibration chart for remote probes.
We check sequentially cascade after cascade in accordance with the scheme. What do we see?
Let's continue ...


Hi Valdimir, I'm not totally sure what you mean but I've been through the probe calibration procedure on both channels and this is what I got:
The test signal that is output from the DVM sockets is a 3 volt square wave at about 550 hzs before the calibration starts. On channel B when the cal is started this goes to a steady 3 volts, after a couple of seconds there are two pulses up to 3.8 volts then after 13 seconds the cal completes. Channel A is the same accept that after the two pulses it just remains at 3 volts and never completes the calibration. ( it times out after a few minuits ).
During the probe cal settup both SCLK and SDAT look the same for both channels. They sit at +3.6 volts and pulse down to 0. Also whenever the range buttons are pressed there are the same pulses on both channels. I think these pulses are in fact a series of very fast pulses in groups.
I tried to look at the HF and LF inputs and outputs to the two ASICs but no joy.
These checks have been done with a scope as I don't have a logic analyzer.
Any other thoughts?
Thanks again.


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