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Fluke ScopeMeter 199 no Channel A trace.

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DaveMX5mk1:
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.
Dave.

asis:
Hi,
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.

Vladimir.

DaveMX5mk1:
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.
Dave.

asis:
Hi,

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 ...

Vladimir.

DaveMX5mk1:
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.
Dave.

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