Fluke Scopemeter 199C - stange fault - ideas appreciated - Page 1
Forum section: electronics / testgear

author: matherp
author_rank: Contributor
timestamp: December 20, 2020, 05:35:01 pm
content: My Scopemaster 199C has a strange fault and I would appreciate any ideas what to look for.Channel A works perfectly, the fault is on channel B.If I apply a DC voltage to channel B then I get an accurate trace at the input voltage. I also get a clean sine wave up to about 1KHz. Above 1KHz I start to see a drop off in amplitude and at 10KHz the amplitude is at about 75% of the actual input. At 100KHz amplitude is down to 20% and at 500KHz there is basically no signal left. At 5MHz I see the attached (input is sine wave @ 0.1V PK @ 50ohm).Result is the same with AC or DC coupling. If I wind the output of the signal generator up to maximum (20V) then I can see the faintest hint of the signal and the frequency is correct.So, at low frequencies the ADC is giving correct readings and responds correctly to changes in sensitivity. It is being clocked correctly as far as I can see. The voltages on the isolated supply for the channel are the same as the other channel. I haven't got a circuit and there isn't one in the service manual.I've remade many of the solder joints although there was nothing obvious wrong.Help and ideas appreciated

author: bdunham7
author_rank: Super Contributor
timestamp: December 20, 2020, 07:51:09 pm
content: Is the attenuation the same in different ranges? For example, can you give it 5kHz and 500kHz at 100mV, 1V and 10V and see the same rolloff?If you have no service info, perhaps you can take it apart and post good hi-res photos of the front end, starting from the BNCs and up to the ADC if you can find it. That will let us see what to suggest. It is possible that there is a problem with some protection component on the input circuitry. Also, does the 199C have a bandwidth filter? If so, try activating it on Channel A and then compare the responses of A and B. If they are similar, perhaps something has happened to keep the BW limiter connected on B.

author: TurboTom
author_rank: Super Contributor
timestamp: December 20, 2020, 08:19:21 pm
content: First thing I'ld do is measuring the input resistance with a multimeter directly at the BNC with the scope in DC mode and compare the two inputs. Maybe there's some series-resistance that got disconnected/burnt out, forming a low-pass with adjacent capacitances.But especially older design analog front ends of oscilloscopes were designed as high bandwidth AC amplifiers with a "DC restoration channel" connected "in parallel". If this is the case with your Scopemeter, just the AC path may be defective. In that case, it may be difficult to spot the problem without a schematic.

author: asis
author_rank: Frequent Contributor
timestamp: December 22, 2020, 06:09:40 am
content: Hi,There are details here.https://www.eevblog.com/forum/repair/fluke-scopemeter-199-no-channel-a-trace/

author: smaultre
author_rank: Regular Contributor
timestamp: December 22, 2020, 05:41:26 pm
content: There are two different principles of transmitting the values of frequency for low and high ranges.Maybe high-freq part of ASIC may be damaged, or HF transmission part is bad.You can start the pre-calibration procedure(selftest), if it goes without errors, maybe it need calibration only.

author: matherp
author_rank: Contributor
timestamp: December 24, 2020, 07:05:04 pm
content: Thanks for the comments and particularly the circuit diagram. The fact that the low frequency and high frequency paths are separate now makes a lot of sense and gives more things to testOne more question: was the OQ0260 ASIC also used on the older 99B model? These seem to appear fairly cheap so could make a donor if it uses the same part

author: matherp
author_rank: Contributor
timestamp: December 24, 2020, 08:04:00 pm
content: FOUND IT.The circuit diagram gave the answer. The signal was good up until the transformer T1 and then instead of being balanced on the other side of the transformer was only showing on pin 6. The answer was a broken 4r64 resistor R1318. I moved one from the other channel and the fault moved with it. Just now need to buy a single new 4R64 0805 resistor and all will be well.Happy days!!!

author: asis
author_rank: Frequent Contributor
timestamp: December 25, 2020, 02:04:49 am
content: Hi,Happy for you.Merry Christmas!Good luck!