Instek GOS-6112 - No signal on either channel - Page 1
Forum section: electronics / repair

author: JamieArbor
author_rank: Contributor
timestamp: December 05, 2018, 10:34:20 pm
content: I just bought a GW Instek GOS-6112 at a reduced price on the basis that it had only been tested by powering on, which showed a flat trace.Unfortunately it only shows a flat trace on both channels when the probes are connected to the test signal, using the recommended settings from the user manual (which I downloaded). I have tried adjusting all settings through all ranges (although almost certainly not all combinations!).I thought it might be that the test signal was not working, but my digital scope can see the wave form from the 6112 test connection.The BNC socket for channel 1 seemed loose, so I took it apart and found the internal connection to channel 1 had broken, probably due to the movement. I tightened it up, resoldered the connection and checked the resistance between the BNC and the board, which now matches channel two (approx 10 ohms).However the test signal still just shows a flat line. I feel like I'm either missing something obvious or perhaps there is some particular area I should look in when the controls all work but the line is always flat on both channels? Since my minor repair I have not reassembled the scope and am testing it with the input board resting on the upside down scope.I am a newbie in electronic repair, though I hope to learn. I have previously used a scope to check for interference on comms lines, as I am a software developer "by trade". I liked the look of a simple analogue scope for learning more about repair work - though I wasn't planning to start with the scope itself!The scope seems in great condition inside and fair condition outside ... apart from the minor issue of not working!Any suggestions of where I might start or if I may have missed something simple would be appreciated - thanks!

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 06, 2018, 12:21:45 am
content: Hi!The GOS–6112 is a very sophisticated press–button microprocessor controlled oscilloscope, however, it's great redeeming feature is it has a conventional linear psu fed from a mains transformer!Study these diagrams first:–https://elektrotanya.com/gw_instek_gos_6103_6112_sch.pdf/download.html. . .then begin with a check of ALL the low to medium voltage power supply rails (NOT the high voltage CRT supplies as you have a trace, so the CRT's E.H.T. (extra high–tension) can be ruled out) – please note that two of them (+10.5V and –5V) are used specifically for a number of '4094 CMOS chips that control large parts of the ch1/ch2 switching functions, and a failure here will leave large chunks inoperative!The '4094 chips themselves can fail in odd ways causing loss of, or otherwise tempremental operation of their switching functions, but careful voltage–tests with a DMM on their output pins whilst you operate the front–panel buttons should help locate a duffy one!Make sure the H.T. + 55V and +110V rails are correct – either one missing will cause the common Vertical Output Amplifier to be inoperative.Note:– Quite a few component values are only marked with an asterisk – they are either different between two models, in which case there's a lookup table, or select–on–test, in which case you should only fit the value you had before!It might be worth printing out the diagrams, then looking at the asterisked values and writing their values on the drawings if you feel confident enough with colour–codes, etc!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 06, 2018, 08:08:56 am
content: Hi Chris,Thank you for the advice, it's much appreciated.I have printed out the diagrams, as you suggested - this is in at the deep end for me!One potential issue is that I am worried about trying to remove the board with the front panel controls, should I need to test it. The knobs are so tight that I suspect I might break something if I pull hard enough to remove them!Thanks again for your help,Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 06, 2018, 12:00:51 pm
content: Hi!Don't worry about trying to remove the front-panel at this stage as it may not be necessary!Rule out the power-supply voltages first, use the diagrams to trace the various ch1/ch2 attenuator and control lines back to the main or vertical amp ch1/ch2 PCBs then check they're changing state when you operate various buttons, etc!If you find from voltage tests, etc., there DOES appear to be a front-panel fault, and the knobs seem to be stuck, check they're not collet-secured - there will be a removable cap in the knob if there is - otherwise, wrap some thin string round the back of the knob, around it's shaft, and try pulling gently but steadily!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 06, 2018, 12:20:36 pm
content: Hi Chris,Thanks again.There does not appear to be a removable cap on any of the knobs. Though as you suggest, I won't cross that bridge until I come to it. Will try the string if I do.I have checked the power out from J1012 & J1014 and they all seem within bounds, but I haven't traced the path through the other boards yet.One thing I have noticed, which might mean something: Although I don't normally get any vertical movement on the trace, such as by touching or tapping the inputs (which I would expect to show interference) . Yet when I turn the "volts/div" knob sometimes the range change causes a fleeting glimpse of a wave trace. At least this shows that it is capable of drawing a waveform, but is it significant, or does it rule out some possible faults?BTW: The unit makes two short beeps when I turn it on, which is not mentioned in the manual. I first assumed this was part of the power on self test, but as I don't have a control unit to compare it to I now wonder if perhaps it means something.Best regards,Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 06, 2018, 02:03:21 pm
content: Hi!Here's a couple of quickie checks you can try:–1) Locate J1101 on the Y Amp PCB OS08P05F, and connect 1.5 or 3V battery in series with a 1K resistor –(you can safely remove plug J1101 for this as there are bias–return components in the output stage!) between Pins 1 and Pins 3 of J1101 – if you apply positive to J1101/3 and neg. to J1101/1 the trace should jump a small amount upwards and vice–versa for reverse battery polarity!If you can't see it clearly you might notice the jump better with a 9V PP3/1604–etc., battery!No movement of the trace indicates a fault further forward from J1101, whilst a jump indicates the signal may be getting lost in the common switching prior to J1101!2) Put your ear near to the input attenuator relays (carefully!!!!) and operate the ch1 and ch2 volt/div controls rapidly up and down – you should hear the relays clatter away in sympathy – if not, you have a control–switching or relay–supply fault, and I'll explain how to go about this in due course if necessary!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 07, 2018, 11:55:19 am
content: Hi Chris,Bit of a delay this time - both work and family commitments trying to spoil my fun!OK, I can hear the relays on both channels and the 1.5v battery makes the display jump!Is that a good sign?I feel like I'm waiting for test results from the doctor ...Thanks (again),Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 07, 2018, 12:53:52 pm
content: Hi!Afternoon Jamie!The jump of the trace when you connect a battery to J1101 proves the Y–final amplifier, common to both ch1 and ch 2 is capable of functioning – it's direct–coupled throughout – and also the relay supply feeding the relays is OK!That is a good sign as almost certainly the WinBond W78–etc., control CPU is perfectly OK and any problems will be down to ordinary bits that can be replaced like–for–like or substituted!We're looking at a problem in the common switching circuits from ch1/ch2 signal preamp outputs to the connector plug J1101 – I'll print a copy of the diagrams and get back to you on this, as this circuitry is intricate and not obvious how it works!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 07, 2018, 01:25:17 pm
content: Hi Chris,Phew. You are a star!After 30 years of helping other people (professionally and personally) with computer problems I'm starting to appreciate what it's like being on the other end of technical support!I welcome any further guidance on where to look. I must say it was very exciting to see the trace jump when I applied the battery!All the best,Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 07, 2018, 03:59:27 pm
content: Hi!Like the "Terminator!!!!", l'll be back presently after I've done a little spot of homework on the diagrams!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 07, 2018, 04:13:27 pm
content: ... OK, thanks, ... but if the next step involves Uzis or explosives I might have to take a rain check!

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 10, 2018, 01:20:49 am
content: Evening Jamie!It doesn't involve Uzis, explosives or doing nasty gruesome things with scalpels but there is a few spots of careful checking & measuring involved!Unfortunately I've been ranted and raved at all weekend and not had a proper chance to study the circuits but rest assured I will & get back to you tomorrow!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 10, 2018, 09:23:39 am
content: Hi Chris,The universe is a cruel place indeed when a helpful chap such as yourself gets ranted and raved at.I'm relieved to hear that this won't be like one of Arnie's more typical missions! Much happier with the careful checking and measuring.I was planning on showing some initiative, by trying to get to grips with what comes before and after the place I inserted the battery current on the schematic. But as it turned out my weekend was evenly divided between the needs of my elderly mother and those of my young children, so no initiative has so far been demonstrated.Best regards,Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 10, 2018, 10:53:30 am
content: Hi!Going back to your oscilloscope, the next step is to a make up a simple variable voltage–source, a 1K preset across a 9V battery will do for this!Connect your meter to the slider of the pot., and adjust to give +50–60mV dc between the slider and battery negative, then examine page 3 of your diagrams, identify the junction of R178/R179/Q117 base in ch1, R278/R279/Q217 in ch2, then connect your 50mV to this point in each channel, battery negative to scope 0V chasis.If the D.C. amplifiers and coupling networks forward of Q117/Q217 bases are OK, the trace should move approximately five divisions upwards, or approx five divisions downwards if you reverse the battery polarity!The technique I am applying is called the "half–split" technique, where a test is made about half-way along each section to narrow down the fault area!No movement suggests a fault in the common switching, page 4, whilst a good movement approx 5 div suggests possibly the variable–gain FETs Q115/Q215 or the switching ICs U102/U202 are blocked due to another problem!Try the above test and report back, then I'll know which direction to advise next!Chris Willams

author: JamieArbor
author_rank: Contributor
timestamp: December 10, 2018, 01:59:42 pm
content: Thanks again Chris!I have ordered a pack of various potentiometers, as I don't seem to have a 1K.In the meantime, would be safe to use my bench power supply? It can output either 52mV or 64mV according to my meter.It's one of these:https://www.amazon.co.uk/gp/product/B01E7OGHJM/ref=oh_aui_detailpage_o06_s00?ie=UTF8&psc=1All the best,Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 10, 2018, 11:45:35 pm
content: Hi!A bench power supply will do for the tests I describe no problem, but still add a 1k or similar resistor in the non–earthed output lead – this is a safety precaution simply to prevent unknown, and possibly relatively large, currents flowing into semiconductor junctions, etc., and giving a misleading result!It was for this reason I suggested a small 9V battery as the internal resistance of this will help prevent unwanted currents wandering about anywhere – the points I suggest to try connect a test–voltage are high–resistance from a d.c. point of view, but there's no reason to tempt fate!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 11, 2018, 01:05:57 pm
content: Hi Chris,Thanks for the clarification.Using my bench supply through a 1k resistor, showing 52.3mV on my meter I see the same result on both the Q117 and Q217 junction points. Hopefully I am applying the voltage to the base of the correct leg!What I see is a rise of two of the small divisions along the centre line of the scope, with what appears to be increased flickering.Does that sound believable, or might I be making a mistake?By the way, your "half-split" technique sounds like it might be what we call a "binary chop" in programming.Thanks,Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 11, 2018, 04:06:03 pm
content: Hi!According to the scale–factor given on page 4 of the circuit–diagram, the gain at that point is about 11mV/div, so two divisions at 52mV input is NOT right!We need to begin on the common items in both channels working back from J1101 towards Q117/Q217 base, so begin by locating the delay line, which will be a large hank of coiled twin or thin co–axial cable secured to the chassis–metalwork – it is needed because the rise–time of the input–amplifiers is less than the rise–time of the amplifier/output stages after J1101, and it ensures there are no H.F. peaks or troughs in the overall scope's response, which would badly distort waveforms approaching the bandwidth limit of the instrument!Test the conductors of the delay line with a continuity meter, and also those of the v.h.f. chokes in the lattice network on a small pcb after this (see page 4) – then let me know what you find!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 11, 2018, 06:32:58 pm
content: Hi Chris,The three wires on the delay line beeped as continuity on my meter (around 6 ohms).I have attached a photo of the board with the chokes on it, as mine is OS08P05F-6 and the schematic is for OS08P05C-6.I was expecting to see a visible component at L305 and L306? The apparently empty pads each show a resistance of 73.8 ohms on my meter.Is this what you would expect?Thanks,Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 11, 2018, 06:54:51 pm
content: Hi!The suffixes of C–6 and F–6 between the diagram and PCB aren't of significance, the difference is simply to help GW Instek's engineer's distinguish the files in their design documentation the number "OS08P05" identifies the diagram and PCB!What does matter are the empty pads on L305 and L306 – me thinks there should be zero–ohm links there in L305 & L306 positions there – inductors are fitted in the "6103" variant to suit it's lower bandwidth!Fit 2 x 0805–size zero–ohm links to the empty pads and try your 'scope again, and I wouldn't be surprised if you now find the signal gets through ok now!(I would advise proper smd links rather than just wire, as the stray capacitances to earth are most important at the potential max frequency the 'scope is designed to handle!)The 73 ohms you're reading comes from the termination resistors after J1101 and the various components in the vertical amp. delay line driver!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 11, 2018, 08:42:42 pm
content: Hi Chris,Well I don't have any surface mount 0 ohm resistors, so I have tried soldering a wire link across each gap to test this.The result is that both channels lock together in a single trace that sits just below the centre horizontal line and the position controls for both channels stop functioning.It looks to me is if some components have been deliberately removed from these locations - see attached photos. The one with the wire is L306 just before I soldered it.Thanks as always,Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 12, 2018, 03:52:18 am
content: Hi!No problem, leave the pads blank as L305/L306 obviously need to be unpopulated in the 6112!Do you have one of those small Chinese Component Testers with the green ZIF socket and the colour TFT display, LCR–TC1 type?I would advise getting one if you've not got one, also one of those digital IC testers about the same price range, but for now I'll have another read of the diagrams and get back later!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 12, 2018, 08:31:15 am
content: Hi Chris,OK, links now removed and behaviour as before!I didn't have either of the devices you mentioned, but I have now ordered one of these :-https://www.amazon.co.uk/dp/B075F7MXCR/ref=pe_3187911_185740111_TE_item... and one of these ...https://www.ebay.co.uk/itm/192729493379I could have gone cheaper, but with much longer delivery.Thanks,Jamie

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 14, 2018, 11:51:12 pm
content: Hi!Those are the devices I use every day in my Industrial Electronics Repair employment and they are simple to use and reliable – I haven't had any "false–positive" indications where they've passed an obviously defective device!DO be careful to make sure ALL capacitors are discharged (electrolytic types!) before you attempt to test them in the Amazon tester, it has no input voltage protection (it would be unable to operate with protective circuits!) – a charged capacitor will destroy the microcontroller's inputs!A Google Search for "Karl Heinz AVR Transistor Tester" and this Forum will get you a 114 page manual that describes these operation and use of these devices in intimate detail down to full schematics, by the way!I'll be back later with further 'scope fault–finding hints in due course once I've had another read of the diagrams – the fault is known to be between the bases of Q117/Q217 and the delay line, so that's not a great deal left!Chris Williams

author: JamieArbor
author_rank: Contributor
timestamp: December 15, 2018, 06:55:27 pm
content: Hi Chris,Well, it’s working! The only problem is that I don’t know if anything I’ve done is responsible or not. I have been looking around the areas you suggested with a jewellers loupe, trying to find any visible damage to any component or dodgy looking solder joint.I found a couple of solder joints that looked a little suspicious and tried moving the components to see if they seemed firm and I found a couple of small foreign bodies on the board, which may or may not have been conductive, which I removed.When I turned on the scope I thought I could see some slight movement on one channel. By this point the controls were all over the place. I adjusted everything to the suggested test settings and I can now see the standard test signal.So I don’t know for certain if there was a problem (other than the broken connection which I fixed), or I have made some rookie mistake. Although I was pretty confident that I have tried the exact same settings. Perhaps one of those joints really was dodgy, in which case the fault may return. Or perhaps I have removed something that was conductive, in which case I am safe. It may even have been that one of the leads I removed during testing had a bad connection.Whatever the case I can’t thank you enough for your patience and guidance. You have increased my enthusiasm to learn to better understand schematic diagrams and it has been fascinating to relate the diagram to the actual PCB.All the best,Jamie

author: JamieArbor
author_rank: Contributor
timestamp: December 15, 2018, 06:57:16 pm
content: PS: Just seen the "say thanks" button, so have thanked you that way as well as in writing!

author: Chris56000
author_rank: Frequent Contributor
timestamp: December 17, 2018, 09:32:09 pm
content: Hi!Evening Jamie!Pleased you've got your '6112 up and running!Make sure all those little white multi–plugs connecting the cable–forms to the PCBs are firmly pushed home, have a look at the front–panel pcb to male sure no plugs, connectors, switches, buttons,, etc., etc., have got dry or yakky joints, going over anything iffy (carefully!) with your iron prior to wrapping the 'scope up and putting into service!The two testers you've got will serve you well for development and repair purposes by the way!Please feel free to come back to the Forum any time, that's what we're all here for!Good luck!Chris Williams

author: Shopy
author_rank: Contributor
timestamp: May 07, 2020, 03:31:51 pm
content: HiI like to ask for your help.I have this osciloscope gos 6112And chanel 2 volt div is not working chanel 1 is okWhen i turn the knod i do not hear the relay only on chanel 2What can be the problemThanks

author: Chris56000
author_rank: Frequent Contributor
timestamp: May 08, 2020, 12:31:46 pm
content: Hi!I'll have to download and print the drawings again but I'll be back shortly with fault–finding advice!Chris Williams

author: Shopy
author_rank: Contributor
timestamp: May 08, 2020, 03:57:39 pm
content: Thank you chrisWaiting for you kind advice

author: Shopy
author_rank: Contributor
timestamp: May 08, 2020, 04:02:26 pm
content: Hi chris i understand that you dl the drawings .is it posible to get this link so i can dl this drawings toThanks

author: Chris56000
author_rank: Frequent Contributor
timestamp: May 08, 2020, 10:57:43 pm
content: Hi!I posted a link to the circuit diagram on the second or third post of the first page ot this thread – you can get them free from Elektrotanya!Chris Williams

author: Shopy
author_rank: Contributor
timestamp: May 09, 2020, 09:50:36 am
content: hi Christhank you for the link but there is only the inputs drawings of the scope is there more somewhere like the other boards of this scope???can you please help with the problem I have with channel 2?thanks

author: Shopy
author_rank: Contributor
timestamp: May 09, 2020, 01:46:40 pm
content: hi chrisMeanwhile, i did some voltage tests on the shift registersI found that there is a big different betwine them and I attached the resultsI do not know why is that buy it shows wיy one channel is working and the other one is notI try to contact GW INSTEK and ask about this problem but I did not get much help from them.but they say that the problem is related to the rotary switch sb09 can be faulty .sb09 encoder ic or ch2 attenuation selector ic 406what you think about that?

author: Chris56000
author_rank: Frequent Contributor
timestamp: May 09, 2020, 02:04:59 pm
content: Hi!I can assure you all the manual is there, honestly – you should have 21 pages in total!Did you wait for Elektrotanya's 30 second timeout before the download link appears?'4094 CMOS series/parallel shift registers are notorious for causing obscure faults, and in this case, where several are used together with U406, I strongly suggest you bin the lot shown on the appropriate diagram sheet and replace them all with new, also check all round that panel for dry–joints, plugs not pushed fully home into connectors, etc!The fact you're getting Ch 1 operational with relay clattering when you turn the ch1 gain control suggests the common main relay supply is present and correct!The Ch 2 Volts/Division Up/Down button SB09 operates lines S5, S6 and S7 which are multiplexed by UB02 ('74HC138/'74LS138) on the front panel PCB – make sure the inputs of UB02 are being operated by SB09, and if you're doubtful, replace SB09 and UB02 together as a pair!You should be able to obtain replacement tact switches similar to what GW Instek used from fleabay!Chris Williams

author: Shopy
author_rank: Contributor
timestamp: May 10, 2020, 02:34:42 pm
content: hi Christhanks for the advicei did some checking on the front panel and the mainboard. tell you the truth im confused.the input from SB09 ch2 and SB08ch1 are entering to UB02 decoder so if ch1 is working and ch2 not i assume this chip works or is it possible only half of it works? I'm not sure. then the output of UB02 goes to UB03 microcontroller. maybe this one i defective on ch2 input?i swap betwine sb09 and sb08 to eliminate this as the problem .so the problem remains .it is not the buttons defective.i really do not what to trace? i try to trace the signals with a logic probe i have and did not find much.what i notice is that when i try to put a signal in ch2 ,i can see the signal but it is like sucked in a constant voltage and is not changing.do you have any suggestions for me what to look for?thanks

author: Shopy
author_rank: Contributor
timestamp: May 13, 2020, 02:27:38 pm
content: hi chriscan you please help with this?when I connect the probe to CAL point I do not get 2v p/p I get about 1.67v on screenI like to change the vertical to fix it.what will be the right pot number to do so?see attachedthanks