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Thread: DSE Q1803 10Mhz Oscilloscope Repair

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    Default DSE Q1803 10Mhz Oscilloscope Repair

    Hi all,

    I have a Dick Smith Q1803 Oscilloscope which has served me well for the last 5 or so years as a basic CRO. I have had the unit in storage for about 3 years and decided to fire it up a few days ago. I have run into some unusual behaviour and hope someone can help point me in the right direction!

    Notes;

    Screen takes 10+ minutes to appear when switched on from cold. I don't think this is any slower than it had always been.
    When the trace appears, vertically it appears OK and has full deflection to the top and bottom.
    Horizontally the trace appears about mid screen but only on the right half of the screen. It is as if the unit is unable to achieve full horizontal deflection.
    The low voltage rails appear good (+12v and -12v OK), from what I can see of the trace it appears the trigger and front panel controls are working as expected.
    I have checked the electrolytic caps and they are in spec.
    I have re soldered a few suspicious looking dry joints to no avail.

    Now I know this is a $99 special and I have had a good few years of use, but if there is any chance I can repair it with some help / advice - it's worth a go.

    Being a cheapie a service manual appears non existent, any help would be greatly appreciated.

    Firewire.



Look Here ->
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    Wink .....dse 1803 cro.



    This may be of some use. Will keep looking.

    DSE Q1803 $128 CRO
    44 posts by 19 authors


    Phil Allison


    28/05/2006



    Hi to all,Picked up one of these 10MHz, single beam CROs from my local DSE store last
    Tuesday - there were none on display and the staff seemed unaware of its
    existence. The unit comes in a carry box, well packed with IEC mains lead,
    10:1 probe and small handbook written by DSE staff.
    Using a bench sine/ square generator and frequency counter when back at
    home, I found my unit met published accuracy specs - time base speeds
    being particularly accurate.
    The *green* trace on the 75m dia tube face is particularly sharp and
    geometry is very good all over the (external) graticule.
    The vertical and horizontal positioning pots allow the pattern to be moved
    right off the screen without visible limiting - so the deflection
    amplifiers have adequate headroom.
    The internal (Y) synch appears to function well as does the external and
    LINE synch inputs. There is an X-Y mode that works fine too. There is no
    "trace rotate " control, but two screws on the rear can be loosened to
    allow the tube and hence trace to be lined up with the graticule OK.
    Now for the insides:

    With the vinyl coated steel sleeve slid off, there is a nice surprise.
    This little CRO is *very* well made - reminiscent of the way Aussie firm
    BWD built their famous CROs.
    There are no SMD, all parts are "garden variety" - ie TO92 pack
    transistors, TO220 regulators, a couple of fets and 3 bog standard ICs.
    There is one large PCB for the PSU, deflection and synch circuits while a
    smaller one at the front covers the vertical attenuator, input pre-amp and
    time base circuitry.
    Both sides of these (single sided, through hole) PCBs are fully accessible
    for servicing !
    A very neat, 20 VA, R-core transformer powers the CRO with a ferrite HF
    inverter supplying 1200 volt DC to the tube. All internal voltages remain
    regulated down to 200 volts AC input.
    The 75mm tube is covered in a black, very snug fitting, full length
    MAGNETIC SHIELD !! This is practically unheard of in a low cost CRO and
    means the unit can be used, without annoying trace jitter, quite close to AC
    power transformers used by other items on the bench or the equipment under
    test.
    The AC current draw was only 84mA rms ( 20 VA ) - so operation from a
    small 12volt / 240 volt inverter supply is possible.
    One anomaly I found - the CRO comes fitted with a 0.75 amp AC fuse. The
    DSE handbook and the back label on the CRO both say this is the intended
    value - which is nonsense since it offers no protection for the 20 VA
    transformer. Best replace it with a 160 mA, slo-blo type.


    You may get a lot more here:-

    Last edited by beer4life; 11-02-14 at 05:57 PM.

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    Thanks beer4life - that's a good start. There is a logo on the logic board which states GF 0510.

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    Success!!

    I have found a copy of the schematic diagram - If anyone needs a copy, feel free to drop me a note. I'm happy to share the document.

    Beer4life - thanks again for the help.

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    Hi all,

    I have had my DSE Q1803 for quite a few years and it has just failed, was wondering if anyone could help me with obtaining a circuit diagram please.

    Regards
    Cherokeecruiser

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    Quote Originally Posted by beer4life View Post
    "This little CRO is *very* well made - reminiscent of the way Aussie firm
    BWD built their famous CROs."
    Reminds me of the song by the Mutton Birds:

    I own a Q1802. Brought it in 1997 for $1500 when I was on tech wages of $11.60. I'm guessing in today's money, that would be $5000 ?. Agree, very good quality for a low end instrument.

    Firewire: Is the horizontal driven off the fly back transformer or is it separate ?. If seperate, half of deflection sound likes like the dual power supply op amp driving the horizontal is missing a power supply or is damaged. Locate the power pins on the amp and measure.

    J

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    Re the Dick Smith Q1803 CRO, I had a fault that caused the screen not to come on. I did not have a circuit diagram, so I did a lot of searching on-line for one. I learned that the same scope was also branded 'Digimess', model MO10. It looks exactly the same as the Q1803. Other similar model numbers are AT7016, and CSV10V. I was able to locate the full circuit diagrams.
    For the benefit of others who have the same CRO and get a blank screen, I traced the problem to the EHT circuit, which should be around 1200Vdc, but was struggling to get to 100V. The EHT voltage is derived from a switch mode circuit output transformer, and has an 8kV single diode rectifier type 2CL-8kV (V109). Immediately following that diode is a filter network comprising of 3 x 2kV rated capacitors connected to Vcc, and two series resistors.
    I checked the 3 HV caps (C113, C114 and C115) and found that they were not shorted, they had high resistance, and also that they all read the correct value (0.1uF). But the output from the filter network was still only 100V. I disconnected the output end of the EHT diode, and read 1242V on its free end. After scratching my head for a while, I decided to buy new caps, and to cut a long story short, when I replaced these, the CRO display came back on ok, which was great. Out of curiosity, I then decided to test the removed caps with a Kyoritsu insulation tester that I have. which has a high range output of 1000Vdc. Two of the caps were ok, but the third one now measured 100Kohms (only with 1000Vdc applied to it). The cap's 100kohm fault resistance had been enough to drag the EHT down to around 100V, so that the trace disappeared. But now - the EHT and the CRO are working beautifully again!

    Aussie Fred

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    Quote Originally Posted by Aussie Fred View Post
    Re the Dick Smith Q1803 CRO, I had a fault that caused the screen not to come on. I did not have a circuit diagram, so I did a lot of searching on-line for one. I learned that the same scope was also branded 'Digimess', model MO10. It looks exactly the same as the Q1803. Other similar model numbers are AT7016, and CSV10V. I was able to locate the full circuit diagrams.
    For the benefit of others who have the same CRO and get a blank screen, I traced the problem to the EHT circuit, which should be around 1200Vdc, but was struggling to get to 100V. The EHT voltage is derived from a switch mode circuit output transformer, and has an 8kV single diode rectifier type 2CL-8kV (V109). Immediately following that diode is a filter network comprising of 3 x 2kV rated capacitors connected to Vcc, and two series resistors.
    I checked the 3 HV caps (C113, C114 and C115) and found that they were not shorted, they had high resistance, and also that they all read the correct value (0.1uF). But the output from the filter network was still only 100V. I disconnected the output end of the EHT diode, and read 1242V on its free end. After scratching my head for a while, I decided to buy new caps, and to cut a long story short, when I replaced these, the CRO display came back on ok, which was great. Out of curiosity, I then decided to test the removed caps with a Kyoritsu insulation tester that I have. which has a high range output of 1000Vdc. Two of the caps were ok, but the third one now measured 100Kohms (only with 1000Vdc applied to it). The cap's 100kohm fault resistance had been enough to drag the EHT down to around 100V, so that the trace disappeared. But now - the EHT and the CRO are working beautifully again!

    Aussie Fred
    Thanks Fred, I had the same symptoms. Tested the caps with a Megger, all good. Went back to the diddlygram (for the MO10) which I found with your help and found D109 V119 open circuit. A 1000V ultrafast diode (UF4007) from Jaycar had me up and running in no time. My scope was a Tricky Dicky model that I got on special for $99 10 years ago. I'm amazed that the same model with a bit of badge engineering is still available, and for serious $$$, like GBP180. Thats nearly 300 South Pacific Pesos (AUD)!

    My scope had lots of use and when I took it apart I noticed a fair bit of screen burn (and a few scratches from when it was put together), and I've noticed that the tube, an 8SJ31J pops up ebay now and again for around USD30. Anyway, just saying, so Fred, thanks again mate!

    Lungs McGee
    Last edited by Lungs McGee; 24-03-17 at 09:42 PM. Reason: Changed D109 to V119

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    An AVE fan there Lungs?

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    Default Circuit Diagram

    Where did you find the Circuit Diagram??







    Quote Originally Posted by Aussie Fred View Post
    Re the Dick Smith Q1803 CRO, I had a fault that caused the screen not to come on. I did not have a circuit diagram, so I did a lot of searching on-line for one. I learned that the same scope was also branded 'Digimess', model MO10. It looks exactly the same as the Q1803. Other similar model numbers are AT7016, and CSV10V. I was able to locate the full circuit diagrams.
    For the benefit of others who have the same CRO and get a blank screen, I traced the problem to the EHT circuit, which should be around 1200Vdc, but was struggling to get to 100V. The EHT voltage is derived from a switch mode circuit output transformer, and has an 8kV single diode rectifier type 2CL-8kV (V109). Immediately following that diode is a filter network comprising of 3 x 2kV rated capacitors connected to Vcc, and two series resistors.
    I checked the 3 HV caps (C113, C114 and C115) and found that they were not shorted, they had high resistance, and also that they all read the correct value (0.1uF). But the output from the filter network was still only 100V. I disconnected the output end of the EHT diode, and read 1242V on its free end. After scratching my head for a while, I decided to buy new caps, and to cut a long story short, when I replaced these, the CRO display came back on ok, which was great. Out of curiosity, I then decided to test the removed caps with a Kyoritsu insulation tester that I have. which has a high range output of 1000Vdc. Two of the caps were ok, but the third one now measured 100Kohms (only with 1000Vdc applied to it). The cap's 100kohm fault resistance had been enough to drag the EHT down to around 100V, so that the trace disappeared. But now - the EHT and the CRO are working beautifully again!

    Aussie Fred

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    Where did you find the Circuit Diagram. A copy would be appreciated.

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    jose0412, please be patient.

    Two posts within six minutes will NOT get you what you want.

    Yes, 38 members have read your post but I'm sure that if any of them was able to help you, they would have done so.

    While you are waiting, it would be a good idea to go to the section of the forum and tell us all something about yourself....your interests, where you are from, etc.

    It might take several days or more before someone with the answer to your query sees your posts and replies.

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    Default robmuso - Looking for the Circuit Diagram for the Dick Smith Q1803 Oscilloscope

    Quote Originally Posted by Firewire View Post
    Success!!

    I have found a copy of the schematic diagram - If anyone needs a copy, feel free to drop me a note. I'm happy to share the document.

    Beer4life - thanks again for the help.


    Hi,

    I bought the Dick Smith Q1803 Oscilloscope some years ago and it is now hardly showing anything on the display.

    You mentioned above that you found the circuit diagram for this model and that you are possibly willing to share it.

    I would very much appreciate a copy of the circuit if it is possible for you to email it to me.

    My email address is xxxxxx

    Thank you for your support.

    Take care.

    Regards,

    robmuso


    EDIT: email address removed - breaches self-promotion rule
    Last edited by mtv; 27-01-21 at 10:54 PM. Reason: EDIT: email address removed - breaches self-promotion rule

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