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IFR-1200 Super S Spectrum Analyzer blowing fuse

Discussion in 'General Technical Questions and Answers' started by KG5TKF, Jan 22, 2021.

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  1. KG5TKF

    KG5TKF Ham Member QRZ Page

    Hello fellow Hams. I have been fortunate enough to acquire a IFR-1200 Super S spectrum analyzer. My good fortune did not last more than about 20mins after powering on. Everything seemed to be functioning in this brief window as scope/crt was on with sweep, digital read outs were functional and lit etc. After a short while the unit suddenly powered off and small wisp of smoke escaped thru the vents of unit. Troubleshooting revealed blown fuse F1 which is a 5A fuse on the Battery Charger board and that the source voltage for the Inverter supply flows thru. Replacing the fuse with a smaller 3A fuse experienced an immediate blowing of the new fuse. Using an ohm meter I looked for shorts on the output of the Inverter supply but could not see any dead shorts. The lowest resistance I found was for the +12vdc output from the Inverter supply that was approx 40 ohms. I went about unplugging all circuit boards and connectors to see if I could trace down the low resistance but it seemed that as I unplugged boards and connectors the resistance steadily climbed which was actually a good indication. Seeing as I was not making progress finding a short I kept as many boards and connectors disconnected as possible and powered on the unit. Fuse did not blow. I incrementally added circuits/boards until everything was plugged except the Scope control and power supply. Plugging this assembly in produced a blown fuse. I removed this assembly and upon visual inspection it was apparent that Mosfet Q5 within the high voltage section had been very hot and was physically damaged. I replaced this Mosfet with one from ebay :-/ and for another 20 glorious minutes the scope and analyzer was alive and well. But alas... once again the fuse blew and I once again dismantled the scope assembly. Q5 and Q6 Mosfets are both making me suspicious but I'm really not sure. Q6 now looks like it might have gotten warm but I don't see any damage. I don't know if something beyond these two mosfets is at fault or if I am yoyoing between bad mosfets... the later doesn't seem likely as the scope worked for a good while so both would have been good upon replacement of Q5. I can't find any shorted electrolytic capacitors or any visible damage. I have measure the windings of the T1 hv transform and all windings seem to measure with normal continuity without any shorts between windings. No diodes are shorted or open. My final test was to reconnect the connectors going to the scope assembly allowing the inverter supply to power the scope power assembly but I disconnected the connector with the 12vdc going to the HV section of the Scope assembly (there is a HV circuit board that is inside of a metal cage below the CRT. Leaving only this connector disconnected keeps the fuse from blowing which seems to only leave the HV card and the CRT. At this point I'm waiting on a couple of capacitors (C4 and C5) and new Mosfets so I can replace both Q5 and Q6 at the same time. After that replacement I'm not really sure how to proceed. I was thinking I might disconnect the CRT and with the HV circuit connected to see what happens. I can't imagine the CRT could be bad but I'm really fumbling my way thru this. Any help is appreciated. The schematics I found online aren't exactly the same but they are close:

    http://www.repeater-builder.com/tes...0/ifr-fm-am-1200a-ifr1200s-service-manual.pdf
     

    Attached Files:

  2. W1NB

    W1NB XML Subscriber QRZ Page

    I would check the ripple on the 12v supply. C4 and C5 may not be shorted but if their capacitance is reduced there may be significant ripple. Similarly, I’d look at all the other supplies. Leaky capacitors will increase the current load on their supply. The other thing I would check is the waveform on the gates of the MOSFETs. If it is too asymmetrical it may be causing the MOSFETs to conduct over a longer portion of their cycle which may overheat them.
     
  3. CX3CP

    CX3CP Ham Member QRZ Page

    hi, if you have determined that the eventually overload coming from the T1 fly-back transformer, you need to put it to the test and the associated components of the CRT block. Firstly you can disconnect the CRT neck conector and leave it unplugged to see if the fault happens again .
    If the circuit works as should then re-connect the CRT and check the nearly associated components like CR1,2 C6, C7 and measure the voltages as the schematics say.
    Focus -1500VDC
    Chatode -1900VDC
    Grid -2000VDC
    All negative voltages referenced to chassis.

    Better if you have some oscilloscope to make an DC voltage analysis, looking for ripple and/or leaks.

    All of this is a very simply circuit scheme, it will not be difficult to find the problem.

    good luck !!

    upload_2021-1-22_20-26-53.png
     
  4. W9GB

    W9GB Ham Member QRZ Page

    Kurt Graber is a good resource (parts, service) for the IFR 500 and 1200 series.
    He restored my IFR-500A almost 20 years ago (including repairing/repainting case)
    BTW Replacement CRTs for the IFR 1200 series are becoming harder to find.

    eMail best contact method (telephone calls interrupt service bench time).
    He has IFR 1200 serial #1 (Prototype build) in his collection.
    ==
    KG Electronics is the leader in servicing the IFR 1200 or 500 Service Monitors.

    With over 30 years of Experience resulting to Multiple thousands of repairs and calibrations.
    Kurt Graber has by far repaired more service monitors than anyone on the planet.
    Kurt started his Electronics career at the IFR factory service department (Wichita) in 1989.
    https://kgelectronics.com/service/
    ==
    KG Electronics
    https://kgelectronics.com/

    KG Electronics
    12502 W. Hardtner
    Wichita, KS 67235. USA
    Shop Phone : +1 316-773-0948
    Email: kurtgraber@yahoo.com.
     
    Last edited: Jan 23, 2021
    KG5TKF and N2HUN like this.
  5. N2HUN

    N2HUN Ham Member QRZ Page

    I had Kurt Graber calibrate my IFR-500A, he did a great job, fast service, and reasonable cost.
     
    KG5TKF likes this.
  6. KG5TKF

    KG5TKF Ham Member QRZ Page

    As this is the High Voltage section of the circuit with 1000+ volts I'm not going to be probing around in that circuit. My only option is to disconnect circuits and to perform de-energized measurements on this part of the scope supply. It is making it difficult to troubleshoot.
     
  7. KG5TKF

    KG5TKF Ham Member QRZ Page


    I've never actually sent a piece of equipment off for repair... Kurt has great reviews and comes highly recommended. My intent here is to repair it myself as that is all my budget allows for but thanks for the reference. If I end up hogtied I might consider it.
    thanks Dayton - KG5TKF
     
  8. KG5TKF

    KG5TKF Ham Member QRZ Page

    Thank you. As soon as I get my replacement Mosfets (at lease one of the installed is now blown) my intention is as you say to disconnect the crt neck connector to isolate CRT from circuit and see what happens. I'm trying really hard to avoid having to do voltage measurements on this section as it is impossible the circuit while installed and it would be difficult to extend the connections to have the circuit on a bench. I'll have to look more at that and see where i can measure safely. All diodes have been tested good in this circuit and there are not any shorted caps that i can find in this circuit. It is not a dead short though so I think it is possible that a capacitor is most like culprit as what else can go bad on it's own just from being stored many years? Also I wonder if I have overlooked where the output of the bridge rectifier is going... perhaps by disconnecting this circuit I have disconnected a bad circuit that is not part of the CRT?
     
  9. W9GB

    W9GB Ham Member QRZ Page

    You could’ve-mail or telephone him ...
    Save Troubleshooting Time and Avoid Further Damage (as I stated CRT becoming unobtainable).
    Last time I e-mailed Kurt (2018), he had no spare CRTs left.
    So I passed on an IFR 1200S with a bad CRT.

    ==
    Buddy’s Video Notes
     
    Last edited: Jan 26, 2021

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