TS-940SAT is deaf above 13.999 MHz

Discussion in 'General Technical Questions and Answers' started by N2ZDH, Jan 15, 2020.

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  1. N2ZDH

    N2ZDH Ham Member QRZ Page

    The problem I need to correct is reception 20 meters and above. The receiver goes deaf at 14.000 MHz. It does not matter the different ways I try to go beyond 13.999 MHz. Spin the VFO, no good. Use the MHz up button, no good. Any direct entry button 14 and higher, no good. The rig was mfg 3Q1989, so many short comings in production were overcome. Pins and connectors appear to be solid.
    Now, I need a starting point for this obstacle. I love the radio.
     
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I haven't dug into the service manual or schematics, but that sounds like an output filtering board problem, likely a bad relay that switches in the filters as soon as the rig passes 13.999 MHz.

    Take a look at the service manual and especially the filtering board just after the finals stage. Gut feel is that there's a bank of relay switched filters that cover different frequency ranges and one of them specifically kicks in at 14.000 MHz. If so, there will be a pair of relays surrounding some passive LC components that make up that filter and it's likely that one of those relays or associated control circuit has failed. It could be a component itself or an electrical connection (e.g. cracked solder joint, bad crimp on a header connector) that's actually failed.

    Maybe it's something different but I'd start by looking at the documentation surrounding the output filtering and see if that would explain what you're seeing.
     
    W1BR likes this.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    So here's the relevant section of the schematic. Kenwood didn't use relays but instead used switching diodes to select the appropriate band pass filter for each frequency range and yes one of the filters kicks in at exactly 14.000 MHz. If it's just that filter (or more likely the switching diodes) that has failed you'd still expect the rig to operate on 15 meters as a different BPF is switched in once you go over 20 MHz.

    TS-940SAT BPF.jpg

    The filter bank is on the RF board and this is taken from page 91 of the service manual: https://www.qrzcq.com/pub/RADIO_MANUALS/KENWOOD/KENWOOD--TS-940-Serv-Manual.pdf

    This description of the filters from page 12 of the manual also helps to narrow it down:
    upload_2020-1-15_9-4-50.png

    As shown above, BPF H is selected when the rig is tuned between 14.000 and 19.999 MHz.
     
    N2ZDH likes this.
  4. KB9DT

    KB9DT Platinum Subscriber Platinum Subscriber QRZ Page

    I'd start by checking RB3 to see if it goes high when 14-20MHz is selected
     
    N2ZDH likes this.
  5. KA5IPF

    KA5IPF Ham Member QRZ Page

    The solution is probably in the RX BPF chart K7TRF posted. That data comes in on CN-7 and if incorrect some bands will not work. They had problems with the physical connectors used for CN-7. They started out tin plated and then switched to gold and still had problems. Then they recommended bypassing the connector and hard wiring.

    Compare the levels at the connector and at the IC.
     
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  6. N2ZDH

    N2ZDH Ham Member QRZ Page

    I’m grateful to you (David, David, Clif) all for your interest and response. I will carefully take this advice forward.
    73, de N2ZDH Raoul
     
  7. N2ZDH

    N2ZDH Ham Member QRZ Page

    I want to keep my activity in perspective.
    I've gone through the table (Table 8) for each "Rx BPF" row, to understand voltage presented, for each zero & one.
    All rows were tested at connector 7 (CN-7), RB0 through RB3, Letters A through I.
    I sampled the voltage on the board going to IC-1. All pins from CN-7 presented voltage on the board. So connector and pins OK.
    All zeros in the table presented a value of 0.2 VDC and all ones presented a value of 3.65 VDC.
    The receiver is deaf for rows H and I; that is 14.000 through 30.000 MHz.
    I uploaded a PDF of the table used to scratch tested values.
    Where do I continue this pursuit?
     

    Attached Files:

  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Are pins 7 and 8 of IC1 going low when those H and I band ranges are selected. The schematic shown above shows voltages when band group I is selected. If those voltages out of IC1 are correct when those frequencies are dialed into the rig then you have a problem with the diode switching or the diodes themselves. If the voltages on pins 7 and 8 of IC1 don’t switch appropriately then the problem is with IC1 or the logic lines feeding it.
     
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    [Edit] Correction to my post above made on my phone... The schematic above shows voltages when Band ground H is selected not Band group I. Also, the two pins of IC1 that should go low (0 volts) when Band group H and I are selected are pins 10 and 9 respectively which correspond with BCD decoding 7 and 8 not pins 7 and 8.

    In terms of further troubleshooting, it might help to understand just what this circuit is doing and how it works to figure out what is not happening correctly.

    IC1 decodes binary inputs and translates them to switch one of 9 outputs. IC1 uses open collector switches so it doesn't output any signals at all but just shorts one of the 9 lines connected to its output side to ground through a transistor switch. RB1 is a resistor pack that supplies DC voltage(pull up voltage) through 9 independent resistors and through a common 680 ohm resistor R143 to each of the open collector outputs of IC1. RB1 assures that each output of IC1 is in the OFF state until it is asserted by the inputs to IC1. If IC1 is receiving its binary input signals on pins 12 to 15 through R139 to R142 and via connector 7 then one of the 9 possible receive filters will be selected and the corresponding output pin of IC1 will go low as its open collector output transistor is switched on.

    So taking the case of filter group H selected (which covers 14.000 through 19.999 MHz)

    - Pin 12 of IC1 (the binary coded input side) should go high (approximately 3.6V) assuming the input to the IC is good and R142 is good with good connections along that line.
    - Pin 10 of IC1 should go low (roughly 0 volts) as that output line is asserted
    - That draws current through R20 and R21 which also draws current through diodes D17 and D18 forward biasing those two signal switching diodes
    - When the signal switching diodes (D17 and D18 in this case) are forward biased the RF signals flow through them as well and that filter bank is switched in circuit

    So there's a number of troubleshooting steps you can take such as comparing the voltages on all the points shown in the schematic posted above when you tune the rig between 14 and 19.999 MHz. But there's other steps you could take to validate various parts of the circuit.

    For instance, if you tune the rig between 14.000 and 19.999 MHz and use a clip lead to short pin 10 of IC1 to ground you're testing R20, R21, D17, D18, the filter itself and the positive supply lines feeding D17 and D18. If the receiver comes alive when you do short pin 10 of IC1 to ground then you know all the filter and filter switching components and circuit connections are good. If the receiver doesn't come alive when you short pin 10 of IC1 to ground then you likely have a problem in the circuitry surrounding D17, D18, R20, R21 the filter itself or connectivity around those components.

    Similarly you could measure what goes into IC1 vs what output lines are switched to see if there's a circuit connectivity or perhaps a failure of R139 to R142 or perhaps a bad connection in connector 7 which brings the binary coded control lines to IC1.

    Here's the circuit again showing the DC conduction path(in red) when filter bank H (14.000 to 19.999 MHz) is selected:
    upload_2020-1-20_17-1-7.png
     
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  10. N2ZDH

    N2ZDH Ham Member QRZ Page

    Thank you for the update and extensive coaching. I need it as I am no where near as experienced with rig repair and restoration as you must be.
    I will be digesting and applying the science and keeping our forum updated.
     
    N2ZD likes this.

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