Kenwood TS-440SAT low power, swr meter pegged!

Discussion in 'Radio Circuits, Repair & Performance' started by W1MM, Jun 13, 2018.

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  1. W1MM

    W1MM Ham Member QRZ Page

    Hi guys and gals. I've got a TS-440SAT that I bought several months ago. It has been working absolutely perfect until last weekend. Now, all of a sudden, it won't put out much power at all. Power measurements were conducted in CW and AM mode. SSB is also affected. All bands are affected. Here are the symptoms.
    • Internal PWR measurement shows about 10-15W into 50 ohm load from the SO-239.
    • External power meter into same 50 ohm load shows about 50W.
    • Internal meter switched to SWR pegs the meter! HARD!
    If I tap power from the input side of the TX filter board I get the full 100-200W on all bands. This rules out the PA stage. Measuring RF through the TX filter board but before the auto tuner is when I run into trouble. With the AT off, power is the same directly at the TX filter output as at the SO-239. This rules out the AT and any of the coax jumpers. If do I try to use the AT, it can't find resonance, even into 50 ohm resistive. All the filter relays check out okay. D1 and D2 check okay.

    My gut says something wrong with the metering circuit throwing the ALC into full protection mode. However, symptoms still seem to point to something on the TX filter board. Has anyone else experienced this? If so, can you give me a hint where to look?

    Mark, W1MM
     
  2. CX3CP

    CX3CP Ham Member QRZ Page

    Hi, maybe the equipment is presenting an anomaly in the VSWR bridge and therefore this affects the ALC control, you should check the LPF section where the VSWR bridge is, and all the associated components with it.

    Disconnect J1 on LPF assy and try TX.

    Sorry I have this low res SM ...

    upload_2018-6-12_23-6-3.png
     
    Last edited: Jun 13, 2018
  3. W1MM

    W1MM Ham Member QRZ Page

    Thanks. I have the same service manual, and several others. I've already looked at the vswr bridge. That's why I stated D1 & D2 checked out. Still hunting......
     
  4. KA5IPF

    KA5IPF Ham Member QRZ Page

    Let me see if I'm following correctly. Power out of final is OK. Power out of filter unit is ???? Bypass AT for the time being and see what happens, not just take it out of line. If the power out of the filter unit is low then bypass the T/R relay on the back of the SO239.

    The internal SWR meter only sees 2 things. The tuner and it's relay and the T/R relay on the small PCB behind the SO239. And then of course what is hooked to the SO-239. All of that can be checked with an ohm-meter.
     
  5. W1MM

    W1MM Ham Member QRZ Page

    Thanks for chiming in, Clif. You're preaching to the choir a bit here. This is an odd one. I take the AT and TR relay completely out of the equation by measuring power out through the TX filter with a 50 ohm load directly at the output port on the TX filter. Pretty sure it's a metering circuit malfunction somehow sending bad information to the ALC. I think my next step is to start probing around for voltages to see if I find anything way out of spec.
     
  6. KA5IPF

    KA5IPF Ham Member QRZ Page

    Ok, so low power out of Filter unit. What is the current draw? What are the VSF and VSR voltages going from the filter unit to the control unit? VSF should be several volts and VSR should be very low(less than 1 v).
     
  7. AA7QQ

    AA7QQ Ham Member QRZ Page

    I am just hypothesizing that a filter set is locked on, somehow. Check output on every band. Post the results.
    If you need a LPF board, I might have one for sale, as I had a change in plans for an SDR.

    Another thought, if the BCD chips are not getting the right 4 bit signal, it will not change LPFs proprely.

    Ed
     
  8. W1MM

    W1MM Ham Member QRZ Page

    No, as stated above, all LPF relays are functioning properly. I'll be checking the VSF, VSR and other voltages later tonight.
     
  9. AA7QQ

    AA7QQ Ham Member QRZ Page

    Could a contact be fused together?

    Ed
     
  10. W1MM

    W1MM Ham Member QRZ Page

    Okay, the transformers L42 & L43 float at 5V above ground. Therefore, DC signal measurements are measured referenced to J1 pin 4. At max power with 25W at the TX filter output connector into 50 ohms, VSF=0.94 VDC, VSR=0.76 VDC. R1,2,3,4 are all in spec. Interestingly, even with display unit S55 in the ALC position, I get almost full meter deflection pretty much as soon as there's any forward power at all, similar to SWR but not as violent.

    One of the service manuals I downloaded has a bunch of voltages on it but it's very difficult to read the labels to know if the rig is in TX or RX for those readings.
     

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