Kenwood TS-870S TX GAIN adjustment not working

Discussion in 'Radio Circuits, Repair & Performance' started by EB5AGV, Nov 9, 2018.

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  1. EB5AGV

    EB5AGV Ham Member QRZ Page

    Hi all,

    I am dealing with a curious problem on a TS-870S. It generates too much power output. I can control it but, for example, on 14MHz, minimum is 10W but maximum goes to 150W. On other bands, power reaches close to 200W.

    So I have adjusted, on service menu A1, ALC reference voltage level to 2.7V (was already really close), have set power output (A7, A8, A9, AA) to 100, 50, 25 and 10W. All OK. Only 100W setting was out of adjustment, and was at about 150W. But then, when I try to adjust TX GAIN for the different bands (parameters AB to B3), there is not power output when I press SEND, so the unit keeps trying to find the correct adjustment endlessly. I have done this adjustment before on other TS-870S so I know what to expect.

    I have checked the -6Vdc supply, which is rock solid with any power setting. I have also checked the power measurement diodes and the VSF and VSR voltages, which are also OK.

    One hint is that the ALC indication on the display, at any power setting on CW is always maximum. And, on SSB, with any minimum audio input at the MIC, it also goes to fastly to max.

    Any hint on what could be happening?


    73 Jose EB5AGV
  2. KA5IPF

    KA5IPF Ham Member QRZ Page

    I just went thru alignment on an 870. Before entering the alignment menu the Car and Pwr are at max. After setting each power level (100, 50, 25, 10) did you push the UP button to transfer from right to left in the display? Then go to the band settings and after the beep on each band push the UP button. Once done go to BA and push UP again to save. When one has been seriously Golden Screwdriver'ed I have had to go thru that sequence several times before it all straightened out. Don't forget the ALC meter setting after the power levels are set on the bands.
  3. EB5AGV

    EB5AGV Ham Member QRZ Page

    Hello Clif,

    Thanks for your message. Yes, I did as you say (I have done this on several TS-870S before), but there is zero power output and the gain setting keeps going up and then cycle again, so there is no stop beeps. I am working on the IC7 (DAC) area of the RF UNIT. I am analyzing how that D/A converter (IC8) and the associated OPAMPs (IC7) should work to be able to find which is the culprit. I hope it is not the control of them, as it is direct from the CPU

    The block diagram shows that IC8(3/4), driven by IC7 (8/8 and 7/8) manage D49 (GAIN ADJ). This should be constant at each band, depending on AB to B3 settings. The ALC acts on Q39 gain. There are quite a number of components which could influence it, so I am checking what each one does.


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