1. You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.
    Dismiss Notice

Midland 13-893 AM/SSB - Low output power issue

Discussion in 'General CB Services Discussion' started by FarmBoy72, Jul 21, 2019.

  1. FarmBoy72

    FarmBoy72 New Member

    Joined:
    Sep 27, 2018
    Messages:
    5
    Likes Received:
    2
    I have a Midland 13-893 AM/SSB 23 channel crystal-synthesized radio that I'm having an issue with and I'm a bit stumped. It has a 4watt carrier on AM, but talking into the MIC and applying audio barely modulates the output, maybe gets 0.5 watts over the carrier with a loud whistle. In SSB its a similar story, has about 4watts max output when using the MIC. (Side note, this came from a CB shop that closed in the mid-late 1980s and the this radio was in the box of units with issues, so this radio has sat for probably 30 years)

    To start, I went through and aligned the synthesizers, one of them had a very weak signal and the oscillator wasn't receiving full power, which lead me to my first discovered problem, a protection diode on one of the power rails was split open, so at some point somebody has flipped the power..... I replaced the diode and now all the oscillators are working properly. A few of the mixing crystals had drifted low over time and I had to swap out a few of the padding capacitors to get them in range of their adjustments. After that mess it now receives on AM/SSB. The receive doesn't seem to be as strong as it should be, the Photofact says to inject a 0.5uV signal for one adjustment but I had to bump that to 10uV to get output through the receiver. But I have no idea how calibrated my old Anritsu MG3601A RF signal generator is or if the output is still working that low......



    So next, to try and track down the low output issue, I started at the microphone and followed the audio trail through both the AM modulation path and SSB modulation path. Everything seems O.K. all the way through, the signal was increasing through the AF amplifers. But I'm quite new to this so I don't have a great feel for the level the signal should be at in each point. I did hook up an 8 ohm speaker to the PA jack and the PA function drives a speaker and the audio doesn't sound garbled or off.

    Other things I have tried is adjusting VR15, the RF power adjustment, and it doesn't make a noticeable difference in output on AM, but I can get about 8-9 watts in SSB whistling into the MIC with the MIC gain at maximum and VR15 at max travel.

    Looking at the demodulated output and RF on an oscilloscope (through a home-made sampler) there doesn't look to be any distortion.

    So that gets me to where I'm at now, I'm stumped. Any ideas??
    Thank you


    CB tricks has the schematic here
     

  2. nomadradio

    nomadradio Analog Retentive

    Joined:
    Apr 3, 2005
    Messages:
    2,778
    Likes Received:
    2,563
    The alignment on this radio is just a little wacky. One set of tuning slugs in the crystal synthesizer is peaked for AM and LSB, the other set for USB only, IIRC.

    Seems likely that the resonant peak setting for a handful of tuning slugs has drifted over the last 40-plus years.

    Only step-by-step guide to alignment will be in Howard Sams volume CB-69.

    73
     
    kaos513 likes this.
  3. FarmBoy72

    FarmBoy72 New Member

    Joined:
    Sep 27, 2018
    Messages:
    5
    Likes Received:
    2
    Oops, I didn't explain myself well. I do have the pages from CB69 on alignment. I was able to follow them (after fixing the blown diode issue) and the 19mhz chain for AM/LSB and the 34mhz chain for USB are adjusted.
    I also changed out most of the electrolytic, a large filter cap near the blown diode had some scorch Mark's on it. And quite a few 1uf caps are used to couple signals between different sections. Knowing this radio was in non cooled storage I felt it best to change those.


    The radio receives on-frequency when I use a 30% modulated signal from my generator. Before alignment of the synthesizers, I had to max out the clarifier to get it to start receiving. All the Crystal's had drifted low and the slugs were a tad off. And I agree, the mixing scheme to get SSB on this is quite wacky.

    I looked at aligning the 27mhx chain, and was able to follow the steps. I'll double check again, some of the slugs dont make a noticeable difference in output power but they did affect some of the spurious signals nearby. I have a spectrum analyzer, but it's only got a minimum 100khz/div with, I think, a 9khz bandwidth. So I can't see what the side bands look like and tried to adjust them to minimize spurious emissions. I dont believe I moved any slugs more than 1/8 turn.

    Thank you for the help! (My end goal is to get this thing working on 27mhz, then set it up with an arduino and DDS to use on the 10m band for ssb and to learn in the process)
     
    #3 FarmBoy72, Jul 22, 2019
    Last edited: Jul 22, 2019

Share This Page

  • About Us

    The WorldwideDX Radio Forum was originally established in 2001. We pride ourselves on welcoming Radio Hobby enthusiasts of all types, while offering unbiased, informative, and friendly discussion among the members. We are working every day to make sure our community is the best Radio Hobbyist's site.
  • Like us on Facebook

  • Premium VIP Member

    The management works very hard to make sure the community is running the best software, best designs, and all the other bells and whistles. Care to buy us a beer? We'd really appreciate it!

    Donate to us!