PA board issue on Heathkit HW99

Discussion in 'Radio Circuits, Repair & Performance' started by N5OLA, Sep 10, 2019.

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  1. N5OLA

    N5OLA Ham Member QRZ Page

    I'm building a new Heathkit HW99 CW transceiver. It sat in a box for 30 years but was still in great condition. Before building, I replaced the electrolytic caps and checked all resistor values (replaced a few).

    I haven't yet put the boards to the chassis, still running checks, but am hitting some snags.

    1. On the PA board, the initial test should get at least 100 ohms between the 12V wire (see bottom left of schematic) and ground. I'm not getting even a twitch on the multimeter.
    2. Should get "at least 1500 ohms" between 30V wire and ground. I'm getting about 1320, which is not quite up to spec.

    I did a thorough check for cold solder joints and solder bridges - found none.

    I would treasure any insights from this group. I got back into ham radio after a 30 year absence, hoping I'd spend more time learning the fundamentals of electronics this time around. Already I feel out of my depth.

    And by the way, there are likely to be more questions as this kit comes to fruition, since grandpa isn't around for me to call anymore and the Heathkit tech support folks have moved on to other careers.

    Best to all,
    N5OLA
     

    Attached Files:

  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Try reversing the leads of the DMM for the 12-volt test. You are actually reading through the transistor and the junction is just like a diode. That is, an open with the DMM leads in one position and a relatively low resistance in the other direction. If your DMM has a diode option in resistance, try that and reverse the leads to see if you get a reading in one direction or the other.

    If that doesn't work, check T-303 and make sure it shows a low resistance. Then, check resistance from the 12-volt input to the collector of Q-302. That should be the same resistance reading as across T-303. Next, check from the emitter of Q-302 to ground. This should be about 7.5-ohms. Finally, check across Q-302 from the collector to the emitter reversing the leads and use the diode function if there is one. If you don't get a reading across Q-302 then the transistor is bad.

    Make sure the soldering connections are good! Did you clean the circuit boards before soldering the components? The boards are definitely going to have corrosion since it has been at least 32-years since they were built.

    Glen, K9STH
     
    WQ4G likes this.
  3. N7EKU

    N7EKU Ham Member QRZ Page

    Hi,

    For sure. Check from the collector of Q302 to the +12V supply line. It should be zero ohms.
    Resistance from the emitter of Q302 to ground should be about 0.75 ohms not 7.5 ohms.

    If you get an open circuit on the collector reading, check you have the wires in the proper holes on T303.

    73,


    Mark.

    PS: Hope you didn't toss all those replaced electrolytics. I know you were being extra careful, but electrolytics caps made in those days were very high quality are rarely go bad, so they should be quite useful for homebrew circuits that you might want to build later.
     
  4. N5OLA

    N5OLA Ham Member QRZ Page

    Thanks, Mark. I didn't toss them, and if you can use them, let me know. Hate for them to go to waste.
     
  5. N5OLA

    N5OLA Ham Member QRZ Page

    Many thanks, Glen. I wish I could read a schematic like that. Much appreciated.
     
  6. N7EKU

    N7EKU Ham Member QRZ Page

    Sure Glen,

    I'll shoot you a PM about shipping. Might be a good experiment, as I have an ESR meter so I can check what kind of shape they are in!

    73,


    Mark.
     
  7. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    EKU:

    I could not see the decimal point in the 1.5-ohms and read them as 15-ohms.

    Yes, 2-each 1.5-ohm resistors in parallel will read 0.75-ohms.

    Glen, K9STH
     
  8. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    OLA:

    I learned how to read schematics when in junior high school almost 65-years ago. Was taught by Orville Hartle a graduate EE who was quite the character. His "day job" was handing out hand tools to employees at the local Allis Chalmers harvester plant in LaPorte, Indiana. At night, and weekends, he had a garage shop TV repair business located less than a block from my parent's house. He also wrote books and chased UFOs (was the chairman of NICAP Indiana #2)

    Orville kept me in as many old TV chassis to tear apart and salvage parts that I could haul off. He also gave me a television set for my bedroom that was better than the one in my parent's living room. In addition, Orville encouraged me to go to the best engineering college that I could find. I ended up at Georgia Tech which, at the time, was rated in the top 3 engineering schools in the U.S. with the other 2 being M.I.T. and Cal Tech.

    Glen, K9STH
     
    N5OLA likes this.
  9. N5OLA

    N5OLA Ham Member QRZ Page

    Looks like it's the transistor Q302. And it looks like it's shorted. I tested between the base and emitter and got a reading either way I connected the +/- probes, which I think means the thing is shot. As it happens, I've got a second unbuilt HW99 arriving tomorrow (long story), so am going to test the Q302 from that kit and if the readings look good, will pop it in and order a new one. You guys are great!
     
  10. N5OLA

    N5OLA Ham Member QRZ Page

    Ignore the above msg. When I removed Q302 from the board it tests fine.

    Glen: When I reversed the leads on the 12V test I got 1000 ohms.

    T303 does have low resistance (1 ohm) and it's the same across T303. From Q302E to gnd I'm getting 5 ohms.

    Mark: Q302C to 12V was 4.5 ohms, not zero. T303 wires are in the right holes.

    The search continues....
     

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