Newbie questions on RF circuit

Discussion in 'General Electronics Chat' started by n9jcv, Apr 16, 2016.

  1. n9jcv

    Thread Starter New Member

    Apr 14, 2016
    Hello everyone, I am a ham radio operator. I have tinkered with electronics but never took any formal training. Now trying to learn from books. I have a real life troubleshooting case with a HF radio. So many questions.

    Below are links to 3 items.
    #1 - Schematic (partial) of circuit I am suspect of
    #2 - scope photo of oscillator driver output with keydown(transmitting)
    #3 - scope photo of oscillator driver output - not transmitting

    First, I do have a bk precision 15mhz scope, the circuit is now running about 7mhz. The problem is that I get no output, or lets say less than 200mw output, cause the driver is outputting. The following is not to explain the circuit to you all, as I know you probably know these things, but it is to ask for your confirmation on how I believe it works. One note, with my dvm, I was able to detect that ZD1 was defective, I pulled it and sure enough it was shorted, both ways, so I have replaced that already. My understanding is ZD1 is there to block reflected rf (when the swr is high on the antenna). So if rf comes back down the antenna, it will not kill the finals. Well ZD1 was a dead short. I can't explain what caused it to short. But since it was shorted, I am almost certain there is other damage. And the finals still do not transmit. On the circuit diagram, I have circled and highlighted in pink my measured voltages, all DC, no rf probe used as I don't have one. Any way starting with the driver Q5, it has voltages, and they are within reason of the marked voltages from Heath. I took a pic of the oscillator output. It looks fairly good to me, it is about .5vpp (volt peak to peak if that is how you say it). Then when I transmit(key down), it goes to 3vpp. So I am going to say, I think the oscillator is ok. I did adjust the 40m driver cap for max output at R31 while keeping it clean. At the base of Q6 and Q7, there is no voltage and no oscillator signal. the collectors of q6/q7 are 0 volts. The voltage on the other side of RFC2 is 12.2v. My thought now is that both the final transistors are bad. I am thinking they are open and not shorted. If they were shorted, I think I would have bigger issues basically pulling all the current my power supply could feed.

    So am I anywhere on track with this? I am not necessarily asking you to fix it for me. More I would like confirmation that what I have done for troubleshooting is logical and correct. And is my hypothesis that the finals are bad correct. I know I could pull one and test it, but I do not know if I can test it in circuit.

    Any help would be greatly appreciated, and by the way I just found this forum and love it. I could read here for months or years and learn a lot I am sure.

    Pic of schematic with voltages marked - Pink are what I actually observed, numbers in rectangular boxes next to them are transmit values from heath.

    Pic of driver when not transmitting .5vpp at q5 collector

    pic of oscillator output at q5 collector when transmitting about 3vpp

    Moderators note: uploaded pictures and inserted them in as full image.
    Last edited by a moderator: Apr 16, 2016
  2. DickCappels


    Aug 21, 2008
    If I am reading this correctly, you have 12.2 volts across RFC2. It is quite likely that when the Zener shorted it opened RFC2.

    While you are in there with an ohmmeter, you might want to check to see whether there is a short between the bases and collectors of the output transistors too.
  3. n9jcv

    Thread Starter New Member

    Apr 14, 2016
    I have a mpsu05 that is bad. They are no longer made. There are some on ebay for big $$ or NTE which is also big money. I am looking for a cheaper generic substitue. Would a 2SC1678 serve my needs. The intended circuit is for RF amplification in the final transmitter stage. A 3 Watt QRP radio 3.5mhz to 21mhz. The relevant part of the circuit can be seen below. Transistors are Q6 and Q7.
  4. n9jcv

    Thread Starter New Member

    Apr 14, 2016
    Thanks for the input Dick. It appears the RFC is open. I will need to replace that. In measuring the transistors from the base to the collector, both ways, it is near infinite resistance. I will need to pull them to verify, but I am guessing they are bad as well. I am thinking perhaps someone reversed the power input and that is what caused the damage.
  5. bertus


    Apr 5, 2008
  6. GopherT

    AAC Fanatic!

    Nov 23, 2012
    I have a pair of MRF477 you could use. Should be close.
    Vintage Motorola new old stock.
  7. ian field

    Distinguished Member

    Oct 27, 2012
    That package style fell by the wayside long ago, but you should be able to find an equally capable part in TO126.

    You could probably wriggle a TO220 part in - but I think the pinout might be back to front.