Radiogram Power Supply

Discussion in 'The Projects Forum' started by redrooster01, Sep 23, 2014.

  1. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
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    I'm having problems with this old power supply giving out the wrong voltages. I was asked to try and fix it by a friend but I have no experience of these radiograms at all and I'm hoping I can get some help with it here ? It seems that I can only upload one file at a time,Ive uploaded a photo so I will post another post Astor Power Supply.jpg Astor Power Supply.jpg with the schematic. I have replaced the 2 x 1000uf electrolytics and 2 x poly caps 1 x 223k and 1 x 274j greencap,otherwise I haven't changed anything. Obviously their is something wrong but I just cant see it? Any help will be greatly appreciated.
     
  2. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    Here is the schematic.The radio is getting 9.6v instead of 14.5v? and the driver boards are getting 24.7v instead of 22v? The tone boards are pretty well good. Astor PS.png Astor PS.png Astor PS.png
     
  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    I would suspect the resistor 28, and switch 73 contacts may need cleaning, otherwise the fault is on the RF board 44.
     
  4. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    Hi Dodgy,Sorry for the late reply,I didn't tick the email alert? Duh! Anyway I desoldered the radio power line wire from the RF board 44 at point J Pos, it read 24.7vdc and then I measured from the switch 73 it read 24.7vdc? Does that sound right to you?
     
  5. #12

    Expert

    Nov 30, 2010
    16,257
    6,757
    That pretty much says the RF board is using too much current.
     
  6. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
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    Ive just finished replacing all the out of spec resistors on the RF board and Ive found one of the three AT319 IF amplifier transistors is no good it reads 850 from base to collector and 850 from base to emitter,otherwise everything else looks OK but its still measures 9volts? I wouldn't have thought that a bad transistor would cause the voltage to drop like this. I tried to upload some photos but I couldn't for some reason?I clicked the upload a file button a few times and it didn't work.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,032
    ?? A bad transistor can completely disable a device.
     
  8. MrChips

    Moderator

    Oct 2, 2009
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    ??? A bad transistor can ruin your whole day.
     
  9. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    can you upload the RF board 44 diagram?
     
  10. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    Here is a photo of the RF board and its schematic. Ive replaced everything that was out of spec except for one of the IN295 diodes that measures 340 forward voltage compared to a reading of 440 on the other IN295 using the diode setting on my DMM. I measured resistor #30 3.9k ohms on the power supply out of circuit about a week ago and it was OK. Ive just measured it out of circuit again and it has gone high to 4.39k ohms? Where I live is fairly remote and Ive had to order the 3.9k resistor in,so that will take about two weeks to get here. Actually the radio was working on 9v,its stopped working now because of the transistor I suppose.
    Astor PS#1 003.JPG Astor RF Board.jpg
     
  11. Alec_t

    AAC Fanatic!

    Sep 17, 2013
    5,776
    1,103
    Did you check resistor 28 (as per post #3)? If it's gone high resistance it could account for the voltage drop. Check also for dry joints in that area.
     
  12. MrChips

    Moderator

    Oct 2, 2009
    12,421
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    Check for leaky power supply bypass capacitors.
     
  13. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
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    Yes I replaced number 28 with a 1 watt 2.7k ohm 1% tolerance resistor and every other resistor that was out of spec on the RF board.Ive also re-flowed all the solder joints. Ive done this to all the boards in this radio.That's why Ive asked for help here because I'm stuck on what the problem is?
     
  14. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    Yes Ive replaced them with new ones. Ive spent $85 on new parts,anything even slightly out of spec has been replaced. Could it be the choke? According to the SM schematic the power transformer has 18vAC across it which is right when I measured it it was 18.3vAC,then after the choke instead of being 22vDC it is 24.8vDC? The choke also reads 3.9 ohms I don't know if that is significant or not but it looks high compared to the power transformers 0.6 ohms.
     
    Last edited: Sep 25, 2014
  15. MrChips

    Moderator

    Oct 2, 2009
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    I was referring to the filter capacitors in the RF module.

    I assume you are referring to the choke (54) in the power supply.
    If you are still uncertain about the current capability of the power supply, disconnect the connection entering the RF board.
    Put a load resistor, from 3k to 5k-ohm, of known resistance, from that supply point to ground and measure the voltage across the resistor.
     
  16. bertus

    Administrator

    Apr 5, 2008
    15,645
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    Hello,

    I would check the capacitor marked 19 in your RF schematic.
    Also check capacitors marked 4 and 18.

    Bertus
     
  17. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    Capacitors 18 & 19 are new, cap 4 is a bit hard to get at to measure so Ill de-solder one of the legs tomorrow and check it.Ill check the power supply as well. Its 12.30 am here and I had better get some sleep.Thanks for the ideas fellas its much appreciated.
     
  18. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    On the RF PCB,try unsoldering resistor 31, and then 37, 39, 43, and check the 14v supply voltage at each removal.
     
  19. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    I used a 4.7k ohm 10 watt resistor from the RF power supply to chassis ground and it read 41mv across the resistor?
    [
     
  20. redrooster01

    Thread Starter Member

    Jul 15, 2014
    84
    1
    I lifted one leg each on resistor 31 & 37, I measured the input voltage at 9v still but when I measured 39 with one leg lifted it reads 18.6v at the power input? I haven't done resistor 43 yet,I thought I had better tell you about this reading first.
     
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