Image Map Image Map
Page 6 of 8 FirstFirst ... 2345678 LastLast
Results 51 to 60 of 79

Thread: PET 4016 power in from transformer to Jumper 8

  1. #51
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,553

    Default

    The transformer AC voltages look absolutely fine.

    The 7.5 Volts DC (however) is really marginal...

    The datasheet for the LM340 regulator states the 'drop out' voltage is 2 Volts (typical). This means that a 5 Volt regulator is 'on the edge' at 5+2 = 7 Volts. And this is a typical value...

    I suspect one of the following possibilities:

    1. You have a faulty bridge rectifier (CR2).
    2. You have a faulty smoothing capacitor (the one off the main board by the transformer).
    3. A high resistance joint on connector J8.

    I have put them in order of what I think the fault could be.

    Do you know how to check a bridge rectifier for health?

    Dave

  2. #52

    Default

    Quote Originally Posted by daver2 View Post
    The transformer AC voltages look absolutely fine.

    The 7.5 Volts DC (however) is really marginal...

    The datasheet for the LM340 regulator states the 'drop out' voltage is 2 Volts (typical). This means that a 5 Volt regulator is 'on the edge' at 5+2 = 7 Volts. And this is a typical value...

    I suspect one of the following possibilities:

    1. You have a faulty bridge rectifier (CR2).
    2. You have a faulty smoothing capacitor (the one off the main board by the transformer).
    3. A high resistance joint on connector J8.

    I have put them in order of what I think the fault could be.

    Do you know how to check a bridge rectifier for health?

    Dave
    excellent.

    I've checked the bridges using diode check. bridge CR1 checks good in circuit. bridge CR2 shows BAD in circuit but shows good when i remove it. i did this a few days ago and will verify this. any thoughts on the in-circuit vs out of circuit difference?

    I have a replacement 23k mf 15v filter cap on order and it will be here tomorrow.

    I'll check J8 solder. i have found two cracked/cold joints on this pet so far.

    thanks again!

  3. #53
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,553

    Default

    CR1 and CR2?

    Should these be CR2 and CR4 - or have I got the schematic for the wrong board?

    "Bad" as in an open circuit or a short circuit?

    Dave

  4. #54

    Default

    sorry. CR2 and CR4.

    I just rechecked and they test OK per my multimeter diode check function. I have always checked the outside pins and flip the probes and a good diode shows a voltage in one direction and not the other. center pins show OL, same check on the other outer pins.

    CR2 and CR4 show good using this method. note that when i got this pet capacitor C6 1mfd was blown and i replaced it earlier this week. maybe that was hosing up my bridge reading?

    do you recommend another bridge test to do?

  5. #55

    Default

    CR4 is used as a full wave bridge recifier
    CR2 is not used this way. It is used as 2 transformer center tapped rectifiers. I don't believe this can be test in circuit with the transformer still connected.
    This is why I said you need to look at the ripple with a scope!
    If the bridge had 60 cycle ripple it would mean a diode is open. That would cause, as we are currently seeing, voltage. It should have 120 cycle ripple as it is a two diode full wave. The other half of the bridge is to make the negative rail.
    If one of the diode paths is open, a diode meter will show the other one as being there with the transformer connected. Try your diode test with the transformer disconnected ( J8 ).
    Dwight

  6. #56

    Default

    Quote Originally Posted by Dwight Elvey View Post
    This is why I said you need to look at the ripple with a scope!
    Thank you so much. I am not ignoring your previous advice on scoping, examining waves, i just am not sure exactly how to do that.

    In order to do what you're recommending, i would scope at the outputs AFTER regulation (right hand side on below schematic) and look at the waveforms of the +5, +12, -5 regulated voltages?

    If so, I'll scope and post the output and maybe you could give me some advice.

    rsz_pet_voltage.png

  7. #57
    Join Date
    Jun 2012
    Location
    UK - Worcester
    Posts
    3,553

    Default

    Er no.

    We want you to scope the +9 Volts unregulated rail with respect to 0V/GND.

    It would, however, also be interesting to note what the oscilloscope indicates for the +5 Volt rail(s) after the regulator(s) as well.

    Dave

  8. #58

    Default

    First look at the rectified and filtered side. That should have small 120Hz ripple. You need to measure the frequency. 60 Hz bad. 120 Hz better.The regulated side should have almost no ripple.
    Dwight

  9. #59

    Default

    Another beautiful morning, and a few brain cells recharged for knowledge.

    The oscilloscope screenshot below is the +9 unregulated taken at J11 pin 6.

    I see f=120hz, DC voltage = 7.08.

    what does this tell me?

    https://imgur.com/sv6Xyyj

  10. #60

    Default

    What is the vertical scale? What is the vertical volts per division?
    It looks like both diodes are there but i don't see a what the volts per division is.
    Is it 2 volts per division?
    Is this measured on the large capacitor?
    Dwight
    Last edited by Dwight Elvey; February 22nd, 2020 at 05:20 AM.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •