High Gid Current

Discussion in 'Amateur Radio Amplifiers' started by NE7X, Jun 5, 2011.

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  1. W8JI

    W8JI Ham Member QRZ Page

    OK, that's grid current.


    OK, that's a problem. Your grid meter appears to be reading plate current.

    That's another problem. Grid current is NOT read in series with the zener. Cathode current is read at that point, and it would include plate current. This really just verifies the grid meter is reading the wrong thing.

    [​IMG]


    Yes, something is probably shorting the B- to the chassis. If the power supply B- shorts to the chassis, the grid meter will be in parallel with the plate meter. Since someone worked on this amp for months, it is hard to guess what the problem might really be. It could have had a simple problem, and he might have moved some wires around and really screwed things up. It is a PITA to have to undo additional problems.

    For the grid meter to read forward and read cathode current, the B- rail of the amplifier has to be somehow tied to the chassis. Does the plate current meter read correctly??

    73 Tom
     
  2. NE7X

    NE7X Ham Member QRZ Page

    The person who I purchased this basket case from told me not once, ten times, that he has gone through the meter circuit and everything checked perfect. So I trusted him and assumed the problem was not in the meter circuit. Anyway tonight after reading everyone's comments here on this forum, and the only circuit which I did not check, I started tracing meter circuit wires per the schematic and guess what? I found two wiring differences and four different resistor values per the factory schematic. Maybe this is some sort of modification or upgrade, I have no idea. However at this time I am totally disassembling the entire meter circuit on the back of the front panel and I am rebuilding/rewiring it 100% back to original per the factory schematic. So with any luck (keep your fingers crossed) by tomorrow this problem will be resolved. As soon as I complete the rewiring, I will post my finding to this forum.

    Isn’t it fun fixing other peoples’ mess? Two good things is coming from this:

    1) I paid just about nothing for it and once I’m done I am going to have myself a very clean looking and working Henry amplifier.

    2) I am learning a lot about grounded grid RF amplifiers and biasing.

    73s, Thomas NE7X…
    http://ne7x.com
     
  3. NE7X

    NE7X Ham Member QRZ Page

    Here is the Henry 2K Classic meter circuit. I am in the process of re-wiring the meters to this schematic, however in doing so I noticed one original factory harness wire connect which does not match this schematic.

    The wire coming from the high side of D107/R119 connected to the HV supply. The schematic shows it connected direct to R108/C112/D105 and to the high side of R109. However the factory harness wire actually is connected to the low side of R109 (diode D106 side). Should I leave it on the low side or move it to the high side like the schematic shows?

    [​IMG]

    Does the rest of this schematic look correct? I am a bit concerned, I have been informed by others Henry schematics have been known to have wiring errors. I do not know if this maybe why the meter circuit was modified and wired differently and the guy kept telling me there was nothing wrong with the meter wiring. I don’t want to rewire the meter circuit only to create new issues.

    73s, Thomas NE7X…
    http://ne7x.com
     
  4. W1QJ

    W1QJ Ham Member QRZ Page

    It appears to me that the low side (as you call it) of c112 and R108 is the "actual" B- connection. and that is where you would want the meter to be connected. It seems to me if it was connected to the bottom of R109 which is the clamp resistor it would be the wrong spot to have it. I might be wrong but I seem to agree with the schematic. Also, did you check D106 to see if it was OK?
     
  5. NE7X

    NE7X Ham Member QRZ Page

    I tested D106 in-circuit twice and it appeared to be good. However with you being persistent, I decided to actually remove it this time and test it again out of circuit and put the question to rest. Well what do you know! BINGO! You were correct! D106 is dead shorted, zero ohms both directions. Per the Henry manual, D106 is a 100V PIV 3A diode. I believe I have substitute in my junk box.

    So now I have the meter circuit totally disassembled (Heathkit style) and it needs to all be put back together. No bigly, however I want to make sure I re-wire it correctly. You indicate the wire coming from D107/R119 going to should not be tied to D106/R109/C114, it should be tied to R108/R109/C112/D106, correct?

    How about rest of the metering circuit? Does it check? Am I OK to rewire everything per the schematic?

    73s, Thomas NE7X…
    http://ne7x.com
     
  6. W8JI

    W8JI Ham Member QRZ Page

    What values are the resistors???

    That would make a difference on which side of R109 you connect. Also, the schematic you posted is not enough to answer your questions. Need values, need more schematic showing more of the system.

    Odds are you fixed it by replacing the diode. That probably was the whole problem.
     
  7. W1QJ

    W1QJ Ham Member QRZ Page

    Madonna Mia, I said days ago to lift one leg of D106 and a couple of other diodes in that area. Glad I was so persistant!! Just sometimes I do know what I am talking about :) I hope this is the fix. At least we found some problem that needed fixing, maybe we are getting close if not maybe we nailed it??? Pray Tell
     
  8. NE7X

    NE7X Ham Member QRZ Page

    You are most likely correct, the diode being shorted was most likely the root cause of all my problems. However when I checked the meter circuit wiring, it was not wired correctly per the schematic, that is why I disassembled it. So now I need to put it all back together and I want to do it the correct way.

    The only question I have right now, originally D107/R119 (150 ohm) tied to R109 (25 ohm / 25 watt) where D106/C114 ties. The schematic shows D107/R119 going to C112/R108/R109/D105. Which one is correct?

    I think other than this, I will just wire the rest of the meter circuit per the schematic, keep my fingers crossed, and power up the amp.

    73s, Thomas NE7X…
    http://ne7x.com
     
  9. W1QJ

    W1QJ Ham Member QRZ Page

    R109 appears to be the clamp resistor and the top part of it seems to be the B- proper. I think the schematic is correct and I would wire it per the schematic.
     
  10. NE7X

    NE7X Ham Member QRZ Page

    Good News / Bad News

    I finished re-wiring the meter circuit per the schematic. I used all the shut values (.2 ohm) as indicated in the parts list.

    Good News
    Now when I key, no drive, HV meter reads 3000V and Ig reads zero.
    When I keyed with 50 watts drive. I get 200-250ma Ig reading.

    Bad News
    The plate Ip meter now pins. I tried moving the wire I had in question on both sides of R109, no difference.

    One thing I noticed, the guy before me wired the Ip meter negative side in series with the 150 ohm resistor and D107 to ground. This was one of the differences I noticed and why I removed all the meter wires. However Ip meter worked before.

    Any suggestions what I need to do to get the Ip meter working again?

    73s, Thomas NE7X…
    http://ne7x.com
     
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