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Author Topic: TL922 Grid Current  (Read 18335 times)
K6JEY
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« on: March 04, 2014, 12:03:51 PM »

Here is a strange one. I have a Kenwood TL922. It had a number of problems which were all fixed. At this point it operates normally except that the grid current pins the meter (I figure 400ma indicated) when the input power, output power, plate current and plate voltage all are normal.
    I know you are thinking it is the meter shunt. Changed and measured: no difference.
    The diode off of the shunt: changed: no difference. Old one was ok.
    The plate voltage indicates normally, so one assumes the grid metering circuit is ok.
       I have run out of logical ideas or parts to fiddle with. Anyone have a good idea? 
     Thanks Doug K6JEY  (drzarkof56atyahoo.com)
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N4ATS
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« Reply #1 on: March 04, 2014, 12:50:14 PM »

Your sure D-2, D-7 are OK?
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W8JI
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« Reply #2 on: March 04, 2014, 01:13:18 PM »

A bad ground or open ground trace path on the board with the grid shunt, X54-1300-10, commonly causes your problem. It will cause very high grid current readings and not affect anything else.

Ohm across the grid meter with it in the grid position and you should see about .8 ohms both directions. This is the shunt and wiring and switch contact resistances in series, so this will vary. Or, better yet, measure each end of R5 on that board to the chassis. One end should be .7 ohms and the other zero.

That is a design flaw. It is an improper way to implement a shunt, because it makes the shunt resistance very unreliable. If you want to make the grid reading reliable, the grid meter really has to ground to the foil on X54-1300-10, and then that board is grounded. The way it is now sets you up for problems like this, and damage to the meter. 
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K6JEY
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« Reply #3 on: March 04, 2014, 02:55:59 PM »

Hi, Great idea. I will do a thorough job of checking it. Doug K6JEY
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N4ATS
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« Reply #4 on: March 04, 2014, 05:35:24 PM »

That is not a poor design by any means, the TL-922 is a major work horse and is reliable. You are sure D-2, D-7 are OK? I'll bet one is open.
« Last Edit: March 04, 2014, 05:54:50 PM by N4ATS » Logged
W8JI
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« Reply #5 on: March 04, 2014, 05:56:32 PM »

That is not a poor design by any means, the TL-922 is a major work horse and is reliable. You are sure D-2, D-7 are OK? I'll bet one is shorted.

The grid meter circuit is a poor design. A current meter with external shunt meter should NEVER ground independently of the meter shunt. That is just asking for problems.

 
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N4ATS
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« Reply #6 on: March 04, 2014, 07:11:56 PM »

Amazing , the 100 plus TL-922's that have been thru here over the past 30 years have never had an HV reading issue that I know of (he is a first) . The original question was "You are sure D-2, D-7 are OK?"

Awaiting an answer...
« Last Edit: March 04, 2014, 07:24:19 PM by N4ATS » Logged
NJ1K
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« Reply #7 on: March 04, 2014, 08:18:44 PM »

Amazing , the 100 plus TL-922's that have been thru here over the past 30 years have never had an HV reading issue that I know of (he is a first) . The original question was "You are sure D-2, D-7 are OK?"

Awaiting an answer...

He wasn't talking about a HV reading issue... He was talking about a grid current reading issue...
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G3RZP
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« Reply #8 on: March 05, 2014, 12:17:07 AM »

As a general rule of good practice, connecting to a shunt should be independent conductors - making a Kelvin connection. Frequently isn't done that way, though.
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N4ATS
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« Reply #9 on: March 05, 2014, 04:10:15 AM »

Ahh, My bad , I jumped from the "pegged meter" thread to here, Wrong thread. You are all right...Continue (sorry)
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W8JI
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« Reply #10 on: March 05, 2014, 04:16:38 AM »

Ahh, My bad , I jumped from the "pegged meter" thread to here, Wrong thread. You are all right...Continue (sorry)

Lighten up and it won't happen.

All I said was meter shunts should have the meter across the shunt, not some other path. With a meter grounded to some other point, all it takes is a loose screw or high resistance connection and the meter gets whacked.

I never said is was the end of the world, just a bad meter wiring practice. Everyone should either know that, or learn that. It isn't about a person being right or wrong. It is useful in electronics life.
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W1QJ
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« Reply #11 on: March 05, 2014, 05:21:53 AM »

I'm sure loads of Kenwood TL-922 and TL-922A owners are reading this thread.  So while we are talking about the shortcomings of these amps I just like to mention that just in the last week I have had 2 TL-922A amps in for repairs.  Both amps had blown grid chokes.  Not just Kenwood but several other MFG's opted to float the grids.  In both cases I directly grounded the grids as I always do with SB-220's as well.  Tom discusses this on his web page.  Both needed the 10 meter mod done as well, so that was also performed.  Adding a negative rail clamp diode also done per JI.  Soft keys also added.  I see no problem with input swr after the direct grounding.  Amp is easier to drive. makes more power, and performs much nicer on 10 meters.  In addition, the self (resistor) cutoff bias mod was done also.  It's funny how back then the amp MFG's copied poor designs.  Thanks to Tom for picking up the grid meter grounding shortcoming.
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K6JEY
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« Reply #12 on: March 05, 2014, 03:00:00 PM »

Hi,
   Here's the results from the suggestions. The shunt resistor shows .68 ohms on one end and zero on the other (HP 34401a DMM 2wire and nulled). The grid meter in the IG position shows .8ohms each way.
   Both diodes replaced. The meter was rebuilt (HV indicates normally)
    In operation the resting plate current is 50ma on cw and 100ma on phone. That says that the diode is biasing the tubes fairly well. The tubes are used. Do I have a bizarre tube? (Input power and output power and Ip are all in range)
   Does that lead me to accepting that the grid current reading is real? How would it do that and not flatten the tube?
      and I am really stumped and I am very appreciative of your help and suggestions   Doug K6JEY
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W8JI
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« Reply #13 on: March 05, 2014, 04:55:38 PM »

Hi,
   Here's the results from the suggestions. The shunt resistor shows .68 ohms on one end and zero on the other (HP 34401a DMM 2wire and nulled). The grid meter in the IG position shows .8ohms each way.
   Both diodes replaced. The meter was rebuilt (HV indicates normally)
    In operation the resting plate current is 50ma on cw and 100ma on phone. That says that the diode is biasing the tubes fairly well. The tubes are used. Do I have a bizarre tube? (Input power and output power and Ip are all in range)
   Does that lead me to accepting that the grid current reading is real? How would it do that and not flatten the tube?
      and I am really stumped and I am very appreciative of your help and suggestions   Doug K6JEY

Can you build a current source to test the metering system?

Do you know how to do that?
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W1QJ
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« Reply #14 on: March 05, 2014, 05:35:18 PM »

What do you mean the meter was rebuilt?
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