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Author Topic: TL922 Grid Current  (Read 18324 times)
K6JEY
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« Reply #15 on: March 05, 2014, 05:52:32 PM »

Part of the original damage was that the multifunctional meter went open. Originally the RF choke shorted and the tubes blew when the RF went down the B+ path. Doug
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W1QJ
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« Reply #16 on: March 05, 2014, 09:05:16 PM »

Part of the original damage was that the multifunctional meter went open. Originally the RF choke shorted and the tubes blew when the RF went down the B+ path. Doug

Doug, if the meter opened then it was destroyed.  How did you repace that bad meter?  Where did you get it the replacement?  You said it was "rebuilt" who rebuilt it?
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K6JEY
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« Reply #17 on: March 06, 2014, 12:01:59 AM »

The previous owner who rebuilt most of the amp had it rebuilt. My assumption is that if it reads high voltage properly, it should read grid current properly. Yes?  Doug
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W1QJ
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« Reply #18 on: March 06, 2014, 03:30:14 AM »

The previous owner who rebuilt most of the amp had it rebuilt. My assumption is that if it reads high voltage properly, it should read grid current properly. Yes?  Doug

The original meters in those amps are unobtainable.  If someone substituted a different meter movement for the original meter movement the grid shunt resistor would have to be changed.  Since HV is usually read by making a string of resistors in the HV line to pass a current through the meter that may have already been done but not the grid shunt resistor.  About a year or so ago myself and Gordon N6WK (now SK a couple months) talked on the telephone at length and we came up with a source of meters that were identical in size and with the same arc swing to the meter with the same size scale and even a red needle.  He would scan an original meter scale on good paper stock, and glue it to the face of the mule meter.  He would sell these along with a resistor that needed to be changed in he amp.  It is imperative that this resistor be changed or else it would not read correctly.  It may read the HV correctly if the current meters were the same but not the grid current unless that resistor was changed.  We were in touch several times over this until he tested the system with the resistor we had calculated.  Also, as I recall we had to change a resistor inside the meter as well.  We found that the original meters were inconsistent and the resistor inside had to be determined on a unit basis.  The difference was about 5 ohms or so and so he stocked several in the range and used the resistor required.  I did not get much involved with he sale of the meters.  I suppose he sold several and thenI  noticed Martin Tippen W0AKG was buying them in quantity from Gordon and then reselling them.  I suppose what I am getting at is, if that meter is a "look alike" it may very well need some circuit change to make it work properly.  As I recall there is a "meter series" resistor up on the board next to the shunts.  That resistor should now be 1500 ohms as I recall.  If it is a knock off meter.   Now I don't remember if we just did this for the Ip meter.  If we did and someone used the knock off meter as the multimeter then it may not have worked as is seen in your case.  This is why the amp history is important.  I really don't remember but I think we just made the meter for Ip.  See what I am getting at?  To fix your problem the meter you have needs to be determined what the internal resistance is and the FS currennt is then we can reshunt the grid resistor and you will be in business.  Consider someone may have put any meter in there for a sale of the amp at a flea market as long as the meter passed the "look test".  I have fabricated many meters for vintage amps where they are no longer made.  This is what I know about these Kenwood meters. Lou
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W1QJ
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« Reply #19 on: March 06, 2014, 04:18:01 AM »

The more I think about this I think we just cloned the Ip meter.  Normally both the meters in the amp have the same movement and can be interchanged.  As I said, if the meter is a rebuild it probably has different specs to it and since we don't know the exact history of the amp or about the meter it is possible more work is needed in the circuit to make that meter work properly.  I am willing to bet this is the problem.  You need to determine the FS current and the internal resistance.  We then an calculate a new grid shunt resistor for you.  It is very hard to measure a grid shunt resistor as they are only fractions of an ohm.  I would suggest removing the wires from the meter, measure the resistance of the meter, then set up a current source and put a 1ma FS meter in line and read the FS current of the meter.  Then we calculate a new grid shunt resistor.
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W8JI
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« Reply #20 on: March 06, 2014, 05:10:25 AM »

What do you mean the meter was rebuilt?
  Good question Lou.

When reading HV the meter functions as a current meter.

When reading grid current, the meter functions as a voltmeter.

If the meter was altered in resistance in the process of rebuilding, it can read correct HV but read incorrect current.

http://www.w8ji.com/metering_amplifier.htm
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W1QJ
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« Reply #21 on: March 06, 2014, 05:25:59 AM »

What do you mean the meter was rebuilt?
  Good question Lou.

When reading HV the meter functions as a current meter.

When reading grid current, the meter functions as a voltmeter.

If the meter was altered in resistance in the process of rebuilding, it can read correct HV but read incorrect current.

http://www.w8ji.com/metering_amplifier.htm

EXACTLY WHAT I MEAN     If he can measure the resistance of the meter and check the FS current we can get a shunt that will fix him up.  I'm sure this is the problem.
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W8JI
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« Reply #22 on: March 06, 2014, 05:30:35 AM »

He doesn't need to change the shunt. All he has to do is stick a resistor in series with the meter.
It will only change the current reading.

The article I linked tells him how and why, and he will learn what to do and not to do when designing metering systems..

http://www.w8ji.com/metering_amplifier.htm
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W1QJ
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« Reply #23 on: March 06, 2014, 06:15:27 AM »

He doesn't need to change the shunt. All he has to do is stick a resistor in series with the meter.
It will only change the current reading.

The article I linked tells him how and why, and he will learn what to do and not to do when designing metering systems..

http://www.w8ji.com/metering_amplifier.htm

I don't think there is a series in the grid circuit, if he needs one it will have to be outboard of the meter.
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W8JI
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« Reply #24 on: March 06, 2014, 09:24:47 AM »

He doesn't need to change the shunt. All he has to do is stick a resistor in series with the meter.
It will only change the current reading.

The article I linked tells him how and why, and he will learn what to do and not to do when designing metering systems..

http://www.w8ji.com/metering_amplifier.htm

I don't think there is a series in the grid circuit, if he needs one it will have to be outboard of the meter.

That's not what I am saying.

He said the meter reads OK high voltage. When the meter reads HV, the panel meter functions as a **current** meter. So if reads OK, his movement has the correct FS current.

He says it reads wrong on grid current. When reading current, the panel meter functions as a voltmeter. This means it is the wrong FS voltage.

Since we know current calibration is correct (because HV scale reads correct), and we know it reads wrong on grid current where the meter becomes a voltmeter across the shunt voltage, this must means he has the wrong resistance movement.

The fix is to add a resistor in series with the meter so the FS voltage comes up to the correct FS value.

All he has to do is add a single resistor in series with the meter and the problems will go away on all scales.

73 Tom
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K6JEY
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« Reply #25 on: March 06, 2014, 11:22:55 AM »

 Hi, sounds like the easiest and best solution. I'll add a little pot in series with the grid meter position and adjust to the right reading.
       What's the right reading? I have a perfect plate meter and could substitute it with clip leads, tune the amp up to 100ma of grid current (250ma of plate current indicated on the substituted plate meter), change meters (unplug and discharge everything) and set the faulty meter with the pot to the same scale deflection. Should be 100ma. 
    Version two- I have a clamp on milliameter and could clamp along the IG lead and read the actual current and then set the meter.
    BTW, I have been using an HP34401A DVM that measures accurately to .001ohm. I also have a 1ohm lab standard resistor.
    The clamp on meter is interesting and quite accurate. It is an ES687.
   
    Thanks to all the excellent postings, I have a good idea of what the problem is. In the next few days, I'll work on the above plan and post back here what the results are.
     Again, thanks very much for your help. Doug K6JEY
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W8JI
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« Reply #26 on: March 06, 2014, 11:46:36 AM »

Hi, sounds like the easiest and best solution. I'll add a little pot in series with the grid meter position and adjust to the right reading.
       What's the right reading? I have a perfect plate meter and could substitute it with clip leads, tune the amp up to 100ma of grid current (250ma of plate current indicated on the substituted plate meter), change meters (unplug and discharge everything) and set the faulty meter with the pot to the same scale deflection. Should be 100ma. 

Doug,

The page I liked tells you exactly what to do.

If you know the meter movement FS current and resistance, and you know the shunt, the rest is easily calculated.

For example, you have .68 ohms for a shunt. That shunt's voltage is .68 times the grid current FS.

If the meter is 300 mA, the shunt voltage is .3 times .68 = .204 volts. This means you need a 204 mV meter to read 300 mA full scale. With a 1 mA movement the meter TOTAL resistance has to be .204 / .001 = 204 ohms.

If you measure the movement and it is a standard 50 mV 1 mA movement, you would have a 50 ohm movement. The series resistor would be 204-50 = 146 ohms. A 150 ohm would be close enough.

Without using a pot and a current source, and without messing around, you could calculate the resistance and have a result closer than anything you could do. If you tell me:

Quote
FS meter scale grid current you desire on the scale

Shunt value (.68 ohms)

along with any two of the three following:

Quote
Meter movement FS current

Meter movement FS voltage at the meter terminals

Meter movement resistance at the meter terminals

I can tell you exactly what resistance to add.

The reason I put that metering page up is a buddy of mine from Australia who build and sells amplifiers was having problems with this, and a popular guy on QRZ forums was telling people how to do metering incorrectly. He was getting ground loops in the meter wiring and advising others to do the same mistake. It isn't a rare thing to do metering systems incorrectly, so I understand the problem you are having, but it is actually faster and better to learn how to do it.

When reading current with a shunt the panel meter actually has to be calibrated to a certain VOLTAGE.
When reading voltage with an external multiplier the panel meter has to be calibrated to a certain CURRENT.   

All of this is easy to calculate once we get our heads around that.

73 Tom
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G3RZP
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« Reply #27 on: March 06, 2014, 11:53:42 AM »

One problem I have seen is that where the meter can be switched to measure the HV, when switched to anything else, the series resistor to HV is connected to a switch tag and the tag floats up to the HV. Example is the Collins 30L1. Since the switch is a phenolic wafer not rated for 1500 volts or so, it is a bit dodgy even though the 30L1 seems to get away with it. The 30L1 isn't the only amp that does this. A 100k or so resistor from the switch tag to ground removes the possibility of arcing.
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W8JI
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« Reply #28 on: March 06, 2014, 12:30:25 PM »

One problem I have seen is that where the meter can be switched to measure the HV, when switched to anything else, the series resistor to HV is connected to a switch tag and the tag floats up to the HV. Example is the Collins 30L1. Since the switch is a phenolic wafer not rated for 1500 volts or so, it is a bit dodgy even though the 30L1 seems to get away with it. The 30L1 isn't the only amp that does this. A 100k or so resistor from the switch tag to ground removes the possibility of arcing.

The 922 is OK there, Peter. Unlike the Collins, the 922 has a shunt resistor on the HV divider. That part is just fine.

The TL922 has a ground loop in the grid current metering system, but if the screws are all tight on the shunt board it will be OK.

I think in this case what someone did is change the meter movement, and they used the wrong sensitivity. It has the correct current, but the resistance is wrong. They probably used a 50 mV movement.

That is just my assumption.

I don't want to sound funny about all this, but it is frustrating to see manufacturers not know how to metering systems properly. I understand why it confuses the typical person who never thinks about the systems, but not someone who works in electronics at a major manufacturer.
« Last Edit: March 06, 2014, 12:38:25 PM by W8JI » Logged
K6JEY
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« Reply #29 on: March 06, 2014, 01:07:15 PM »

Hi Tom,
   Thanks very much.  I'll pass on the numbers. Probably tomorrow.
    can I get your email? Mine is drzarkof56 atyahoodotcom 
      Doug
       
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