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Author Topic: AL-811H grid current meter  (Read 2526 times)
N3QE
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Posts: 2070




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« on: December 09, 2012, 12:21:44 PM »

Got a used 811H. It seems to amplify just fine.

But... grid current meter sticks negative whenever power is on, whether the amp is keyed or not. When I remove power, the grid current meter relaxes back to zero just about like the HV decays. (More below).

Took out the tubes. Grid current meter still sticks negative with power on.

Checked the back PC board MOV's with DC supply, no problem. Installed W8JI mods (remove grid resistors, ground grids, add GDT's). Replaced a dead-short D16 diode on the metering board. Put DC voltage on the filaments with an external current-limited DC supply, no leakage from filament circuit to ground. Still, grid current meter sticks negative.

I am thinking, from the way the grid current meter relaxes back to zero at about same rate as HV decays when power removed, that the grid current meter is reading the current through the plate HV meter divider circuit. The 1M resistors are 1M (at least on the ohmmeter, I didn't test at a kV, but the HV meter reads about 1650V at idle which has to be right). The 180 ohm resistor is 180 ohms, the 330 ohm resistor is 330 ohms, the 0.6 ohm resistor is 0.6 ohms, the 1.5 ohm resistor is 1.5 ohms. RLY1B contacts on the back board work fine (seems to be part of bias network.) Checked and cleaned all the grounds to/from the metering board.

What am I missing? I have a hard time wrapping my head around the grid current metering circuit. I swear at least once while wiggling the front panel back and forth the grid circuit meter worked correctly. Is it possible... there's a short internal to the grid current meter, maybe to the lamp circuit? May have to tear it apart.
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N3QE
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« Reply #1 on: December 09, 2012, 12:34:43 PM »

AH-HA!!!

Turns out "thinking out loud" in my first post put me on the right track. I undid the meter lamp positive wire, and now the grid current meter movement works correctly. I may dig into the grid current meter and figure out what's wrong inside of it but for now I have a working (if unlighted) grid current meter!!!
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KE3WD
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Posts: 5694




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« Reply #2 on: December 09, 2012, 05:25:38 PM »

...I swear at least once while wiggling the front panel back and forth the grid circuit meter worked correctly. Is it possible... there's a short internal to the grid current meter, maybe to the lamp circuit? May have to tear it apart.

That is part and parcel of troubleshooting. 

Never overlook something like that, for in effect, it means that you have pinpointed the area of the problem. 

Good on ya for going in there and investigating the clue. 

Or, as I like to say on the test bench, "She's talkin' to me..."

Then all I need do is figure out what it is she's tryin' to SAY. 

All these decades of repair, I've learned that it is great to have lots of neat test equipment, schematics and service manuals plus the experience and knowledge to use them well, but if
asked to think about it, I'd say a goodly number of successful troubleshooting and repair came
down to using the human senses, mechanical intrusions to see what happens when you flex, poke or even lightly drop it back onto the bench from an inch or two, just this sort of thing as found here.  Eyesight, sense of smell, ability to sense excess heat without actually touching parts that may have high voltages riding on them.  Plus the reasoning of just plain common sense.  You did well. 

I love it when the plan comes together. 

Good Troubleshooting!

73
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W1QJ
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« Reply #3 on: December 10, 2012, 04:20:39 AM »

ineresting, I would like to know what the fix is for this or perhaps how the voltage for the lamp is getting into the grid circuit of the meter?  On those amps doesn't the lamp just hing in a hole in the meter?  I don't see how there is an electrical connection involved.  Will you be going any further to find out why?  Let us know the fix.
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N3QE
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Posts: 2070




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« Reply #4 on: December 10, 2012, 06:22:27 AM »

ineresting, I would like to know what the fix is for this or perhaps how the voltage for the lamp is getting into the grid circuit of the meter?  On those amps doesn't the lamp just hing in a hole in the meter?  I don't see how there is an electrical connection involved.  Will you be going any further to find out why?  Let us know the fix.

You are right, the lamp circuit is supposed to be isolated from the meter circuit. But in my case, it wasn't!!!! Lamp current was taking a path through the meter movement and pinning it to the left as long as the amp was on.

I think the Ameritron big boy amps have little lamp cartridges that are isolated by plastic from the meter movement. But the 811H has a lamp wire isolated only by a few millimeters of air at best.

See the Ameritron 811H meter at http://www.ameritron.com/Product.php?productid=400-2600A the lamp and movement are kinda clouded by the transluscent case.

If you look in the pic above the zero set screw, you can see (through transluscent plastic) the tubular lamp. Wires emerge from each end of the lamp and pass right in front of the screws on the movement. In my case one of the lamp wires curly-Q'ed around in a way that had it touch the movement.

In the end the fix was trivial... just bend the lamp wire away from the movement screw. Maybe I could have re-engineered the meter lamp so it was actually separated by an insulator other than air but I didn't :-).

Tim.
« Last Edit: December 10, 2012, 06:31:09 AM by N3QE » Logged
KE3WD
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Posts: 5694




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« Reply #5 on: December 10, 2012, 09:22:33 AM »


Another "Good Onya" plus one Attaboy for resisting the rather common urge to "overfix" a found problem. 

Bending that terminal well out of the way should last a long, long time. 

After all, how many of those meters are in service vs how many might exhibit this particular problem?  (the answer would be a statistically insignificant number...)

I shudder nearly every other day at the testbench when opening up some unit for repair only to discover somebody's notion of a need for re-engineering that which does not need it. 

"Who put all this glue and crap inside the meter?  And why?"


73
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W1QJ
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Posts: 1439




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« Reply #6 on: December 10, 2012, 11:30:22 AM »

So happens I have an 811H here for repairs, I will have a look at this.  Thanks
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N4CR
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Posts: 1650




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« Reply #7 on: December 10, 2012, 04:23:29 PM »

So happens I have an 811H here for repairs, I will have a look at this.  Thanks

I have one for repair here, too. I'll also look at how the lighting wires intersect the meter wires. Or not.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
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