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lets break it down
all the following with CRT anode off to take the CRT out of it. shunt cap off 30kv at anode lead shunt cap on 19kv to 22kv HV setting to max so the shunt is drawing current with pot a the max HV setting two things can cause it to draw excessive current 1)bad shunt tube (gassy/grid emissions), replace with a KNOWN working one. 2)incorrect bias (voltage divider resistors/pot) leakying cathode/grid cap on shunt tube (upsetting the bias as well). It has to be one of the above. Did you alter any wiring around the shunt? does the cathode resistance to ground change the the HV pot? do you have resistance checks for the tube pins on the shunt. did you REPLACE the grid to cathode cap on the shunt? if so did you make sure the replacement cap was good? Last edited by DaveWM; 01-10-2015 at 10:44 AM. |
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I wonder if your HV reg tube socket is making proper contact with the tube pins? If not that would explain correct voltage at the socket, but incorrect behavior of the tube.
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Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4 |
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is it possible the wire burned inside the insulation ? if it did then i would pull the wire slightly and if it stretches then that could be it so ill look into that. the pins look very good and the tube fits tight in the socket, but i could check that again.
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19kv with the HV pot maxed and the cry disconnected means a shunt regulator problem, does it have one of those 100meg resistors in the circuit somewhere? Might have drifted. You need to take readings of the regulator tube cathode and grid, post them here. The bias on the regulator is certainly funny...it's dragging your HV down for some reason.
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I think you should try switching places of those 2 1.5meg ohm resistors......
Turn the horiz centering control.... Does it do anything....? What exactly does it do....? Turn the focus control, do you have a range on it....? Does it do a lot, or not....? .
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Yes you can call me "Squirrel boy" Last edited by Username1; 01-12-2015 at 07:03 PM. |
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A long shot but there is usually a low ohm resistor
in the HV rect filament inside the HV cup. Also if you unplug the CRT socket does the HV come up ? May have been covered but the thread is getting long ! And dont forget the H. blanking tube, causes all sorts of odd problems. 73 Zeno |
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the horiz centering control is just about all the way to one side it will swing from one side to another but to center the screen the pot is all the way to one side. the focus does have range although it could be a bit clearer. and the 1.5 meg resistors i was going to try 2- 1 meg resistors but that i think will only kind of change the pot position from maybe maxed to the half mark.
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Well changing the resistors from 1.5 to 1.0 would not do much.... I figured if they
were off, like the one you said was 1.5 and the other 1.6 and the pot .5 meg, then it may have changed the range of HV if it shifted the bias on the grid of HV Reg. So, with no plug on end of crt, hv changes by how much...? And no HV lead attached to tube HV goes to how much...? Can you make both of those readings again with no other work done in between...? And one more.... HV connected, Plug on tube.... Brightness at cutoff - Dark Screen..... What id HV.....? Make each of these tests with the HV adjusted to MAX..... And write out each reading, and condition for the reading.... Acceptable answers are not "same as before" and or "they are good". We need the condition, and the value - please. And the cap between the grid and cathode of HV reg, you replaced that one....? Anyone got an objection to running the set without that cap for a test...? Just in case it's not working at that high voltage.... As a matter of fact, you should make these tests on both your sets with the same chassis, or allow someone with the same set chassis to do them and have an ongoing comparison..... And for each of these tests, please also record the B+ Boost..... I think it's 390V..... .
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Yes you can call me "Squirrel boy" Last edited by Username1; 01-13-2015 at 08:50 AM. |
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And what are the voltages at pin 1 and pin 5 of the 6BK4 Shunt when the HV adjust
is at min, and max....? .
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Yes you can call me "Squirrel boy" |
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6bk4 reg tube pin 1 402 volts and pin 5 390 volts but i dont remember if the bright was down when i had checked it as i hope this helps.hv current dark screen hv pot low 1.64 pot high 0.93 bright up pot low 1.16 pot high 0.20
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If you mean .93ma with a dark screen and HV set to max, that's your problem. Most of the Sams I've read for roundies specify .8ma with a dark screen, HV set to whatever the schematic calls for. If you're pulling any more than that, chances are you're getting close to melting the 6BK4 as you're likely exceeding it's dissipation. You see HV schematic information has more to to with the regulator tube than it does the chassis.
For example, the 6BK4B (which is what most will have in their chassis) is rated at 40 maximum plate watts. Taking into account a 20% tolerance, this tube can be expected to dissipate somewhere around 32 watts continuously. Typically it comes to around 25 watts though, RCA designed the shunt circuit conservatively to maximize the shunt tube's life. So for your example with a dark screen and the HV pot at 'low' and drawing 1.64ma (assuming a 23kv anode), that comes to 37.7 watts- easily above what RCA would have designed it to do, and dangerously close to the tube's maximum dissipation. I strongly suspect that if you look at the tube's structure under those conditions with a darkened room, you'll see the plate glowing which can be seen reflected off the bottom part of its structure (the part containing the grid and cathode, it's usually shiny so you can see stuff in it). For the other example: .93ma@23kv=21 watts which is much better but again, outside what I would consider normal for a shunt tube which should be closer to .8ma. Obviously whenever you have something on screen, shunt current should drop as the CRT beam current picks up where the regulator left off. You should check to make sure the shunt tube's cathode current drops as brightness increases, if not the regulator circuit is suspect. Also take a look at your B+boost voltages, as that's where your regulator is taking its reference voltage from. If B+boost is jacked, so will regulator cathode current as the grid receives it's marching orders from whatever B+boost is doing.
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