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 Post subject: screen voltage issue
PostPosted: Oct Fri 23, 2020 6:40 pm 
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Joined: Sep Sun 27, 2020 9:28 am
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I cannot find other references to this particular problem with my Hickok 600A tube tester. P4 does not work. I have replaced the 5Y3 and 83 tubes. No help. I checked plate and heater voltages. They are ok. However, the screen voltage is only 8 volts. I checked the bias and English pots. They are ok. I set the bias pot fully clockwise and measured the dc voltage at pin 5 (setting for 6V6 tube w/o tube in place). Reading should be around -40 vdc. Readjusted bias control to read 22 and checked voltage again. Voltage should read -3 vdc. Instead, the reading is -.05 vdc. Almost no voltage. Do I have a transformer problem or perhaps something else. How do I verify? audacity for windows


Last edited by heywantgold on Oct Sat 24, 2020 11:01 pm, edited 1 time in total.

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 Post subject: Re: screen voltage issue
PostPosted: Oct Fri 23, 2020 8:38 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 8459
Location: Sunbury, Ohio 43074
A lot of the time, this sort of problem can be due to dirty, or oxidized, switches or even tube sockets. Before you go spraying a ton of cleaner into any switches or pots, please search this forum for threads on exactly this type of procedure in tube testers. You can easily create leakage paths in the switches that may be very difficult to un-do.

Might be enough to just exercise every switch about a million times to "self clean" the contacts. That works a lot of the time, and you can then elect to apply some DeOxit D5 via a Q tip to only the contact surfaces of the switches in question.

Otherwise, deeper troubleshooting is going to be needed. Start by putting the original tubes back in. No need to add more variables just yet.

_________________
Preserving the hist. of electronics, one boat anchor at a time! :)
https://www.bbtvtestequipment.com


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 Post subject: Re: screen voltage issue
PostPosted: Oct Sat 24, 2020 12:35 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 423
Location: Green Valley, AZ, USA
Check the both pots for continuity and resolder the connections as you might have a cold solder joint. Years ago I had a similar problem and discovered a cold solder joint that looked perfect.
Hank


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 Post subject: Re: screen voltage issue
PostPosted: Oct Mon 26, 2020 5:05 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 4642
Location: Lincoln City, OR 97367
Greetings to heywantgold and the Forum:

Your indications are consistent with a problem in the screen supply. The bias voltage is developed from the negative side of the screen supply (center tap of the 5Y3 plate winding). Note that all voltage measurements must be made with respect to pin 8 (cathode) of the octal socket when the tube tester is set up to test a 6L6 (standard test and calibration setup; JR-53472).

Quote:
They are ok. I set the bias pot fully clockwise and measured the dc voltage at pin 5 (setting for 6V6 tube w/o tube in place). Reading should be around -40 vdc.


You do not mention what voltage you see under this condition; only what it should be, which is about -40 volts. If you actually see -40 volts at a setting of 100 and it drops to near zero at a setting of 22, then either there is a problem in the grid circuit or the bias pot is open. If you do not see -40 volts with the bias pot at 100, then you should check the bias fuse (if your tester is so equipped). Bear in mind that the most negative point in the screen supply connects through the grid signal winding and R27 to the grid directly. The bias pot wiper actually moves the cathode of the tube under test away from the most negative point of the supply. Therefore, if your troubles persist, you should perhaps measure from the center tap of the 5Y3 filament winding (negative probe) to the center tap of the 5Y3 plate winding (positive probe). You should see approximately -130 volts (low Z meter). If you then move the positive probe to the center (wiper) terminal of the bias pot, you should be able to vary the voltage on the wiper from about -130 (bias control at zero) to about -96 volts (bias control at 100). If good, follow the negative voltage through the grid signal winding and on to the cathode of the tube under test (pin eight).

Somewhere in here you should come to the reason for the loss of both voltages (grid and screen).

Good Luck,

_________________
Jim T.
KB6GM


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