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#1
05-27-2013, 08:32 PM
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Dumont RA-109A help needed - very little high voltage
Its been recapped and finaly ready to go , well the sound works but no raster or sound of horz sweep , i tried the probe and theres maybe 1kv on the anode , tried the 1x2 that is v31 it has a connection from pin 1 according to sams it goes to a doorknob and to ground , it has more voltage when doorknob disconnected (2kv ) i tried replacing the doorknobs it made no change , sams shows a doorknob from pin 9 of v32 (1x2) to pin 1 of v31 (1x2) and there is none in the set nor signs there ever was one , also observed v31 gets hot fast , replaced the 1x2 tubes and no change , if you measure pin 1 of v31 when set turned on it shoots up and almost gets to 5kv before it quickly drops to 1kv , fly seems ok , any ideas where to go from here ? horz out or osc area ?
i did notice i didn't hear vertical roll or anything like that , will double check thanks mike 
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#2
05-28-2013, 12:50 AM
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the vertical is working i can hear it when i rotate the vert hold , starting to wonder if the horz freq and hold control/transformer could be at fault ? or the horz lin or horz size ? this would cause not enough voltage on the anode right ? i ask because i did go over all the resistors and replaced any that where not right , there is one other thing , horz reactance tube v25 in sams it says 6BC5 is installed in the set , 6AG5 is listed as an alternative and theres a 6AG5 in the set , i wonder if i were to put a 6BC5 in if it would correct this ?
mike
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#3
05-28-2013, 09:08 AM
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If the 1X2 is getting hot I would suspect something is loading the high voltage down. Have you checked the picture tube. If it is "gone to air" it may be loading down the high voltage. What is the normal high voltage (KV) on this set?
Are you sure the doorknob caps are OK? They do tend to breakdown under load. Check the 1X2's after you have run them hot. They are not a robust tube and die fairly easy if loaded hard.
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#4
05-28-2013, 09:11 AM
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If one or more of your 1X2s are getting hot, you have either a bad 1X2 or a overload on the hot tubes output (filament).
If a 1X2 tests good on a tube checker, it can still be bad under actual operating conditions.
__________________
John Folsom
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#5
05-28-2013, 10:50 AM
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i tried changing the 1x2 tubes and no change , the crt has not gone to air , i will have to take voltage readings on the horz out tubes which wont be easy since the set has the crt and yoke mounted in the cabinet of the tv.
mike
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#6
05-28-2013, 11:25 AM
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I have the Same set and it's the A version, and I would agree with the other 2 members about about something loading it down if one of the 1x2 tubes get hot,
try measuring HV with the CRT 2nd Anode disconected, and probe right into the connector coming out of the HV cage. I am due for open surgery this week so I cant pull the set out and compare for you.. (open hernia surgery sucks,I tend to lift way over my limit and I'm trim and fit !)
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#7
05-28-2013, 12:13 PM
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Years ago to check to see where the overload was, we would remove the cap from the 1X2 to the flyback and let it hang in mid air. Using a well insulated screwdriver NOT connected to ground, bringing it near the cap should make a fairly long spark to jump to the screwdriver. If the spark goes way down when the cap is reconnected, it means the overload could be from the 1X2 on to the CRT. If I remember right, this set has a high voltage doubler so arc over of the flyback may not be as possible as with some half wave setups. Don't leave it on long, just enough to test the spark. Check for pinholes in the 1X2 filament windings. This usually makes a burning smell unless they are carbonized badly and are really loading the high voltage down. From your description, as horizontal output starts working the 1X2 filament starts warming up and a leak path to ground will get worse as the tube warms up.
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#9
05-29-2013, 08:43 AM
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remove the caps from the 1X2 tubes. One of them will have the RF from the sweep circuit on it. Using a well(!) insulated screwdriver, just go near the cap from the flyback and if everything back from the flyback is good a spark should jump to the screwdriver tip. If this spark is less than 1/4 inch I would look to horizontal/damper/yoke circuit. If it is good, look for troble in the 1X2 circuitry.
Don't ground any part of the screwdriver. Hold the plastic end and just go near the cap with the tip. Don't leave the set on a long time as the flyback will be unloaded at the time of testing.
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#10
05-29-2013, 12:51 PM
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with the anode disconnected from the crt the voltage shoots up to around 14kv , so its obvious the metal part of the crt is somehow being grounded ? does that make sense ? so time to try to figure out how , if theres something wrong with the crt itself it will be a shame because it has good emissions
mike
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#12
05-29-2013, 02:08 PM
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there is a ground strap that sort of curls upward on the lower left side of the CRT, this grounds metal parts around the front of the cabinet, and there are some sort of insulators between the brackets that hold the Yoke and hold the CRT in place.
somehow one or all of them are not assembled correct and it is shorting the lip of the CRT to ground . I would remove the CRT and or see what the HV connector is touching on the CRT itself..
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#13
05-29-2013, 03:55 PM
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I assume this is a metal cone CRT?
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#14
05-29-2013, 04:06 PM
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the yoke is on the glass part of the bell (?). Possibly dirt/grease has become conductive and leaking to the metal bell.
Also, does the emission start slowly and rise rapidly when you test the tube? My T-933 CRT checked incredibly well but when the set was turned on the neck went purple 
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#15
05-29-2013, 04:06 PM
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loosed the yoke bracket , moved the yoke back and no resistance , so i put the chassis in i have a picture , i do have an arcing sizziling issue in the hv section that stops if brightness turned up all the ay , so i have to figure out whats happening there and how the yoke is making contact with the cone , no its not the ground straps .... well this will answer the clean or not to clean post - no choice
mike
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