05-29-2019, 08:34 PM
Since pulling the tubes did not matter. That, says the excess load is coming from some location from a tube socket back to the feed (source) side of the speaker field coil. I'm still thinking it is a solder blob or solder flow in between a wafer socket or snipped end of a component wire shorting.
It is preferable to measure the resistance of the power supply B+ that is the load as I stated if it is greater than 8K it is probably O.K., if it is lower that is the indication of the heavy load short.
Why is it better to use a resistance measurement? Because the load is so heavy there is risk burning out the field coil or the power transformer.
Follow the B+ as it feeds each stage, the output stage is via the output transformer and a direct wire to the screen of the output tube. To isolate the stage lift the screen wire and the B+ feeding the output transformer.
If the resistance reading rises dramatically that is the area of the short...
If not move on to R24, then R34, then R14, to isolate the if's one has to actually remove the B+ wire from the IF termination.
The voltage dropping network of three resistors totals 86K, that sets up the minimum current flow for the B+. I do not know the design of these resistors. Some types can short to the steel case if the happens then the current will go up and B+ voltage drop... That 86K should yield 1ma approx based on the highest plate voltage. Not gonna call up the NA schematic, again...
So, if there is no problem on the B+ placing the ohmmeter + on C45 positive and chassis ground negative respectively should yield 86K after the caps have charged, if not then do the "lifting" (de-soldering) and disconnecting a wire to a stage. Do so until the resistance is 86K then check that divider if it is truly is 86K
Note if there is no other leakage, the resistance across the "B" supply will be this 86K, if it is any lower that is what the problem is...
Look at R14 the 32K resistor at the far left, that lower buss travels across to the white wire, all the connections to that white wire have to be lifted, one at a time to locate that short circuit. Since you already know when you lift the white wire at the field coil the high current flow stops.
The B+ sink is any tube and the voltage divider resistors.
Note there are a couple of small bypass caps on the B+ be sure they are O.K. but they will show up if the load removal process is done.
Use an analog ohmmeter, some DVM's don't like inductors like a field coil as a resistance.
Got It?
Fair warning cheat the test process and results will not be found. There is no "easy way out"...
GL
Chas
Embarrassing question: Did this radio work to some extent before it was refurbished?
It is preferable to measure the resistance of the power supply B+ that is the load as I stated if it is greater than 8K it is probably O.K., if it is lower that is the indication of the heavy load short.
Why is it better to use a resistance measurement? Because the load is so heavy there is risk burning out the field coil or the power transformer.
Follow the B+ as it feeds each stage, the output stage is via the output transformer and a direct wire to the screen of the output tube. To isolate the stage lift the screen wire and the B+ feeding the output transformer.
If the resistance reading rises dramatically that is the area of the short...
If not move on to R24, then R34, then R14, to isolate the if's one has to actually remove the B+ wire from the IF termination.
The voltage dropping network of three resistors totals 86K, that sets up the minimum current flow for the B+. I do not know the design of these resistors. Some types can short to the steel case if the happens then the current will go up and B+ voltage drop... That 86K should yield 1ma approx based on the highest plate voltage. Not gonna call up the NA schematic, again...
So, if there is no problem on the B+ placing the ohmmeter + on C45 positive and chassis ground negative respectively should yield 86K after the caps have charged, if not then do the "lifting" (de-soldering) and disconnecting a wire to a stage. Do so until the resistance is 86K then check that divider if it is truly is 86K
Note if there is no other leakage, the resistance across the "B" supply will be this 86K, if it is any lower that is what the problem is...
Look at R14 the 32K resistor at the far left, that lower buss travels across to the white wire, all the connections to that white wire have to be lifted, one at a time to locate that short circuit. Since you already know when you lift the white wire at the field coil the high current flow stops.
The B+ sink is any tube and the voltage divider resistors.
Note there are a couple of small bypass caps on the B+ be sure they are O.K. but they will show up if the load removal process is done.
Use an analog ohmmeter, some DVM's don't like inductors like a field coil as a resistance.
Got It?
Fair warning cheat the test process and results will not be found. There is no "easy way out"...
GL
Chas
Embarrassing question: Did this radio work to some extent before it was refurbished?
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
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