03-12-2019, 06:08 PM
Do not let production changes guide the sensible, analytical, stage by stage repair of this radio...
Begin by resolving the B+ issue, is it really low, is it bad measuring technique or what. Then fix it...
Jumping in without factual, accurate measurements will result in good components replaced or irreplaceable components ruined. Continue and it is better to toss the chassis into the scrap...
The Riders print is very comprehensive, a lot of data in just a few pages.
Since my test questions were not answered, I will assume the power transformer is good. Have one or more 84 rectifiers been tried, tested? it is possible to have several bad tubes of any type and come to a bad conclusion...
You can check the total current consumption in the B circuit of the radio by measuring the voltage across a resistor, install a 1 ohm resistor in-between the chassis ground and Cap 59 and resistor 59 a 1 or 2 watt will be fine carbon or whatever. Operate the set and measure the voltage drop across the resistor. Use ohms law and get the current flow. Do correct for the actual value of the resistor and make measurements with the same VOM...
The current should be some 10% less than the actual cathode current of the 84 tube, if is is larger then there is a problem downstream eating current. Look up the current capacity of the 84 it is on the web...
By following the schematic, taking the actual value of the B dropping resistors it is possible to find the offending branch of the B circuit. Simply measure the voltage drop across that known value.
Brute force testing? Sure go ahead its your radio substitute some 3 amp diodes for the 84 and let out the smoke. That will find the short made by rewiring the radio...
Oh, are you using a 1000ohm/volt VOM or messing about with a DVM?
FYI the caps are part of the trimmer structure, leaves of mica and sheets of tin plated brass, they never fail...
This radio will test your analytical prowess, either you have got it or not...
It will not respond to shotgunning or fishing for someone who had the same problem like "change R273 and the problem is solved"...
YMMV
Chas
Begin by resolving the B+ issue, is it really low, is it bad measuring technique or what. Then fix it...
Jumping in without factual, accurate measurements will result in good components replaced or irreplaceable components ruined. Continue and it is better to toss the chassis into the scrap...
The Riders print is very comprehensive, a lot of data in just a few pages.
Since my test questions were not answered, I will assume the power transformer is good. Have one or more 84 rectifiers been tried, tested? it is possible to have several bad tubes of any type and come to a bad conclusion...
You can check the total current consumption in the B circuit of the radio by measuring the voltage across a resistor, install a 1 ohm resistor in-between the chassis ground and Cap 59 and resistor 59 a 1 or 2 watt will be fine carbon or whatever. Operate the set and measure the voltage drop across the resistor. Use ohms law and get the current flow. Do correct for the actual value of the resistor and make measurements with the same VOM...
The current should be some 10% less than the actual cathode current of the 84 tube, if is is larger then there is a problem downstream eating current. Look up the current capacity of the 84 it is on the web...
By following the schematic, taking the actual value of the B dropping resistors it is possible to find the offending branch of the B circuit. Simply measure the voltage drop across that known value.
Brute force testing? Sure go ahead its your radio substitute some 3 amp diodes for the 84 and let out the smoke. That will find the short made by rewiring the radio...
Oh, are you using a 1000ohm/volt VOM or messing about with a DVM?
FYI the caps are part of the trimmer structure, leaves of mica and sheets of tin plated brass, they never fail...
This radio will test your analytical prowess, either you have got it or not...
It will not respond to shotgunning or fishing for someone who had the same problem like "change R273 and the problem is solved"...
YMMV
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
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