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Detrola 571A filament string
3/3/2006 4:44:34 PMStan L
This Detrola is the one with 35Z5 50L6 12SA7 12Sk7 12SQ7 lineup. I was working on it today and had it on...it was playing but had distortion, so I was chasing down bad resistors (it has already been recapped). I turned off the power (iso and variac), found R3 to be way high in value, so I replaced it with a 220,000 ohm resistor. Flipped the power on and the pilot lamp went out and the tubes won't light. I tested all the tubes (all good), checked the field coil (333 ohms) and started measuring voltages. All the AC filament string voltages are measuring line voltage. So why wont the tubes light? (I apologize for always asking for advice and never giving any...but that's because I have not found any posts that I can help with so far...just a schematic here of there...most of you know SO MUCH it's amazing...maybe one day I'll be able to help also. Thanks for always being there for us rookies!!!!
3/3/2006 5:17:37 PMStan L
:This Detrola is the one with 35Z5 50L6 12SA7 12Sk7 12SQ7 lineup. I was working on it today and had it on...it was playing but had distortion, so I was chasing down bad resistors (it has already been recapped). I turned off the power (iso and variac), found R3 to be way high in value, so I replaced it with a 220,000 ohm resistor. Flipped the power on and the pilot lamp went out and the tubes won't light. I tested all the tubes (all good), checked the field coil (333 ohms) and started measuring voltages. All the AC filament string voltages are measuring line voltage. So why wont the tubes light? (I apologize for always asking for advice and never giving any...but that's because I have not found any posts that I can help with so far...just a schematic here of there...most of you know SO MUCH it's amazing...maybe one day I'll be able to help also. Thanks for always being there for us rookies!!!!
:PS...I know the heater voltages should add up to the line voltage (approx)..so nothing is consuming any power.
3/3/2006 5:27:21 PMMark
Often what happens in a AC/DC radio when you shut it off and turn it back on is you get a surge of current. You must have blown out one of the filaments. Measure the voltage across each filament. The one with the line voltage across it should be the burnt out one. You can verify it being burnt out by measuring the filament continuity. Remove the suspect tube and measure its' resistance.
MRO
3/3/2006 5:24:57 PMThomas Dermody
I usually give advice and don't ask for any, so we're even here. R-3 really has no electrical current purpose, so changing it probably affected little. It may have cleared up your distortion if there was a shielding problem. R-3 and its paralleling capacitor simply link the chassis to B-, which is one side of the AC line. They prevent the chassis from being connected directly to the line--this way you or whatever else that may touch it won't get a full supply of juice. R-3 helps keep its paralleling capacitor from charging up with any stray charges that may accumulate on the chassis. Sometimes when a charge builds up on the chassis, it can throw off grid biasing on the sensitive control grids. This can especially affect the RF stages (like the oscillator). Sometimes the radio will completely pop out of operation due to one of these charges.

Anyway, the reason why your radio is dead is due to something other than your replacing that resistor. Either the pilot lamp shut filament within the 35Z5 opened up, or a connection to this filament opened up, or you may have a short in the power supply after the rectifier. This would have blow the shunt filament and pilot light, both of which supply both the filament string and the B circuit.

If you have replaced all capacitors, connect your multi-meter's plus lead to the cathode (pin 8) of the 35Z5. Connect the negative lead to B-. This would be the cathode of the 12SQ7 (pin 3) or any other point at this reference (lots of things connect here). Use the X10,000 resistance scale (or the most sensitive one) of your meter. The needle should swing up towards zero and then slowly fall back down due to the charging up of the electrolytics. It should approach infinity. If it doesn't fall all the way down, there is leakage in the B circuit. If it stays towards zero, change meter settings. If you go all the way down to the X1 setting and the resistance is still low, then there's a heavy short in the B circuit, which is probably what caused your pilot lamp to blow. It probably also blew the shunt filament in your 35Z5, which is why your tubes won't light. Find the short and mend it. Then replace the 35Z5 and the pilot lamp. If no short can be found, assume that the shunt filament in the 35Z5 opened up on its own, or that its associated wiring is faulty and opened up (perhaps a dirty pin connection or poor solder joint at one of the pins--the shunt uses pins 2 and 3).

If you don't have a spare 35Z5 on hand and wish to continue listening to the radio even before a replacement can be obtained, temporarily shunt a 47 ohm 2 watt resistor across pins 2 and 3 of the 35Z5. Replace the pilot lamp with a new #47 bulb. Both of these items may be found at Radio Shack. When you replace the 35Z5, be sure to remove the 47 ohm resistor. If you wish to "use up" the old 35Z5, continue using it with the resistor until it becomes weak (you may get old and die before this happens). Just be sure that whenever you replace the tube that you remove the 47 ohm resistor. Leaving it in will leave you with a dim pilot lamp.

In either case, whether you have replaced the 35Z5 or are using it with a 47 ohm resistor, NEVER EVER run the radio without its pilot lamp in place. Not having the pilot lamp across the shunt filament (or the 47 ohm resistor) allows too much current to flow through the shunt filament. This will severely shorten its life. Also, though you can effectively operate the radio in the following way, never shunt a wire across pins 2 and 3. Yes, you can use the radio this way, even though your pilot lamp won't light, but the pilot lamp and shunt filament within the 35Z5 act as a sort of fuse. They'll blow if something goes wrong in the supply. I suppose you could shunt across pins 2 and 3 with a .25 ampere fuse (also found at Radio Shack). This will readily blow if a short occurs within the radio. Your pilot lamp will not light because there is no voltage drop across the fuse.

Thomas

3/3/2006 5:27:55 PMThomas Dermody
Also, once you get the set running again, if you still have distortion problems, make absolutely sure that you replaced the audio bypass condensers--the ones that either feed audio to the grid of the 12SQ7 or the 50L6. If you are sure that you replaced them, and still can't clear up distortion, you may have either a bad 50L6 or a bad 12SQ7. This set also has an unusual Automatic Volume Control circuit, which may cause distortion if not operating properly. Incidently, when does the distortion occur? Does it occur on strong stations only? Does it occur at all volume levels? Does it occur on weak stations?

Thomas

3/6/2006 4:11:29 PMStan L
:Now that the switch is fixed, I am getting distortion at all volumes weak and strong stations (althought strong stations produce a little less). The subject caps have been replaced. I have subbed out all the tubes one by one to no avail. I also must point out that I had to restring the ferrite cores of the tuner (thanks to Dennis the Detrola man's instructions), but I confess it is not perfect...a little more slack than it probably should have. I think that just makes the tuning sloppy, but wanted to point it out. I also mended a 1/4 in tear in the speaker with speaker fabric glue...couldn't find any more tears...maybe I should try another speaker? Thanks for the help!
:Also, once you get the set running again, if you still have distortion problems, make absolutely sure that you replaced the audio bypass condensers--the ones that either feed audio to the grid of the 12SQ7 or the 50L6. If you are sure that you replaced them, and still can't clear up distortion, you may have either a bad 50L6 or a bad 12SQ7. This set also has an unusual Automatic Volume Control circuit, which may cause distortion if not operating properly. Incidently, when does the distortion occur? Does it occur on strong stations only? Does it occur at all volume levels? Does it occur on weak stations?
:
:Thomas
3/6/2006 5:15:16 PMThomas Dermody
Check all resistors. Sub another speaker to see if the distortion might be in the speaker. Be sure that the AVC condenser isn't leaky (that is, unless you replaced it with a new one).

Thomas

3/3/2006 5:44:55 PMDoug Criner
Here's Stan's schematic: http://www.nostalgiaair.org/PagesByModel/075/M0004075.pdf This is an AC/DC set with all tube heaters in series across 120V.

Examine how the pilot lamp is connected to the 35Z5 recitifier. The tube's heater is tapped for the pilot lamp. If just a portion of the heater opens, then the lamp will burn out and the heater string will go dead.

Check for continuity between pins 2 and 7 and between 2 and 3.

I can't explain why properly replacing R3 would have anything to do with this, besides coincidence.

I'm not sure what you mean by this statement: "All the AC filament string voltages are measuring line voltage." Measure ACROSS the heater pins of each tube's socket. If one tube is burned out, then you should measure line AC voltage across that one tube; you should measure zero voltage across the heaters of the other tubes. I can't imagine how 120V could get across the heater of EVERY tube, but if it did, certainly all tubes would be burned out in an instant.

:This Detrola is the one with 35Z5 50L6 12SA7 12Sk7 12SQ7 lineup. I was working on it today and had it on...it was playing but had distortion, so I was chasing down bad resistors (it has already been recapped). I turned off the power (iso and variac), found R3 to be way high in value, so I replaced it with a 220,000 ohm resistor. Flipped the power on and the pilot lamp went out and the tubes won't light. I tested all the tubes (all good), checked the field coil (333 ohms) and started measuring voltages. All the AC filament string voltages are measuring line voltage. So why wont the tubes light? (I apologize for always asking for advice and never giving any...but that's because I have not found any posts that I can help with so far...just a schematic here of there...most of you know SO MUCH it's amazing...maybe one day I'll be able to help also. Thanks for always being there for us rookies!!!!

3/3/2006 6:03:12 PMStan L
:Here's Stan's schematic: http://www.nostalgiaair.org/PagesByModel/075/M0004075.pdf This is an AC/DC set with all tube heaters in series across 120V.
:
:Examine how the pilot lamp is connected to the 35Z5 recitifier. The tube's heater is tapped for the pilot lamp. If just a portion of the heater opens, then the lamp will burn out and the heater string will go dead.
:
Thanks Thomas Mark and Doug...I'll follow your suggestions this weekend. I'm headed to the Atlanta swap meet tomorrow :>)
:Check for continuity between pins 2 and 7 and between 2 and 3.
:
:I can't explain why properly replacing R3 would have anything to do with this, besides coincidence.
:
:I'm not sure what you mean by this statement: "All the AC filament string voltages are measuring line voltage." Measure ACROSS the heater pins of each tube's socket. If one tube is burned out, then you should measure line AC voltage across that one tube; you should measure zero voltage across the heaters of the other tubes. I can't imagine how 120V could get across the heater of EVERY tube, but if it did, certainly all tubes would be burned out in an instant.
:
::This Detrola is the one with 35Z5 50L6 12SA7 12Sk7 12SQ7 lineup. I was working on it today and had it on...it was playing but had distortion, so I was chasing down bad resistors (it has already been recapped). I turned off the power (iso and variac), found R3 to be way high in value, so I replaced it with a 220,000 ohm resistor. Flipped the power on and the pilot lamp went out and the tubes won't light. I tested all the tubes (all good), checked the field coil (333 ohms) and started measuring voltages. All the AC filament string voltages are measuring line voltage. So why wont the tubes light? (I apologize for always asking for advice and never giving any...but that's because I have not found any posts that I can help with so far...just a schematic here of there...most of you know SO MUCH it's amazing...maybe one day I'll be able to help also. Thanks for always being there for us rookies!!!!
3/4/2006 12:17:10 PMStan L
:OK...here's what I know after reading the reply posts. The pilot lamp did not blow. The tubes and continuity checks all good. I get esentially zero voltage readings between heater pins. Each Heater pin shows 117 volts when checked against the AC line that runs to the on/off switch. When checked against B-, they are nothing. Seems like no DC is getting into the radio...I jumped new electrolytics across the 20 and 40 and nothing happened. I must be missing something really obvious here...
:Here's Stan's schematic: http://www.nostalgiaair.org/PagesByModel/075/M0004075.pdf This is an AC/DC set with all tube heaters in series across 120V.
:
:Examine how the pilot lamp is connected to the 35Z5 recitifier. The tube's heater is tapped for the pilot lamp. If just a portion of the heater opens, then the lamp will burn out and the heater string will go dead.
:
:Check for continuity between pins 2 and 7 and between 2 and 3.
:
:I can't explain why properly replacing R3 would have anything to do with this, besides coincidence.
:
:I'm not sure what you mean by this statement: "All the AC filament string voltages are measuring line voltage." Measure ACROSS the heater pins of each tube's socket. If one tube is burned out, then you should measure line AC voltage across that one tube; you should measure zero voltage across the heaters of the other tubes. I can't imagine how 120V could get across the heater of EVERY tube, but if it did, certainly all tubes would be burned out in an instant.
:
::This Detrola is the one with 35Z5 50L6 12SA7 12Sk7 12SQ7 lineup. I was working on it today and had it on...it was playing but had distortion, so I was chasing down bad resistors (it has already been recapped). I turned off the power (iso and variac), found R3 to be way high in value, so I replaced it with a 220,000 ohm resistor. Flipped the power on and the pilot lamp went out and the tubes won't light. I tested all the tubes (all good), checked the field coil (333 ohms) and started measuring voltages. All the AC filament string voltages are measuring line voltage. So why wont the tubes light? (I apologize for always asking for advice and never giving any...but that's because I have not found any posts that I can help with so far...just a schematic here of there...most of you know SO MUCH it's amazing...maybe one day I'll be able to help also. Thanks for always being there for us rookies!!!!
3/4/2006 1:43:17 PMDoug Criner
Stan, I'm having trouble reconciling your findings with the schematic.

Pull the 35Z5, and tell us the resistance you measure between pins 1 & 7 and between pins 2 & 3. Then pull each of the other tubes and measure the resistance between the two pins labeled "H" on the schematic. Write down the resistances and report here.

Also, with the radio unplugged, measure the reistance across the power switch (with the switch in the ON position).

Do all this very methodically and write down your results.

I noticed your statement: "Seems like no DC is getting into the radio..." If the tube filaments don't light, then the rectifier won't work, and there can be no DC. We need to get your filaments to light first.


3/4/2006 1:44:37 PMDoug Criner
Stan, I'm having trouble reconciling your findings with the schematic.

Pull the 35Z5, and tell us the resistance you measure between pins 1 & 7 and between pins 2 & 3. Then pull each of the other tubes and measure the resistance between the two pins labeled "H" on the schematic. Write down the resistances and report here.

Also, with the radio unplugged, measure the reistance across the power switch (with the switch in the ON position).

Do all this very methodically and write down your results.

I noticed your statement: "Seems like no DC is getting into the radio..." If the tube filaments don't light, then the rectifier won't work, and there can be no DC. We need to get your filaments to light first.


3/4/2006 1:46:36 PMDoug Criner
Sorry, for the 35Z5 I misspoke. Check the resistance between Pins 2 & 7 and between Pins 2 & 3.

:Stan, I'm having trouble reconciling your findings with the schematic.
:
:Pull the 35Z5, and tell us the resistance you measure between pins 1 & 7 and between pins 2 & 3. Then pull each of the other tubes and measure the resistance between the two pins labeled "H" on the schematic. Write down the resistances and report here.
:
:Also, with the radio unplugged, measure the reistance across the power switch (with the switch in the ON position).
:
:Do all this very methodically and write down your results.
:
:I noticed your statement: "Seems like no DC is getting into the radio..." If the tube filaments don't light, then the rectifier won't work, and there can be no DC. We need to get your filaments to light first.
:
:
:

3/4/2006 5:00:06 PMStan L
:OK...here are the readings
35z5 pin 2 to 7 = 32; 2 to 3 = 7.7
50L6 = 46.1
12sa7 = 12.6
12sq7 = 12.3
12sk7 = 12.3

Measuring across the switch, I get infinite both on and off, so something is wrong. I checked another switch and got an infinite reading in off and a very low reading in on, as I would expect. Call me a rocket scientist, but I notice that the heater resistances look like the voltage reading should look. That is probably not a coincidence, right? Thanks for helping me!

:Sorry, for the 35Z5 I misspoke. Check the resistance between Pins 2 & 7 and between Pins 2 & 3.
:
::Stan, I'm having trouble reconciling your findings with the schematic.
::
::Pull the 35Z5, and tell us the resistance you measure between pins 1 & 7 and between pins 2 & 3. Then pull each of the other tubes and measure the resistance between the two pins labeled "H" on the schematic. Write down the resistances and report here.
::
::Also, with the radio unplugged, measure the reistance across the power switch (with the switch in the ON position).
::
::Do all this very methodically and write down your results.
::
::I noticed your statement: "Seems like no DC is getting into the radio..." If the tube filaments don't light, then the rectifier won't work, and there can be no DC. We need to get your filaments to light first.
::
::
::

3/4/2006 5:57:47 PMDoug Criner
Stan, it appears that your power switch is bad.

You can try squirting some contact cleaner or WD-40 into the contacts and maybe the switch will start working. Or, you can temporarily jumper around the switch until you come up with a permanent fix or replacement. That would enable you to keep working on your radio.

It appears that the power switch and the 500K volume control are on the same shaft? RadioDaze R-SPA-500K ($1.69)audio pot should work along with S-POT1 ($0.79), which is a power switch that mounts on the rear of the pot. The pot has a 2" shaft, which you can cut shorter with a hack saw.

3/4/2006 8:40:38 PMThomas Dermody
Sorry, sounded like you said that the pilot light burned out. Guess it just went out with the tubes like you said. Since it went out, it does sound like the switch could be saved with WD-40, as Doug said. I suggest that before you replace the control (should WD-40 not cure the problem), you open it and examine the switch. The switch can be removed from the back of the control after the main shell is removed from the control--the switch is attached to this shell from inside on most controls. On others it is riveted on. With these, gently drill out (or file off) the rivets. Then remove the switch. For re-assembly, place switch over rivets and solder in place. If a spring or lever has fallen out of place, often things can be straightened out and the switch can be saved. Improvise.

THomas

3/5/2006 5:15:26 PMStan L
:Bingo...took the switch off the back and cleaned it with contact cleaner...tested continuity and it now powers up fine. In about 60 radios, this is my first bad switch. Thanks very much guys!!!
Stan

:Sorry, sounded like you said that the pilot light burned out. Guess it just went out with the tubes like you said. Since it went out, it does sound like the switch could be saved with WD-40, as Doug said. I suggest that before you replace the control (should WD-40 not cure the problem), you open it and examine the switch. The switch can be removed from the back of the control after the main shell is removed from the control--the switch is attached to this shell from inside on most controls. On others it is riveted on. With these, gently drill out (or file off) the rivets. Then remove the switch. For re-assembly, place switch over rivets and solder in place. If a spring or lever has fallen out of place, often things can be straightened out and the switch can be saved. Improvise.
:
:THomas



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