Schematic: http://www.nostalgiaair.org/PagesByModel/623/M0010623.pdf
Be sure both sides of the push pull output are working. Pull 1 - 6F6 at a time and see if volume is about the same for each. A radio will operate with only 1 side of a push pull amplifier but may distort.
Measure voltage on pin #5 of each 6F6 tube from pin #8. Voltage on pin #5 should be around -15 to -20 volts. Both tubes should have about the same voltage.
Norm
:Finished a Majestic model 850. Complete cap/ecap and resistor changeout. Micas were not changed. Alignment performed multiple times. Radio has good sensitivity and receives across the dial. The issue is that the clarity at the speaker is slightly less than desirable. Tone and volume controls are working well. Speaker is in very good condition. This unit has push/pull power output. This radio has an interesting unshielded series coil arrangement under the chassis. Any ideas on how I might "crisp" up the sound?
:
:Schematic: http://www.nostalgiaair.org/PagesByModel/623/M0010623.pdf
:
:
: Be sure both sides of the push pull output are working. Pull 1 - 6F6 at a time and see if volume is about the same for each. A radio will operate with only 1 side of a push pull amplifier but may distort.
:
: Measure voltage on pin #5 of each 6F6 tube from pin #8. Voltage on pin #5 should be around -15 to -20 volts. Both tubes should have about the same voltage.
:
:Norm
:
::Finished a Majestic model 850. Complete cap/ecap and resistor changeout. Micas were not changed. Alignment performed multiple times. Radio has good sensitivity and receives across the dial. The issue is that the clarity at the speaker is slightly less than desirable. Tone and volume controls are working well. Speaker is in very good condition. This unit has push/pull power output. This radio has an interesting unshielded series coil arrangement under the chassis. Any ideas on how I might "crisp" up the sound?
::
::Schematic: http://www.nostalgiaair.org/PagesByModel/623/M0010623.pdf
::
:
:
Hi, had one recently that did the same thing changed all of the caps and resistors but wasn't happy with the sound. I changed the 3 Micas and now it sounds great. I checked the tolerance of the micas and they were way off same with the resistors. Even went as far as installing a new transformer and speaker. Sometimes you just have to get midevil on them I guess. Good Luck, George T.
:
: Be sure both sides of the push pull output are working. Pull 1 - 6F6 at a time and see if volume is about the same for each. A radio will operate with only 1 side of a push pull amplifier but may distort.
:
: Measure voltage on pin #5 of each 6F6 tube from pin #8. Voltage on pin #5 should be around -15 to -20 volts. Both tubes should have about the same voltage.
:
:Norm
:
::Finished a Majestic model 850. Complete cap/ecap and resistor changeout. Micas were not changed. Alignment performed multiple times. Radio has good sensitivity and receives across the dial. The issue is that the clarity at the speaker is slightly less than desirable. Tone and volume controls are working well. Speaker is in very good condition. This unit has push/pull power output. This radio has an interesting unshielded series coil arrangement under the chassis. Any ideas on how I might "crisp" up the sound?
::
::Schematic: http://www.nostalgiaair.org/PagesByModel/623/M0010623.pdf
::
:
:
Hi, had one recently that did the same thing changed all of the caps and resistors but wasn't happy with the sound. I changed the 3 Micas and now it sounds great. I checked the tolerance of the micas and they were way off same with the resistors. Even went as far as installing a new transformer and speaker. Sometimes you just have to get midevil on them I guess. Good Luck, George T.
....Anyway, let us know what you come up with, and then if you're still not satisfied, I can give you some other simple but great ideas.
T.
The neg voltage on the 5 and 8 pins of 6f6g's is -14.
Pulled each 6f6g tube separately as Norm suggested and volume was about the same.
Jumped the micas with capacitor box and no improvement.
So then started hearing a slight pop sound through the speaker with volume getting slightly lower after the pop. Lowered the variac voltage to about 75v and then back to 115v and sometimes the pop and lower volume would repeat. Checked plate voltage and running about 40v above spec.
Then pop and no audio signal at all.
Checked voltages coming out of power supply...OK.
No plate voltage...
Have 200v going into pin 3 of speaker but nothing on pin 4? Speaker field coil...getting hot.
Turned everything off. Unplugged speaker and ohm check on field coil is 820ohms.....should be 680ohms.
Probable short? Was plate voltage high because of field coil shorting?
You have a cap from plate of each output tube to ground, C31 and C33. One of these has shorted since you have no plate voltage and speaker field is getting hot.
Speaker field isn't a problem. It's getting hot since the circuit is drawing too much current.
The other thing that will cause no plate voltage is an open output transformer. In this case a radio will draw less current so not as much heat on the field.
Norm
:Follow up to tests.
:The neg voltage on the 5 and 8 pins of 6f6g's is -14.
:Pulled each 6f6g tube separately as Norm suggested and volume was about the same.
:Jumped the micas with capacitor box and no improvement.
:So then started hearing a slight pop sound through the speaker with volume getting slightly lower after the pop. Lowered the variac voltage to about 75v and then back to 115v and sometimes the pop and lower volume would repeat. Checked plate voltage and running about 40v above spec.
:Then pop and no audio signal at all.
:Checked voltages coming out of power supply...OK.
:No plate voltage...
:Have 200v going into pin 3 of speaker but nothing on pin 4? Speaker field coil...getting hot.
:Turned everything off. Unplugged speaker and ohm check on field coil is 820ohms.....should be 680ohms.
:Probable short? Was plate voltage high because of field coil shorting?
:
There is still 200v at pin 3 of speaker socket which feeds field coil.
Am I correct in thinking that the plate voltage for the tube sockets is coming through the speaker socket?
Also need to correct ohm reading for field coil. It is supposed to be 680ohm and measures 705ohm.
:Scott
:
: You have a cap from plate of each output tube to ground, C31 and C33. One of these has shorted since you have no plate voltage and speaker field is getting hot.
:
: Speaker field isn't a problem. It's getting hot since the circuit is drawing too much current.
:
: The other thing that will cause no plate voltage is an open output transformer. In this case a radio will draw less current so not as much heat on the field.
:
:Norm
:
::Follow up to tests.
::The neg voltage on the 5 and 8 pins of 6f6g's is -14.
::Pulled each 6f6g tube separately as Norm suggested and volume was about the same.
::Jumped the micas with capacitor box and no improvement.
::So then started hearing a slight pop sound through the speaker with volume getting slightly lower after the pop. Lowered the variac voltage to about 75v and then back to 115v and sometimes the pop and lower volume would repeat. Checked plate voltage and running about 40v above spec.
::Then pop and no audio signal at all.
::Checked voltages coming out of power supply...OK.
::No plate voltage...
::Have 200v going into pin 3 of speaker but nothing on pin 4? Speaker field coil...getting hot.
::Turned everything off. Unplugged speaker and ohm check on field coil is 820ohms.....should be 680ohms.
::Probable short? Was plate voltage high because of field coil shorting?
::
:
:
Voltage is supplied through the field coil for all power to tubes in your radio. Resistance of the field isn't critical. It will vary with temperature.
Did you replace filter caps? C28 could be shorted.
Norm
:Changed out two .005 mica caps c31/c33 with called for .002 value. Still no plate voltage.
:There is still 200v at pin 3 of speaker socket which feeds field coil.
:Am I correct in thinking that the plate voltage for the tube sockets is coming through the speaker socket?
:Also need to correct ohm reading for field coil. It is supposed to be 680ohm and measures 705ohm.
:
:
:
:
:
:
:
::Scott
::
:: You have a cap from plate of each output tube to ground, C31 and C33. One of these has shorted since you have no plate voltage and speaker field is getting hot.
::
:: Speaker field isn't a problem. It's getting hot since the circuit is drawing too much current.
::
:: The other thing that will cause no plate voltage is an open output transformer. In this case a radio will draw less current so not as much heat on the field.
::
::Norm
::
:::Follow up to tests.
:::The neg voltage on the 5 and 8 pins of 6f6g's is -14.
:::Pulled each 6f6g tube separately as Norm suggested and volume was about the same.
:::Jumped the micas with capacitor box and no improvement.
:::So then started hearing a slight pop sound through the speaker with volume getting slightly lower after the pop. Lowered the variac voltage to about 75v and then back to 115v and sometimes the pop and lower volume would repeat. Checked plate voltage and running about 40v above spec.
:::Then pop and no audio signal at all.
:::Checked voltages coming out of power supply...OK.
:::No plate voltage...
:::Have 200v going into pin 3 of speaker but nothing on pin 4? Speaker field coil...getting hot.
:::Turned everything off. Unplugged speaker and ohm check on field coil is 820ohms.....should be 680ohms.
:::Probable short? Was plate voltage high because of field coil shorting?
:::
::
::
:
:
Any ideas?
Also check for shorts across the field coil and the negative bias resistor network during this occurrance.
T
:Norm....you nailed it. C28 was shorted out. Jumped in another 10uf/350 and radio came alive. Now here is new concern. The plate voltage is running high as stated earlier. Example 6a8 spec is 225v. At turn on the voltage is 267v and then after 1 minute warm up there is that pop sound followed by a lower volume and the plate voltage jumps to 289v. Something amiss.
:Any ideas?
:
:Regarding voltage spike and abnormally high voltages to begin with, I would check all cathode, screen, and plate voltages when the voltage jumps higher. A screen supply can open up, and the radio might still perform okay, but with the reduced current draw, voltages will climb. Also, one side of the push-pull output could be opening up. Suspect resistors, wires, solder connections, transformers, and even tubes. Occasionally an internal connection in a tube will go bad and open up from thermal stress.
:
:Also check for shorts across the field coil and the negative bias resistor network during this occurrance.
:
:T
:
::Norm....you nailed it. C28 was shorted out. Jumped in another 10uf/350 and radio came alive. Now here is new concern. The plate voltage is running high as stated earlier. Example 6a8 spec is 225v. At turn on the voltage is 267v and then after 1 minute warm up there is that pop sound followed by a lower volume and the plate voltage jumps to 289v. Something amiss.
::Any ideas?
::
:
:
:
:Also check for shorts across the field coil and the negative bias resistor network during this occurrance.
:
:T
:
::Norm....you nailed it. C28 was shorted out. Jumped in another 10uf/350 and radio came alive. Now here is new concern. The plate voltage is running high as stated earlier. Example 6a8 spec is 225v. At turn on the voltage is 267v and then after 1 minute warm up there is that pop sound followed by a lower volume and the plate voltage jumps to 289v. Something amiss.
::Any ideas?
::
:
:
Hi Scott, Norm and Thomas,
I agree, but want to add, that I have been running into intermittant tubes a lot latley. Just coincidence I am sure, but they seem to be all around.
Tube testes usually don't catch them, although I saw one of the intermittant buggers go off on the tube tester. On a tube tester it is easy to attribute that to a bad socket, though.
Best Regards,
Bill Grimm
Also worth noting that the radio is not coming on gradually like a normal tube radio but pops on after brief warm up.
The voltage is missing on cathods of 3 tubes.
The voltage is higher prior to popping sound on cathodes of 6f6's and plate of 6q7. Not sure what that means.
It takes about a minute or 2 after turn on for popping sound and resulting decrease in volume and increase in voltages to occur.
Checked all contact solder joints except resistor in eye tube. Used VTVM after not getting volt readings on cathods with DMM.
Thanks to all for comments. Any additional insights appreciated.
:Also worth noting that the radio is not coming on gradually like a normal tube radio but pops on after brief warm up.
:The voltage is missing on cathods of 3 tubes.
:The voltage is higher prior to popping sound on cathodes of 6f6's and plate of 6q7. Not sure what that means.
:It takes about a minute or 2 after turn on for popping sound and resulting decrease in volume and increase in voltages to occur.
:Checked all contact solder joints except resistor in eye tube. Used VTVM after not getting volt readings on cathods with DMM.
:Thanks to all for comments. Any additional insights appreciated.
:
:
The clue being it pops on. Sometimes a tube has an open cathode connection. As the filament heats cathode metal expands and can either make a connection or open. When a tube causes this it's usually repeatable. Let the radio cool and it will happen again.
Since your volume changes but doesn't disappear might be a 6F6 tube? All other tubes are required for receiving any signals. If you don't have more 6F6's try a set of 6V6's or 6K6's.
Sometimes top caps of tubes have an oxidized solder connection. Try remelting solder on 6A8, 6K7 and 6Q7 tubes.
Norm
::Here is an update from voltage checks.
::Also worth noting that the radio is not coming on gradually like a normal tube radio but pops on after brief warm up.
::The voltage is missing on cathods of 3 tubes.
::The voltage is higher prior to popping sound on cathodes of 6f6's and plate of 6q7. Not sure what that means.
::It takes about a minute or 2 after turn on for popping sound and resulting decrease in volume and increase in voltages to occur.
::Checked all contact solder joints except resistor in eye tube. Used VTVM after not getting volt readings on cathods with DMM.
::Thanks to all for comments. Any additional insights appreciated.
::
::
:
:
So now the popping stopped and I am back to constant voltages that I posted and sound that is very slightly distorted.
What are your thoughts on the high and low voltages?
Don't see any voltage in your chart that should give distortion. Sure don't know how someone expects 3 volts on cathodes when they are grounded in your radio.
Is distortion only heard on strong stations?
Norm
:Right you are. Based on Bill's previous comments I was rechecking tubes in tester. But I let them set for a minute and then one of the 6f6's dropped to 0 on the meter. Repeated and same result. Replaced in the set and good to go.
:So now the popping stopped and I am back to constant voltages that I posted and sound that is very slightly distorted.
:What are your thoughts on the high and low voltages?
:
Warren- Don't have any speakers that are compatible.
Anyway...thanks for all the help.
If you can find a station that doesn't broadcast digital in the sidebands (heard as a rushing sound on each side of the station), its fidelity may be better. A better test of the amplifier would be to pipe in music through the audio amplifier or through a home-made transmitter. Of course if you are piping in high fidelity music through the amplifier, and you truly want to hear all of the detail, all of the high filtering tone capacitors that are in place to remove all of the scratch and heterodyning of typical AM must be disconnected. If you do pipe in music through the amplifier from a reliable source, and are still not satisfied with the audio quality, let me know, and I can tell you of a few circuits that will make the audio more natural and pleasing, as well as crisp.
If the distortion is heard on strong local stations but not distant ones, you might consider having a closer look at the AVC circuit and all RF biasing circuits. If any capacitors are leaky in the AVC circuit, this will keep it from operating, and distortion will result. Improper bias in any of the RF stages due to other reasons will also cause distortion. You show cathode voltages of the RF tubes as being 0, when they should be between 2 and 3 volts. This should be with reference to the control grid of each tube, and not to chassis ground. The cathodes are tied to chassis ground, and will, of course, measure 0 volts if referenced there. If referenced to the respective control grids, be sure to use a VTVM, as impedances are sometimes high here. If bias is off, then there is a fault in the bias network between the chassis and the center tap of the H-V winding of the power transformer. Be sure that no signal is being received, as this would add AVC voltage to some of the grids, and throw off your bias reading.
If distortion is present at all volume levels, consider the amplifier. Be sure that all audio bypass capacitors are new and leak-free, especially when under high voltage. The capacitor at the volume control is critical, too, even though it doesn't block any high voltages. Any leakage, however, will pass AVC bias voltage to the 1st audio grid, and throw off its bias, and cause distortion. All capacitors, when checked on a Solar capacitor analyzer with the shorts test in the 'paper' position, should not flash the neon light at all, even after 10 or 20 seconds, if in good condition. This is critical for all parts of the audio circuit and the AVC circuit. Most new metalized film capacitors are either good or bad, but do not usually develop the very high resistance leakage that paper capacitors develop, but if you are at all in question, test them anyway.
Incidentally, resistors 9 and 10 develop bias not only for the RF circuits but also for the audio circuits, and so if any shorts or faults lie around these resistors, distortion can be introduced into any of the stages biased by these resistors.
T.
Glad I could be of help in finding the popping problem.
I have found the audio quality of these radios to be excptional withing the limits of AM. Audio distortion is abnormal, even in an old radio.
Although I agree with trying another speaker and the other ideas, a weak emission tube needs to be considered also as a possibility.
I have run into disotortion sounding like 'cotton in the mounth' being caused by low emission power output tubes. these tubes will test fine on a tester but change them for a good one and the audio perks right up.
All the Best,
Bill
Also R12 on schematic shows tapped info center of candohm but connection of R12 is to r9 side of candohm which terminates at pin 6 of 6q7 and joins 3 meg r11.
R10 side of candohm is neg side of ecap c27, HV center tap and 1meg r16 to pin 5 of 6q7.
Onward.
What Bill said about tubes is very true. I have had some output tubes, especially when used in push-pull, that would distort. In push-pull, when one was removed, the distortion would go away. In some the distortion was constant, and in others it would appear shortly after the set was turned on. This could be from any number of problems, including gas and partial shorting of the control grid to the cathode or one of the other elements, most likely due to thermal movement or oxide flaking from the cathode. Sometimes, if this is the fault, it can be corrected by jolting the cathode and grid with the highest current setting of the plate supply on your tube tester. Sparks will fly and the problem will go away (often, but not always). Tubes can also test good and have an emission hot-spot on an otherwise so-so cathode. ...Etc. Etc...
Also, in one of my radios the 1st audio tube was faulty. Not sure if it was gassy or what, as it didn't seem gassy, but the radio sounded distorted, badly, and changing out the tube cured the problem. It tested good, but wouldn't work in the radio.
You may be able to isolate the distortion by piping in music from your computer or iPod or any other line-level audio source. If solid state, you will get better results if you connect the headphone jack through a 6.3 volt filament transformer, like those sold at Radio Shack, and then connect the 120 volt part to the end terminals of the volume control of your radio. Adjust the audio source so that it does not distort, and then listen to the radio throughout its volume range and see if you hear any distortion at normal listening levels (very quiet to comfortably loud). If no distortion exists, then likely the distortion you hear when receiving broadcast is due either to trouble in the RF circuitry or to the signal being received.
T.