I just recapped and have new tubes that check good. I also subbed other known good tubes during this test. The following was noted:

On power up the radio had very loud static, but it could bring in stations all along the dial the way it should. I suspected a noisy vol control or tone control and changed the volume control after checking it (was falling apart and measured 3Meg side to side and should have been 1 Meg). I noticed the tone control was scraping internally like it was broken so I took it apart and it was in pieces, so I jumped a 50K resistor in its place as I do not have a replacement yet and powered up again.

All the static was gone and it sounds great as long as the volume is low. As I turn it up past about 1/4 turn it begins a motorboat sound and squeals, once it's at highest volume, the motorboating sound is gone but the squeal stays and is loud. This happens on strong and weak stations and on all bands.

I am not sure how to troubleshoot from here. Does antone have any ideas? Thanks ahead for your advice.

Thanks Edd.

I did replace all the tubes with metal ones except the rectifier and the 6F6. The 6F6 was from a working radio and the rectifier was the original one. All the other tubes are new metal and were changed out a couple times to troubleshoot.

All caps were replaced and are new except the micas.

I did not check all resistors, but did find an out of tolerance R9, which i replaced with a 3 Meg,1/2W 5% one.

I did clean all the tube pins, but not the sockets (except I did use an electronics wash. They are hard to get to the inside of).

I will try the tapping and rubber glove test you recommend. I assume you are looking a dressing and possible other problems this way. I'll try shielding by hand each tube as well in case there is a bad ground on one.

Thanks. I'll get back during this weekend.

:
:

: : : : : : : :Sir Steve . . . . . : : :Sounds fine on the volume control replacement and the " fixed" tone is a viable workaround until the control :can be located. : :That second problem , when the volume hits a cetain threshold, could be a tricky thing to :isolate and pinpoint without checking several aspects. ( Unless it could be a microphonic 6Q7 which should :quench the effect when you grasp the tube envelope to cease its sensitivity to vibration.) : : :Initially the tube lineup tends to suggest all metal tubes being used, any chance that sometime in its :lifetime that a tube replacement ended up with a GT . . glass envelope tube(s) version ended up getting :placed in the unit in RF positions instead of the use of a metal / shielded tube ? : : :That possibly might account for unwanted interstage coupling . . .feedback . . and oscillation. If that is the :case, you might further confirm that by bringing up the volume right to that effects onset and then clasping :a hand around the glass tube to see if that then changes the frequency, if a squeal, or speed of the problem, :if a motorboating situation. : : :At least that clues in as to that area being sensitive, suspect and possibly related to the problem. When doing :this, have the fingertips touching the chassis, or you might actually worsen the conditon by not having your :hand grounded to the chassis. : : : :Also down in the power supply, you assuredly have replaced the C15 and C16 electrolytics and installed properly : if having no fireworks, but can you also confirm that you have also changed out C8 . :Also that the RF stages screen bypass capacitor C2 got changed out . : : : :Also wouldn't hurt to do a tube plugging in and unplugging multiple times to scrape contacts clean. Then, do an :individual tube tapping, lastly, tap the frame of the tuning condenser to see if it might have time onsetted connectivity :problems. : : : :Last thought is related to the two different thresholds involved, what I will do is sheath my hand in one of the heavier :duty Playtex kitchen gloves of Mamma Mias, get the set right at the ONSET of either (eventually both) of the problem :conditions and start grasping components and wiring . : : :Move selectively across the bottom of the set and you may find an area which enacts a change , and that is getting you : closer to some idea of the problem area is. : : :Specifically, is this going to be a feedback / shielding or PS decoupling problem. : : : : : : :73's de Edd : : : :

:
:

I had more time this afternon and after trying your suggestions, the noise reduced a little when I sheilded the 6F6 and 6Q7 with my hand.

So I started poking around the wires near them under the chassis and noticed that when I touched the capacitor that is connected to the rear tap on the volume control, the noise changed a lot. I can't find this one on the schematic though (i am still new to this). It was connected to the resistance side of the tone control (now that 10k resistor) on my set where the plate connection to the 6F6 is and it is .005mf. That's how I found it when I recapped the set this week.

Someone did work on this set before, and the tapped terminal on the old volume control was broken and was soldered to the volume control case where it came out, so it was may have been grounded then however the same problem occurred then also, making me wonder why the ground either did not work, ie. the solder joint itself was bad.

Then I disconnected that .005 cap from the center tap on the volume control and the motor boating was gone, but still noisy when the volume went up. I cant find a .005mf on the parts list, I am not sure what it should be or if it should have even been there. After grounding it to the chassis, it is even quieter with no squeal except at about 3/4 the way up the volume control. It then goes away altogether at the max level. I noticed a cap C17 on the schematic that is identical to this arrangement, so it appears correct now except for the value. I may sub a .002 like the schematic says later on.

Should I just leave the rear tap of the volume control unconnected? I noticed that the volume control I am using now (rebuilt one from another Belmont, a 778A) doesn't seem grounded where it comes out of the case. The set does play quite well now. I'll never use it at the 3/4 level as it is too loud anyway. Do you know what criteria I should use to find another tone control?
Thanks again, your experience is invaluable to me.

:Thanks Edd.
:
:I did replace all the tubes with metal ones except the rectifier and the 6F6. The 6F6 was from a working radio and the rectifier was the original one. All the other tubes are new metal and were changed out a couple times to troubleshoot.
:
:All caps were replaced and are new except the micas.
:
:I did not check all resistors, but did find an out of tolerance R9, which i replaced with a 3 Meg,1/2W 5% one.
:
:I did clean all the tube pins, but not the sockets (except I did use an electronics wash. They are hard to get to the inside of).
:
:I will try the tapping and rubber glove test you recommend. I assume you are looking a dressing and possible other problems this way. I'll try shielding by hand each tube as well in case there is a bad ground on one.
:
:Thanks. I'll get back during this weekend.
:
::
::

:: :: :: :: :: :: :: ::Sir Steve . . . . . :: :: ::Sounds fine on the volume control replacement and the " fixed" tone is a viable workaround until the control ::can be located. :: ::That second problem , when the volume hits a cetain threshold, could be a tricky thing to ::isolate and pinpoint without checking several aspects. ( Unless it could be a microphonic 6Q7 which should ::quench the effect when you grasp the tube envelope to cease its sensitivity to vibration.) :: :: ::Initially the tube lineup tends to suggest all metal tubes being used, any chance that sometime in its ::lifetime that a tube replacement ended up with a GT . . glass envelope tube(s) version ended up getting ::placed in the unit in RF positions instead of the use of a metal / shielded tube ? :: :: ::That possibly might account for unwanted interstage coupling . . .feedback . . and oscillation. If that is the ::case, you might further confirm that by bringing up the volume right to that effects onset and then clasping ::a hand around the glass tube to see if that then changes the frequency, if a squeal, or speed of the problem, ::if a motorboating situation. :: :: ::At least that clues in as to that area being sensitive, suspect and possibly related to the problem. When doing ::this, have the fingertips touching the chassis, or you might actually worsen the conditon by not having your ::hand grounded to the chassis. :: :: :: ::Also down in the power supply, you assuredly have replaced the C15 and C16 electrolytics and installed properly :: if having no fireworks, but can you also confirm that you have also changed out C8 . ::Also that the RF stages screen bypass capacitor C2 got changed out . :: :: :: ::Also wouldn't hurt to do a tube plugging in and unplugging multiple times to scrape contacts clean. Then, do an ::individual tube tapping, lastly, tap the frame of the tuning condenser to see if it might have time onsetted connectivity ::problems. :: :: :: ::Last thought is related to the two different thresholds involved, what I will do is sheath my hand in one of the heavier ::duty Playtex kitchen gloves of Mamma Mias, get the set right at the ONSET of either (eventually both) of the problem ::conditions and start grasping components and wiring . :: :: ::Move selectively across the bottom of the set and you may find an area which enacts a change , and that is getting you :: closer to some idea of the problem area is. :: :: ::Specifically, is this going to be a feedback / shielding or PS decoupling problem. :: :: :: :: :: :: ::73's de Edd :: :: :: ::

::
::
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