[bobclausen]

https://philcoradio.com/library/download...l.%201.pdf

I am working on this 3116X and it does work quite well. I’ve changed all the caps, out of range resistors and the tubes test well. It picks up the only local am stations near me and the sound is good. However, it does have this rumbling noise coming thru the speaker. 

The rumbling diminishes as it warms up but doesn’t totally go away. Also if I touch the grid cap of the 1st audio tube, a 77, it sounds like a firecracker going off and seems like the speaker diaphragm is going to blow off. Again I don’t know where to begin troubleshooting this as well nor do I know if this is normal.

I’m not experienced enough to know where to start troubleshooting either of these two issues.If anyone knows where I should start and what to check for it would be greatly appreciated. Any and all advice/suggestions are welcome as I have much to learn.

[morzh]

Have you t ried to see if the grid cap needs re-seating? Touch the grid cap with a plastic rod (anything plastic), see if the cracker appears. That owuld mean it needs re-soldering. It could be either tthe tube's cap, or the lead cap, or both.

[bobclausen]

Thanks Morzh. I’ll try that and see what happens.

[bobclausen]

I tried it with a plastic rod and the cracker noise didn’t show up.

[morzh]

Did you touch or tap?

I'd prefer tapping, but if this yields no result then this is not the problem.

[bobclausen]

Touch and tap, no cracker noise.

[tbone]

If you think the issue is with the 77 tube, you could swap the two around to see if the problem clears.

You could remove the 37 detector to see if the problem is truly in the audio section, or deeper in the radio. The 42 driver could also be removed as a process of isolation.

Tubes can do funny things at their age! I know I do at my age

I know tube shields are also critical, but in theory, a set warming up and getting better would not generally be a shield. Still worth checking.

[bobclausen]

Tony

I don’t know if the 77 is the issue. I was just surprised at how loud it was when I touched it. I did swap out the 77 with another. Same noise. The radio works well but I would like to know if that loud of noise is expected when you touch it.

I haven’t removed the 37 or 42 yet. What should I be looking for when I do that?

[morzh]

I am not sure I understood: so, the radio now works well and the noise only appears when you touch the 77' grid cap, or, as you described it in your first post, it comes all the time but goes down as the radio warms up.

It is expected to have a fairly loud hum and buzz when a grid of a 1st audio is touched.

You could also film the performance and the noises, put it on youtube and give us the link.

[Brad Winder]

I have experience the "rumble" noise while rebuilding a couple different sets and both times I traced it down to faulty mica caps. The one time it happened, the mica turned out to be a paper cap in disguise.

Both times I used cold spray to find the problem.

[radiorich]

Hello guys,
I also came across what Brad Winter mentioned about "faulty mica caps." One was an RCA AA5 that had a mica capacitor in the audio section, which was causing some strange noises.

Sincerely Richard

[Arran]

It could be the grid cap lead, some of those become flaky right where they solder onto the clip, and those can be effected by heat or vibration. I was just watching Don's Radio Shed and he was working on an Australian Briton radio (rebadged Phillips) and it had a down under version of a Micamold paper cap, it did more then cause noise though, it decided, periodically, to "transition" from being a paper bypass cap into a resistor. When I spotted it under the chassis whilst he was recapping I though "I hope Don checks that one out too, it looks like a Micamold" well it wasn't a Micamold, but it was a Micamold clone.
Real mica caps seldom cause trouble, well most of them don't, except for a certain series of post war Aerovox brand ones, that are all peanut butter brown instead of dark brown, I'm not sure why they fail but a lot of post war/Korean war era sets have them. What is strange is that I have also had them fail in very low voltage circuits, like coupling the loop antenna to the mixer section of the tuning condenser so the dielectric is clearly deteriorating due to age.
Regards
Arran

[bobclausen]

Arran thanks! I resoldered the grid caps and the rumbling went away. I don’t know specifically which one it was because I resoldered all of them.

Many thanks for all the help from the Forum members.