[k9uw]

Hello, everyone!

I recently replaced all capacitors in a Philco 14B and my confidence as a relative newbie got a boost when, upon power up, the radio came to life.  Both bands are active.  Tonight, the broadcast band is wall-to-wall with stations, WWV is where it belongs at 2.5 MHz, and the Shadow Meter works!

Unfortunately, that power up and resulting reception of stations revealed a problem with the audio.  If I advance the volume beyond a modest level (25% of the control’s range) the audio sounds distorted and raspy as though the speaker cone was torn.  I pulled the speaker from the cabinet and found a cone with two minor tears on the outer edge, but the cone looks as though it got wet at some point and whatever solids were in that liquid reached the voice coil and dried out.  The cabinet’s grill cloth was torn, and I presume someone dumped liquid on the speaker cone through the hole in the grill cloth.  Photos can be found at the bottom of this web page:
http://k9uw.weebly.com/current-and-future-projects.html

At lower volume levels, the audio sounds fine.  A typical listener would prefer more volume and would then experience the distortion.

What, if anything, can I do to deal with this problem?  Is this “gunk” a potential source of the audio distortion reported above?  If so, does anyone have a suggestion as to how to remove it?  Is it reasonable to think that this stuff is contributing to the distortion?  I have not replaced any resistors yet, so perhaps there are component-related issues that are impacting the audio quality?

Finally, I noticed that someone had wrapped a loop of heavy wire around the speaker’s magnet and audio output transformer, as if to hold things together.  Things seemed solid enough so I poked around and found that the winding within the speaker’s magnet could be turned a bit if I pressed with my fingers.  Could this be an issue?

Thank you, in advance, for any suggestions.

Mike, K9UW
Amherst, WI

[morzh]

1. Check continuity of the output (secondary) of the transformer 46 (audio diving the grids of 42 tubes).
2. Check the primary of the audio output transformer.
3. Have you checked all your tubes? Check the 42s for being good.
4. Check their plate voltages.

[k9uw]

1. Check continuity of the output (secondary) of the transformer 46 (audio diving the grids of 42 tubes).
2. Check the primary of the audio output transformer.
3. Have you checked all your tubes? Check the 42s for being good.
4. Check their plate voltages.

Thank you, morzh!

I will pursue your suggestions and report back. As you know, this radio has three 42's and I know one of them is weak. Moving the weak one between the three locations did not seem to matter but I'll pay more attention and try it again.

[morzh]

No, weak is not a likely culprit.

It may be that half of your output is not working. But when signal is weak and the tube is not clipping the bottom, it might sound OK, but with large signal hou might have 1/2 the waveform.

This can be due to one tube being bad, or one half of either of the transformers bing bad.

[k9uw]

Thought I’d provide an update on the status of the Model 14B distorted audio problem reported earlier (see below).

Based on the great advice I got here, tubes were tested and several were replaced as needed.  The audio transformer was also checked and proved to be OK.  Tube pin voltages were below spec in some cases by about 20% so I suspected that some original resistors had gone high.  New resistors were installed but the distorted audio persisted.

My local mentor stopped by yesterday to have a look for himself.  He examined the speaker, pressed down on the cone and said he could hear the cone rubbing on the voice coil.  Though he described what he was hearing and expressed confidence that the problem had been found, I could not hear what he was describing.  After he left I also examined the speaker but, instead of pressing down evenly on the cone, I applied outward pressure to the side of the cone where the stain and dried crud was present.  BINGO!  The distortion disappeared.  

Photos of the speaker are available at the bottom of this page: http://k9uw.weebly.com/current-and-future-projects.html

This discovery creates a new question.  Is there a way to fix this problem without resorting to a re-coning?  I do have a good source for re-coning if that is the best approach.

Some day I hope to have the experience to be helpful to others on this forum but, for now, I’m here to capture your advice.  Thank you, in advance, for your suggestions!