Expander issue in philharmonic

[Kenneth Chaffin]

Hello, I got my Philharmonic all back together and it is working with plenty of volume but I have 2 issues and they might be related--there is a lot of microphonic noise & rumble in the background and tapping the 6L7G tubes in the expander changes the noise. I subbed the tubes but the microphonic noise and rumble still is present also, I removed the 6L7 tubes and the noise goes away but no audio as they are used as part of the audio amplifier circuit--the expander control has no effect on the audio-like it doesn't work at all. The rubber mounts on the plate that has the 2 6L7 tubes are rotted out and the assembly is loose. I was curious if anyone else has encountered this issue.  Thanks,  Ken Chaffin

[David C. Poland]

Yes - I am dealing with both now - low frequency audio hum and a very sensitive microphonic chassis. I can tap any number of places on the left side of the receiver chassis, not just tubes, and it comes through the speaker loud. I reduced microphonic condition somewhat when I discovered the scratch suppressor 6J7 tube grid cap wire metal braid ground solder had broken, and re-soldered the ground connection.

Pull the 1st audio and inverter 6J5's and my 1940 BOL Philly is stone quiet, unless I mess with the 6L7s.

Now back from vacation, I have just dismounted the IF diode to re-check it as it is in the vicinity of the worst microphonic area.

The expander 6L7s are very sensitive, hence the isolation rubber mounts. Scott instruction is to try several 6L7s for minimum hum. I did replace the rubber isolation pads of the expander 6L7's sub chassis - used a pair of grommets for each of the 4 mounting bolts - just snug, not tight. 

Be sure your C- voltage divider is all good (negative 27 volts from the amp and progresses down to about negative  3 volt in 4 steps.

I recently recapped my pointer dial Philly and it works great, including the volume expander.

As for your expander not working, did you replace the 4 caps in the bass tone reactor (topside chrome box between the expander 6L7s and the 3 rd audio 6J5 pair? are both chokes inside ok? is your expander control pot and switch ok?

And did you replace the 7 caps (.5 MFD) inside the soldered box on the side apron below the expander tubes?

[Kenneth Chaffin]

Hi David, are the 4 caps inside the bass tone reactor box? I replaced any paper capacitor that was in that area. I recapped the tuner except the 7 capacitor box as I didn't have any .5 MFD caps (I do have plenty of .1 MFD caps though). I'll double check the expander pot & switch to be on the safe side & the C- voltages.  The microphonic & rumble noises come and go but no hum is present. Other than that the radio works very well and is quite loud. I do have some grommets--I'll see tomorrow if they will fit.  Thanks,  Ken

[Kenneth Chaffin]

Hi David, replaced the 7 caps & the 4 caps in the tone reactor block and that fixed the expander and the microphonics are greatly reduced and the bass control is working now--before there was little bass when the bass control was turned clockwise all the way.  Thanks for the help,  Ken 

[David C. Poland]

Ken - Glad you re-built the bass tone reactor - cuz cap leakage could cost you the 2 unit bass control pots.

Not obvious there are 4 caps inside. Ditto the 7 caps in that box.

With speaker in a good cabinet, should have considerable bass response now. 

Did you replace the cap under the big cover of the tuning capacitor? 

[Kenneth Chaffin]

Good morning David, I did replace the 2 paper capacitors under the tuning capacitor cover when I had it off to clean & lubricate the tuning capacitor. I have the Warrington cabinet

David C. Poland said:

Ken - Glad you re-built the bass tone reactor - cuz cap leakage could cost you the 2 unit bass control pots.

Not obvious there are 4 caps inside. Ditto the 7 caps in that box.

With speaker in a good cabinet, should have considerable bass response now. 

Did you replace the cap under the big cover of the tuning capacitor? 

[David C. Poland]

The way I replace the grommet where the set cable enters the chassis:

Get a larger grommet (1/2 inch hole) at a hardware or Ace store. Cut it, so you can wrap in around the cable. Start at the cut and work it into the chassis hole. With a small screw driver,  keep working it into the hole until completed. be careful not to damage the cloth braid. The cut is not a problem after it is installed.

Same process for the set and speaker cables at the plugs, using a smaller grommet.