[Hot Rod]

Well I think we've confirmed that the bass section is not functioning correctly.  I pulled the 6K7 bass amp tube and there was no change in output.  I de-soldered the high path circuit and although there was some sound, it was very very weak, distorted, and almost inaudible.  I re-soldered it and back to square one.  I poked around a lot about the 6K7 and didn't find anything.  My only question is the ground on the audio cable.  Is that a grounded shield?  Should cap 136 go to ground? Mine does not.

On a positive note, I'm picking up stations on the 690 from EVERYWHERE!  Once the sun goes down it's non stop from one end of the dial to the other.  Unbelievably sensitive!

Even though my 690 is still sick, I could not have gotten to this point of the diagnosis without your help. Thank you SOOOOO MUCH!!!

BTW I've got a line on an Airline model 62-418 movie dial at the local online auction.  So many radios, so little time! 

Thanks Mike!

[morzh]

We don't know if your base does not function properly. As I said, in my case it is too not super strong and only becomes somewhat strong at good overall volume level, but in your case as you said the volume is weak as a whole.

Cap #136 is the DC blocking cap for the audio out from the 2nd audio to the power amp, it goes from the 2nd audio plate to the output cable and then to the 6F6 driver tube grid. That is, it is the audio signal path from the tuner chassis to the output amp.

I have to see if the 6J5 bass AVC tube in the amp chassis affects the bass level; the second pot attached to the volume pot regulates it.

[Hot Rod]

Yes, the bass circuit may be working but something is not right.  It sounds terrible.  The only way I can describe it is that it sounds like the diaphragm is torn, which it is not.  You get that very distorted rattling sound at higher bass levels.  From mid-range frequencies on up, it's very clean.  But like I said, overall volume level is low.  And one thing that was confusing me is that the bass control increases bass CW.  The treble decreases CW.  

So I checked the woofer VC at the ouput trans (with it still connected) and I'm getting maybe 1 ohm.  I'm not finding VC resistance on the schematic.  Also my muli-meter is starting to act flaky at near zero.  I know the VC may only be a few ohms but I'll check again.  Any chance it's shorted?  I didn't think it would work at all.  The output trans resistance is right on (checking at the connector side).

And just for the heck of it I pulled the 6J5 and tested it again.  It checks fine on my tester.  It's been replaced with a GT.

[morzh]

I am not sure but I think the resistance of the VC room of the 690 us 3.9 ohm.

[Hot Rod]

I disconnected the woofer VC at the output trans.  VC is 3.3 ohms.  Output trans is 0.4 ohm.  I don't see a resistance number for the speaker side of the trans on the schematic, but that seems low.  I connected a PM speaker to the output trans and still the same garbled, distorted sound.  So I think my VC is ok.

[morzh]

It's ok, the output wnding is extremely low resistance.

Start looking at the output amp. Check 6F6 driver.
Also try to pull IF AVC tube.

[Hot Rod]

I found both the tweeter and woofer FCs wired to #175 choke and the 2k resistor that goes to the 6L6.  It's only supposed to be the tweeter FC. Also cap #164A was a 16uF instead of an 8.  I knew somebody had been in there before me because a few lytics had been replaced.  It seems like I gained volume from the woofer but it still distorts when you turn up the bass gain control.  So I've got more searching to do.  I ran out of time.  4 am comes early.

[morzh]

The tweeter FC and the centertap of the output xfmr plus the 2K resistor are wired to the 175 choke right side.
Then the other side of the tweeter FC wires to the woofer FC, the interstage xfmr primary and the Pin 6 of the 10-pin cable panel and then via the cable to the #69 500 ohm resistor in the tuner chassis.