[RodB]

I'm looking for help with my Bendix 0526 AA5 restore. I replaced all the electrolytic and paper caps, speaker and all 5 tubes. I've been through the alignment several times monitoring the AVC line but all I get is very strong stations and the audio is garbled, sounds like it is detuned. The station at 1500 is somewhere down around 700. My freq monitor says the LO is running very low so I swapped out a resistor and 47pf cap with a small improvement in LO freq. Here is the schematic:

http://www.nostalgiaair.org/Resources/316/M0002316.htm

Any ideas?

[KCMike]

Just a couple simple ideas. Is your antenna connected? Is the tuning gang capacitor not grounded as per the note on the schematic? I'm sure you will get more help if this doesn't help.

[Mondial]

Rod, you might check the wiring of the oscillator coil as something makes no sense on the schematic. As shown, there is no DC continuity of the cathode pin 6 of the 6SA7 through the osc coil to the common negative bus. There seems to be two versions of the set with C5 connected differently.

In any case, one end of the cathode osc coil winding must connect to the common negative bus for the oscillator to work properly. Where do you have C5 connected in your radio?

[Paul Philco322]

These are decent little sets, you will get it going.

paul

[RodB]

Thanks Guys.

KCMike - Antenna is connected, Gang capacitor measures 3.4 meg to chassis (looks good).

Mondial - Yes, the osc coil is wired as the schematic. This is the "O" chassis (note P) and C5 is the one on the left. The C5 on the right is for "R" chassis and is a wire to B- on mine.

One more thing I left out, I checked the audio with a signal, 200 - 900 Hz, into the grid of the first audio stage and it was sweet music. It was C4 and R2 that I replaced. I'm going to see if I have another converter tube and try that.

Paul - Yes, this was a barn resident and the bakelite case looked the part. I used shoe polish after removing all the gunk and there are a few minor surface scratches at the corners otherwise it shines.

[RodB]

I wanted to look at the LO freq. again for stability and it started at 928.55 kHz and slowly drifted to 928.33. Maybe there is a leaky grid or cathode in the tube.

Rod

[Mondial]

What is the radio dial frequency setting where you get the 928 KHz LO output? 

What is the range of oscillator frequencies you measure as you tune the radio from 540 to 1600 KHz?

[RodB]

The LO measures 928.3 kHz to 1.8867 mHz. The freq meter loads the oscillator and has about a 10 kHz shift (I tested this on another operational radio).

I don'have another tube available. It's hard to imagine that 2 tubes would give the same symptoms.

Beyond finding another LO coil I am stumped.

Rod

[Mondial]

Rod, where are you connecting your frequency counter? If you are connecting across the LO tuned circuit at the tuning cap, the loading effect will be much greater, especially at the high end of the band.  You might try connecting the counter to the cathode pin 6  of the 12SA7, where the loading effect will be much less and you can get a more accurate reading.

I don't think the osc coil is the problem. It is practically impossible for the coil to increase in inductance, which would be required to make the LO run low in freq. More likely there is some additional capacitance in parallel with the tuning cap, causing it to run low

From your original description, you mentioned that the sound is garbled and you received 1500  at 700 on the dial. It might be there is a problem with the antenna loop (open?) and it is not actually tuned to the dial freq. You could be receiving the 1500 signal as an image (1500 - LO freq = 455 IF) or by mixing with the second harmonic of the LO if the antenna circuit is not actually resonant at 700 KHz.

What is the resistance and voltage reading you get from pin 8 of the 12SA7 to ground?

[RodB]

The freq counter is connected to pin 6 of the 12SA7 (cathode). The antenna measures 1.3 ohms. The resistance of pin 8 to B- is 3.2 meg and the voltage was -1.3v to -4.0v depending on the tuning of the gang cap. That second harmonic occurred to me too because it might be closer to 750 kHz which is half of the 1500 I tuned to. I can't get an accurate location of the tuned station because the dial face is still in the cabinet. I'm glad you think its not the coil!

I'm going to check out the area around the tube socket and maybe the osc section of the gang cap. The circuit is pretty simple, not many components.

[RodB]

Hi Guys - I found the problem. After poking around the chassis and finding nothing of significance I decided to scrub the gang cap. plates with a pipe cleaner and lacquer thinner. dried it, applied power and the freq. monitor went to over 2 MHz. I then went through the alignment twice and now it picks up all the stations in the right place. I guess the spray bath I gave it during the rebuild wasn't enough. Too much barn juice. The buzzy garble is a rubbing speaker voice coil. I connected a 12" PM speaker and it sounds great.

I have to thank all for your assistance and verifying that I was on the right path. Feels great to have this help.

Rod Basham

[Paul Philco322]

Fix that speaker or recone and start from scratch and you are done!

Paul

[RodB]

Here's a photo of the near complete AA5.

Rod

[Paul Philco322]

Case looks nice, chassis looks rust free from the part visible. How bout a chassis pic?

Paul

[RodB]

Case looks nice, chassis looks rust free from the part visible. How bout a chassis pic?

Paul

Chassis pic.