• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Valve AM tuner fault

ok, was just curious if this was a design where the resting position was at full, and it drops down from there when the circuit is conducting. I've had things that worked that way.
 
Well, I picked up 4 used 6BE6's They were supposedly all Brimar but I have one Brimar, a russian Sovtek I think, a mullard and another one with no name.

Tried all of them with no change in behavior of the AM tuner...
 
Wait... looking at Pin 2 of V14 on the scope now, moving the tuning knob, I can find one slight tiny lull or bump in the waveform near the top of the scale. Is this important?

Using the Mullard 6BE6 instead now, the waveforms on Pin 2 and Pin 7 are much larger.
 
Last edited:
I have a number of sets that appear to have silver mica disease. This can kill all reception. The condition is where the silver ions migrate in the direction of voltage so they eventually short out. In the past, Zenith and Silvertone were famous for this.
 
Which Coils? I ask because a lot of the initial coils are not in cans and mounted in the chassis. The L's not the T's
 
On the sample schematic I posted, yes. Tuning thru on AM should show some output peaking at IF frequency, if the AM front end is working.
 
can usually get an idea if its bad caps in the IF trafo by injecting signal at the IF frequency. 455kc is usual but it could be something near that. Confirm with the manual. A few loops of wire around the 6BE6 should be enough to do it, or a tube shield that isn't pushed down far enough to contact the chassis. Clip the signal generator + to that, and - to ground. Set the generator to whatever the proper frequency is, and see if it peaks. Good caps will result in a sharp peak. Bad ones will give a very wide area with slightly higher response. If the meter won't read at all, you can just do this by ear, peak it for max volume.


T1 and T3 are your IF transformers for AM in this. T2 is FM.
 
Ok

Wrapped wire around V14 and connected it to the signal generator. I put scope probe on T3 Pin 1.I get the waveform from the signal generator. It seems to peak when I adjust the signal generator to 471kHz. At this point I actually start to hear some tiny scrap of reception on AM and I get some tuning whistles. LW also works on one tiny portion of the band - ie just noise and interference.

If I drop signal generator output to 0 the waveform is gone. Turning the tuning knob changes nothing.Do I need try probing somewhere else?
 
Running the AM I've managed to tune in a station incredibly weak but can just hear spoken word. Found it on another AM tuner nearby. Its roughly in the right place on the scale. Very weak and howling.
 
What does the service manual say the IF frequency should be? Would be in the alignment instructions if its not spelled out anywhere else.
 
Problem I have here is that I couldn't find a service manual for the LX-420A anywhere - only a schematic. Through some digging however, I did find the service manual for the ER-420. This looks to be very similar but with only AM and FM and no LW. In the alignment section for the ER-420, the AM is said to be tuned to 455kc (kHz). Hopefully there are enough similarities that the alignment procedure should be similar if not the same.
 
You must log in or register to reply here.
Back
Top Bottom