Vintage Radio Strong Station Distortion

[berkemo]

Working on a Motorola Model: 13KT15CW Chassis: Ch= HS-677. Just finished recapping the filter caps, all paper caps, and cleaning tube leads. The amp sounds great but the radio needs some tweaking. Generally it works pretty well but the issue at hand is that I've got a couple of strong signal stations on both AM and FM that really distort at max signal. As you approach and leave max, the station is legible but not clean. I've been searching the forums for ideas and it looks like the issue might be centered around AVC/AGC?

My question is: are there any 'top level' troubleshooting steps a newbie could do to address this issue? Things like: other caps I should look at? Tubes I should replace? Antenna issues? Specific resistors to look at?

Help! Thanks.

 

[gadget73]

AM or AM and FM ? Does the eye tube close all the way? the eye runs off the AGC line, so if that is closing all the way, it implies there ought to be enough voltage for the circuit to do it's thing. If the eye isn't real responsive, that would make me think its not creating enough voltage, or something is bad and leaking to ground. The stronger the signal, the more negative it should be, but if some component has failed it might prevent the voltage from swinging down enough to keep things under control.

 

[berkemo]

AM or AM and FM ? Does the eye tube close all the way? the eye runs off the AGC line, so if that is closing all the way, it implies there ought to be enough voltage for the circuit to do it's thing. If the eye isn't real responsive, that would make me think its not creating enough voltage, or something is bad and leaking to ground. The stronger the signal, the more negative it should be, but if some component has failed it might prevent the voltage from swinging down enough to keep things under control.

Thanks for the analysis! The distortion is on both AM and FM. The eye tube does not close all the way either AM or FM. However, it is much more responsive on FM. Even though AM pulls in stations fairly well (other than the distorted strong station) the eye tube only moves a little bit on AM. I'm not sure how to identify the AGC line is on my machine. Can you suggest some components to look at in that area?

 

[dr*audio]

Sounds to me like the alignment is off. You'll need a scope and an rf generator that does AM and FM.

 

[berkemo]

AM or AM and FM ? Does the eye tube close all the way? the eye runs off the AGC line, so if that is closing all the way, it implies there ought to be enough voltage for the circuit to do it's thing. If the eye isn't real responsive, that would make me think its not creating enough voltage, or something is bad and leaking to ground. The stronger the signal, the more negative it should be, but if some component has failed it might prevent the voltage from swinging down enough to keep things under control.[/

FM works really well now after I moved the 6BA6s around. Strong signal still clips a little but the magic eye is responding well. AM reception is fair and magic eye is feeble at best. Question: which tube/components are most suspect to cause this on AM? Thanks!

 

[gadget73]

Probably the rf amp. Can't read the number but it's the bottom left 6ba6 on the schematic. A weak 6be6 may also do this. Swap the 6ba6 with the 1st fm if tube and see if it changes anything. May also be able to peak the antenna trimmers on am if the tubes are good. Probably will be the last step on the alignment instructions. I use radio stations near the frequency specified. Guessing it wants 600 and 1500.

 

[berkemo]

Probably the rf amp. Can't read the number but it's the bottom left 6ba6 on the schematic. A weak 6be6 may also do this. Swap the 6ba6 with the 1st fm if tube and see if it changes anything. May also be able to peak the antenna trimmers on am if the tubes are good. Probably will be the last step on the alignment instructions. I use radio stations near the frequency specified. Guessing it wants 600 and 1500.

Thanks Gadget73. The 6ba6 swap didn't help the AM magic eye response or AM quality. It does pull in stations but still little to no magic eye response. Do you think it's worth picking up a new 6be6 as a next step? I'm still a bit nervous to start playing with the trimmers!

 

[gadget73]

not likely to hurt if you don't have a tester. They are cheap enough.

If this is a design that cuts plate voltage to the tuner when not in use, its possible the converter tube was damaged from it. They don't like to sit for long periods with no plate supply. I have a Pilot tuner/preamp that does that and the RF amp and converter tube in the AM section were pretty much dead. I fixed it by putting a resistor across the contacts on the switch so it gets 10 volts or so even when switched off. Its not enough for the tuner to work, but its enough to give the electrons somewhere to go rather than poisoning the cathode.

The antenna trimmers are pretty safe to mess with. I can't read the schematic enough to tell you which they are but it should be in the service manual. I can make out that there is a loop antenna, and what looks like a trimmer right next to it with both a part number and a number in a box. That should only be done when everything is known to be working right though.