Hi All ,
I got this from a buddy who was going to put this beauty out to the road. it is a Model SL2505 I think it is from 1963.
It plays great except on AM all it does is motorboat , I have replaced the electrolytics on it only.
My Jackson 648 does not have a listing for the receiver tubes so I can't test them. Cannot find a schematic on line. The FM and phono play Great! would a tube cause the motorboat just on AM only? Any help is appreciated.
Thanks

Yes a tube could cause it , also a capacitor or resistor could

Most likely the AGC cap is bad, along with any other paper caps.

I agree with Repair Tech. Check the AGC cap. Also, check for high-value resistors in the AGC circuit that have begun to open up. Also, check for loose or poorly-soldered grounds. The nature of FM is such that poor filtering in the power supply will be less likely to induce "motorboating" than in a similar AM stage.

And do find a tube checker. Some TV shop in your town should have one. Tubes from the early 1960's are very common. At my repair shop, when someone brings in a set made in 1963, nearly always there are a couple of bad tubes.

Fred
repairing audio gear since the Pleistocene

Well , I had the story backward , that's what I get for working on several at a time, I fired it up again and it is the AM that is dead , and the FM motorboats. There is only 1 paper cap under the chassis, the rest are disc type, the tube compliment is as follows 1-12TD8 , 2- 12BA6, 1- 12AU6, 1-12BE6, 1-19GQ7, and the 35W4. my tester does not have settings for the 19GQ7 , others test OK. I do not have a spare 19GQ7 so I will order another just to try. But other than that , has anyone come across a radio with similar problem like this? Any suggestions are appreciated.
Thanks

Could be a bad tube.
Also it maybe a cap, just because they are disk type doesnt mean they dont go bad, remember that unit is pushing 50 years old.
Am totally dead good be a bad rf coil, u need to get out a meter and do some checking.

has anyone come across a radio with similar problem like this? Any suggestions are appreciated.

Gee wizz ..... after all of us mentioned a capacitor or resistor or tube would cause that , how else can we help you Maybe if you rifinish the cabinet it would fix it .

GREAT answer, Ken! Putting a doily on top for the cat to sit on would help, too...........GREG

Yea how could I be so stupid ... sure it could be a resistor, tube or cap or even a coil! I'm sure one of these items is the cause of every radio not working, DUH!!! I just thought someone may have come across this same problem and might pass along what they found , thought it might narrow it down to a particular stage or something .... sorry for pissing everyone off!

Next time i dont think i will bother answering.

As I said Sorry, my reply was directed at the sarcasm, I have no experiance in FM sets , someone mentioned the AGC cap , which I appreciate, trouble is with no schematic I don't know where to find it , as I said all the caps are disc type , and are only color coded , none show any obvious shorts or leakage. I did find a resistor that had went up in value from 33k to almost 100k on the plate of the 19GQ7 tube , but did not seem to make a difference. So I guess my next step will be to refinish and put a doiley on top as suggested
Thanks

Not sure if this helps you but here is the schematic for the L701 tuner; it also uses a 19GQ7 tube.
Different overall tube lineup than yours but maybe this gets you a little closer. This tuner was paired with a solid state amplifier. I recently had and sold one of these.

Dang, I can't upload the .pdf but here is where you'll find it, in the 1965 Beitman's manual. You'll need the djvu plugin
http://makearadio.com/beitmans/files/1965.djvu