Hello, Altho this radio is not exactely an antique, it is a tube set, and I have a question about it, as it is not working properly. It's an Am/FM with a phono jack Motorola. Kind of neat, because it has a "tuning-eye" as well. Anyway..... after the radio is on for about 1/2 hour it gets really "garbled" I've checked all the tubes and had 1 bad one (6SQ7). Replaced that and it still does it. Testing and replacing tubes is about the extent of my repair knowledge. Do you think it's a bad cap??? The radio has no "hum" or anything like that, so I wasn't sure, and haven't run into this problem. Any advice would be helpful. Thanks!

Could be a coupler capacitor changing values as it warms up. If you have a hum, it comes from the electrolytic capacitors (the main caps). Wouldn't hurt to just change them all. :)

Do you have a photo of it? Sounds like an interesting set. Motorolas have always had a interesting style, and are usually easy to pick out in a crowd without seeing the nameplate because of their distinctive styling.

Charlie's on the right track. Also replace the rectifier tube and the output tube to be on the safe side. In most cases, it's caps that have gone bad and look for any charred resistors. They go way off tolerance due to heat damage.

Curious... what is the tube line-up in this set? You said you replaced a 6SQ7. Isn't that a metal envelope 8 pin? I didn't realize they were still using these in 1958 radios. By then, I thought most tubes in radios were 7 and 9 pin miniatures.

i was wrong on the tube. the tube I replaced was a 6BQ7. Also changed the 6BE6. All the others tested fine. It's a Motorola table-top, in a blonde cabinet, and even has a tuning eye. The model number is 10T28B The tube line-up is: 6BQ7- RF Amp, 6BA6 AM-FM IF,6BA6 FM IF, 6BE6-AM ConV, 6AU6- FM Lin, 6AL5 FM Ratio, 6AV6- AM Det, 6AQ5-Pwr Amp , 6X4-Rect and finally the EM81 tuning eye tube. The radio has no hum and works great, but after 45 minutes gets so garbled you can't understand anything. Have re-tested all tubes when it does this, and they still test fine.

Yeah, you have a coupler cap or resistor changing values. My bet is on a coupler cap.

change the cap to g1 of the 6aq5.
might be a gassy 6aq5 too.
btw does it distort on am and fm?