I left my fender frontman 65r on for over a year and yesterday a gentle hum for a few seconds then a loud hum for a few seconds more then no more power to the unit. I opened it and troubleshooted it around the power supply, fuse 3.15A is ok, diodes are ok, the power resistors are ok, the power supply tansformer's primary winding is open. I substituted it for a similar one and powered the amp up and again the smooth hum for 2-3 seconds and then the louder hum. I turned it off. I had my guitar plugged in with it's volume turnd up but no amplification at all while the amp and its volume turned up. I assume there is a short in the power unit but the output transistors seemed ok.

SOURCE: Fender Frontman 65R Solid State guitar amp.

There is a protection thermistor in the circuit to keep it from over-heating, this is why it works again when it cools. Check and make sure you are not covering any ventilation holes, or if the unit has a fan, make sure it is working.

SOURCE: When I plug in my

Yes, there are several possibilities. First thing is to disconnect your guitar and then plug in the cord to the amp. Expect a bit of hum. If it screeches, then problem is not feedback via the guitar. If it doesn't MAYBE your guitar is "hearing the amp" which the guitar is then a microphone and can feedback. Moving the cable generates electrical noise which can trigger the feedback to start.
If you get the screech without the guitar connected, then TRY a new guitar cable. Make sure it is an INSTRUMENT cable... I have seen some try to use speaker cables or junky 1/4 inch cables intended for home stereo... these will NOT work and can cause the feedback. The next possibility is a broken jack on the amp. This would require repair. The common cause of this is to fail to run the cord through the handle of the amp... one trip on the cord or step on it pulling the plug sideways in the amp and the jack is broken and it is a trip to the shop as well. While on the subject ALWAYS run the cord at the guitar end either through the strap or your belt loop. Cords left dangling keep rotating and soon wear out the plug and also the jack in the guitar and then that has to go to the shop.