The thing is, with the gear you have, finding replacements is almost a waste of time. Because you have an AM radio, any gear you replace would be old school to be compatible. You will not gain any performance and spend similar money to replace as you would to upgrade to newer, 'fm and digital compatible' gear. You can buy a Flysky radio system for low cost. It is a basic system BUT it gets good reviews from a LOT of ppl on this forum! A replacement servo should be cheap enough to find. Replacing the esc COULD be cheap if you can find one.
As far as diagnosis... For starters, verify you have new batteries in your radio. If your radio uses chrystals, verify they are in good shape. If you have an extra pair, swap them out and test again.
If all that looks good, unplug the steering servo and see if your problem goes away. Next, unplug the motor from the esc and see if the issue stops. Verify that if there are caps on the motor, that the haven't blown up (check the tops of them as well as the leads to them making sure they haven't crossed or come loose from the component). Again, while unplugged, test the system.
Next, check and repair any and all wiring in the srvo and esc. Make sure the plugs to the receiver (and on the receiver itself) are in good shape, free of loose contacts and that there is no corrosion.
If you have a battery pack avail (if it's rechargeable, be 100% sure it is FULLY charged, if AA batteries, be sure they are new) Plug that in and unplug the esc. Again, power on, test the system and see if the problem goes away. This should at least narrow down what your issue is. However, irrelevant of the issue, replacing the the radio for instance, you would have to verify that the servo and the esc will be compatible with the replacement. A digital radio will not work with the old servo and esc unless the radio can be switched between digital and analog. (this is not an uncommon feature but one that is important in this case) the same thing goes for the servo or the esc. (Your equipment is analog)The only other thing that I can think of in this case is that the power switch on the receiver side of the system has somehow partially allowed SOME power to flow, but not enough to power correctly. This could be due to prior damage and/or corrosion. Some of those switches are easily replaceable but I would personally just eliminate it and get used to unplugging the battery. Hopefully this gets you to at least identify the issue. Repost with your results and we'll see what we can find to help get you moving again with the least expensive and most logical repair. Good luck!!!
