I recently purchased a model 51f at a swap meet for $20 that did not spin, I thought even if it can't be fixed the parts are easily worth more. Everything else worked perfectly, so I was encouraged that I could find the root cause. A great deal of research indicated most people recommend replacing all the capacitors to fix all sorts of problems with these complicated microprocessor controlled turntables. That approach is ok for antique tube radios and even tube equipment from the 50's, but could cause more problems with equipment from the 70's and later due to the compactness of the circuit boards.In addition it is so easy to inadvertently install an electrolytic capacitor with the polarity reversed when doing so many. Then once done, it may not solve the problem anyway and you may have made matters worse.
So, please don't do that without properly troubleshooting this circuitry first. These absolutely can be fixed, but some knowledge of electronics is needed along with good problem solving steps. Here was my diagnostic approach and ultimate resolution.
Step 1 is to print off the motor and tonearm control board schematics. Review those and learn the individual systems and sensors driving each operation such as tonearm start, return, motor speed control etc. It is helpful to use different colored highlighters to indicate where grounds and positives are.
Step 2, devise an approach to troubleshooting by using the operation manual to understand exactly how it should work when working properly. I used Youtube videos to see when the platter should start spinning.
Step 3 is to document a plan to begin testing voltages. Since the platter was not spinning at all, I decided to start with the motor and noticed there was no voltage reaching it. Ok, that is a problem, but what is the root cause.
Step 4, I checked all the power supply voltages and they were good.
Step 5, next checked voltages from the tonearm control board to the motor control board and found those to be as expected based on the schematic. This was important information because it eliminated the whole tonearm section and associated electronics as the root cause.
Step 6, now time to troubleshoot the motor control board. The voltages coming out of the optical coupler ICs did not seem quite right. So after troubleshooting I determined that the general area of the problem is between here and the motor. As a start, I was able to order replacement ICs and replaced both of them but it did not fix the problem.
Step 7, all that was left after this were four transistors and and two bridge diodes (and yes, some capacitors and resistors). On a hunch, I decided to check the power transistors since they generate a great deal of heat and because of that can be susceptible to failure. I desoldered them and found one to be shorted and the other to be not as one would expect a good transistor to be. I ordered new ones, replaced them and now had voltage at the motor! But still not spinning.
Step 8, I tested continuity on the motor, three terminals. I did not have continuity from terminal 1 to terminal 2 and 3, but I should have so this told me the motor was bad. It is difficult to find motors for these as I found out, so I decided to open it up and see if it was anything obvious like a broken lead or bad solder joint. What I found was terminal 1 lead goes into a heat shrink tube and connects to the coils of the motor from there. I did a little very careful surgery and cut the tube open. I found a little component in there that turned out to be a thermal fuse. It was open. Probably failed when the power transistor shorted. I went to a local electronics store, Frys, and found a compatible NTE 8226 fuse and carefully replaced it. You must put this back in the exact same position to ensure correct temperature reading and avoid any shorts to the case or rotor. You can do this, just take your time and be careful.
Step 9 was time to try it once again. When the platter began to spin I was pretty happy and tried to convey how great this was to my wife, she said it was very nice. Oh well.
Last words - to test the voltages on the turntable is a bit of a challenge because you need to do it in operation. To do this, I removed the bottom of course and used two tables to set the turntable on in order two be able to put a record on it and press start. I had to lay on my back to test voltages. Think safety first! Between each voltage test, I unplugged the turntable and then attached the alligator clips to the next point. Usually one clip can remain connected to a ground point and just the other needs to be moved. If you don't feel comfortable troubleshooting, take it to a professional. The only thing that cannot be fixed on these are the two microprocessor chips because they are Denon specific. Good luck, be SAFE, and happy listening!!