Did you clean the head and capstans with a cotton swab and IPA?
Check in service mode with a pre-recorded tape and see what channels give an error.
Try recording a DCC tape on the 730 and play it on a DCC player with a 100% working head. See if that works. This way you can see what channels/tracks are bad when recording.
Your head may have a defect on at least 1 channel.
The speed a bit out of tune but that was not the problem. I cleaned it very good. On the precorded tapes I have just one dead channel (20) and this is the AUX channel. So no tape info etc.
I tried a recording. Playing in on another DCC I have no sound. And no sound on the 730 too and four dead channels.
I hate these 3rd generation models. The first and the second generation are so reliable.
Yeah, with 730 and 951, dry solder joints in the power supply can cause trouble and unstable voltage to the internal parts. I suspect it very well may end in head failures. Apart from that, they are vulnerable to static discharge too.
Also if the glue around the power supply capacitors turns dark brown, it starts conducting, causing shorts and current leaks, or overvoltage in in the long run.
But the looks are good and they are very easy to maintain.