Try this.. Use a new clean good quality tape. With the tape loaded and running lightly press your finger on the back side of the tape against the spinning drum. You will feel the heads bumping against your finger. That will usually clear a stubborn head. If that doesn't work you may have to remove the upper drum and re-solder each individual head. It sounds to me that you are missing the output of one head. If you have a known good recording you can monitor the RF envelope. Make sure you sync the scope with the RF switching pulse. It's been a while since I worked with this technology myself.