Today I will be showing Tank Girl at the video machine… but yesterday during testing the tape exhibited a particular kind of a fault:

  • CTL counter is not advancing (which means the servo system is not sensing CTL pulses)
  • The image is unstable (it works fine for a bit, but then snow covers the screen for a moment)
  • Tracking meter is unstable (wobbly and unsteady suggesting that FM envelope is very bad)

The image being unstable is the consequence of servo system not having CTL signals as the input - the system does not know where video tracks begin and end actually, so as the playback continues the video head paths keeps drifting at a slight offset relative to the video tracks on the tape, periodically falling into the space between the tracks where there is no video data.

My initial hypothesis is related to one item I have not adjusted back when performing re-alignment of this rebuilt VCR: the tilt of the audio/control head.

In the VHS system, the A/C head reads linear audio (a form of low quality audio recorded along the length of the tape - unlike Hi-Fi which is mixed with video data) and it reads the control track - which stores pulses indicating exactly where each video track begins.

If this control track cannot be read, the servo system will not be able to synchronize properly to the video tracks on the tape and will result in the sort of symptoms that I got.

This head can be rotated in various axes in order to better conform to the flow of the tape. My initial adjustment set tilt only very coarsely and has not updated it since - so decided to perform the fine adjustment on top and see if that recovers playback of the problematic tape (spoilers: it did).

The tilt of the A/C head on the JVC BR-S800U is adjusted by four screws/bolts:

  • ① Forward tilt - the adjustment most relevant here
  • ② Azimuth adjustment - side-to-side lean relative to the tape (rotation on axis perpendicular to the plane of the tape)
  • ③ Height adjustment - shifts the A/C head up and down
  • ④ Taper nut for X-value adjustment - longitudinal position of the A/C head, only relevant for fine-tuning audio-video sync/the coarse component of the distance between the video drum and the A/C head

So… I went and did it. This was the methodology I came up with the day prior for this specific situation and this specific JVC BR-S800U:

  1. First check tape transport for any visible issues (make sure of the initial state of the alignment). Tape flow checks are implicit after each of the next steps
  2. Connect oscilloscope to linear audio R output and to CTL pulses, synchronize it on CTL pulses
  3. Record voltage magnitude of the CTL pulses and linear audio R output in the initial condition.
  4. Adjust forward tilt ① of A/C head and monitor change in waveform. Desired: find balance where CTL and audio are both at their highest values
  5. At this point the head is approximately more correctly tilted than before. Re-connect oscilloscope to linear audio R & L outputs
  6. Perform head height adjustment by turning hex nut ③. Since the reference has a linear stereo audio recording, attempt to equalize amplitude of two waveforms
  7. Perform azimuth adjustment ④ to match phase and maximize output level of two waveforms, correcting the residual error
  8. Re-connect oscilloscope to linear audio R output and to the CTL pulses, synchronize on CTL pulses. Check magnitude of the CTL pulses and record it
  9. Go back to step 4 and repeat forward tilt adjustment again while monitoring the change in waveforms. Perform at least two iterations of this process, then decide whether to continue it based on those results
  10. After these adjustments perform a full tape transport check and ensure that tape flows smoothly

I did the procedure, though it turns out a very small tilt adjustment was enough to increase CTL strength and no other actions in this methodology had any further strong effect (however, it was important to do them to ensure that iteration converges).

Checked the result on many tapes - both test tapes and the known problematic tapes. There is definitely a minimum threshold for CTL track signal level beyond which BR-S800U just completely gives up and this threshold is higher than that of other VCR’s I have (makes sense - it’s an editing VCR).

The specific values for CTL signal level (measured at test point TP2 of the SERVO/M-CTL board) for the same section of the reference tape with known good CTL pulses:

  • Initial: 0.34 V
  • Adjusted: 0.37 V (~9% increase)

With this adjustment, the previously problematic tape now plays just fine. No issues with tracking whatsoever, no issues with servo lock, no issues with Hi-Fi sound. In my experiments, it seemed that the threshold for CTL pulses to be considered valid in BR-S800U seems to be around ~0.30 V. Issue can be considered solved!