Editing Sinclair QL Microdrive Troubleshooting Guide (section)

== Signal test procedure (Appendix D) ==

Sinclair's '''SIGNAL TEST''' program writes a steady 100 kHz tone to a cartridge (erasing simultaneously) for 8 seconds, then switches to read, so the electronics and mechanics can be assessed against a stable waveform. A two-channel oscilloscope is required, set to add CH1 and CH2 with CH2 inverted to measure the differential signal.<ref name="sm" />

Load the program, remove the program cartridge, insert a known-good blank, and select the drive. The QL beeps when it switches from writing to reading; the space bar stops and starts the process.<ref name="sm" />

=== Write check ===

While writing, check the TTL data lines at ULA pins 19 (24). If those are correct, look at pins 4 and 5 (14 and 15). '''If the signal is not present there, the fault is a faulty ULA or head.'''<ref name="sm" />

=== Read check ===

After writing, switch to read mode:<ref name="sm" />

* Check data lines at ULA pins 19 (24). '''A waveform that is very unstable in the X axis indicates a mechanical fault.'''
* If the waveform is poor or absent, check the peak-to-peak signal at pins 4 and 5 (14 and 15).
* '''Signal present and correct there''' — the fault is in the intermediate read stages.
* '''No signal at all, with the write check correct''' — the head is faulty.
* '''Low voltage at this point (for example 150 mV pk-pk)''' indicates a worn head, which causes intermittent soft read errors rather than outright failure.
* '''A waveform very unstable in the Y axis''' (widely differing pk-pk levels) points to the erase function — or simply to a poor tape.

=== Erase check ===

Erase problems show as residual magnetism from previous recordings not being removed during writing. '''The microswitch is usually the culprit''' and can be checked with a multimeter with the power off. To verify with the signal test program, measure at '''HBC pin 11''':<ref name="sm" />

{| class="wikitable styled-table" style="width:80%; text-align:left;"
|+'''Erase check — HBC pin 11'''
! Condition !! Not write-protected !! Write-protected
|-
| During write/erase cycle || approx. 5.6 V || approx. 0.8 V
|-
| During read || approx. 9.0 V || approx. 0.1 V
|}