[DMcMahon]

Before connecting the tape deck decided to do some more checks to try and establish where the horrible loud ticking noise is originating from.

Found that the faint ticking sound (with volume turned right down) that I previously thought was coming from the power supply, was in fact coming from the speaker (speaker siting right next to the power supply).

Scope measured the PB/Record head socket input and saw the same pulsed noise signal as seen at the EF 86 control grid.

With the EF 86 removed and also with the EF 86 back in with its grid coupling capacitor to next valve (ECC 83) disconnected, the noise stops, supporting the fact that the noise signal is pre ECC 83.

Tried another EF 86 and also fitted a screening can to it but no change. Closely checked all the components/wiring around the EF 86. A Hunts 4uF/150 volt electrolytic de-coupling the EF 86 screen grid, only measured 0.2uF with high ESR (140 Ohms). Replaced it with a NOS 4.7uF/100 volt but no change.

The Series 2 schematic shows the capacitor to be 8uF but the 4uF looked original, the same capacitor shown in the Mk. 5 schematic is only a 0.5uF

Found two 27k resistors in the HT chain measure around 35k so 30% high, will replace those later.

Disconnecting the 0.05uF capacitor that connects the PB/Record head amplifier socket through (via switch contacts) to the control grid of the EF 86, stops the noise, supporting theory that the unterminated input (no head connection) is the root cause of the noise, causing the EF 86 to effectively self oscillate.

Connected tape deck and no more horrible noise. With volume wound up there is quite a bit of audible mains hum at the speaker, will check that out more, once tape deck operation checked out and tape playback working.