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Old 8th Oct 2020, 11:24 pm   #48
DMcMahon
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Join Date: Apr 2019
Location: Worthing, West Sussex, UK.
Posts: 6,587
Default Re: Brenell Purchases

Checked out the 3 grid coupling capacitors on the Mk. 5 Series 2, two were fairly leaky so renewed, the third (good one) was a 22,000pF (0.022uF) yellow mustard type.

The 2 bad ones (0.1uF) were Hunts, one a black AD51 and the other a nice looking red G51. Also replaced another red G51 (0.1uF) the coupling capacitor to the external amplifier output socket, this one was very leaky (passing a lot of DC volts).

There are another two Hunts 0.1uF capacitors (one black AD51 and one red G51) but they do not see DC volts, but I will replace them later.

On the Series 2 schematic none of the resistors and capacitors are identified component number wise. Looking at the Series 5 schematic for comparison (which does provide component identification numbers) could see that even though very similar, there are quite a few component value differences and some circuitry connection differences, compared to the Series 2.

Borrowed an ECC 83 (from Grundig TK 120) and an EL 84 (from Simon Minstrelle) so that could power up with all valves fitted. With the Variac set to 240 volts mains output, the DC +HT supply now reduces down from its previous 450 volts to around 330 volts (at 40mA drain).

Schematic shows 285 volts for the +HT, so the HT is still somewhat high, to get HT down to around 285 volts, have to reduce the output of the Variac down to around 205 volts.

With amplifier switched to Playback, there is a horrible loud fast ticking sound from the loudspeaker as soon as the volume control is adjusted away from its minimum setting. With amplifier switched to Record or Amplifier there is no sound from the speaker.

There is also a faint fast ticking sound heard from the power supply, even at zero volume setting, HT output at this time is still mains ripple free.

Had a quick look at the electronics with scope, and when the volume control is moved away from zero, there is a very clean regular spikey pulsed signal all over the electronics at 60mS (16Hz). With scope connected to the input of the pre-amp EF 86 (Control Grid pin 9) there is a constant 32 Hz (30mS) pulsed signal even at zero volume setting, the scope connection is obviously modifying the pulsed signal because the sound from the loudspeaker changes.

Currently do not have the tape deck connected, so the PB head is not connected into the EF 86 (when switched to Playback), so wondering if the EF 86 is breaking into oscillation due to no head connection (seems odd though).

Tomorrow will connect the tape deck and see what happens.
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Last edited by DMcMahon; 8th Oct 2020 at 11:34 pm. Reason: Typo correction
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