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DMcMahon · 28th Mar 2020, 11:10 am

Did some more tests and 100% confident that the issue must be the valve itself.

Fitted the EL 42 from the Bias Oscillator position into the audio output position and this valve does not exhibit the noise issue, taping this valve (quite hard) just gives a bit of hiss type noise of a few volts at the anode. The bad valve when lightly tapped outputs > 600 volts peak to peak noise spikes.

Can only assume that something internal within the valve is shorting intermittently which can be induced by taping it.

Need to try the valve out to make sure that playback still works, first have to tidy up various wiring that has been disconnected during the various tests.

Have ordered a NOS EL 42.

Have one other EL 42 that I can borrow in the meantime (from my TK 819) to fit in the Bias Oscillator position to be able to check out recording again.

28th Mar 2020, 11:10 am	#45
DMcMahon Dekatron Join Date: Apr 2019 Location: Worthing, West Sussex, UK. Posts: 6,604	Re: Grundig TK 5 Restoration Did some more tests and 100% confident that the issue must be the valve itself. Fitted the EL 42 from the Bias Oscillator position into the audio output position and this valve does not exhibit the noise issue, taping this valve (quite hard) just gives a bit of hiss type noise of a few volts at the anode. The bad valve when lightly tapped outputs > 600 volts peak to peak noise spikes. Can only assume that something internal within the valve is shorting intermittently which can be induced by taping it. Need to try the valve out to make sure that playback still works, first have to tidy up various wiring that has been disconnected during the various tests. Have ordered a NOS EL 42. Have one other EL 42 that I can borrow in the meantime (from my TK 819) to fit in the Bias Oscillator position to be able to check out recording again.