Dropout in RTP3 phono stage - diyAudio
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Old 18th August 2012, 08:42 PM   #1
Alex M is offline Alex M  United Kingdom
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Location: Hampshire, UK
Default Dropout in RTP3 phono stage

My DIY version of Allen Wright's RTP3C has been showing some odd behaviour. From time to time, the right-hand channel of the phono stage becomes very faint. There is no crackling, popping or other noise - just sometimes all I can hear is the left channel.

I have swapped the single 6922 valve over for a spare one, with no improvement. Swapping the signal polarity at the source select doesn't make a difference; I have reformed almost all the soldered connections, and I have checked the continuity of the tonearm cable.

The phono stage is the left-hand portion of the schematic here: RTP3C circuit. It's such a simple circuit that I would have thought it would be easy to find a fault...

I hate intermittent faults! Has anyone else come across this kind of problem?

Thanks,

Alex

Last edited by Alex M; 18th August 2012 at 10:07 PM.
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Old 20th May 2013, 07:43 PM   #2
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Hi Alex,

Did you find the root cause?
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Old 20th May 2013, 10:35 PM   #3
Alex M is offline Alex M  United Kingdom
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Hi Nelson,

As it happens, I posted an update to a different thread, since I had no responses to this one!

It turned out to be due to a partial failure of the shunt regulator for that channel, which resulted in a droop of the HT down to 260V after an hour or so. The current sink on the emitters of the input transistors means that a constant 240V is dropped across each 25K cathode resistor, so the HT voltage sag is enough to starve the input stage of voltage - hence the signal dropout.

As I mentioned in the other thread, I replaced the HPHV regulator with the much simpler Salas HPHV regulator, which is still working fine.

Alex
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Old 22nd May 2013, 05:59 AM   #4
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Great! Thx for your reply. :-)
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