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Quad FM1 Question

R

rvw67

pfm Member
I have a quad fm 1 that I had serviced a few years back by Quad. I break it out every once and while to use. The last time I did this I noticed that when tuning
a station in I used to just obtain even brightness with the tuning indicators and get a very quiet signal, now that isnt always the case, sometimes I will have just one indicator lit for the clearest signal. Is this a tube issue? is there something that I can just adjust a bit to solve this?
Thanks, John
 
Don't worry, my FM1 has proved exactly the same.

Looking at the circuit again it's probably down to (1) differential aging effects in two (very) different valves which indirectly feed either leg, one a dual triode, one a pentode, doing very different jobs; or less likely (2) mu differences between the two triodes in the one envelope which light the neons ( one in each leg of a LTP based around valve 7, a 12ax7 or ECC83)

One could possibly null the brightness variance by fiddling with R27, a 2700ohm resister (replace with a 1w trimmer?); but since it's a side effect of valve aging, not a reflection on RF performance, I'm leaving mine alone. HTH
 
Martin you clearly are the bees knees but I hope you are also getting out enough, anyone who knows this much about a pair of pentodes in a piece of kit from 1960 needs to check in everynow and again. However Im easily impressed.

PS I think my wife will let me into the attic to get the rega box - as the decorations are beside it!
 
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