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Old 27th Jul 2009, 12:20 pm   #2
Leon Crampin
Octode
 
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,869
Default Re: Bush VHF61 Quiet on VHF

My first point of investigation would be both EF89s. These valves have a shorter life than the others (even the ECC85) and fail with very low emission. Then check the ECC85 by substitution and the EABC80 for ratio detector diode balance. A poor diode is surprisingly common and makes the set impossible to align correctly.

When working correctly, on a reasonable signal (these sets have lots of gain on FM) the audio level should be at least as high as that on AM. The eye will also overlap on a strong signal - I fit a 3:1 attenuator in the FM DC feed to the eye to make it more useful as a tuning aid.

The AM performance of these sets is very poor indeed, even with a good set of valves and correct alignment. A fixed rod aerial is a joke on a big mains set and the external AM aerial provision (bottom coupling to a single tuned circuit) is hopeless.

The Bush sets of this era (last of the "horizontal chassis" AC only FM sets) are my favourites - the extra EF89 really transforms the FM performance. A correctly set up VHF64 with a working tweeter and a couple of anti-hum mods is the best of them all.

Leon.
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