N Cook wrote in message
...
Had been in a loft for years
Original Mullard EL84 tested good gain,no leakage, and all else checked
out.
On powering there was amp hiss and crackle on turning the worn out vol
control.
But stroking the stylus with fingertip produced nothing.
I did not think to feed an external signal into the tape input and now it
is
back apart.
Wiring between pu and vol control is fine.
Putting the output of either pu LP or 45 to a scope and touching either
stylus produces absolutely no signal at 2mV per division.
Make of rotatable pu looks like
Fulfi (Googling produces nothing)
, Made in England, number TC8S , and TC8C and TC8RS for the stylii.
Does it need an excitation voltage ? its only connected to the valve grid
via 1M vol pot.
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
Let this be a lesson to anyone in the UK -
DO NOT store Rochelle crystal cartridges in lofts or sheds.
The crystal must be hygroscopic.
After removing the copper rivits (used flute section of small end mill to
avoid the rotating rivet problem with drilling out rivets in plastic) and
separating the 2 main parts, the problem was obvious.
I just managed to move the parts apart enough to photograph before the
active part fell to bits, lightly probing with a pin it was the consistency
of dusty paste.
(1 mm graph paper)
http://home.graffiti.net/diverse:gra...et/ful-fi1.jpg
Left section is the cover, slid sideways, and the R section, on a nylon nut
for focus, is inverted, part of the "crystal" with angled ground strip that
touches the central pu pin (common to the other pu) to the outside, along
with the signal pin for that pick up.
The other pickup , not seen yet, but will be the same state, is under the
central view.
The brown part is the rubbery material that engages with the crystals and
stylus shafts.
Plenty of copper carbonate corrosion inside.
The remaining parts of the yoke, styli etc
http://home.graffiti.net/diverse:gra...et/ful-fi2.jpg
Anyone reckon I can rob the crystal from a piezo ceramic pickup to fudge a
functional and "as original" repair?
I assume piezo-ceramic is not prone to this problem over 40 years.
--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/