[Boatanchors] TV-7D/U with faulty wafers... from the manufacturing line!
Jose V. Gavila
eb5agv at amsat.org
Thu Aug 27 17:55:02 EDT 2015
I was happy to have found a reasonably priced TV-7D/U here in Spain. I
own a TV-7C/U (the Canadian TV-7/U made by Stark) but wanted a 'D'
version for a long time.
So, when today I have got the package, I have immediately opened it,
checked visually (looked fine) and then, powered it up. I had around a
6BN8 tube, and have set the selectors to check the triode section... it
didn't work!. But, checking a 7 MINI tube (6BA6), readings were OK. I
have also checked a 83 and it also checked fine... So there was
something on the NOVAL socket which was not right.
After some struggling, following the schematic diagram something was
plainly wrong on NOVAL PIN 9 circuit (CATHODE on that setting). If you
check the schematics, from PIN 9 of CATHODE SELECTOR S102, if SCREEN is
at 0 (as with the 6BN8 triode), wire goes to S103 SECT E, to what seems
like a contact which is always present, and then goes to S103 PIN 9. But
there was not continuity.
And here comes the surprise: checking the wafer, on my TV-7D/U there is
a gap!. It should be a reduced contact, as pictured on the schematics. I
have opened the TV-7C/U and it is the way is shown on the schematics. It
is not a case of a broken part, but all the similar wafer assemblies
(S106, S103, S107, S104 and S105) have exactly the same fault!. The top
wafer (SECT E) on the FRONT part has a wrong contact, so it has that
gap. That position corresponds to 0 so, whenever 0 position is selected
on any of the faulty wafer assemblies, the unit could not work fine (it
depends on other tube settings)
So I wonder if someone has some spare wafer assemblies for a TV-7D. They
are ceramic; perhaps other models used them also. The plain TV-7 uses
phenolic ones. As the faulty wafers are the ones on the top, they can be
replaced with a reasonable amount of work.
I wonder if someone else has found a similar problem. I am really
amazed, as this unit was manufactured with that defect and has never
worked as it should.
73 EB5AGV - JOSE V. GAVILA - La Canyada - Valencia (SPAIN)
http://jvgavila.com Personal WEB
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