[Heathkit] IT-11 difficulties, continued

[Mike McCarty]

Michael Tortorella wrote:
> Thanks to all who replied to my plea for help with the IT-11 cap checker.  I
> still have not found the problem(s).  Some more clues: the voltages at the
> tube pins are all in the ballpark except for 6BN8 pin 1 which should be
> -8VDC and is instead -1VDC, pin 7 should be 68 and is instead 51, and pin 8
> should be -0.22 and is instead -0.38.  But the really weird part is the
> bridge transformer.  When the int/ext generator switch is in the ext
> position, there is 6.5VAC on the switch "int" terminals and on all the tube
> filaments.  However, when you switch to the int position, the filament
> voltage on the 6BN8 drops to 3.5VAC as does the voltage on the bridge
> transformer primary.  And to make things worse, the filament voltage on the
> 6AX4 remains at 6.5 VAC.  So between the 6AX4 and the 6BN8 there is a
> voltage drop of 3 VAC...across a pair of wires.  I checked the resistance of
> the bridge transformer primary and secondary and the primary is about 2.9
> ohms and the secondary is about 6.9 ohms.  But the bridge transformer is
> behaving almost like a dead short.  But I have not been able to pull all
> this together to make a conclusion.  Can anyone help with this?

You obviously are not actually losing that voltage through some wires.
In the first place, the 6AX4 still has good voltage on it. So, something
else is wrong. I don't have the physical layout to hand, but it's
possible that R41 and/or R42 may be mis-installed, leading to one or
both of them being in series with the 6BN8 heater and the transformer
depending upon the GEN.SWITCH setting. I'd make resistance checks from
the transforner leads to the GEN.SWITCH terminals showing connection to
X and R11. Also, R11 may be accidentally mis-wired into the circuit,
dropping things down. Perhaps the switch deck supposed to short out
R11 has a problem with a bad contact. I don't see how that would affect
V2s heater, though.

I always try to bear in mind that the physical circuit may not be the
same as the schematic diagram. Sometimes the differences are minor,
as when ECOs have been made without getting into the schematic (rare
in Heath equipment, though not so in others) or due to miswiring
or solder splashes (not rare in Heath kits).

Mike
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