[ARC5] T-22 Woes

[Neil]

> I've got a working T-22 down here and one that isn't working; the non
> working unit is stumping me.
>
> The working one is putting out about 40 to 50 watts via a series 67 pF
> capacitor to a 4:1 UNUN thence onto a dummy load.  The non-working one is
> putting out nothing into the same output network, and I think the problem
> is in the screen circuit.
>
> Voltages for the working and non-working T-22 are pretty much the same
> except for the screen voltages on the 1625s.   The non-working one only
> develops about 200 to 220V on the screens while the working on has about
> 300V.  The Grids of both are about -50V as expected.
>
> R-90 is 540 ohms which is a bit low.  I tacked in a 680 ohm resistor, as
> well as  replaced C-64, with no results.
>
> Since I'm getting -50V on pin 4 (and also the hot end of R-74),  that 
> tells
> me I'm getting sufficient drive to the finals.
>
> With a pickup loop on the finals, I can tune the output coil to resonance
> but not enough to move the watt meter.
>
> The oscillator waveform on the non working unit appears to be not as good
> as the one on the working unit.  Also, I can hear the oscillator of the
> non-working one on my Icom, and it doesn't sound pure.  I don't know how
> else to explain it.
>
> So, I think I'm getting sufficient drive,  I've replaced R-90 and C-64, 
> and
> I'm still getting only 200 volts or so on the screens.
>
> I do have continuity between the tap on L-52 and ground.
==========


There's a distinct possibility of an open circuit in the +500 V supply to
the plates of the 1625's.

Turn off the power.
Check the continuity of choke L53, resistor R76 (20 ohms), and the two
parasitic suppressors RL-50 and RL-51.

Since the screens are at 200 V or so, they appear to be drawing current,
so the selector relay contacts in the cathode circuit of the 1625's appear
to be OK, but it wouldn't hurt to check that they are making anyway.

If either L53 or R76 are open circuit, make sure you check that C72?
(the 0.0004 uF plate coupling capacitor) is not short circuited before
replacing them. If this capacitor is short circuited, either L53 or R76 gets
destroyed by current flowing through the 1625 tank coil.

73 de Neil ZL1ANM