[ARC5] T-22 Woes

[Mark K3MSB]

Hi Neal

I'm getting full plate voltage - I forgot to mention that.  I also
substituted tubes from my working unit.

Since I'm getting the expected Ep the coupling cap isn't shorted - but I
didn't check to see if it was open.

73 Mark K3MSB

Sent from my Android phone
On Jun 11, 2013 4:10 AM, "Neil" <neilb at ihug.co.nz> wrote:

> 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
>
>
>