[ARC5] T-19 Update

[Neil]

> As posted earlier this year I’m working on my first T-19 project.
>
> I started testing it this week, but there are a few issues that I’m
> stumbling with.
>
> Here are my voltage measurements:
>
> Oscillator Voltage OFF
>
>          Key Up   Key Dwn
>
> Plates:   550       440
> Screens:  550       275
> Grids:      0       -18 DC
>            0       0.1 AC

The 1625's should NEVER be keyed with the oscillator off. The RF drive
supplied by it sets their grid bias. They may be ruined if operated in that
manner.


> Oscillator Voltage ON
>
>          Key Up   Key Dwn
>
> Plates:   550       510
> Screens:  550       150
> Grids:      0       -40 DC
>            2.1      2  AC
>
> As you can see, the screen voltage is about 100V too low under key down
> conditions.  R-90 and the 20K tap resistor in my power supply (as per Dave
> Stinson) measure OK.

You should not be taking voltage readings at the grids with an ordinary
DC voltmeter. The capacitance to ground of the meter leads will severely
reduce the drive to the 1625's.
As to the low screen voltage, were you perhaps measuring it with one
meter while measuring the grid voltage with another? That may produce
the symptom you describe.

The proper way to check the level of drive is to measure the DC volts
across R74, the 15K grid bias resistor. You can measure that with the
1625's keyed or unkeyed and it should hardly vary from one to the other.
For two 1625's, grid current should be in the vicinity of 5-7 mA, exactly
as Kenneth Gordon says elsewhere, so Ohms Law says you should see
75 or more volts across R74.
Measure R74 first to eliminate it as a problem. It's not unheard of for 70
year old resistors to go high.


> I’m wondering if C-64 could be the issue;  I was going to unground one end
> of it to experiment;  I can’t see any danger in dong that.

C-64 is the screen decoupler, 0.002 uF. It may indeed be the problem.
If it's bad, it may be giving misleading DC voltage readings due to presence
of RF at the screen terminals. If you leave it disconnected, these high RF
voltages may find their way into other parts of the transmitter, causing
undesired effects. Better to temporarily replace it.


> I’m getting about 1W out of the transmitter.  Antenna Coupling is at max,
> and Antenna Inductance is almost 0 (expected).
>
>
> I’m using a small air variable cap at the output of the transmitter to give
> me a series capacitance of about 50 pF,  the other end of the cap goes into
> an SO-239 which goes to a W-4 Wattmeter and finally into a Cantanna Dummy
> load.

73 de Neil ZL1ANM