FT901.2

From: LA8AK@LA9K.XND.K.NOR.EU
To  : MODS@WW

10) PB-1718/1720B: Push-buttons.
After some years in use, the push-buttons will not function properly,
the problem is that some small springs, easily seen from above, will
not move too well in the tracks, some thin oil improves this.

11) PB-1715 PA-unit (2x 6146B): PA-valves protection.
To protect the power-supply against anode-to-screengrid voltage flash-
over, a small 1N4148 type diode is connected in series with screen-
grid supply to the 6146B's. This is not shown on the circuit diagram.
This may be good for power supply, but it may be fatal for the valves
in case of some secondary emission due to hard operation. Some com-
promise is therefore needed. The diode must be paralelled with a
10KW 0.5W resistor. To protect the screen-grid circuit against flash-
over I have connected a pair of SIEMENS varistors, SIOV 14K150, they
draw 1mA current @ 250V DC.

12) PB-1715 PA-unit, PB-1708 RECT-A-unit:  BIAS-circuit improvements.
To stabilize grid supply against drift-off due to gassy valves, a
diode is connected in paralell with R05, a 1N4007 is preferred because
it is a poor RF rectifier. To avoid that the grid voltage should
dissappear because of bad contact in BIAS POTMETER VR01 a 100KW
resistor is connected from negative side to center. Often this occur
if you have left the potmeter in same position for years, and when
one needs to adjust the idle-current, the trouble sometimes happen.

13) PB-1708 RECT-A, PB-1717 RECT-C: IMPROVING THE CW-KEYING (Normal
CW). Orignally the CW-keying is bad, lots of key-clicks. For HF CW ca.
4-6ms delay is optimum. A single capacitor cannot solve this problem,
because of different charge and discharge current. A suitable
compromise was made with simple capacitor/diode/resistor combinations
see figure ----

14) PB-1705 AF-Unit, PB-1846 VOX/MARK: AUDIO LINE OUTPUT.
For packetradio/RTTY/CW-meteorscatter it is desired to use an in-
dependent AUDIO LINE OUTPUT. Surprisingly, there was an unused 1/4-
LM324 on the AF board. There was marked but unused component-designa-
tions around this, one point was used only to connect two components,
they were removed from this common point and connected in the air.
Then the components as shown were mounted, the un-numbered components
are mounted below PCB or which is practical. It was an unused cable
from AF 'MJ6' to VOX 'MJ7', and further to J17 SIDETONE out on rear
side. I could not see any practical use for this TONE output, so I
decided to use this for AUDIO LINE OUTPUT. The 4k7 resistor is changed
to 1KW or removed, IN VOX UNIT R52 is loosened in one end.

CONT IN FT901.3