Frequency Change or RF Realignment. Your TC70-20 is aligned at the factory to accept crystals (Fo/4) for any frequency you specify
from 426.25 to 439.25. P.e.p. power should not be more than 10% different between these two frequencies. If you plug in a new crystal,
or one frequency seems not to come on or is intermittent, first push the crystal down and then raise it back up just a little to make sure its
case is not shorting out on the socket. Then, with a DC voltmeter connected to the 100 Ohm resistor oscillator test point, slowly peak for
max voltage (typically >.6 Vdc) with the highest frequency crystal. Then switch to the lowest frequency crystal and check to see that the
voltage is within .1 Vdc of the higher frequency crystal reading. Slowly rotate C1 with an insulated tuning tool toward more capacity to bring
it in if necessary. You should not have to peak any of the other variable capacitors, but if the power out of the TC70-20 (not an external
amp) is different by more than 10% you may want to. In this case, you will have to unplug any video and set the pedestal pot to maximum
power output. Then starting at C2 & 3 peak at its multiplier test point with a voltmeter for maximum on the highest frequency, and then bring
the lower frequency up to with in 10% without going below 1.4 Vdc. Then peak C4 & C5 at its doubler Test point (>.3 Vdc), followed by C6
& C7 peaking at the RF TP for negative voltage. Then reset the RF level and Pedestal pot per the previous procedure.
TXA5-70 Exciter Modulator -
Read this completely before touching any adjustments.
There are two pots on this board that you may have occasion to readjust. The blanking pedestal pot is common to all the P. C. Electronics
transmitters. It is used to accurately set the video to sync ratio whether running barefoot or through an external amplifier or by itself. The
sync stretcher circuit clamps to the video blanking pedestal to keep a constant sync and blanking level regardless of video gain and
average picture level. To set the blanking pedestal, disconnect any video input, and connect a RF power meter (Bird with 25E / 100E slug
or Diamond SX1000) in the antenna line after any amp. Turn the 1K pedestal pot full CCW. Turn on the transmitter and quickly (<20 sec.)
set the peak power output with the 250 Ohm RF Level pot (to no more than 90% of maximum output for external amps; multiply max power
noted by .9). This will be your constant peak envelope power. Do not exceed 150 Watts if driving a Teletec DXP-U150. If driving other
amps, make sure you don’t exceed the maximum rated input drive at any time by measuring and presetting the TC70-20’s power level
before connecting the amp. Next reduce the p.e.p. power by 60% (multiply the peak envelope power just set by .6) using the 1K pedestal
pot. This will be the constant blanking pedestal power. Now you can connect video and adjust the front panel video gain pot for best
picture just before white limiting. Do not adjust any board pots or trimmer caps with video connected, and only adjust power out with the
RF level pot, never any of the trimmer capacitors - it could cause instability and poor linearity in the video.
TVC-2G
FMA5-F
TXA5-70
PA5
DMTR
3 A
3AG
fuse
Osc Peak C1
>.5 Vdc
1st Doubler
Peak C2 & C3
>1.4 Vdc
2nd Doubler
Peak C4 & C5
>.2 Vdc
Note: Voltages noted
are with 13.8 Vdc
applied, pedestal full
ccw; 12Vdc will be
less.
Final Peak C6 &
C7 -1 to -3 Vdc
To FMA5 sound board
RG174
F2
F1
+T
5 MHz Low-
pass Filter
Video gain
100 ohm
To DMTR
Video
In
To PA5 amp
RF power
output pot
Blanking Pedestal pot
7
7
7
7
7
(c) 1999 P. C. Electronics
2 5 0
>.3V
MPS911
2 2
RF TP
-1to -3V
2N4124
Level
2 2 0
4.7K
2 2
+
3 3
3 3
F
r e l a y
. 1
2N2219A
Modulator
P
RF1
RF2
RF3
RF4
5 6
1 0
. 2 2
1 0 0
1st Doubler
2nd Doubler
Final
t p
t p
osc
t p
Xtal
1N5711
OSC
2N5770
L2
. 0 0 1
L1
1 0
1 0
1 0 0
4.7K
4.7K
68K
1 0 0
3 3 0
10K
. 0 0 1
Sync Stretcher
4.5 MHz
Sound
Input
2 2 0
Sync
VIDEO
INPUT
V
1K
1 0 0
3 3
A d j
1 0 0
1N914
1N914
1N914
Blanking
Pedistal
1K
2.2K
2N2907
2N2222
2N2222
1N751
5.1Vz
1K
2 2 0
2 2 0
4.7K
3 3 0
. 0 5
1 M
1 M
22K
. 0 0 1
2 2 0
1K
22K
4.7K
1 0
+
+
1 0 0
Video
gain
C1
C2
2N5770
5 6
C3
1K
15K
1 8
. 0 0 1
2 2
2 2 0
1 0 0
2N5770
2 2
2 2 0
L4
C4
3.3K
C5
10
C6
C7
2 7 0
2 2 0
RF OUT
50 Ohms
+
1 0 0
2 2 0
+12 to
14.0 Vdc
+
-
1N4745
16Vz
>.5V
L6
>1.4V
TXA5-70b ATV TRANSMITTER
© 1997
L3
L5
5A
3AG
Fuse