Vintronics AM20 User manual

VINTRONICS AM20 20-Watt AM BROADCAST TRANSMITTERS
The transmitter has been designed to cover the AM Medium Wave band. Tuneable across the entire band
and to maximise the output power, pre-set tuning components are selectable in circuit.
Features of the AM20:
• Digital IC design
• 28 Volt 2Amp DC powered with UK adapter included
• 9KHz channel spacing for EU operation
• Inherently stable
• Audio input with automatic Modulation circuit (volume) via 2 RCA phono sockets
(stereo input)
• Low Distortion Analogue Linear modulator Amplifier.
• SO259 Aerial Socket on rear of unit
• average 20W RF output (80W peak) into 50 Ohm load
• 531 to 1638KHz selectable from front panel
• Tuning Meter LED
• Ready-made, almost ready to go! (frequency pre-setting using internal links)

This AM Transmitter is a self-contained unit housed in a steel box of dimensions 220 mm by 210 mm by 80mm high.
It covers frequencies between 531Khz to 1638 KHz. Powered by an external power adapter providing 28V DC, it is
plugged into the rear socket. A power on-off switch is located on the front panel. A Hi-Lo switch is located on the
rear panel, this is used to aid aerial tuning at half power.
Audio input is via 2 RCA Phono sockets also on the front which mono’s up a stereo feed. This in turn drives an
automatic gain control circuit which enhances modulation depth and at the same time reduces the effect of over-
modulation. If your audio feed is already mono, just feed it into either RCA socket. The pre-amp feeds into the power
modulator amplifier. The output of the modulator feeds the MOSFET PA stage which has a floating DC offset of half
the supply voltage. The A version uses a standard Analogue Audio Amplifier. The audio frequency response is 50Hz
to 5KHz.
The Phase-Locked Loop oscillator frequency is controlled by the lever switches on the front panel.

The frequency assignment is determined by the lookup table further below. It is set as multiples of 9Khz spacing,
from 531Khz to 1638KHz.
The PA output of the transmitter is very rugged, utilising 2 high power MOSFETS which are therefore significantly de-
rated. The output power is maximised by adjusting the output capacitors via the set of links. A table shows the
settings for maximum matching/output. There are adjustments on the matching coils for low band operation,
together with adjustments to various capacitances. The design uses air-spaced output coils for the matching, as their
inductance can be altered marginally by means of squeezing the turns together or spreading apart to maximise
output power.
A link is used on L2 which is used for band matching (between high and low frequencies) as shown in the table (LK2)
by linking either left or right. LK1 is the output capacitor and links are made as per table 1 to maximise output level.
The output is designed to match a 50-ohm load. Therefore, a suitable aerial arrangement is required to match your
aerial for 50 ohms.
It is expected that the output power across the Medium Waveband will vary due to the matching arrangement
design compromises, but broadly speaking the output averages around 20 watts RMS into 50 ohms, that’s a healthy
80 Watts PEP.
Table 1. Tuning adjustments and setup.
FREQUENCY
SWITCH
SETTING
Output
CAPACITOR
LK2
L1
L2
OUTPUT
LEVEL
585
01000000
LK1, 2, 3, 4, 5
left
Squeeze in
Apart
16W
666
01001001
LK2, 3, 4, 5
left
Squeeze in
Apart
18W
720
01001111
LK2, 3, 4, 5
right
Squeeze in
Squeeze in
20W
801
01011000
LK2, 3, 4
right
Normal
Squeeze in
22W
882
01100001
LK2, 3, 5
right
Apart
Apart
20W
963
01101010
LK2, 3
right
Squeeze in
Normal
20W
1053
01110100
LK1, 2
right
Squeeze in
Squeeze in
20W
1269
10001100
LK1, 2
right
Apart
Squeeze in
13W
1350
10010101
LK2, 5
right
Apart
Apart
16W
1431
10011110
LK1, 2
right
Apart
Apart
18W
1557
10101100
LK2
right
Apart
Apart
16W
NB. The frequencies used in the above table are an example of settings required for frequencies of nearby
frequencies. Although a certain amount of experimentation might improve output level.
SETUP and Tuning
Method: Select quiet AM Channel, if aerial is short (i.e. below 25M) best to use top end of AM band. Pre-set links as
in the above table and switch on transmitter. It is very advisable to use a SWR meter between TX output and ATU
input. Aim for lowest SWR but maximum RF out. A field Strength meter is also invaluable in order to realise the
signal strength emitted from the aerial. For stability, keep the AM20 well away from the ATU and high voltage RF
line.
Set up the ATU with your aerial tester for as near as 50 ohms and zero reactance. Set to ‘LO’ power on rear. (this
limits output to roughly 10 Watts). Switch Transmitter on and observe SWR for minimum. If happy, switch to ‘HI’
power and again check SWR/Power output. Apply Audio signal and check all is good!

MATCHING ARRANGEMENT
A suitable ‘long-wire’ aerial can be matched to this transmitter by using the Vintronics ATU, which is highly
recommended as it is fully compatible.
Other arrangements can be used. There is a lot of information available on-line regarding aerials and matching
circuits. Be sure to try and get an efficient aerial system which provides a low SWR in order to maximise the transfer
of power into the aerial.
PLEASE NOTE, damage may occur to the transmitter if it is not matched correctly. Its not as forgiving as a Valve TX!
16 Watts at 1557KHz into 50 Ohm load. 20 Watts at 954KHz into 50 Ohm load.

SPECIFICATIONS.
RF Output: 20 Watts RMS nominal as measured into a Bird Termaline via MFJ-836 Power meter. (also AVAIR
AV-201)
AF Input: -20dB to 0dB (auto levelling circuit will adjust to correct level)
Audio Frequency response: 40Hz to 6Khz
Power output: nominal 20 Watts RMS as per chart 1.
Size: 220mm wide, 210mm depth, 80mm height
Weight: 1.3Kg (2.8Lbs)
Chart 1 – Output Power verses PA volts into 50 Ohm dummy load.
10 10 10
15 15 15
10
12 12 12 12
14
16
18
20
22
20 20
13
16 16
18
17
16
0
5
10
15
20
25
550 600 700 800 900 1000 1100 1200 1300 1400 1500 1600
Output Power vs Supply Voltage
24V 28V

TRANSMITTER ARRANGEMENT USING THE VINTRONICS ATU.
Arrangement of Transmitter and Aerial Tuning unit. A coax cable is used to connect both together, but the ATU is
situated away from the Transmitter in order to improve stability. Pay special attention to audio feed, it is best to
keep this away as much a possible from the aerial feed as this will have very high voltages, it can cause audio
instability and the frequency counter to flicker.
The ATU is a ‘L – C’ matching arrangement comprising of a 60 turn airwound inductor and a 375pF 750v Variable
Capacitor, as seen below. It has a LED signal strength display in order to aid maximising signal into the aerial.
Good results have been obtained using a 18M long wire, at frequencies from 800KHz and the top end of the MW
Band.
.

FREQUENCY SETTING (kHz)(Metres)
(Shown set on 1557KHz)
Preset S1 S2 S3 S4 S5 S6 S7 S8
531 565 00111010 621 483 01000100 711 422 01001110
540 555 00111011 630 476 01000101 720 416 01001111
549 546 00111100 639 469 01000110 729 411 01010000
558 537 00111101 648 463 01000111 738 406 01010001
567 529 00111110 657 456 01001000 747 441 01010010
576 520 00111111 666 450 01001001 756 397 01010011
585 512 01000000 675 444 01001010 765 392 01011100
594 505 01000001 684 438 01001011 774 387 01010101
603 497 01000010 693 433 01001100 783 383 01010110
612 480 01000011 702 427 01001101 792 378 01010111
801 374 01011000 1062 282 01110101 1314 228 10010001
810 370 01011001 1071 280 01110110 1323 227 10010010
819 366 01011010 1080 278 01110111 1332 225 10010011
828 362 01011011 1089 275 01111000 1341 224 10010100
837 358 01011100 1098 273 01111001 1350 222 10010101
846 354 01011101 1107 271 01111010 1359 221 10010110
855 351 01011110 1116 269 01111011 1368 219 10010111
873 343 01100000 1125 266 01111100 1377 218 10011000
882 340 01100001 1134 264 01111101 1386 216 10011001
891 336 01100010 1143 252 01111110 1395 215 10011010
900 333 01100011 1152 260 01111111 1404 214 10011011
909 330 01100100 1161 258 10000000 1413 212 10011100
918 327 01100101 1170 256 10000001 1422 211 10011101
927 323 01100110 1179 254 10000010 1431 209 10011110

936 320 01100111 1188 252 10000011 1440 208 11001111
945 317 01101000 1197 250 10000100 1449 207 10100000
954 314 01101001 1206 249 10000101 1458 206 10100001
963 311 01101010 1215 247 10000110 1467 204 10100010
972 308 01101011 1224 245 10000111 1476 203 10100011
981 306 01101100 1233 243 10001000 1485 202 10100100
990 303 01101101 1242 241 10001001 1494 201 10100101
999 300 01101110 1251 240 10001010 1503 199 10100110
1008 297 01101111 1260 238 10001011 1512 198 10100111
1017 295 01110000 1269 236 10001100 1521 197 10101000
1026 292 01110001 1278 235 10001101 1530 196 10101001
1035 290 01110010 1287 233 10001110 1539 195 10101010
1044 287 01110011 1296 231 10011111 1548 194 10101011
1053 285 01110100 1305 230 10010000 1557 193 10101100
1566 191 10101101 1593 188 10110000 1620 185 10110011
1575 190 10101110 1602 187 10110001 1629 184 10110100
1584 189 10111111 1611 186 10110010 1638 183 10110101
FREQUENCY METER RESET.
If the frequency meter misreads or has become unstable when the transmitter is adversely matched, the
meter can be reset.
To reset the frequency meter, there is a tiny switch behind this reset hole on the front panel as shown:
Use a pin to scroll through the settings.

Press pin sequentially until ‘zero’ is shown, then hold for 3 seconds until a rapid flash, then release. This
resets the display to show correct frequency.