DSE K-7620 Spectrum Analyzer Kitset Modifications


This note describes the method of upgrading the DSE K-7620 Spectrum Analyzer Kitset to include full 40MHz to 1800 MHz coverage, consisting of -:

Band I 40 - 140 MHz

Band III 120 - 460 MHz

Band IV / V 440 - 800 MHz

Satellite 750 - 1800 MHz

And increasing the ease of use of the front panel controls by replacing multi-pole switches with Multi-turn Pots.


The existing tuner covers 45 - 150, 90 - 240, and 440 - 870 MHz. (K7620 Tuner) The new tuner from a 10" colour TV, cat no. G7230 (G7230 Tuner), replaces the K7620 Tuner, and adds the hyperband. The Centre Frequency and Sweep Width switches are replaced with 10 turn pots to give finer control over their operation.


The K7620 Tuner runs on 9 volts DC, while the G7230 Tuner runs on 12 Volts. The pin-outs for both tuners are identical, although the pin numbering is reversed - i.e. the K7620 Tuner has pin 1 for the IF, whereas the G7230 Tuner calls it pin 8. The supply can be changed for the G7230 Tuner by replacing voltage regulator U2 with a wire link from input to output, and optionally removing R8, 820ohm, C15 2.2uF, and Z1 4.7 V Zener

K7620 Tuner Pinouts.


Pin 9 is UHF band select

Pin 8 is Tuning Volts

Pin 7 is VHF High band select

Pin 6 is AGC (RF Gain)

Pin 5 is VHF Low band select

Pin 4 is AFC (not used)

Pin 3 is +12 Supply

Pin 2 is not used

Pin 1 is IF output (38 MHz)

Satellite Tuner Pinouts.

'F' Connector

Pin 1 of the Sat tuner is LNB supply,

Pin 2 is RF AGC.

Pin 3 is +12 Supply

Pin 4 is Tune RF

Space / missing pin

Pin 5 is Tune Osc - connected to 4 via a 100k resistor.

Pin 6 is +12 Supply

Space / missing pin

Pin 7 is RF AGC.

470 MHz IF comes out the back end.

The Satellite Tuner (Sat-tuner) has 12 volt applied only when the range switch is switched to the satellite band, as the 470 MHz IF output of this module is connected to the input of the G7230 Tuner via a 4.7pf Capacitor. The G7230 Tuner has the UHF band selected, and has it's tune volts set to tune the Sat-tuner's IF frequencies by means of a 100k resistor and a 50k pot placed across a regulated supply rail and switched in to the G7230 Tuner's tune rail. The tuning voltage here is approximately 1.3 volts. The tuning sweep rail is the band switched to a 56k resistor, which in turn connects to pin 4 of the Sat-tuner. A 100k resistor connects between Pin 4 and Pin 5 of the Sat-tuner.

The RF gain pot on the front panel is replaced with a 10k dual log pot R7606. 12V fed via a 12k resistor to the top end of the pot, and the bottom end is grounded as with the original. The wiper of the pot connects to the Sat-tuner's RF AGC pin 2 (See drawing).

The band switching can be implemented by either using the existing range switch and moving the unit to a larger box, which will allow the Sat-tuner 'F' connector to protude out the front panel. At the frequencies that this tuner operates at, even a small amount of interconnect cable can have a detrimental effect on performance. The alternative is to remove the range switch and place switches on the rear panel, this allows the tuner head 'F' connector to come out where the power LED is.

A 25k-log dual gang pot (R7608), and the Centre Freq Fine pot is replaced by a 10k replace the Centre Freq Coarse switch, 10 turn 2W multi-turn pot. (R6960). Points 9, 10, 11, 12, 13, 14, and 15 are disconnected from the existing switch and are no longer used (See original assembly manual). The top end of the 25k dual pot, section A, connects via a 4.7k resistor to the 12 volt rail, (point 15) and the bottom end connects to ground (point 8). The top end of the 25k dual pot, section B, connects to the 12 volt rail,(point 15) and the bottom end connects to via a 4.7k resistor to ground (point 8). The 10k, 10 turn pot ends connects to the 2 wipers of the 25k dual pot. The wiper of the 10k, 10-turn pot connects to point 17 on the PCB.

The Sweep Width Switch is replaced by a 100k 10-turn 2W multi-turn pot (R6970). Points 20, 21, 22, and 23 are disconnected from the existing switch and are no longer used (See original assembly manual). The wiper of the 10-turn pot connects to point 18, one end of the pot to point 19, and the other end to point 24. Unfortunately this part of the modification removes the muting of the RX Audio output, as there is no switch attached to the Sweep Width pot. This is the shielded cable that comes from points 33 and 34. The 22k resistor is moved to the rear audio output socket. An additional switch can be added to act as a mute switch

X and Y in to Video out Mod

I come up with this idea after I tried to reline one of my Satellite dishes. I needed away of displaying the Spectrum Analyzer as a video signal. To do this you need cut the track between pins 2 and 6 on the NE555 timer IC (K-7620). This so that you supply a trigger pulse to pin 2, this is to start the sweep at the right point in time.

The LM1881 provides this timing pulse from a reference video source. The op-amp adjust level of bias to the FET, this sets amount of defection on the screen. 4528 gives delayed output after a trigger is received, the amount of this delay is set by a RC network on pins 1 and 2. In this configuration the FET replace the R and this will very the amount of delay on each line. Pin 5 of the LM1881 outputs the line and field syncs, and pin 6 of the 4528 provides the luminance. These to output are then mixed in at the video output transistor (BC337). To get the correct video levels you need to set the two 10k trim-pots.

LM1881 Datasheet

Below there are two images showing what the video display looks like

Intelsat 8

Optus D1