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Wavetek 3ST Operation Manual: How to Use the System Sweep Transmitter



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Wavetek 3ST Operation Manual



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It was relatively cheap to buy (under 200 in 2002 when it was 11 years old) considering the build quality and performance. The complete user manual can be seen here Below is a key to the controls. It's quite user friendly but it needed a handbook to let me know most of the functions. When the power is turned on (button 1) the generator runs through a self test routine which gets more worrying as the equipment ages. I've had two problems so far. One, immediately after buying it and turning it on for the first time, which turned out to be a short-circuit protection diode and the second quite recently which was again the loss of an internal power rail. This was easily fixed although the self test gave a strange fault report which I imagine was because the guys writing the test firmware must have imagined the power supplies would never fail?


When I was testing my DST100 receiver the Wavetech broke down. It took me a little time to discover the fact because the receiver has a few intermittent faults. When AM was selected the received signal kept cutting out for brief moments before re-appearing almost instantly, but connecting the generator to an oscilloscope showed it was the Wavetech not the receiver. RF output was OK but switching on modulation resulted in an intermittent RF signal. The manual points to modulation problems being in a specific area, that of the AM/DIV module so that's where I started. After removing two dozen screws the top cover can be detached revealing at the side of the unit the view below. The AM/DIV module has two circuit boards mounted in an aluminium sleeve which can be removed after unplugging various RF plumbing, including one at the rear of the unit connecting to a power amplifier module. At each end of the sleeve you need to remove the end plates by taking out their four fixing screws plus unscrewing the nuts securing the RF plugs. Below, pictures of the circuit board carrying a lot of the final RF circuitry. Modulation is carried out using a mathematical process controlled by one of the units two microprocessors, but as with all the circuit boards in the signal generator local power supplies are fitted in order to provide accurate voltages. The top view does have a clue to the fault I'm investigating but the underside view reveals clearly what's happening. Here's the fault: three dry joints at the pins of a power regulator transistor Q905 an MJE253. Although the designers included a hole for fitting a heatsink, this wasn't fitted so after replacing the solder I fitted a small brass bush which should help keep the transistor temperature down a little. In fact, as I tracked down faults later on I found the overheating was due to chemistry. Everyone knows about Mullard AF117 problems but I for one hadn't heard about Motorola transistor problems. Read on.... Back in business after the Wavetech had gone successfully through its diagnostic tests. After using the Wavetech on and off for a week or so I noticed a sizzling noise on the output. I looked again at the repaired output board and found another dry joint and a discoloured resistor. I resoldered the joint and fitted a new resistor. After assembly the sizzling noise was still present. The equipment cannot be tested by usual means so I decided to detach all the circuit boards and apply 18 volts to each. This is the standard supply to all the boards and on each one a 15 volt regulator provided on-board power. Starting with the output board I noticed the new 68 ohm resistor I'd fitted was very dark in colour and sure enough, powering the board showed the resistor was getting very warm. The 18 volt current was over 400mA and across the 68 ohm resistor I measured about 11 volts. This represents a power dissipation of about 1.75watts although the handbook states it's a half watt component so something is wrong. The chief feed from the resistor is the collector of an MRF571 RF transistor and this measured as a pair of diodes on my transistor tester. I substituted a BFR90 for the MRF571 and the resistor ran cool with 11 volts on the transistor collector intead of the previous 4 volts, but after reassembling everything there was still a sizzling noise on the RF output. Maybe the transistor had failed due to a problem with its components and this is still giving trouble? A little more about this saga. Having diagnosed a faulty drive transistor I eventually decided to splash out and buy a proper MRF571 (see above) because the output wasn't quite right. It was difficult to put one's finger on the problem, but using a scope I determined the RF was jumping around amplitude-wise and frequency-wise. I found a source of the rare device and ordered it. In fact it came as a set of six transistors from Poland although I'll probably never use the other five (but see later). After dismantling the assemblies I fitted the new transistor and the RF problem seemed to have cleared up... or had it? Above right to left...driver transistor Q900 (note the crossed zeroes which look like eights!), with Q901 bias set for Q900, Q905 current regulator for Q902, and RF output Q902. Q903 sets the bias voltage for Q902. Below Q905 (centre) is a voltage reference diode LM336Z-2.5. The MRF839F is a variant of an MRF839 fitted to a Type 319-07 base. This is a view of the underside showing the heatsink. After replacing the temporary BFR90 with a new MRF571 it turned out the problem was still present albeit not as bad as before. Why was it that sometimes the RF was as clean as a whistle and at other times infuratingly crackly? By experimenting I discovered that any RF output greater than 137.5Mz was faultless but below that I found the RF was sort of wobbly. Time to re-read the repair manual and study the circuit diagrams. This explained the reason for the transition frequency. Above the frequency of 137.5MHz the RF amplifiers, feeding the output socket via a programmable attenuator, are driven directly from a VFO but below that a mixer is involved. This mixer combines a second RF signal of precisely 512MHz with the VFO to derive any frequency from 10KHz to 137.5MHz. This being so, any problem in the mixer circuit or in the generation of the fixed frequency of 512MHz could be responsible for the crackly output.


Time to study the repair manual circuits once more. These are poorly scanned in my copy making it a time consuming job to identify the parts (not to mention the use of a crossed zero making a nought look like an eight). Before tackling the 512MHz oscillator which is on another circuit board I looked at the RF amplifier transistors. I'd already replaced the driver transistor because it had a leak of a couple of hundred ohms between its collector and base so I looked at the output device. This is Q902, an MRF839F fitted on a heatsink. It has an odd-looking base listed as Type 319-07 and I'd noticed several years ago when I had a problem with crackly RF output that the bias circuitry had been running very hot leaving the Q905 power transistor dry-jointed. I'd resoldered it and for a short time all seemed well, but why had this happened? 2ff7e9595c


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