Help: JUMA PA100 powering itself off suddenly

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Help: JUMA PA100 powering itself off suddenly

Postby OH5YU » 20 Sep 2015 14:41

This is the problem I run into on this contest week end: when increasing the output to somewhat over 50 watts and keying my JUMA PA100-D on cw, the power goes off all of a sudden. As long as the output is kept at about 50 watts or less, all is well. You can drive the amp to 100 watts using the tune function button of the exciter. However, after a few characters of cw the amp turns it self off without a warning if the output is over 50-60 watts.

I have upgraded the firmware to version 2.0 and calibrated all of the other parameters but the ammeter. I don't have the equipment for the current calibration.

One thing under suspicion is the power source I am using. The voltage drops considerably under load, from 14 to 11 volts or so. Because of that, I have turned the low voltage alarms off on the amp. Does anyone have suggestions as to what the problem might be? I am grateful for any possible help.

73, Juhani OH5YU
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Re: Help: JUMA PA100 powering itself off suddenly

Postby 5B4AIY » 23 Sep 2015 13:21

Hi, Juhani,

From your description, I assume that there are no alarms present. In the alarm logic the over-current, over-voltage, and high SWR alarms will all force the amplifier to the STANDBY state, however the under-voltage alarm will not do this, it will simply display the low voltage alarm.

Your description states that the amplifier powers down, i.e, it shuts off. This can certainly happen if the supply voltage drops to such a low value that the microprocessor experiences a transient reset event, in which case the logic level that holds the power switching logic on will be lost, and the amplifier will indeed shut off.

If you examine the schematic of the Juma-PA100 Control Board, PSU & Relay Drivers, Page 1, in the top left-hand corner you will see that Q2, the main switch transistor, is turned on by Q1 whenever the PWR button is pressed, and then held on by the logic signal from the microprocessor, PWR_ON. When the PWR button is released, if your lose PWR_ON, even momentarily, Q2 will immediately switch off, and the amplifier will shut down. You will also notice that Q2 can be bridged by a jumper, J4, labelled TEST POWER ON. To determine if your power supply is causing this, you can bridge this jumper. If the amplifier now stays on when you key at 100W, then it is almost certainly a transient low voltage that is causing this, momentarily resetting the microprocessor and dropping the PWR_ON signal. If you do this test, and a transient reset occurs, if the reset is incomplete, then you may see strange displays, the amplifier may behave in an odd way, so be ready to switch the main power off. You will not be able to switch off with the PWR button because Q2 will be bypassed.

Bear in mind that when keying 100W output the amplifier can be drawing 18A - 20A or more of input current, and thus any resistance in the power leads, as well as internal resistance in the power supply or battery will severely affect operation. Also, when operating at low voltages in order to achieve 100W the input current will need to be higher, and this can easily trip the 24A over-current protection circuitry. You need to use a much better regulated power supply.

Regards,
Adrian, 5B4AIY
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Re: Help: JUMA PA100 powering itself off suddenly

Postby 5B4AIY » 24 Sep 2015 12:30

Further to my previous post regarding your poor power supply regulation, the following data may be of interest. I set up my Juma TRX-2 and PA-100D connected to a well-matched 50 ohm dummy load. The TRX-2 and PA-100D were powered by the same power supply, a MyDel switch-mode PSU, model MP30SWIV.

Both the TRX-2 and the PA-100D measurement systems had been calibrated with a Fluke 8502A 6-digit laboratory grade multimeter.

In the TUNE mode, with an output power of 100W the power supply voltage dropped from 13.7V to 13.6V, and the total current drawn by the TRX-2 and PA-100D was 18.0A. The voltage displayed on the TRX-2 and PA-100D was 13.27V.

Using the TRX-2 in the CW mode, with the PA-100D gain set to maximum, and keying the output with 20wpm dots, the PA-100D was supplying 120W to the dummy load. The PA-100D ammeter registered 17.0A. The voltmeter reading dropped from 13.72V to 13.50V.

Using a gain setting of G3, the output power was 90W, and the PA-100D current was 15.0A. The voltmeter reading dropped from 13.72V to 13.58V.

The key was held closed as the PA-100D heat-sink temperature reading increased from ambient 27C, to 42C.

This is the sort of regulation that is demanded in order that the transceiver and amplifier can operate satisfactorily at these power levels. This requires relatively short power supply leads, especially for the PA-100D, in my case 90cms. Even so, an oscilloscope connected to the power supply leads at the input socket will still show significant keying transients caused by the series inductance of these leads, and these keying spikes, even though steps are taken in the amplifier to suppress/minimise them, can certainly cause the amplifier to behave erratically if the input voltage is too low.

Hopefully this data will be of some use and interest to you.

73, Adrian, 5B4AIY
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Re: Help: JUMA PA100 powering itself off suddenly

Postby OH5YU » 27 Sep 2015 17:44

The source of the problem was the power supply. Using another power pack with less voltage drop under load, the JUMA amp works faultlessly. Many thanks to Adrian again for a thorough analysis and very valuable information. Your help is much appreciated :D

While on the subject of power supplies, what would you recommend for 20 amp continuous and some more intermittently? After this exciting(!) experience, good regulation is a must. Another important criterium is as little RFI on receive (hash or birdies) as possible. A third one is availability in Europe, 230 V mains voltage. From what I have read and experienced, switching power packs are much less heavy and smaller than linear ones, but demand careful engineering in order to avoid RFI.

73, Juhani OH5YU
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Re: Help: JUMA PA100 powering itself off suddenly

Postby 5B4AIY » 28 Sep 2015 08:42

Hi, Juhani,
Glad to be of assistance. Power supplies, OK, let's discuss what is required. There are two characteristics that govern the operation of a regulated power supply - the static regulation, and the dynamic regulation.

Static Regulation
This parameter determines how well the power supply maintains the desired output voltage under steady-state conditions. This factor is primarily set by the closed loop gain of the regulator amplifier. A typical method of measuring this is to use the TUNE mode of the transceiver and output a steady carrier into a dummy load and measure the change in voltage from the idle to the full-load state.

At the power supply terminals the change in output voltage should be a very small percentage of the preset output voltage, and, for example, a typical power supply might well be specified as less than 0.7V for a 16A change.

Dynamic Regulation
This parameter determines how well the power supply maintains its stated output voltage as the load current changes, and is largely determined by the transient response of the error amplifier. Obviously a fast transient response is desirable, but too wide a bandwidth and the power supply may have a tendency to oscillate with certain types of load. This parameter becomes important when keying a transceiver or linear amplifier. For example, when the key is up, the current drawn from the power supply is only the idle current and amplifier bias currents, typically of the order of about 500mA - 800mA, but as soon as you key down the output power is immediately at 100W, and the current drawn immediately increases to about 18A - 20A. If the transient response is lacking, the output voltage will momentarily drop and then recover. Equally, when you release the key the current immediately drops back to the idle current, and again, if the transient response is poor there may be considerable overshoot on the output voltage before it stabilises.

All of this also presupposes that the resistance of the power supply leads is low, and does not further add to the regulation problems. One way to overcome the inherent voltage drop in the power supply leads is to use a feature called remote sensing where an additional pair of wires connects from the load (amplifier) back to the power supply, and the voltage is then sensed directly at the amplifier, thus including the power leads inside the regulation loop and further improving the regulation. Unfortunately I do not know of any amateur power supplies that make use of this feature, but certainly industrial high-current power supplies are equipped to do this.

There are two types of power supplies commonly used for amateur transceivers and linear amplifiers, the linear power supply and the switch-mode power supply. Linear power supplies represent very mature and reliable technology. They use a large mains transformer, bridge rectifier, large filter capacitor, one or more series-pass transistors, and a linear DC amplifier that is the regulator. The output voltage is usually very well smoothed with very low AC ripple, typically of the order of 10mV or less, and excellent static and dynamic regulation. The drawback is that they are usually very bulky and heavy largely because of the size and weight of the mains transformer, and they are not very efficient, generating a fair amount of heat, and have large heat sinks which are often forced air cooled with a fan, and thus might well be quite noisy. One factor that does make them highly desirable for amateur radio applications is that they do not generate any RF noise, and are thus radio quiet.

The second type, switch-mode, use a bridge rectifier to convert the incoming AC line into a 300V - 400V DC voltage which then powers a high-frequency oscillator, using a switching frequency between a few hundred kHz and 1 or 2MHz. The transformer can now be a small ferrite-cored device. The regulator uses a pulse-width modulated switching technique to modulate the average output voltage. There is usually a significant amount of high-frequency ripple on the output voltage, typically about 100mV P-P, but this is fairly easy to eliminate in the transceiver or linear amplifier. The static regulation is somewhat worse than for a good linear supply, but still more than good enough for our purposes. The plus side is that these power supplies are physically much smaller and lighter than their linear counterparts, and much more efficient, often exhibiting full-load efficiencies of 90% - 95%. They are thus quiet, and cool, but the drawback is their tendency to produce RF interference. The switching frequency and its harmonics can be fed back into the mains wiring, which will then radiate it, and this can be quite difficult to satisfactorily eliminate. For amateur applications it pays to choose a switch-mode power supply that is specifically designed for this mode rather than trying to adapt a surplus industrial supply, even though these are often available quite cheaply on eBay for example. An amateur supply will often be constructed in a shielded metal case, with good quality input and output filters, and in some cases there will be a knob on the front panel to shift the frequency of the oscillator so that its spectral components do not interfere with the wanted signal.

A good linear power supply is, for example, the Yaesu FP-1030A, the Kenwood PS-50, or the Palstar PS-30M. One serious drawback of linear power supplies concerns their catastrophic failure mode whereby a series pass transistor fails short-circuit. I have had this happen twice in my professional career, with disastrous consequences. What happens is that the transistor fails short-circuit, and now the full unregulated supply voltage appears at the output terminals. To protect against this most power supplies have an over-voltage protection circuit built-in to either crowbar the supply or otherwise turn it off in this event.

Switch-mode power supplies such as the MyDel MP8230, and the MP30SWIV are now very mature and reliable, and I have found them to be radio-quiet as well. In the case of these types of power supply, a catastrophic failure typically results in an immediate and complete loss of DC output, thus protecting the load.

There are also some more modern switch-mode power supplies that, whilst slightly less efficient, using a resonant transformer technique which is inherently quieter in terms of RF interference than the usual switching technique. One such example is currently being sold by Elecraft for their KXPA100 add-on linear amplifier, and is the Power-Werx SS-30DV, switchable between 120V and 230V operation, and CE certified. (powerwerx.com) This is even smaller than the MyDel power supplies, and is rated at 25A continuous, and 28A for 5 minutes. I have used this power supply as well as the MyDel ones, and have not experienced any significant amounts of RF interference.

Hopefully this answers your questions.

73, Adrian, 5B4AIY
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Re: Help: JUMA PA100 powering itself off suddenly

Postby OH5YU » 28 Sep 2015 19:44

Adrian, thank you for the very informative and educational reply! I am sure many readers benefit from it :)

In my mind, it would do great service to publish this in a forum with an even wider audience such as the eHam.net, for example. -- Oh, I know there are always some besserwissers and nitpickers in well known forums but never mind.

73, Juhani OH5YU
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Re: Help: JUMA PA100 powering itself off suddenly

Postby OH5YU » 23 Nov 2015 20:41

For those interested: the root cause of my problem was a too long and coiled power cord between the power pack and the amplifier. That was the cause for the big voltage drop. Luckily, the power pack turned out to be well regulated.

Once more, thanks to Adrian for explaining why the JUMA amp shuts down immediately if the voltage drops too low. It works as designed, it is not broken.

73, Juhani OH5YU
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