After years of faithful service, I noticed my SPE 1.3k EXPERT PA produced
from time to time some smell of a 'burning resistor', later some 'crackling'
noise, typical of RF flashover situations. Also, in the low and mid power
settings, it refused from time to time to amplify (despite going into TX
mode), unless I switched to high power mode, and reverted to mid or low
power ... very strange !
Anyway, when I opened the PA top cover and the inner cover plate, I immediately ascertained what was going wrong: the LDMOS device was dying ... despite the fact that I did not mistreat the unit (always driving it in RTTY only till 750W about in mid power setting, never reaching the 75°C temperature limit ...).
It seems that one of the LDMOS drain leads was literally 'burning up' :o( See pictures below of the MRFE6VP61K25H device pulled from the amplifier... you can see through the drain terminal !
Very strange manner in going defective ....
In my opinion, fault was due to improper soldering of the LDMOS tabs on the PCB (sorry to say so Gianfranco I0ZY). I once had to repair another 1k3FA of a friend who reported 'intermittent power output' and 'no drain current'. In this unit I found that there was solder applied on the gate tabs upper surface, but they were not soldered & electrically bonded to the PCB ... in fact just merely making contact by pressure. If same problem would be existing on the drain leads of my unit, this could lead to arcing and probably the burn-up pattern of pictures below ?
Other users have reported same problem on their 1K3. Max NG7M serviced a friend's PA and ascertained similar troubles and hereby sent me following pictures ... one of the LDMOS drain tabs was badly damaged by arcing, the second tab was still intact, but not soldered to the PCB. However, there was some solder on the underside of drain tab (pre-tinning).
And further the German DX-pedition to TUVALU T2C had same problems : "Two of our power-amplifiers blew up in the second week, one after the other. We were able to repair one of them by re-soldering the transistors" (quote and picture below ref. EUDXF newsletter 2 • 2024) Exactly the same spot where mine blew up !
By chance, I had a MRFX1K80H (1k8 version, see specs sheet here) on hand, intended as spare for my homebrew 2m PA - so I decided to give it a try with this version. Quite an easy job, performed in less than 1 hour, as the whole PCB can remain in place ! To exchange the LDMOS, temporarily lift the toroid choke coil by desoldering it from the output transformer, as well the two solder lugs between retaining standoffs and PCB. Apply really a minimum of heat paste on the LDMOS, well smeared out evenly along it's complete surface. Be sure the copper heat spreader is clean (use alcohol and a cotton bud) and without any dust / solder particles or fibers of the cotton bud before putting the new LDMOS in place - you can check this with a torchlight illuminating laterally the trench for LDMOS. It is VERY important that both drain tabs are making as well as possible contact with PCB foil, so not only on their outer edges. Therefore, carefully and evenly pre-tin their surface on the underside - this will ensure that they will be optimally soldered to the PCB (which is off course already tinned). Once the LDMOS is in place, start to wiggle it sidewards by applying a firm finger pressure on the ceramic body in order spread the heat conducting paste and remove some excess. In the beginning, the LDMOS will easily move, but after some time it will be like 'sticking' to the copper heat spread = perfect ! Then put back the 2 retaining standoffs, do not over tighten, but equalize gradually the pressure of both. Wait 30 minutes, and check / re-tighten slightly. Only thereafter solder the 4 LDMOS tabs to the PCB. I use for this a wedge type solder head 5mm wide of my Weller station running @ 450°C - it just takes 2 or 3 seconds on each tab to have a perfect joint!
Take care of adjusting the trimpot on the PCB (close to LDMOS) completely CCW before applying any power. This trimmer is the adjustment of idle current through LDMOS (IDq). Then, while in TX mode and LOW power setting, with NO DRIVE at all, watch the current consumption (on the display of PA, it should be 0,1 Amp) and by VERY SLOWLY turning the trimpot clockwise, set it to 1.6 A. Watch out, because beyond this value, it increases quickly (with as final result sure a 'BIG BANG'). Check the idle current on HIGH power in the same manner, it should be about 1.9 A.
Now you can test the PA, first on LOW power, then MID and finally HIGH power. After some tests to ensure all is OK, check and retighten if necessary the 2 retaining standoffs.
The MRFX1K80H is designed for output power of 1k8 continuous @ a supply voltage of 65V, so in the SPE amp (max 1,3 kW @ 50v ) it feels very 'comfortable'. In my howebrew PA for 2m I had to replace as well the 1k25 version (which went SK) by the 1k8 type, and found that this version was somewhat 'lazier' (less gain), because circuit components need some fine-tuning when operating at this high frequency. But on HF bands, like in the SPE, there is no difference, 1k8 version is considered as a 'plug&play' replacement, and I experienced that you will more easily reach 1k3 or 1k4 peak (SEE UPDATE BELOW !) In fact, now I usually work in 'mid power' setting, and it delivers 1 to 1,2 kW with very limited heat dissipation, where with former 1k25 LDMOS I had to run in high power mode. To minimize stress on the LDMOS, always try to run on LOW or MID power and apply enough drive to operate PA in saturation (=ALC limiting output), this will ensure best efficiency (= minimal heat dissipation) .
The following graph - provided by NXP - clearly shows that the MRFX1K80H is more efficient running on 50v @ 1k2 output power on 27 MHz than the MRFE6VP61k25 version, without retuning of any components:
It seems that SPE is now factory fitting the 1k5 LDMOS version instead of 1k25 and soldering it to the copper heat spreader....
After one year of intense but trouble free operation with the 1k8 device, during the CQWW contest of October 2021, while operating full power in SSB on 160m, the PA suddenly delivered no more output power.... While going in TX mode, no more idle current drawn, and rig output power was limited to 20W due to the ALC feedback of SPE.
Why 20 W ?
On the input circuit of the PA (EU version), there is a T resistive network (2x 22R with 47R), providing 8 dB of attenuation. According to manufacturer's specs, the LDMOS requires 3W (+ 35 dBm) input to provide full output (going into compression) of 1,2 kW @ 50V. Adding 8 dB of the input attenuator, the result is 43 dBm or exactly 20W !
For information, hereby the drive level required (as per SPE information) on the 1.3K FA EU model (not knowing which series ...), specified per band and power setting:
I immediately checked the LDMOS - which was visually in an immaculate condition - and ascertained that in TX mode the gate voltage was only 0,3 V instead of about 2,5 - 2,7V ! So obviously there was a problem with the gates ... most probably they were dead due to overdriving (In general, LDMOS go SK due to 2 reasons : either overheating, or too much input power).
Replaced the LDMOS with another 1k8 device on hand and measured how much power was required to drive it up to 1.3 kW - depending on bands, in RTTY mode about 5W or so... much less than the 20W limit defined by the ALC ! So I decided to attenuate the drive level applied to LDMOS to keep it on the safe side. On the PCB , close to the input transformer feeding the LDMOS and next to the idle current adjustment trimpot, there is a wire jumper which can be removed and replaced by a resistor - I found that a 36 Ohm / 5W out of my junk box did the job ! However, this had the effect of limiting the maximum output power on the 6m band. To compensate for this, I placed a 270 pF dipped mica capacitor over the 36R resistor (which on 6m has a reactance of about 12 Ohm) .
Next step to avoid overdriving was to place some Transient Voltage Suppressor (TVS) diode(s) on the LDMOS gate input circuit. Hereby take into account that a TVS exhibits a high capacitance (in general > 300 pF) and therefore attenuates the RF signal, even when not conducting. So always consider 2 TVS diodes with each in series a signal diodes (e.g. 1N4148 with 4 pF). For limiting the input power to 3W, it is expected that the RF voltage will reach about 12,5 V RMS maximum, or 17 V P-to-P. A TVS of 15V should be fine, like the BZW06-15B.
See picture below.
According to some sources, it is recommended to set the transceiver output power to maximum, and let the ALC do it's work .... In my opinion, it is more prudent to set the maximum output power of TRX to such level where the PA can deliver it's full power on 6m band, but not beyond. Under these conditions, the LDMOS cannot be stressed by excessive power on it's input ... even should the ALC plug on the TRX or PA be removed by inadvertence.
Important remark : According to ITU Rec. ITU-R SM.326-7 (download) for single-sideband single-channel radiotelephone emissions (R3E, J3E, H3E) without a privacy device the acceptable intermodulation level can be taken as –25 dB or less. The intermodulation level considered here is expressed in terms of the ratio, generally in decibels, between the powers of the largest intermodulation component at radio frequency p(F0 +/- f1) – q(F0 +/- f2) - with p, q = 1, 2, 3, etc - and the power of the fundamental component at radio frequency (F0 +/- f1 or F0 +/- f2) produced by either of the two f1 and f2 modulating oscillations applied simultaneously at the input of the transmitter. Because of this, the average or mean power (as you will read from a wattmeter if only one tone present) will be 2x higher (+3dB) and the PEP power (= when the 2 peaks of modulation are in phase) will be 4x higher (+6 dB) than the power of each single tone f1 or f2, therefore 6dB must be ADDED to the difference between one fundamental tone f1/f2 and first IMD product . E.g. in the screenshot below, IMD is better (according to ITU) than 26.42 + 6 = 32,42 dB . (If in doubt about 'power per tone', 'average/mean power' and 'Peak Envelope Power (PEP) see pdf document and pdf document)
Two tone test with bare IC-7600, ALC just coming up, Po 15W mean power
Two tone test with SPE 1.3k PA, LOW power setting, 500W mean power out
Two tone test with SPE 1.3k PA, LOW power setting, 800W mean power out
Two tone test with SPE 1.3k PA, HIGH power setting, 1000W mean power out
This is the third MRFE6VP61K25H device that I am sending into semiconductor's heaven ... I have the impression that, since these very first high power devices went into production, a lot of research and improvement have been made by manufacturers to make them more reliable, resulting in the 1k5 and 1k8 versions.
The repaired PA was stress-tested in 2022 IOTA contest for our running station in CW, operating continuously during 24hrs @ 'medium power' setting, providing effortless 1kW out, under ambient temperatures of up to 25 °C, the displayed temperature never exceeded 39°C (fans programmed for 'contest mode')
Good luck with the repairs !