Need help with burned Low Pass Filter torroid coil (100W Shortwave TRX)

[rg58]

Hello everyone,

Just fixed a shortwave transceiver and when running final testing I found that on 12m and 10m the unit only outputs about 50-60W. Looking at the schematics it was clear that the LPF (or a relay) has a problem because one common LPF is being used for both bands. Telling by the audible switching relay(s) I can tell that this filter is in use from 21.5 to 30 MHz. When taking off the cover of the PA / LPF section I immediately saw the burned torroid coil (L11, see schematics attached).

Here the guesswork starts. The schematic states no values for the coils anywhere, and when looking up the part number in Google nothing useful was returned. All cores have a painted dot of a certain color on them. There is no other coil having the same part number I could use as a reference.

So what I did now was running a simulation on the PREVIOUS filter (used 14.5-21.5MHz) to get a better understanding on this (working one, so I would be able to take measurements later by unsoldering the 2 coils). As all capacitor values are known I managed to find a rather good combination for both coils by trial and error, being 380nH (L9) and 500nH (L10), giving me a cutoff frequency of around 25MHz (-3dB) while being flat until ~22MHz. This filter topology has 2 notches equal to the resonance frequencies of the 2 L/C resonance circuits. I have attached the simulated schematic along with the attenuation plot.

I would appreciate of someone could help me to find a replacement. I know I could also unsolder the other coil L12, meassure that, then run the simulation again and then trying different values for L11. But the points in question remain.

Question #1) Where exactly should I place the cutoff frequency and the 2 notches if I could choose freely? What I did now was placing one at 43MHz (21.5MHz x 2) and the other one at 64.5MHz (21.5MHz x 3) for maximum suppression of the highest frequency (21.5MHz) harmonics. At the same time this seem to give me the lowest VSWR of ~1.4 at 21.5MHz (getting better on lower frequencies) as well as ~50 Ohm on the input.

For 14.5MHz (lowest for this LPF):
14.5MHz -0.1dB (hypothetical frequency)
29.0MHz -13dB (x2) (only -13dB!!)
43.5MHz -66dB (x3)

For 18.1MHz
18.1MHz -0.1dB (17m band)
36.2MHz -32dB (x2) (only -32dB!)
54.3MHz -53dB (x3)

For 21.5MHz (highest for this LPF, I optimized for this frequency, -3dB is at ~25MHz):
21.5MHz -0.3dB
43.0MHz -82dB (x2)
64.5MHz -81dB (x3)

So which would be the best -3dB cutoff frequency and 2 notch frequencies? First harmonic on 17m is only 32dB down, but these inductances of 380nH and 500nH seem to give me the steepest shape overall while being flat up to 21.5MHz. Which would be the best approach?

Question #2) Which core material should I choose for 100W on 21.5-30MHz? What aspects should be considered when choosing a core for high power? This is by far my biggest concern.

Thanks a lot!

[bd139]

Looks like Amidon material 6. Fine for 10-50MHz. I'd measure the diameter, buy a suitable TXX-6 toroid, wind it with the same number of turns and stuff it in. Then run it up at 5-10W and measure harmonic output and return loss. If it's a good 50 ohm match and harmonics are down enough (as per initial specification) then run 50W into it and measure the temperature. Then 75, then 100.

Main thing with core selection is picking a big enough one. My 450D had T50 sized ones in for 100W out which is an indicator of a properly functioning radio. I ran a T68 as a poorly matched balun at 100W out and it got a bit hot so YMMV depending on match conditions as well.

As for the initial cause, that's interesting. Did the previous owner smoke the toroids or did it happen during repair? Possible they ran it into a shitty match at 100W for ages. PA transistors probably got sick too.

[HB9EVI]

yep confirm: Micrometals material 6, either T68-6 or T80-6; often you see T68 or even just T50 in 100W LPF, but from my point of view T80 is a better choice.

[rg58]

Thanks! So I will go for the AMIDON material 6 then. Still need to order it.

@bd139

I will probably follow your procedure, eventhough the AL value of the core could be far off from the original one. But at this point I have not much choice maybe. Although I could measure it first after winding it and then run the whole LPF through the simulation again to see if it is way off.

Regarding the original cause, I don't know what the previous owner did to it, probably had a mismatched antenna. It didn't happen during my repair. The PA transistors still doing well as the unit is working on all other bands with 100W.

@HB9EVI
Danke für die Bestätigung!

Today I took some time and measured the LPFs with my MFJ-259 and a scope, terminated with 50Ohms on its input, later plotted a graph in Excel (attached). The 14.5-21.5 filter looks quite good as expected, the broken filter (21.5-30MHz) looks not as steep although the SWR isn't too bad. One thing I noticed when measuring this is that the output went up and down a bit all the time, as if there is a short or contact problem. Must have to do with that coil. I haven't taken out the PCB of the chassis yet, so probably underneath it will look aweful.

[HB9EVI]

Use the Mini Ringkernrechner to calculate the toroid parameters. If you know the size of the old burnt ones, you can calculate L and transfer it to bigger toroids without much manual calculations.

Normally powdered iron toroids are quite resistant to overload, where ferrite already loose their properties, but here it's over. But for every case check the capacitors as well; my guess is these are 500V silver mica, they are robust, but also not unlimited.

[rg58]

Thanks! I just measured the core which is a little over 12mm in diameter, so I need to get the T50-6 (12.7mm) as a replacement. I can't fit in a bigger one as the cover sits right on top. Next bigger size would be T68 which is already 17.5mm and the cover wouldn't fit anymore. All three cores T50-6, T68-6 and T80-6 have a very similar AL between 45 and 47. But you're right, I will replace C33 (68pF) and C34 (33pF) as well - Justin Case :-)

[bd139]

Sounds like a win. I wouldn’t worry about exact values. The LPFs will still work with a wide range of tolerance I found up to 10% either way which is within the winding spec.

Definitely worth replacing caps. There can be high peak voltages in there especially if something was toasted.

[HB9EVI]

I forgot to post the link to the mini ringkern rechner, just in case; it's a practical tool:

http://www.dl0hst.de/mini-ringkern-rechner.htm#en

[rg58]

I had that one already, but thanks anyway. And yes, it is very handy.

[rg58]

Just FYI: Yesterday I managed to fix the LPF with a T50-6 and 7 turns of 1.2mm copper wire. The original coil had 7 turns as well, made from approx. 0.8mm wire. I had to drill the 2 holes bigger for that. Works like a sharm now with 100W!    Although the wire gets very hot after 1-2min (and bloody hot after 5min straight), but all other wires in other LPFs do as well. Those transceivers are not built for 100W FM continuosly anyway.

Initially I tried with 8 turns (as per simmulation and calculation) but it turned out the input SWR was over 2 and the cut-off frequency just about 30MHz. By reducing the number of turns to 7 the input SWR was 1.2 - 1.4 between 21.5 and 30 MHz and the -3dB cut-off frequency meassured was 36MHz now. Furthermore I replaced both caps as well. The 68pF still meassured OK while the 33pF only had 12pF left.

Thanks for your help again! This core was definately the right choice 

[bd139]

Good to hear it worked out.

Something I heard recently on the subject of transmitter duty cycle is that a lot of the stuff in the 1980s/1990s has had an audio/USB interface (tigertronics etc) whacked in it and then running WSPR or FT8 at 100W out both missing the point of these data modes and the transmission restrictions  . This conversation was someone in my local ham shop having this and the repair bill explained to them

[m3vuv]

was the rig a yaesu by any chance?,cheers m3vuv.