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Hi all, I would like to share my technical experience with my FT980. After many many years of no use I wished to make a deep check and precise alignment of the equipment. I found three blocking failures:
- fail contact of the coax connector J02 on RF UNIT (symptom: very poor rx sensitivity and no output pwr in tx)
- output PA UNIT coax shield disconnection (instability in output spectrum and output pwr, in particular on low bands)
- open capacitor C3067 on AF UNIT (tx spurs of 240 Hz and harmonics in SSB, AM, FM)

After these mayor fixing I investigated on the frontend HAM bandpass filters: after a little retuning of the inductances the resulting behaviour was not so good due to frequency regrowth for frequencies lower than the bandpass regions. Probably this is due to the aging of the electrolytic bypass capacitors used in the pin diode switching band circuit. I added two new tantalum smd 47uF 16V cap and a 330 nF smd ceramic cap in parallel to C212, C130, C125, C120, C114, C109, C104, C97, C91. The results were very good: the spectral regrowths disappeared as you can see from the attached pictures. Only four bands are present on the pictures, but all the bands gain benefits from the additional bypass caps. It is important to use very low ESR capacitors, those used in switching power supplies generally fit the scope. Best if you can trust on specific caps datasheets for the ESR value. No significant difference in the response were noted in the bandpass region nor in the higher band frequencies region after the use of additional bypass caps.?

The 10m bandpass filters is a bit worse in terms of insertion loss and form factor, I'm investigating on it because I plan to develop a multiband transverter that will use the FT980 10m band.

The responses were carried out with AGC off, gain RF max,? input rx -30 dBm, output at TP1 on RF UNIT with a 450 ohm resistor in series to the spectrum analyzer input (it results in an additional loss of 20 dB in measurement, but it will not load significantly the circuit).?

The same check was carried out for the GEN (wide) band pass filters, but in this case adding low ESR caps in parallel of the existing bypass caps didn't result in significant improvements, so I don't recommend it.

Next steps will be a new 11m band pass filter and the complete check and retuning. Also 12.5 KHz FM channel spacing and CTCSS encoder are in my plans.?

Thanks for hosting me, best regards

Enrico IK2XEI

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Hi Enrico,

I'm very interested in your work. Unfortunately your pictures did not make it through. Can you link through dropbox, or maybe try again?

Do you have access to a VNA (like a nanoVNA)? It would be interesting to see a S21 (or S11 into a 50 Ohm load) through the BPF segment. A entire HF sweep would be ideal, one or each of the band selections. I might do it myself next time I have the rig open.?

What is your theory? Without the electrolytic cap bypassing RF when the Filter line is not pulled low, that the BPFs were "self activating" through RF, and therefore multiple BPFs were "open"? Seems plausible, if that is your thinking ...

Kind regards and thanks,

Mat, N8TW




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-Mat Breton, N8TW

Hi Mat, thanks for your kind reply. Sorry for pictures not being uploaded, I'll try again right now.?

I do not have VNA, I'm thinking to buy NanoVNA because I read very positive reviews, but now I use a Siglent SA that is enough for my present needs. Also, a HF RF Bridge prototype is waiting for me for many months?in order to be finalized, let us see next future...

About the modification: the cold side of each bandpass is RF grounded via capacitors, so the bandswitching PIN diodes can act being diven by direct or reverse polarization. But so doing the filter cold side is not really a solid RF ground because the capacitors are not ideal, an a low value resistance is inherently present inside. So the ultimate attenuation values far from the bandpass region is dependant on this residual resistance. You can also imagine the filter as a frequenct selective attenuator, where the ideal ground path has a low value resistor in series to the real ground, so that the desired attenuation out of band can't be achieved. Probably 40 years ago the electrolytic caps within the equipment were good enough for the purpose, but after this long aging the ESR tend to raise. I simulated the original circuit inserting a low value resistor in series to ideal bypass caps obtaining responses thhat are very similar to the real behaviour.?

Yesterday I built a prototype of a 28-30 MHz filter that could replace the original one. Measuring it as a stand alone unit results are very good, next days I''ll try to put it into the rig. With a very little arrangement the new filter can be referred to the real solid gnd and in theory no adverse spur variables will worse the ultimate out of band signals rejection.

Kind regards

Enrico IK2XEI

I wonder if I could drop in an MLCC ceramic in place the electrolytic: TDK?FG14X5R1E226MRT00 for example would have negligible ESR and parasitics well past 100MHz. Definitely added to the list to check out ... I also have a new IF buffer circuit to put in (panadapter), and a new backup battery case.

I wonder if there other pin-diode switched circuits (shared xmit/rcv circuits like the filters maybe) that are also ticking "time bombs"?

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-Mat Breton, N8TW

Dear Mat, sorry I was very busy and weren't able to reply to you soon.

I wasn't able to find the data sheet of the TDK Caps, but your proposal seems better than mine (tantalum caps)! Please note that, according to simulations, the ultimate out of band attenuation is achievable when ESR is less than 20 mohms or so.? Also, using a single 22 uF will preserve the response time during band switching that, in case of 2*47 uF, will become four times larger.

About other "time bombs": I had no time to think about it, I'll try do do so... It is possible, you know aged electrolytic caps may change their characteristics, I found one of them in tx audio path responsibe of tx spurs...

I also tried the new bandpass 28-30 MHz alternative filter but results were not good. Even though the filter was simulated and pre-tested with very good results (my previous picture on stand alone 28-30 MHz filter), once fitted into the equipment the overall response were bad (also modifications of polarization paths were performed and checked before). So I paid better attention to the schematic and I understood that the output impedance termination of the filter (the input impedance of the RF Jfet preamplifier) is NOT 50 ohm!
So, in order to synthetize the correct filter,?I got a Nanovna to measure the real terminating impedance, but I wasn't not able to get good results from it. After calibration the Nanovna reports an impedance of 120 ohm with open circuits, denying any reasonable measure different from 50 ohm. I subscribed to a Nanovna group to understand wether a solution is possible. I'll keep you informed.?

Also, once the full RX performances will be verified, I plan to check in depth the TX modulation paths because I suspect that some AGC behaviour will need attention.

And what about your new IF buffer??

Enrico IK2XEI

On Thu, Jul 1, 2021 at 05:20 AM, <e.tallone@...> wrote:

I plan to check in depth the TX modulation paths because I suspect that some AGC behaviour will need attention.

Enrico,

I have four '980s. All of them exhibit a condition wherein a carrier of, say, more than 30w will cause ALC saturation - and as a result, output power will creep up to the maximum set point (~110w) if the rigs are kept in transmit for more than a few seconds.

All of these rigs have had their ALC systems set per the Technical Supplement. This leads me to believe there's an inherent (unintentional) feedback loop with a long time constant; I don't really see the issue when running the radios on SSB. So yes...I'd have a look at that portion of the transmitter circuit while you're analyzing the radio.

73 - Fred, N8YX

Hi Fred, thank you very much for your comment. I'll consider that when I'll start the tx review! I'll follow the steps present in Service Manual but I also want to check the dynamic behaviour of ALC.
I'll let you know the news.
Ps: my congratulations for your Four '980! Sure you ARE an enthusiastic for this great rig!!

Enrico IK2XEI