Pages: [1]   Go Down

Author Topic: ft991a intermittent no output  (Read 221 times)

KC3NEJ

  • Member
  • Posts: 35
ft991a intermittent no output
« on: June 22, 2022, 09:17:11 AM »

I've been doing ft8 lately and notice that after several hours, the radio doesn't transmit.  Connected a dummy load = nothing.  Sometimes, after a rest, it will start to function again.  It's behaving like a heat problem.  This is all on HF; the VHF and UHF are fine even when  the HF quits.  I'm a bit new to ham radio.  Do bad finals act this way?  I thought they would just fail, sort of like output transistors in an audio power amp.

I turned it on this morning and it worked fine.  So today is a day of trouble shooting, checking fan operation and temperature over time.
Anyone had a similar problem?
Thanks
KC3NEJ
Logged

K6AER

  • Member
  • Posts: 7159
Re: ft991a intermittent no output
« Reply #1 on: June 22, 2022, 09:40:23 AM »

You didn't mention what your power level is or the duty cycle.

Few radios are designed for continuous operation of a high duty cycle transmission modes such as FT-8 or RTTY. I am surprised the finals came back to life. If you want higher power operation with digital modes you need an aftermarket amplifier.
« Last Edit: June 22, 2022, 09:42:42 AM by K6AER »
Logged

KC3NEJ

  • Member
  • Posts: 35
Re: ft991a intermittent no output
« Reply #2 on: June 22, 2022, 10:21:30 AM »

You didn't mention what your power level is or the duty cycle.

Few radios are designed for continuous operation of a high duty cycle transmission modes such as FT-8 or RTTY. I am surprised the finals came back to life. If you want higher power operation with digital modes you need an aftermarket amplifier.
Just the kind of info I need. Power level is typically 30 watts, but I have increased it to as much as 60 watts for one QSO.  I estimate the duty cycle is 50%; there are long periods of monitoring between contacts.  Hope I'm making sense. Appreciate your patience.
KC3NEJ
Logged

KC6RWI

  • Member
  • Posts: 901
Re: ft991a intermittent no output
« Reply #3 on: June 22, 2022, 11:08:53 AM »

Maybe there is a heat sensor on the heat sink of the finals, that will make for a shut down for awhile.
I have no experience with your radio, but I think thats a good possibility.
Logged

W8RMV

  • Member
  • Posts: 35
Re: ft991a intermittent no output
« Reply #4 on: June 22, 2022, 07:28:44 PM »

Make sure the power supply is working properly.  Check the fuses as sometimes they will develop intermittent blade contact.  Measure the voltage while in transmit right at the connector to the radio.
Good luck.
Logged

KC3NEJ

  • Member
  • Posts: 35
Re: ft991a intermittent no output
« Reply #5 on: June 22, 2022, 08:25:25 PM »

Maybe there is a heat sensor on the heat sink of the finals, that will make for a shut down for awhile.
I have no experience with your radio, but I think thats a good possibility.
I did a lot of testing today including measuring the temp at the output of the fan.  The radio transmitted fine all afternoon using ft8 (almost all time at 20w, once going to 60w).  I did pull the unit out from its rather tight living quarters.  I also moved some other gear from above it.  The radio sat under a shelf close to the top of the unit.  When the temp reached its max at the fan (96 degrees), I did see some power reduction like some others have reported in other threads.  I also put a small fan on the air intake openings.
I'm pretty sure that the problem is temp related; not sure that there isn't something wrong with the unit.  Time will tell.
KC3NEJ
Logged

N4ATS

  • Member
  • Posts: 1416
Re: ft991a intermittent no output
« Reply #6 on: July 02, 2022, 11:20:20 AM »

The FT-991A has built in features to reduce or turn off power when constant transmitting happens. FT-8 is very hard on radios and you must stick to recommended power levels and duty cycles. It has sensors in it for low heat and one high heat. The 2nd will shut the power off until it cools down.
Logged

W9IQ

  • Member
  • Posts: 8866
Re: ft991a intermittent no output
« Reply #7 on: July 02, 2022, 11:34:10 AM »

FT-8 is very hard on radios and you must stick to recommended power levels and duty cycles.

I think that should say some or perhaps most radios. The IC-7300 can run FT8 non-stop. I use the radio in the lab for cooking ferrite transformers with full carrier for 30 minutes or more and it doesn't even complain.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.
Pages: [1]   Go Up