Yeasu FT-7 (not B) No CW Transmit - UK Vintage Radio Repair and Restoration Discussion Forum

N5EN_Neal · 6th Sep 2020, 7:22 pm

The results of a google search have led me here.

I am continuing the troubleshooting of a Yeasu FT-7 (not B) transceiver for a friend. The transmitter has no output when in the CW mode, but transmits normally on USB and LSB. He and I ended up at the CW Carrier Oscillator circuit on the IF-Unit board. He thought it was working, but I have a few more test instruments at my disposal. So, I found that the oscillator was working, but the signal level transitions from little to nothing after passing through C419. C419 is 10pF in the radio and on the schematic, however, the BOM calls out a 33pF. If I change C419 to 33pF, CW works as it should on all bands. Should I just leave it as 33pF and call it good or could there be some other problem? I assume that this circuit once worked, so it has me wondering when I change the value to make it work. Also, the manual instructs to adjust TC401 to set pin 2 of the IF-Unit to 75mV (with an RF Probe). This is confusing. TC 401 changes the frequency of the CW carrier oscillator. Should I not set the frequency of the oscillator to 8.9993 MHz with a frequency counter?

Terry_VK5TM · 7th Sep 2020, 1:20 am

Hi Neal.

Check C420 100pF for value shift/leakage and dry joints around Q421 and associated components.

I pulled the IF board out of one of my FT-7's just to confirm what was shown on the schematic (re C419) was what was actually on the pcb and it is 10pF.

Re TC 401 adjustment - it's a compromise between output level and being on frequency, The output level tolerance is +/- 10mV, so tweek closer to frequency rather than voltage but make sure you do have enough output level.

6th Sep 2020, 7:22 pm	#1
N5EN_Neal Diode Join Date: Sep 2020 Location: Houston, Texas, USA. Posts: 1	Yeasu FT-7 (not B) No CW Transmit The results of a google search have led me here. I am continuing the troubleshooting of a Yeasu FT-7 (not B) transceiver for a friend. The transmitter has no output when in the CW mode, but transmits normally on USB and LSB. He and I ended up at the CW Carrier Oscillator circuit on the IF-Unit board. He thought it was working, but I have a few more test instruments at my disposal. So, I found that the oscillator was working, but the signal level transitions from little to nothing after passing through C419. C419 is 10pF in the radio and on the schematic, however, the BOM calls out a 33pF. If I change C419 to 33pF, CW works as it should on all bands. Should I just leave it as 33pF and call it good or could there be some other problem? I assume that this circuit once worked, so it has me wondering when I change the value to make it work. Also, the manual instructs to adjust TC401 to set pin 2 of the IF-Unit to 75mV (with an RF Probe). This is confusing. TC 401 changes the frequency of the CW carrier oscillator. Should I not set the frequency of the oscillator to 8.9993 MHz with a frequency counter?

7th Sep 2020, 1:20 am	#2
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	Re: Yeasu FT-7 (not B) No CW Transmit Hi Neal. Check C420 100pF for value shift/leakage and dry joints around Q421 and associated components. I pulled the IF board out of one of my FT-7's just to confirm what was shown on the schematic (re C419) was what was actually on the pcb and it is 10pF. Re TC 401 adjustment - it's a compromise between output level and being on frequency, The output level tolerance is +/- 10mV, so tweek closer to frequency rather than voltage but make sure you do have enough output level. __________________ Terry VK5TM https://www.vk5tm.com/