IC 746 no output on HF

[Xavierno]

Hello,
I have a buddy who owns an IC 746 that puts out full power on 2 meters but has no output on HF. It receives fine on HF but no transmit. Any ideas?
thanks everyone for the help

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[rainbow]

First download the service manual http://www.radiomanual.info/schemi/ICOM_HF/IC-746_serv.pdf and read the transmission chapter. If the problem arise also disabling the tuner you should start checking the voltage/faulty of the final HF mosfet (Q5 and Q6 on PA unit via RL2).

[msuthar]

Hi,

Check the IC-151 at the Main Unit Board.
It is a known symptom of Missing HF power and full power on VHF Tx ok.
Just change the IC 151.
You will find many videos on youtube for reference.

Best regards.
73
DE VU2EHY.

[geggi1]

As far as i remember from many years ago this radio had some issues with the pre-driver for the PA.
It was something with overheating on the transistors.

[rainbow]

From the service manual:

The RF circuit amplifies the operating (transmitting) frequency to obtain 100 W of RF output.
The signal from the mixer circuit is passed though one of the low-pass or bandpass filters (refer to page 4-1 for RF filter used) after passing through the low-pass filter (RF unit, L285, L286, C287-C289, C291). The filtered RF signal is amplified at the YGR amplifier (RF unit, IC151), then applied to the PA unit.
The signal applied from the RF unit is amplified at the 2 predrive (PA unit, Q1, Q2) and drive (PA unit, Q3, Q4) amplifiers. These amplifiers cover all HF, 50 MHz and 144 MHz band amplifying operation.
When operating on the HF or 50 MHz band, the signal from the drive amplifier is power amplified at the HF/50 MHz power amplifier (PA unit, Q5, Q6) via the band switching relay (PA unit, RL2). However, the 144 MHz signal is power amplified at the 144 MHz power amplifier (PA unit, Q201, Q202) to obtain a stable 100 W of RF output power.
The amplified signal is applied to the desired antenna connector via one of 8 low-pass filters.