If you've been following my latests posts, you'll know that during the last V-UHF contest I detected reduced output power on the 70cm band in my FT-817ND. The output power was only about 60% of the maximum 5W in SSB and CW, but in FM mode it reached 5W. This problem only happened on the 70cm band. On all the other bands, the radio reached 5W output power in every modes. After spending some time studying the service manual, I came to the conclusion that the problem was that TX gain in the UHF band was too low. This is a software calibration parameter, so, in the end, fixing this problem has been rather easy.
Below you can see the relevant part of the block diagram of the FT-817ND. Q1007 is marked in blue. This is an IF amplifier where the FM signal and the signal for SSB and CW join together. Before this stage, the FM signal and the SSB/CW signals follow totally different paths. Q1007 is a dual gate FET. The ALC circuit applies a DC level to its second gate to control the gain of this stage. This ALC circuit is rather complex: it monitors output forward and reverse power and it will reduce the gain of Q1007 under high forward power or high SWR conditions. In fact, the ALC circuit is what limits the output power to 5W, since the finals of the FT-817ND can reach 10W or more.
The signal then passes to Q1002 (marked in green), which is an attenuator controlled by the software calibration parameter TX gain. This parameter is band-dependent (there are 3 HF bands plus VHF and UHF), but mode-independent. Then, the signal is mixed with the local oscillator to convert it to the transmit frequency, filtered and amplified before passing to the PA board.
The key aspect to understand the problem with my radio is something that is not really mentioned in the manual: when reaching Q1007, the FM signal is at higher level than a (full power) SSB/CW signal. Thus, the FM mode will operate under a high level ALC action, which will reduce the gain of Q1007. Even if TX gain is set a bit low, the radio will reach full 5W output power, since the ALC will just reduce a its action to compensate for increased attenuation in Q1002. However, the SSB/CW mode operates under a low level ALC action. The ALC is just used to make the radio reach full output power even if the gain of the finals changes slightly under temperature variation (or similar). Therefore, if TX gain is set a bit low, the ALC level will drop to zero and the radio will not reach full output power. That is precisely the problem I was encountering. It seemed that I just had to adjust TX gain for the UHF band.
The service manual describes a procedure to adjust TX gain: A 1kHz 1mV signal should be injected into the microphone jack. Then TX gain should be adjusted so that the output power is 2.5W. However, this procedure wasn't so easy to do for me, because I would have to hook up a sound card to the microphone jack, and adjust the level of the injected signal using the oscilloscope to monitor. Also, this procedure relies on the correct SSB mic gain being set.
Instead, I used the following procedure. I assumed that the VHF band was more or less well adjusted and I took advantage of the fact that I have fldigi in my computer prepared to do digital modes and hooked up to the radio properly. I generated a 1kHz signal with fldigi and lowered the soundcard volume until the output power in VHF was about 3W (there is an unlabeled mark between 2W and 5W in the cheap power meter I'm using, so I just aimed for that mark). Then I switched to UHF and increased TX gain until the output power was the same as in VHF.
To adjust TX gain, I used FT-817 Commander instead of using the built-in software calibration mode (which is accessed by pressing A, B and C while powering on the radio). FT-817 Commander can read and update calibration parameters over CAT by reading and writing the EEPROM directly. Of course, this is a bit risky in case something weird happens with the serial connection, but it allows me to use the radio normally and adjust the calibration parameters with the computer. Before touching any of the parameters, you should take note of all the factory settings. I've made a gist with the factory settings for my radio.
I've found that the TX gain parameter is rather sensitive. The effect of varying this parameter by a single step is rather noticeable. Hence, it is not very important to use precission lab equipment to adjust this parameter. Even if the measurement error is very low, it will be impossible to adjust this parameter very precisely. In fact, in my case I only needed to increase TX gain from 56 to 58. After making this adjustment, the radio seems to be working properly.