FT-100, Help with setting the bias current on the VHF/UHF Final - Page 1
Forum section: electronics / rf-microwave

author: rjmusto
author_rank: Contributor
timestamp: January 19, 2025, 03:28:59 pm
content: Hi guys,I have an FT100 that is showing a high ALC reading on VHF/UHF (HF is working fine), although it is able to output full power on both bands. I don't see any sign of the infamous crack on the output transistor, so it is hopefully all ok.One puzzle I have though is that the maintenance manual instructions for setting the bias on the driver and the final don't make sense to me, so has anybody here got experience in doing this?The driver section instructions are pretty straightforward, and I was able to set that fine. But for the final it asks to monitor the current going to the set in total, saying to adjust it for 2.3A. The set I have shows 3.5A.But it takes around 1A in receive, so assuming the basic functions in transmit are about the same, and the driver section is taking 1.5A, we're already at 2.5A excluding the final stage. So setting 2.3A makes no sense. But the 3.5A I am seeing does. So what is the correct figure here?Anybody know?Thanks,Ralph

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: January 20, 2025, 12:41:20 am
content: There are two steps for each output. Two separate outputs. one HF (and 50MHZ) and then the VHF output.Follow the instructions as written in the service manual, The first adjust is across the jumper with the jumper removed the second adjust is with the jumper re installed and is in line with the Power Supply. Then repeat for the VHF....Their numbers as far as amps measurement are correct and should be followed.

author: rjmusto
author_rank: Contributor
timestamp: January 21, 2025, 09:32:54 am
content: Thanks for the reply - I'm away for a few days, so will have another go at following this through when I get back.Regards,Ralph

author: rjmusto
author_rank: Contributor
timestamp: January 30, 2025, 05:26:06 pm
content: Have managed to get this FT-100 back on the bench today. As mentioned at the start, the Service Manual instructions for setting the final idle current (VHF), say to messure the total current drawn by the set and adjust VR502 for 2.3A, and this is with the driver stage also connected. But this makes no sense, as the set takes more than this without the finals running. My reading in all this is that the jumper enabling the driver stage should be removed when setting the final - so an error in the manual, as it says it should be connected.With the driver disabled, it is then possible to set 2.3A to the whole set when adjusting the final stage idle current. That is what I have done anyhow.73 to all.

author: Sassy Taste
author_rank: Contributor
timestamp: March 03, 2025, 06:48:53 pm
content: VHF/UHF, very high frequency/ultra high frequency, i didn't think they had this technology around still. Everything is through cable. Transfer guides, fiber optics, ethernet cable. Are they still setting these technology up with the advancement of computers? The technology was from the 1970s and early 1980s.

author: Simon
author_rank: Unknown
timestamp: March 04, 2025, 04:59:59 pm
content: Quote from: Sassy Taste on March 03, 2025, 06:48:53 pmThe wheel was invented in the stone age, why are we still using it?