Yeasu FT-991 Repair - Page 1
Forum section: electronics / repair

author: xrunner
author_rank: Super Contributor
timestamp: August 25, 2021, 11:32:34 am
content: Friend of a friend asked me to look at a Yaesu FT-991. Said VHF power output was only max 3W, other bands fine. I agreed to look at it. I asked if the owner even opened it up to visually inspect it. Of course not!To make a long story short, there is a failed PIN diode on the VHF board. This is where all the bands pass through first - even HF. There is a bank of filters that steer the RF, and a few PIN diodes. After verifying that the power was coming into the board OK, and that the other bands power output was OK, I knew what the problem probably was. Of course the PIN diode I suspected was on the bottom of the V_U PA board, so had to pull it to check. Sure enough, the suspect PIN diode (D4503) showed signs of overheating as you can see in the pic.They are available on RF Parts for $2.95 Sez "New Old Stock "No longer available for export".The friend who is coordinating this will order them. Be back when part is received. Shelved for now.

author: Hamelec
author_rank: Regular Contributor
timestamp: August 25, 2021, 12:20:05 pm
content: sorry, but circuit diagram and especially the pin diode part number is unreadable...

author: xrunner
author_rank: Super Contributor
timestamp: August 25, 2021, 12:34:15 pm
content: Quote from: Hamelec on August 25, 2021, 12:20:05 pmOh come on it's notthatbad. But the PIN part is JDP2S12CR. By the way did you see my response to your post on the hp 436A? You are missing the RIFA at the back of the unit ...

author: Hamelec
author_rank: Regular Contributor
timestamp: August 25, 2021, 12:41:00 pm
content: OT @xrunner:yes, but no second RIFA here at the back panel, checked it again.thanks

author: m3vuv
author_rank: Super Contributor
timestamp: August 25, 2021, 04:24:31 pm
content: i wouldnt touch a yeasu rig with a bargepole,had an ft100,an ft840 and a ft290 mk 1,the 101 was a good rig,the other two were built to a price using marginal spec components,horrid unreliable junk,imho.

author: xrunner
author_rank: Super Contributor
timestamp: August 25, 2021, 05:44:54 pm
content: Quote from: m3vuv on August 25, 2021, 04:24:31 pmThis is the second FT-991 I've repaired, and also a third that needed the HF final idling current adjusted to prevent the HF finals from blowing.Their firmware update procedure is way too complicated also. I've had hams come over that couldn't get it done themselves. It's also like they have three or four software groups and each one has a different little app to accomplish the part of the firmware update they are responsible for. Whereas Icom has a vastly simpler update process. Download the update, put on SD card, insert into radio, tell radio to update. That's about it.

author: xrunner
author_rank: Super Contributor
timestamp: August 25, 2021, 09:49:45 pm
content: Don't have new part yet but decided to remove damaged PIN diode. Discovered one pin was already loose from the heat. The other pin would not unsolder, so had to cut away. Now I have to repair damage. Small wire or just a blob of solder to new diode? Any advice?

author: CJay
author_rank: Super Contributor
timestamp: August 26, 2021, 08:50:40 am
content: Piece of thin, tinned copper wire, bent to match the shape of the missing track between the two remaining components and then solder the new PIN diode on top of it as though it's a pad/track.Also, check the service settings, make sure it's not been 'tuned' or adjusted for more power.