YO-901 Yaesu Multiscope teardown and repair - Page 1
Forum section: electronics / rf-microwave

author: worsthorse
author_rank: Super Contributor
timestamp: October 07, 2019, 02:39:40 am
content: I have mostly old Yaesu rigs, though nothing quite as old as this multiscope, which mates with the FT-901DM from the 70s, and provides a RF monitoring function, bandscope (an option), and two tone generator, along with a limited oscilloscope function. My unit came from a warehouse auction, and I bought it as-is. Here it is, front and back, out of the box:I gave it the once over, including taking the top cover off. It looked to be in very good shape so I powered it up and applied a signal to the vertical input:That's a good start, though the horizontal trace isn't level. Here's what it looks like inside.Notice that the transformer is mounted at an angle. The form factor had to match the FT-901DM in depth and height. This was apparently the only way to meet both the power and space requirements. As I was soon to discover, it wasn't really designed to be worked on. There are no test points, many of the adjustment pots and caps are in hard-to-reach places, and several of the adjustments require the box to be flipped back and forth.For example, two of the vertical amp compensation adjustment access points are under the PCB and unmarked.The user's manual has a maintenance section that describes how to bring the scope back into spec, along with schematics and a theory of operation section. Unfortunately, there's no parts list other than a list of transistors. After checking the power rails, I started working through the alignment procedure. Getting the trace to be horizontal required slightly turning the CRT in its mounting.After that, redoing the vertical amp compensation, and setting the vertical bias correctly, I was rewarded with a nice, level square wave:So far so good. The next step involved testing the two tone generator. Here I ran into trouble... no audio tones on the output jack. I started tracing that back and have isolated the problem, but more on that tomorrow.

author: w2aew
author_rank: Super Contributor
timestamp: October 07, 2019, 04:14:08 pm
content: Nice!The "tilt" in the trace is usually due to nearby magnetic fields. Don't be surprised that if you move the unit to a new location that the tilt returns. Just be prepared that you may have to re-twist the CRT in it's housing when deploying the unit in its operational location.

author: worsthorse
author_rank: Super Contributor
timestamp: October 07, 2019, 04:38:21 pm
content: Quote from: w2aew on October 07, 2019, 04:14:08 pmYes it is. The manual even has a warning to that effect. In this case, though, I think the tilt was due to previous adjustment... as I changed the orientation of the scope, the tilt remained the same. When it goes back into service, at least if the gear it is paired with is of the same age, it will probably need to be adjusted again.Do you have this scope? Back in the day, it was quite coveted. Now it is just another boat anchor.

author: w2aew
author_rank: Super Contributor
timestamp: October 08, 2019, 02:33:29 am
content: No, I don't have that one - I have the Kenwood equivalent, the SM-220. I did a few videos on it:Overview:Cleaning a dirty function switch:

author: worsthorse
author_rank: Super Contributor
timestamp: October 11, 2019, 04:03:56 am
content: Back from a road trip, so I spent the evening finishing up the YO-901 project. There were two apparent problems... the two tone generator output wasn't appearing on the rear connector, and the marker output wasn't stable. Both were easy fixes.I traced the tone generator outputs to the input side of an audio isolation transformer. There was no signal on the output side. Turned out to be a faulty cable from the PCB to the output jack. Replaced that and seeing this on the output:I am actually feeding the two tone output to the the vertical input and using the 901's oscilloscope mode for this image.The other problem... when in bandscope mode, the horizontal trace wasn't stable and the marker would disappear. This turned out to be dirty a dirty pot and a dirty trimmer. Deoxit to the rescue.That's the 10KHz marker, showing up as expected and now clean and stable.I connected my FT-897 to the 901, just to be sure that the RF path is good. It is. I don't have a FT-901, so I can't test the bandscope fully but after poking around on the PCB and looking at signal levels, I have no reason to think it is bad. Going to declare this one done and ready to take to the hamfest next weekend!

author: wevets
author_rank: Newbie
timestamp: March 02, 2020, 12:03:50 am
content: Hi - hoping you can help. I'm looking for the cable wiring requirement between the FT901 and the YO-901. Haven't found it yet.Do you have any information on this. I've acquired a YO-901 and want to start using it!Thanks,Steve KA6S

author: worsthorse
author_rank: Super Contributor
timestamp: March 02, 2020, 03:06:29 am
content: Quote from: wevets on March 02, 2020, 12:03:50 amHi Steve, I sold the YO-901 along with the manual a couple of months ago but I may still have a copy of my notes. I will look tomorrow and let you know.

author: worsthorse
author_rank: Super Contributor
timestamp: March 03, 2020, 07:42:03 pm
content: Here's a link to the YO-901 manual. There is a diagram of the interface cable in there, along with diagrams showing all the interconnects. Have fun!https://www.dropbox.com/s/hmdditd8qq20yq2/yaesu%20yo-901%20manual.pdf?dl=0I will leave the link up for a couple of days. If you missed it, PM me.