Lexicon 300 Display Transformer needed

[mumek]

It would be reasonably simple to wind a new transformer, I guess?

lol If you have that power, yes, it should be pretty simple. Otherwise, I’ll give ya the part numbers for the ones I went with, when I get home.

 

[DoctorQ]

I would like to have these instructions too please!

 

[mumek]

I would like to have these instructions too please!

First off, download a copy of the service manual and schematics, refer to the page on the power supply circuit, read up on VFD driver circuits. Replace all the electrolytic capacitors in the entire power supply. Next, remove the ceramic capacitor between the 35v windings of the small display transformer, it's marked C9 on the board. Next, if your display transformer is not visibly torched to death and crumbling apart, test the windings with a multimeter on continuity. If any of the windings don't pass the beep test, or if the ohm reading for either secondary winding is higher than 100ohms, your transformer is toast. If all the windings test good, replace C9 with a solid quality ceramic or film capacitor with a voltage rating of at least 100v. If one of the windings tests bad, you will have to replace the transformer with two transformers, unless you can wind your own from scratch or rewind the original on new bobbins. I used a Triad Magnetics F36-65 for the center-tapped 36VAC winding, and a Triad Magnetics FS12-090 with the primary winding wired in series to produce as close to 6VAC as I could get. It would up being just under 5VAC. This is not ideal, but I couldn't find any other transformer in my country that would fit inside the 300, alongside another transformer, that produced the right voltage. Do not excede 7VAC for filament winding or you might burn out the VFD. You may want to change diodes CR8 and CR9 in the path of the 35v winding to a diode with a lower voltage drop to get closer to 35VDC. Do not hook the wiring header that feeds the display board from the power supply board to the display board first time you power your 300 back on. Test the voltages coming from whatever solution you try before hooking the display wiring back up to make sure you're getting at least 30VDC and 4.5VAC respectively. The original .06A fuse feeding into the display power circuit should still be fine for this new solution, but if it blows, replace it with no higher than a .1A fuse.
I cannot help you beyond this point unless you're willing to ship your 300 to me and pay me for the labor. This is also a solution I've only been able to test by burn-in for a few weeks now, so I can't vouch for it as a permanent solution. Undertake at your own risk. How you decide to install any replacement transformers will be up to you. If you're not completely comfortable with your skills and knowledge, do not hesitate to take this job to a tech. You can destroy your 300 or harm yourself if you don't take precautions. The way I had to implement this fix was not ideal. I'm working on sourcing smaller transformers with closer voltage ratings that I'll use to develop as close to a drop-in replacement as possible. I'll update this thread when I come up with a better solution, maybe even pre-made boards or a kit you can assemble. It may also be possible to salvage the original transformer if you're versed in how to wind your own. From what I've gathered, if you catch the issue soon enough, it may only be the 35v winding that burns out...

 

[DoctorQ]

Problem solved! Thank's to mumek for his suggestions. I have changed the caps in the psu and reconstructed the display xfmr.

 

[DoctorQ]

Hera are some pictures

 

[mumek]

Hera are some pictures

Hell yeah, looks great Was the original x4mr completely Kentucky fried?

 

[DoctorQ]

The original xfmr looked in perfect conditions apparently....certainly not dark brown toasted like many I' ve seen...but it was giving bad contact...was functioning only putting some pressure on it! Not a possible solution really!