Can anyone ID these ferrite cores?

[dorkshoei]

Neither does the one on my board.

I don't see that you've established that, unless I'm misunderstanding your measurements?  (Could you show a diagram..?)  The dimensions are all out by 0.1mm or more, so the drawing doesn't tell you anything, nor can the parts be relied on to be that consistent.

The diagram is in the datasheet (copied below).    For most of the outside shell w/h measurements I used an undamaged core.    For the wall thickness and wall+core measurement (the later is the most important I think,  red arrow in the diagram) I used 3 different broken pieces and got the same result on all 3,  2.95mm.

Would this pass scientific muster, no.  Do I think there is an air gap, no.

0.21mm doesn't sound like a lot but I'm getting reproducible results to +-0.02mm with calipers.

As has already been said, the fact that with a broken ferrite the VFD still seems to work (note: I only ran the all-pixels test - shorting a test jumper when powered on - rather than programming selected pixels ) probably indicates that the actual value perhaps isn't that critical.

My orderable options seem to be: gapped: N87  ungapped:N49/N97

https://www.tdk-electronics.tdk.com/en/529404/products/product-catalog/ferrites-and-accessories/ferrite-materials; N87/N97 are both listed as f<500khz; N49 >500khz

I'm a digital (firmware) person, not analog so I'm reliant on the opinions of others here.  I'm thankful for yours and floobydust's help (and PMd him to this effect) and am not trying to be argumentative but I'm not in a rush to order,  new year is fine so I figured I'd see what the consensus of thought was.

If I had to pick something today I'd probably go with this: https://www.digikey.com/en/products/detail/epcos-tdk-electronics/B65513J0000R097/3913694

Thank you!

[floobydust]

Something else would be measuring the MB3800 output pulses, they should be full tilt, at max. PW. That is, running the display with no transformer. I think it all runs off 5V?
It might help to estimate the core's inductance. A_L is 10-15X higher for an ungapped core.

[dorkshoei]

I think it all runs off 5V?

Correct.  5V is the sole input.

[floobydust]

If you measure the bobbin's (primary winding) inductance in air, and know PW I think you can do math and infer the ballpark core A_L.

[dorkshoei]

If you measure the bobbin's (primary winding) inductance in air, and know PW I think you can do math and infer the ballpark core A_L.

unfortunately as I said, I'm a digital type, not analog.   given specify directions "measure x across pins y and z" I'm sure I could get it done,  else .....

[T3sl4co1l]

If you have a signal generator, you can do something like this,
https://www.seventransistorlabs.com/Calc/RLC.html#frq
The numbers are exact for sine wave; you can use a square wave still, but the source voltage will be somewhat in error (if it's a 50% duty, bipolar (symmetrical +/- V) square wave, the fundamental peak is 4/pi times the square wave peak (not p-p!) voltage).

Typical values might be 0.1uF and 1kohm.  Don't forget to include source resistance (of the generator) -- or, measure the voltage on both sides of the resistor, when at the resonant peak frequency.

You'll usually need an oscilloscope to do this, but you can still get the frequency right by, well, anything that can measure/infer the frequency, and using a peak detector to qualitatively measure the tank voltage (you just need to find the peak).  (A DMM will not have high enough frequency response to measure the voltage here.)

So, the most basic case could be an Arduino generating a square wave, and you have a console or debug interface to tweak the period up or down as you find the resonance.  A peak detector can be any signal diode, into a parallel RC network to GND, say 10nF and 100k, then measure the DC voltage on it with a DMM.

Tim

[floobydust]

Some manufacturers put the air gap only on one half of the core's two pieces, others half on each half - you measure the center pole at 2.98mm on one, does that match the other?

I was thinking you could measure primary inductance bobbin's pins 1-5 in air with your Sencore LC103, and then estimate that resulting inductance in the various cores.
We don't know # turns but that could cancel in the two equations. Knowing the pulse-width IC6 is making at R18, you could ballpark if a gapped core is reasonable or not.
If you don't want the hassle, then take your best guess...

[dorkshoei]

Some manufacturers put the air gap only on one half of the core's two pieces, others half on each half - you measure the center pole at 2.98mm on one, does that match the other?

I'd managed to lose the lower side of a core but I found it today.     So I have 4 cores, from 2 bobbins.

Of these 2 are definitely tops (as I can see part of the serial# marking) and they measure 2.95 and 2.98.    One is definitely a bottom as it's the one I glued back together and it measures 2.85.  I'd discounted this measurement difference thinking I'd messed it up during gluing/clamping but as I said I found another today, small piece, no markings, which is also 2.85 and I suspect it's the missing lower.

So possible I was wrong and there is a 0.1mm airgap.  Hey it's half way to 0.21 :-)

I have a third VFD with a broken ferrite (upper and lower cores are still intact). Tomorrow I could de-solder the bobbin from the pcb if it would be helpful.

If you don't want the hassle, then take your best guess...

To be honest I'm starting to get a headache.  These inductance and frequency measurements, while well intentioned, all sound a bit vague. I have this nagging feeling that after taking them and doing some math I may still be no better off.

I have a friend who's a smart EE.  I'll ask him for his thoughts and go with what he suggests.    At this point my preference is whatever is least likely to make the board go poof and emit white smoke.  Maybe I should flip a coin LOL.

[T3sl4co1l]

Ah, seeing a correlation between parts is quite encouraging!  Yes, that makes 0.1mm quite plausible, despite the tolerances on the drawing.

Tim

[dorkshoei]

I ended up repairing the boards successfully.

I'd ordered the gapped N87 and ungapped: N97 based on the picture here indicating the air-gap was formed by both halves.     I'd hoped I could therefore use one half of an N87 and one half of an N97 to create the exact air gap

However in the parts I received the bottom half of both the N87 and N97 were identical and unmarked and the air gap was all in the upper half.

I tried both types (I had 4 boards with cracked ferrites) and they seem to work fine with either.   I kind of assumed this may be the case as even the boards with worst damaged ferrite was still working.

Anyways,  thanks for everyones help!