FIP Coil Driver alternative for a fluorescent display - Page 1
Forum section: electronics / projects

author: Midphase
author_rank: Regular Contributor
timestamp: April 16, 2025, 10:16:14 pm
content: I'm working on a vintage synthesizer which utilizes a fluorescent tube display. One of the common failure points for this is the so-called FIP Coil Driver which appears to be a small cylinder that apparently plays an important role in the proper functionality of the display.I can't find any online documentation on what exactly a FIP Coil Driver does, and how could it be replaced with alternate components if it goes bad (especially since it's becoming harder to find one, and the few on sale tend to be rather pricey). In addition, when it does fail, is it repairable?I was wondering if anyone here might have some speculation on the function of the FIP Coil Driver, possible alternatives for one, and whether or it might be repairable?Here is a web site that discusses its impact on display issues for this particular display:https://supersynthprojects.com/super-jx-display-replacement/super-jx-display-failure-modes/

author: pqass
author_rank: Super Contributor
timestamp: April 17, 2025, 01:49:57 am
content: The FIP Coil is a transformer that's part of a DC-to-AC converter. Seethis schematicpage 10 (and is attached below).It has two isolated secondary windings.One winding to drive the VFD filament is center-tapped.It may be 45Vp-p (or 16Vrms with your MM on ACV) (as perthis NEC databookpage 9-4 line item for FIP32B6R.CORRECTION: Althoughthis NEC databookpage 9-4 line item for FIP32B6R says the eb=ec=45Vp-p (or 16Vrms with your MM on ACV), according to the Noritake link below, ec/eb is the cathode to anode/grid potential and is a DC quantity and not the ef or filament voltage. Without a datasheet all you can do is take measurements.The second winding produces three voltages (with respect to T1 pin 6):(a) feedback to keep the transistors oscillating, T1 pin 7(b) two negative voltages (via D2, D3 diodes), T1 pins 8 and 9?If you can document the voltages before it fails, you could wind your own T1 replacement. It'll likely be physically larger.Or, more likely, you can replace the entire circuit to produce the same voltages. Current requirements are likely very modest (a few 10s of mA).If it hasn't failed yet, record the following:(a) MM on ACV: Leads across T1 pins 3 and 5. Or VFD pins 1 and 17 (I think they're the top-left and top-right pins; may see the filament wires attach to pins through the glass).(b) MM on DCV: Leads across C3 cap(c) MM on DCV: Leads across C4 cap(d) measure the frequency across T1 pins 3 and 5 or VFD pins 1 and 17.To understand how to drive VFDs seethis guide.You'll see negative voltages in many designs because it removes the need for the driver IC to have a high voltage supply pin in addition to a +5V logic supply. It's easier just have P-FETs/PNPs in the driver IC switching the VFD anodes+grids and make the VFD cathode (filament) negative via DC-AC converter. Resistor packs RA3, RA4, RA5 pull down the anodes+grids to the most negative voltage (below the cathode) when the transistor is off to make sure no light is emitted.

author: Midphase
author_rank: Regular Contributor
timestamp: April 18, 2025, 06:13:36 am
content: Thank you this is useful information. At the moment the FIP is working, so I can certainly take the measurements.The ideal solution of course would be to just replace the entire assembly with an OLED or LED display which I assume would require an in-between microcontroller to interpret the incoming data from the processor and send it out in a way that the new display would recognize.

author: pqass
author_rank: Super Contributor
timestamp: April 18, 2025, 02:56:57 pm
content: Replacing the entire assembly would be a pain since IC1 is a slave MCU (taking commands via SPI from the main processor). It would've been easier if IC1 was a simple SPI shift register like IC2.But you can treat IC1 and IC2 as having low current capable PNP open collector outputs. So you can replace the VFD with a 32 character, 14+DP segment LED display with drivers taking the load off of IC1+IC2 outputs (eg. ULN2803*2 sink driver for the segments+DP and UDN2981*4 source driver for the common-anode LED digits).Or, you can take a snapshot of the segment+DP (and P80 for sync) outputs with a MCU and feed that to whatever display you like. You could tie 32 diodes (one per anode+grid output) to an interrupt pin on the MCU to take the snapshot but that'll likely be too fast (refresh rate is probably 50Hz*32 per second) so you'd need to divide that line by the exact refresh rate (to maintain sync). You'd need protection diodes and pull-down resistors (to GND) on the segments+DP to keep negative voltages away from your MCU (when the PNPs are off). You'd still need -4V on IC1 Vpre and the Vload input to, say, -12V since we don't have a datasheet telling us otherwise.

author: Midphase
author_rank: Regular Contributor
timestamp: April 18, 2025, 04:59:04 pm
content: These guys managed to do it with what appears to be a small microcontroller. Of course I was interested in seeing if I could figure it out myself...otherwise where's the fun?https://supersynthprojects.com/super-jx-display-replacement/

author: pqass
author_rank: Super Contributor
timestamp: April 18, 2025, 06:41:02 pm
content: That's encouraging!You don't know what you'll face reverse engineering a protocol (I wouldn't say it would be fun). You'd have to be able to trigger all possible events to cause a display change. At best it's tedious. And there's always the chance you've missed something and can't interpret some data element or miss a delimiter. The best you can do is store those oddball records for later recall and keep evolving your code.

author: Midphase
author_rank: Regular Contributor
timestamp: April 30, 2025, 05:44:13 am
content: Quote from: pqass on April 17, 2025, 01:49:57 amSo today I finally got around to recording some of those reading, and I thought I'd post it here to see if it might help, here is what I found (assuming I read everything correctly):MM on ACV: Leads across T1 pins 3 and 5: When I set my multimeter to AC I am not able to get any readings, so I stuck to DC and I got = -34.03v on those two pins.I am not sure of the numbering, so for the FIP I went counter clockwise from bottom right pin:Pin 1 = .03vPin 2 = .03vPin 3 = .00v (ground?)Pin 4 = .00v (ground?)Pin 5 = -34vPin 6 = -34vPin 7 = -34vPin 8 = 10.08vPin 9 = 10.08vOr VFD pins 1 and 17 (I think they're the top-left and top-right pins; may see the filament wires attach to pins through the glass). = Not 100% sure about these, but if I guessed right those would both read -34.6vMM on DCV: Leads across C3 cap = -34.13vMM on DCV: Leads across C4 cap = -41.56vMeasure the frequency across T1 pins 3 = On the scope, the frequency was a bit wobbly, it was hovering around 50khz, sometimes dropping slightly below and at times getting as high as 54.095KhzPossibly other Pin 3 = Similar reading as about around 52.7KhzAnd 5 = Same, around 53.31KhzVFD pins 1 = Also same around 54.390KhzAnd 17 = Also same around 54.559KhzI think I got the right measurements, as they seem to be somewhat consistent. While I was at it, I reflowed all the connectors and replaced the electrolytic caps around the board (although I think the old ones were in pretty decent shape).