Check the two thin multi wire lines that go from heads to main board, on many Teac's they go to a header and it might be loose, have corrosion, cold solder, etc
just a thought as that second head see's nothing (and if even out of line would show something iirc)...
added: and am assuming both drives have exact same jumper settings except for drive ID

Head 1 is the upper one, it's cable is flexed with each opening/closing of the drive door/latch, and also it's more exposed in general. The first thing to check would be the cable (on older models it's a classic multi-wire shielded cable, on newer it's an FFC - flat flex cable) for any mechanical damage. The heads can be tested with a modern ohm meter, preferably set to manual range of 1k to limit the current to safe values. There are no exact values to get, what you do is test both heads (working and non-working) and compare results. If the heads are OK then it might be a problem with the preamp or diode signal switch - this is also something only found on older drives. Modern ones are pretty much 1 or 2 chips with all the stuff integrated within so at that point it's easier to swap entire PCB or just replace the drive.

EDIT: Upper head can be, in a pinch, removed completly for proper inspection. But that will require almost full realigment procedure afterwards so should be done only as last resort if no other obvious issues are found.

Horun wrote on 2023-07-23, 01:20:

Check the two thin multi wire lines that go from heads to main board, on many Teac's they go to a header and it might be loose, have corrosion, cold solder, etc
just a thought as that second head see's nothing (and if even out of line would show something iirc)...
added: and am assuming both drives have exact same jumper settings except for drive ID

I checked this last night. I actually swapped the main board with a working FDD (I have two). No change.

Deunan wrote on 2023-07-23, 10:16:

Head 1 is the upper one, it's cable is flexed with each opening/closing of the drive door/latch, and also it's more exposed in general. The first thing to check would be the cable (on older models it's a classic multi-wire shielded cable, on newer it's an FFC - flat flex cable) for any mechanical damage. The heads can be tested with a modern ohm meter, preferably set to manual range of 1k to limit the current to safe values. There are no exact values to get, what you do is test both heads (working and non-working) and compare results. If the heads are OK then it might be a problem with the preamp or diode signal switch - this is also something only found on older drives. Modern ones are pretty much 1 or 2 chips with all the stuff integrated within so at that point it's easier to swap entire PCB or just replace the drive.

EDIT: Upper head can be, in a pinch, removed completly for proper inspection. But that will require almost full realigment procedure afterwards so should be done only as last resort if no other obvious issues are found.

I performed a test last night and pretty much narrowed the problem from the pin headers that connect to the mainboard to the head itself. Somewhere in between that. Replacing the mainboard with a known good one did not solve the issue.

I lifted the metal cover that is held on by adhesive foam on the top of the head. I compared it with the working on and oddly enough they looked different, even though they were the same model...

I'll have to research where to probe to check the heads. There is a service manual for these but it seems like you need a lot of test equipment to use it. I will also check the flex cable.

Ok. Was hoping it just a simple connection issue. The top head (side 1) is the most common for damage/misalignment. Often it gets warped/ off angle slightly due to being in the raised position most of the time, being lifted only on one side....
There is a bit of info at https://retrocmp.de/fdd/teac/fd55_i.htm which you probably already found... yeah you need a scope to do full maintenance.
For reference here is the head area of 55-BR which is same family (BR is gen 3, BV is gen 2) and use LSI and nearly exact same head mechanisms, only straps/jumpers are a bit different.

PioneerSX1280 wrote on 2023-07-23, 14:26:

I performed a test last night and pretty much narrowed the problem from the pin headers that connect to the mainboard to the head itself. Somewhere in between that. Replacing the mainboard with a known good one did not solve the issue.

Heads can burn out but that is due to the electronics failure, it's rare that one would and the other not on such a modern drive. I'd first suspect the cable before the actual head but you can't just replace it with whatever. It must be the same kind (FFC or shielded) and must be elastic enough and not fragile to withstand all those bending cycles when the head moves. Might be easier swap the entire upper head with the cable, though again that would require a scope and realigment. Replacing just the cable, if you can do it, from a donor drive might be preferable if you'd rather not touch the screws. I've never done but with some patience it doesn't look impossible.

Do you have a multimeter? It would be easy to eliminate cable or head problem. Disconnect the cable header from the main board, then using a needle probe, measure in continuity mode, between each solder joint on the top of the head (see picture). You should get two pairs of shorted joints. Then to check continuity of the cable, measure from each solder joints on the head to the corresponding contact on the header pins on the cable.

Edit: You can do the same on the working drive (if you have one) and compare with the bad one.

I found something with a multimeter when comparing the epoxied soldered wires on the head to an unplugged pin header connector to the main board:

Working drive:

Blue / Red / White / Black all have continuity in various resistance,

Red wire soldered under epoxy to pin header:
Red 0.50 Ohm
Black 2.25 Ohm
Blue 12 Ohm
White 12 Ohm

Bad drive:

Blue / Red / Black all share continuity together share continuity with each other but do not share any continuity with the white wire.

Red wire soldered under epoxy to pin header:
Red 0.50 Ohm
Black 2.25 Ohm
Blue 12 Ohm
White 0 Ohm

White solder joint on head to white wire pin connector: 0.50 Ohm

So the problem must be where the ribbon cable meets the head or an internal failure with the head...

Horun wrote on 2023-07-23, 15:20:

Ok. Was hoping it just a simple connection issue. The top head (side 1) is the most common for damage/misalignment. Often it gets warped/ off angle slightly due to being in the raised position most of the time, being lifted only on one side....
There is a bit of info at https://retrocmp.de/fdd/teac/fd55_i.htm which you probably already found... yeah you need a scope to do full maintenance.
For reference here is the head area of 55-BR which is same family (BR is gen 3, BV is gen 2) and use LSI and nearly exact same head mechanisms, only straps/jumpers are a bit different.

I do have a scope incase I need to re-align, but no IBM DOS pc with a floppy controller 🙁

Zerthimon wrote on 2023-07-23, 17:39:

Do you have a multimeter? It would be easy to eliminate cable or head problem. Disconnect the cable header from the main board, then using a needle probe, measure in continuity mode, between each solder joint on the top of the head (see picture). You should get two pairs of shorted joints. Then to check continuity of the cable, measure from each solder joints on the head to the corresponding contact on the header pins on the cable.
Screenshot_2023-07-23-20-30-12-950-edit_com.android.chrome.jpg
Edit: You can do the same on the working drive (if you have one) and compare with the bad one.

Thank you for the suggestion, above I has posted my results

Deunan wrote on 2023-07-23, 17:20:

PioneerSX1280 wrote on 2023-07-23, 14:26:

I performed a test last night and pretty much narrowed the problem from the pin headers that connect to the mainboard to the head itself. Somewhere in between that. Replacing the mainboard with a known good one did not solve the issue.

Heads can burn out but that is due to the electronics failure, it's rare that one would and the other not on such a modern drive. I'd first suspect the cable before the actual head but you can't just replace it with whatever. It must be the same kind (FFC or shielded) and must be elastic enough and not fragile to withstand all those bending cycles when the head moves. Might be easier swap the entire upper head with the cable, though again that would require a scope and realigment. Replacing just the cable, if you can do it, from a donor drive might be preferable if you'd rather not touch the screws. I've never done but with some patience it doesn't look impossible.

Above I have more info on continuity. At this point I have identified a inconsistenty with the white lead, compared to a working drive. Perhaps to get a final verdict I should remove the top head and inspect? Not sure how much more there is to lose.

PioneerSX1280 wrote on 2023-07-23, 20:21:

So the problem must be where the ribbon cable meets the head or an internal failure with the head...

Exactly.

I took the head off, not too much to lose at this point. No immediate signs of physical damage

Bottom head has a copper surrounding, top is silver

PioneerSX1280 wrote on 2023-07-23, 20:21:

I found something with a multimeter when comparing the epoxied soldered wires on the head to an unplugged pin header connector […]
Show full quote

I found something with a multimeter when comparing the epoxied soldered wires on the head to an unplugged pin header connector to the main board:

Working drive:

Blue / Red / White / Black all have continuity in various resistance,

Red wire soldered under epoxy to pin header:
Red 0.50 Ohm
Black 2.25 Ohm
Blue 12 Ohm
White 12 Ohm

Bad drive:

Blue / Red / Black all share continuity together share continuity with each other but do not share any continuity with the white wire.

Red wire soldered under epoxy to pin header:
Red 0.50 Ohm
Black 2.25 Ohm
Blue 12 Ohm
White 0 Ohm

White solder joint on head to white wire pin connector: 0.50 Ohm
So the problem must be where the ribbon cable meets the head or an internal failure with the head...

Ok so a direct short from Red wire at head to white. Being opposite sides of the solder tabs that is not a good sign. I suspected a possible open on one of the wires to the read head part.....not a short.
The layout of the head coils circuit in the maint manual which covers FD55, FD55-BR and FD55BV shows below. For the gen 2 & 3 BV and BR the Diode Switch is inside the LSI BA6581 chip but is outside on the gen 1.
So the two thin lines on the head are the read and write coils, the fat one is erase coil if I interpret the diagram. So one wire is common, not that it matters but you choose red. I would have assumed it black, and each of the coils has a diff color.
Just thinking out loud... even if black was common you still show a short from red to white... ok. Ideas ?

PioneerSX1280 wrote on 2023-07-23, 20:21:

Red 0.50 Ohm Black 2.25 Ohm Blue 12 Ohm White 0 Ohm […]
Show full quote

Red 0.50 Ohm
Black 2.25 Ohm
Blue 12 Ohm
White 0 Ohm

Welp, that's bad. What you have here is common terminal (that'd be red) and erase head (black) and two sides of R/W head (blue and white). Two sides because it is center-tapped with the common going to center. So it looks like you have no connection to one side of the R/W coil. At this point it's not really something you can fix - even if this was a bad connection and not a burned coil, once you disassemble it you are unlikely to get it glued back like it was, and there's IMO too much risk a tilted head might damage the media. That's completly ignoring the fact that the both heads must come together almost perfectly for the drive to work properly. A head swap is the best bet - or a whole drive if you can, no need to realign it afterwards.

You can, if you want, try to remove the glue that holds the spring, which in turn holds the head assembly. On the back you should have access to the coils - not that you can rewind those, but to inspect. I mean it's dead anyway, might be interesting to look at it.

Horun wrote on 2023-07-23, 22:44:

The layout of the head coils circuit in the maint manual which covers FD55, FD55-BR and FD55BV shows below.

That diagram is wrong, it shows one R/W coil and two erase coils. It's the other way around.

Look again. It shows two arrows from Read Write. one read, one write and one erase.

Oh, that's from the '83 docs, my bad, I didn't click on the picture to make sure. This one is correct, the '85 one is not.

Yeah I looked thru a few and noticed some changes that did not make sense 😀
Any idea what would cause a coil short ? Would think an open be more plausible (power surge, worn head, etc) ???

Horun wrote on 2023-07-23, 23:10:

Any idea what would cause a coil short ? Would think an open be more plausible (power surge, worn head, etc) ???

Usually the coil wire overheats from too high write current, which is something that IC should be limiting but if the driver fails for whatever reason this is what you get. Temperature goes way up, wire insulation fails and you get a short between windings. It could be some weird connection failure external to the coil (dodgy soldering that somehow eventually shorted) but I've never seen that. I did see burned coils though.

The issue, beside the head, is I would not trust that IC anymore. If the internal driver failed somehow and is not properly current limiting then it'll burn another head afer it's installed. It's far more likely that the IC failed than the coil was somehow not up to spec from factory. The only way to test it would be to install ~12ohm dummy loads in place of the coils and measure the current, compare between the 2 coils and possibly between the heads as well. This requires a scope and some external driver for the floppy interface to force write with a logic level you want to test (L/H should energize opposite coils).