Fix Old Cameras: OM-1 Dead Meter / Diode Modification
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- เผยแพร่เมื่อ 9 พ.ย. 2013
- Dead light meter in an Olympus OM-1. This video shows one meter issue and how to correct it along with a diode modification that makes the meter compatible with 1.5V battery.
classic camera repair - www.phototronic.biz
A few tools and books - www.amazon.com/shop/fixoldcam...
+Endra Hanugroho The installation of the appropriate diode between the battery wire and meter circuit will work on the FTn as well as other cameras with a single cell galvanometer based meter.
Thanks for the videos. I followed the instructions and did a full overhaul on my OM-1.
The light meter seems to stay on no matter the switch position but it might be a contact problem so we'll see with a new battery.
Though most of the foam has eroded all over the place, I will probably need to fix the light-sealing.
Wonderful - thank you very much for this. Just did the diode instalment myself. I do not have experience in this type of work. Light meter seems to be spot on now 👌
Good to hear of your successful repair.
Took the bottom plate off my $5 flea market OM-1 and you were right, the nylon screw head had come off. I was luckily able to remove the "stump" with a pair of tweezers. The wire had also snapped off so I had to delicately strip off some insulation with a razor blade. Now I just need to get the diode, solder it in and if I'm lucky I've got a beautiful classic camera for peanuts. Just need to recover it also as the leatherette is just starting to peel off in 1 corner, always wanted to have a go at that, I might try a nice fake snakeskin ! Thank you so much for posting this video.
Great comments and thank you for your interest in Fix Old Cameras.
Hi awesome video, can the same method be applied to Nikkormat FTn?
Tim Powell If the meter in an OM-1 turns on but refuses to respond to light changes than most likely the CDS cell(s) function should be checked. Check the resistance of the cell for linear changes that coincide with the changes of light intensity. Check for open circuit.
Awesome stuff! I have an OM-1 that I am working on. The battery wire has a crusty brown substance on it. Does this mean that the wire needs to be replaced? Or can this be fixed with the Diode method or even some cleaning. Your advice would greatly appreciated!
The wire is corroded to the point where it will probably not take solder. The wire will then need to be replaced. It is also possible the contact will not take solder. Fashion a battery contact from copper or brass plate or salvage one from a donor camera. This video shows how to access the battery wire.
th-cam.com/video/swcuZ9xCFXs/w-d-xo.html
I modified the contact with a spring type contact that was JB welded to the Battery compartment.
Hi, Thanks for the video - great help. I was only able to find a Schottky 1N60P diode. Will this be adequate? I know that it is temperature dependent but only on the milivolt scale?
+Jonathan Malin
The ECG109 germanium diode is probably considered a vintage diode. Finding it or a direct substitutes might be getting more difficult. Not knowing the specs of a schottky 1N60P prevents an accurate comparison. However the diode used is actually not as critical as you might think. As long as you get the desired voltage drop on a linear scale and the physical size fits in the space available it can be used without concern. If the diode you chose does not work, try another, you will not harm the meter.
Your Videos are fantastic, i loooove you!!!!! :-)
Hi all, I would like to share an issue I had with my M6 non TTL. It is a strange issue on the lightmeter. I found that the lightmeter is giving wrong exposure only when shutter speed is at 1/125 s position. And I slightly turned the dial towards to the 1/60 s direction, the exposure was then corrected. I am wondering if anyone encounters the same issue before? and how did you fix this problem? Thank you
Do you do repairs?
I have a wonderfully brassed OM2, with shutter lever that won't wind, and dead meter.
I was just wondering. I have a Vivitar 45 that uses the old Mercury Batteries PX 675. It has the circuit that is reverse polarity. Do you think the diode fix will work with this unit?
@FOC: What do you use at min 2:16 to improve contact of the old solder pad? Thanks in advance.
Rosin Paste Flux www.amazon.com/shop/fixoldcameras?ref=ac_inf_hm_vp
However often the residual corrosion will make the contact nearly impervious to new solder. In such a condition, no amount of burnishing or cleaning will prepare the metal to take solder. In those case a substitute contact will need to be used at this time the battery contact wire will also need to be replaced.
th-cam.com/video/swcuZ9xCFXs/w-d-xo.html
Doing good dude. Do have Minolta spotmetre f parts? I want viewfinder of it. can you help me?
Hi can a simple voltage divider do the same thing?
Old post, I know, but are you using the voltage drop across a forward biased diode to take the effective voltage down by .2V??
Is this procedure the same on a OM-1n? Thanks for this video.
The battery contact is different, but the diode modification is the same.
Question: what would be the modern equivalent of the ecg109? Can't find it online.
Thanks for all that info Mike. I located ECG109's but the company only has 1 in stock, bummer. I emailed the salesman and he'll check for me. Meanwhile I'll try an IN34A if I can find any. Roger on the Mercury content in CFL's. Have you ever noticed that a CFL gets so hot you could fry an egg on it. I've had them melt light fixtures. I also seem to remember an Article in the old "Camera Craftsman" about the diode fix. I used it on my Yashica 124G years ago.I'll have to see if I can find that issue and see what diode it used. I've purchased Wein cells by the box from B&H camera in NYC. I find they only last a month or so.
Great comments as always Tom. Thank you for your continued interest in FOC.
does the ISO dial interact with the operation of the camera?
I have got the OM-1n, The meter reads when off or on and is not effected by changes in light conditions. Anyone able to help? I have had a look in the bottom and it all seems in order.
Trivial I know, but the original screw on many (but not all) OM-1 and 1n's as shown was plastic, not nylon.
And you can easily insert the white nylon screw described without re-tapping the hole -- the original metric threads in the metal casting will easily cut new threads into the nylon of the replacement screw. Once inserted fully, cut off the excess length of screw that protrudes into the film chamber next to the take-up spool flush with the body using a narrow X-acto knife.
Would it be possible for you to do a video about a meter replacement? I’ve done the battery wire replacement to no avail, and have another OM1 that’s in poor condition with a working meter.
The OM-1 will need to be confirmed as to be the same meter version. It might be easier to repair the mechanics if the meter swap out includes the galvanometer itself. The CDS cells or circuit should not give you too much trouble. But the string the links the meter to aperture ring is tricky.
Hi I enjoy repairing cameras, I have an om2 to which I changed pentaprism and switch, I ask you a question.
Is it normal that, by increasing the ISO, the light meter indicator in "auto" mode gives the same shutter speed values? shouldn't they increase? Thanks, Nunzio
In Auto the aperture setting and the ISO should change the shutter speed accordingly.
Great video, I have bought a wein air cell for my Om1, however that brown wire to the battery holder was corroded badly, so I removed it in the hope to replace it.
But I cannot feed a new wire from the base of the camera up to the top, it keeps getting jammed inside the camera!
Any tips on how to feed the new wire through? Is it something you have done before?
The battery wire is glued to the inside of the mirror box cavity. Leave the wire and try to find another route up to the top circuit with your replacement wire. Otherwise your will have to remove the mirror cage assembly to gain access to the existing battery wire.
Thanks for the reply, I managed to remove the old wire a few weeks ago, I wondered what was holding it in place so tight...
I have also managed to route a new wire the same way as the old one, took me a few hours. Exposure meter still not working :( you cant see the needle at all! I am getting a voltage to the top and through the switch though :)
You might remove the prism to better see the galvanometer reaction. Perhaps the needle is physically binding. You may hook the wein air cell directly to the galvanometer to check the meter functionality.
Thanks for the tip, I will give it a go when I have some spare time :)
Hi - I have a Minolta XG-2, and its meter works inconsistently. As in, it has worked for quite a while, but sometimes it gets confused by something and instead of lighting up 1 or 2 LEDs in the viewfinder it will just kind of sweep and light up a bunch of them, and they light up rather nervously instead of solidly as well. So it's like something isn't making contact well... - For example, last time when I was aiming at a bright sky, it would just meter as if it was completely dark. I took the picture, but the shutter stayed open for maybe 25 seconds. Long enough for me to think the mirror got stuck, but it was just a really long shutter-time because of the confused meter. It can als go the other direction, all the way over its maximum speed. - So it's really inconsistent.
I just now gave it another try and while at first I was metering in a poorly lit room at f32, it would just meter beyond 1/1000, which doesn't make sense at all. But then I turned down the aperture and it seemed to display a more realistic reading. I do notice that it freaks out when I turn the aperture-ring, so that might be a hint for you. - Maybe it's even this particular lens, I'm not sure. Though, I don't think my X-700 responds the same, as that one seems to be much more solid. - Do you have any idea what it is and what I could do?
Under the lens mount between the upper mount screws is a ceramic resistor base. Clean the surface of the contacts and the resistor base with alcohol. Make sure the gold wipers are making good contact. Be mindful of the spring attachment as you work.
Thanks for great video. Can You tell me does the SCHOTTKY BAT42 diode will work? My local store only have that one :(
The diode will do no harm to the meter circuit if you want to give it a try.
Fix Old Cameras I did it, but I use Schottky BAT43 diode and V625u Varta battery. So far looking great. BTW it's Olympus 35RC camera.
Good to hear of your successful repair. Keep it going. The Olympus 35RC is a nice camera to use.
Thank You. Keep filming good tutorials. :)
One more question, I have Pentax ME Super and meter constantly underexpose one stop. Therefore film 400ASA I have to treat like 200ASA. Is there possibility of adjusting?
Hi. I have an OM1n, and the meter is not operating correctly, showing greatly overexposed readings. I have purchased the CRIS adaptor and installed the recommended Eveready alkaline 386 battery to compensate its 1.5v. What would you suggest?
I took a reading against a buff colored building and received a f4 aperture with a 1/2sec speed, on a relatively bright but cloudy day, with ISO 400 film aboard. Obviously wrong.
A CdS cell or cells could be faulty or have a broken wire. Confirm CdS
cell condition. Depending on the type of cell used in your camera the
resistance reading should range approximately 30 - 70 K ohms with your
light source at BV4. At a light source of BV16 or higher the resistance
should max out at 1- 3 K ohms. If resistance is infinite, the CdS
probably has a broken lead wire. If the cells are good and the zero
point is still extremely out of position adjust the position of the
meter connective string attached to the galvanometer assembly. In case
of an excessive up swing of the meter needle loosen and turn to the
right. In your case, that of an insufficient swing of the needle, turn
the adjustment to the left. You can make fine adjust via the eccentric
screw that slightly changes the position of the lever on top of the
galvanometer. For non linear meter issues the resistors on the meter
board can also be checked and increased or decreased to affect the
position of the meter needle across the light spectrum. Please note that
resistor service would be quite tedious. Without another OM1n with a
properly working meter of the same sort for reference, it is not
recommended.
@Fix Old Cameras
I have an Olympus om2n with a meter that works fine in auto mode, but once in manual - the meter reacts only to the aperture change, whereas changing the shutter speed does not affect it.
However, once I move the power switch to check/reset, it usually starts recognizing the shutter speed change in manual mode. This happens every time when I turn the camera on and without changing the lens prior to that.
Did you encounter such an issue and could know how to solve it? Many thanks!
Typically checking the ASA cam linkage and aperture ring string to see that the coil is rotating and not binding would help... but because the meter is resetting itself through the power switch makes it look like a process circuit issue.
Fix Old Cameras Thanks!
Changing the shutter speed dial should have no effect on the meter when the camera is set on AUTO mode -- the shutter speed is controlled by the light reflecting off the film into the photo cell below the mirror, not the dial on the front of the camera. Also not that the needle in the finder is an indicator only -- it in no way actually controls the actual shutter speed in Auto.
There are numerous possible faults in the meter that will affect meter movement. Meter needles often stick to the lower stop once that stop gets sticky with age. Or metallic debris may have been attracted to the magnet inside the meter movement, periodically inhibiting the movement of the meter needle. Both require disassembly of the meter to correct, with great risk to the meter movement itself.
hi. great video. i have a quick question. i have an om3 and for a long time now the light meter comes on but doesn't respond to light. it just sits there and only moves when i move the rings. Do you know what could be causing this as i dont want to get rid of this classic and rare camera.. Thanks.
When a camera meter does not respond to light changes, but seems to work otherwise than it is most always caused by the CDS cell or CDS support circuitry.
is that an expensive fix?
The OM-3 does not use CdS cell circuitry. The input of the Silicone blue cell is amplified and input directly to the Cpu. There are numerous circuit faults that may cause this problem. In all likelihood, the camera will need a new main circuit.
I have a question, in order for a meter to function properly and accurately, you will need the exact batteries right? I have an FM2 and the meter seems to be off by one stop higher then the correct exposure. What do you think may be the problem? Is this fixable?
The FM2 was designed to work with 2 of the 1.5 volt button batteries, preferably the silver or lithium type... such as the MS76. You may also you the 3 volt... such as the CR1/3N.
Fix Old Cameras yes I am using the correct batteries then, do u think it may be a matter of calibaration? Or maybe the electronics are messed up?
It is possible you can get by setting up the ISO/ASA to compensate.
The question is what battery to use even after the modification, as the widely available LR and SR batteries drop in voltage over time?
Answer to my own question: Silver Oxide Batteries like the S76 or 357 should be fine.
They show discharge profiles that are almost identical to the ones we find when looking at mercury batteries.
Very good battery of the Silver Oxide choice for the reasons you state. Thank you for you interest in FOC.
Hi FOC, the ECG 109's turned up the other day and unfortunately they are all IN27's packed inside the ECG 109 original packages. The seller claims they are the OEM replacements. Is there any quick easy way to test the voltage drop? Say a resistor in a circuit with the battery and diode? Or must I go ahead and try it on my F1 body? THANKS
Yes you could mock up a test circuit, but if you have a multimeter with a diode test mode it is simpler. (it may share a space on the meter's selection dial with another function) A multimeter’s Diode Test mode produces a small voltage between test leads. The multimeter then displays the voltage drop when the test leads are connected across a diode when forward-biased. A germanium diode has a voltage drop ranging from 0.2 to 0.3 Volts. A silicon diode has a voltage drop ranging from 0.5 to 0.8 Volts. A shorted diode will have the same voltage drop in both directions.
OK, My meter does have a diode test. Most of these IN27 diodes are measuring between .253 & .267. My meter isn't a high end one but it seems these may work OK. Thanks again Tom
Thanks FOC for this useful video. I have the Nikon F and Rollei 35 which uses the 1.35v battery. Is it the same way/approach to fix meter in those cameras like this one ? Thank you in advance.
It is indeed. The issue is to find a location for the diode in the Rollei 35, with care it can be done. Also note that the Photomic meter in the F is a two battery system and will need a diode equivalent as the IN4007 or SK3080.
Will the new replacement battery of 1.5 v cause a malfunction of the light meter in the OM-1? I recently replaced my battery and it quit working. The contacts are clean with no corrosion. Thanks.
Such a battery will not cause the meter to fail. The only damage it can do is corrode if left to discharge over a long period.
How about cameras that use two PX25 mercury batteries. Is there a diode that will drop the 3 volts? Thanks
P.S. How about a how to for a very tight Canon FD 70-210mm lens?
A 1N4007 diode would give the needed voltage drop in cameras designed with the double PX625 battery system. Thank you for the suggestion.
Thanks, I'll let you know which camera I do and my results. I have a Gossen N100 that I'll try an IN4007 in.
Tom Heckhaus Very good.
could this technic be aplied on any other camera?
Yes, and no harm will be done if you install the diode wrong. Simply flip and reinstall.
will throw out the meter by 2 f stops because the diode not only reduces the voltage but also the current (I) and is less than mercury battery current. with this mod you also have to change circuit board resistors to adjust for new current. adjusting the eccentric screw by meter to compensate meter needle is false too because will throw out the zero position of meter (no battery, ASA at 1600, shutter at B f stop at 8= zero position- needle on center between +/-), thus not acurate at different lighting condition.
My meter is not functioning properly with a new Wein Cell 1.35v there is some movement in the meter needle in the viewfinder but its way way underpower - I had a look under the bottom plate and the nylon screw appears to be in tact. There does not appear to be a diode. For example i have asa set to 1600, and shutter speed to 0.5 second looking at the daytime sky to give a good exposure metering. Changing aperture doesnt appear to have any impact in this situation as well. Any advice please? I would love to get this camera working again!
If the meter needle moves when the meter is turned on but is not reacting to light then the CDS cells are probably defective. If the changing the aperture of the lens does not change the resting position of the needle then the aperture link ring needs to be checked.
Thanks for getting back to me on this. Is it difficult/expensive to have the CDS cells changed?
If you are interested in getting an estimate for repair of your classic camera you may go here
www.phototronic.biz/services.html
Or find a camera to salvage and swap them out. The cells are integrated into the eyepiece assembly, replacing the entire eyepiece/ cds unit would be the easiest. You will have to solder the wire leads.
i dont know if anyone would be able to help but i have an OM20 that the light meter has been working perfectly fine on but all of a sudden the LED indicator on the inside is behaving erratically and jumping between all of the numbers. the light meter is still working as i am still getting the correct exposure but the indicators you see through the viewfinder are not showing the correct shutter speed. any ideas what might be causing this? i have new batteries in and still nothing.
Do not know for certain. Might try cleaning the printed contacts of the ASA/ISO or the aperture linkage sweep arm and circuit base.
Hi A few questions. (1) ECG 109's are tough to find do you have a list of substitutes you've actually tried? (2) Is there any inherent problem with using a 1.5 v battery and just resetting the ASA dial? (3) Any plans of doing a Canon F-1 video. These were a true 1970's workhorse camera. A video with the usual CLA, seals, diode etc would be great. If you'd like I can donate a Canon F1 for you. I have a couple. It may have a dented prism and a non working motor drive & a modern FD lens but it'll be worth fixing up. Let me know. THANKS P.S I may even have servicing info, if you need it. I'm pretty sure you have the microfiche for the F1. P.S.2 (If you do a video then I can fix the 1/2 dozen I have. PS3 Have you considered a video on cosmetic repair, such as brassing on enamel and chrome parts? MANY THANKS TOM
The diode 1N34A will work satisfactory when the ECG 109 is not available. ECG 109 is still this camera repairman's first choice, but experimentation has proven the 1N34A a good performer in this application. The ASA dial adjustment works, although the meter will not be linear accurate. Film as enough latitude that most shots will be OK. No harm will come to the meter. Some of the later classic cameras using the 1.35 volt mercury battery
have compensation circuits and are not affected by substituting a silver battery. Your comments present and opportunity to relay some information about battery substitutions in classic cameras.
Mercury oxide batteries typically had a voltage of 1.35 Volts. They kept this voltage constant over their entire lifespan and lasted for years. They could be recycled and even recharged a couple of times. Question… Why is mercury acceptable in CFL light bulbs but not in batteries?
Silver oxide batteries typically have a voltage of 1.55 Volts. They also keep their output voltage quite constant over the entire lifespan of the battery and they have relatively long usable life.
Zinc-air batteries mainly for hearing aids have a constant voltage output too, typically 1.4 V. Since their voltage is close to the voltage of mercury oxide batteries, they can be used as a substitute without making any changes on the meter circuits. They are cheap… but they have a short life span of only 2 to 3 months, not depending on usage. Once their seal is removed, they dry out inevitably, even when not used. When first taken into service, new zinc-air batteries need quite a long time of approx. half an hour up to one hour until they achieve their correct output voltage. In this time they can cause incorrect meter readings.
Wein Cells, available locally from photo equipment dealers, are basically zinc-air batteries with fewer ventilation holes to achieve a longer lifespan (approx. 6 to 12 months), as they do not dry out as quickly as normal hearing aid batteries. They have a smaller maximum power output than zinc-air batteries, but for photographic equipment this is not a problem, as the meters do not need much power. Wein Cells are still expensive and can be difficult to get in some places. They have the same problem with the initial voltage as normal zinc-air batteries.
Alkaline batteries have an output voltage that drops continuously over the lifespan of the battery as it discharges - depending on the battery brand the initial voltage is approx. 1.6 volts - 1.5 volts. They are not a good substitute for the mercury oxide batteries because of the voltage drop over its lifespan.
EGG 109 diodes are not hard to find, at least in the USA.
What kind of batteries does it use?
The 625MRB (1.35 volt zinc air battery) will work perfectly without modification. However it has a short life once it is put into service and it is a little pricey. The 625A will work in an OM-1 with the diode modification but should be changed out once a year because of its sloping discharge rate. The MS76 button will work with the diode modification and can be left in the camera for several years. Due to its smaller size, a small o-ring encircling the MS76 battery will help it stay in the proper position upon installation of the battery cap.
These diodes are readily available online from several sources.
Yes. Thank you for your comment and interest in FOC.
The brown wire on my OM-1 is far too corroded even after attempting to strip it as far back as possible. Is there a way to completely replace this up through the camera?
+mj0188 isg The brown battery wire routes up behind the self-timer and is glued to the chassis. Seep some alcohol down the wire from the bottom and the top. If the glue dissolves to the point that the wire becomes loose you can solder a new wire to the top end of the old wire and pull the new wire down. Chances are you will not be able get the old wire to loosen without accessing directly, which requires removal of the mirror box / front plate assembly. NOT advisable unless you have some previous camera repair experience. The most likely option for you is to route a thin gauge insulated braided wire over the front plate and under the leatherette following the seam grove of the front plate. A Fix Old Camera video will be produced to demonstrated this procedure in the near future.
+Fix Old Cameras Look forward to it! Thanks!
Are you able to obtain the Diodes to fix these cameras?
phototronic.biz/tips - one of several sources
Any one knows a equivalent diode that can be used?
1N34A or SK3090 are possible substitutes. Others might work also.
Have you actually measured the voltage after the Diode install? Oh yes, can you do a meter calibration video on the OM-1?
Absolutely, take a DC volt meter / multi meter reading on the wire side of the diode to the ground and the voltage drop will register if the diode is installed correctly. If installed incorrectly, no voltage will register and the diode will need to be flipped. All suggestion for upcoming videos are greatly appreciated and considered. The list is long, it could be some time until you see your suggestion. There are some meter calibrations that would be suitable for a Fix Old Cameras video on the different meter versions that were introduced over the years in the OM-1 camera models
Got it! I will try that and I hope you can do a video on a simple meter calibration on the OM-1.
***** The alkaline battery is satisfactory. However, after approximately a year of typical usage the alkaline's voltage will fade. The silver or lithium battery cost more but is stable on the voltage output until the end of its life.
As always, very helpful video! Is it possible to calibrate an OM1 meter on 1.5V batt w/o installing the diode? How far can it go +/- ?
@@tulauc In a word, no -- the circuit design of the OM-1 and 1n is such that when using any battery other than a 1.35volt, the error will not be linear. In other words, while the meter may be half a stop off at lower light levels, it may be more than 2 stops off at higher light levels. Some form if recalibration is necessary, either by the described method of "stepping down" the voltage from 1.5 volts, or by insertion of a compensating resistor at or near the meter's circuit.
Hi, I would like to upgrade my OM-1 camera for 1,5V battery, so I bought two pieces of germanium 1N60 and two pieces of schottky 1N5711. Now I don't know, which could be better. I read that 1N5711 should be connected 4 pieces in paralell, else the battery will be overcompensate. But I have only two pieces, so now I tend to 1N60, but I'm not sure..
The ideal diode is the vintage ECG109. A very good substitute 1N34A. In nearly every application tried in a single 1.35v battery meter system one of these diodes gave the required voltage drop when using 1.5v battery substitutes. However, there is no reason that you can not use the diodes you have acquired, they will cause no harm. You will have to experiment with singles and different combinations to get the desired voltage drop.
I would like to ask you, on ebay there are several 1N34A diodes, which came from China. Have you any experience with it? I think that these diodes aren't the same as the vintage 1N34A..
They are a serviceable substitute. You might still be able to get a vintage ECG109 here -
astore.amazon.com/fixoldcam-20/detail/B001ISCAFG
Why a diode? Shouldn't it be a simple resistor to lower the voltage of the battery?
There is almost no power dissipated through the diodes (as opposed to a resistor) and little loss of current. Since we only needed a drop of .15 volts but still want to maintain the current the diode is used. Along the lines of your thinking there are some classic camera meter circuits that can function perfectly on a 1.5 volt battery source even though they were original design to operate on a 1.35 volt battery source.
+Fix Old Cameras Thank you very much for the reply! I have an OM-1n, no one in my city seems to have the original battery or something that physically fits the battery compartment. I'm trying to use a 1.5v (LR1130) button cell, but no dice so far
A diode will drop the voltage reliably, while if you use a resistor, it will drop the voltage as a function of the total resistance of the circuit, which is unknown in this circuit.
The meter of my OM1 isn't working as it should. A correct exposure according to its light meter, is in fact a +2EV overexposure. Do you know what's causing this and if it's possible to fix it?
After the diode modification is completed a linear adjustment can be made via the eccentric screw on top of the galvanometer.
See www.miketrost.com/archive.html and scroll to the bottom of the page for a photo of an OM-1 galvanometer adjustment along with additional details on the procedure.
The nylon screw is 2.15mm diameter.
Not an easy size to find.
Indeed it can be a challenge.
2mm might go if you put tinfoil or paper in to get that fraction more diameter.
Also a bat 43 schottky diode might work to drop the voltage.