How can I safely mount a TO-39 in a TO-92 location? - Page 1
Forum section: electronics / projects

author: Sir Knight
author_rank: Contributor
timestamp: August 04, 2021, 09:00:13 pm
content: A recent build on an amp has me up against a wall. The board is drilled for a TO-92 package, with 3 pins in a straight line. The part to be installed is a TO-39 (TO-205AF) with the 3 leads in a triangle configuration. The prototype I got from a friend has the TO-39 stuck in the relevant spot with the leads bent to fit the inline holes of the original transistor . I'd rather not do something so kludgy. I'm wondering if there is a such thing as a pin adapter/socket that I could mount on the board and then just press my 39's in there. I recall someone telling me a long time ago that there were various sockets that could be added to boards to accommodate something like this. A reasonably thorough search has turned up nothing, but I'm sure if it exists someone here would know.Or should I just grab some hobby board and lay out an adaptor myself? These are for a stereo differential pair, so they're all right next to each other in a line - maybe I should just stand a little phenolic table off the main board and do it that way?Or is there an even simpler way to do this and I'm just not seeing it? Advice, please!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2021, 10:57:20 pm
content: Just form the leads, nothing hacky about that.More to the point is whether the fat can fits around neighboring components, but I guess you aren't having issues with that so it's okay.Another question is, why TO-39 at all? Such devices are largely archaic. A TO-92L or TO-126 would be more modern I suppose. Or if the power isn't needed (as "diff amp" might suggest), plain old TO-92 should do.Tim

author: David Hess
author_rank: Super Contributor
timestamp: August 04, 2021, 11:21:31 pm
content: TO-126/TO-225 parts were never released in as wide a variety as TO-39 parts. Maybe the circuit uses something exotic by modern standards?I have the same problem with my old function generators which use TO-39 output transistors for which no modern replacements exist.

author: EPAIII
author_rank: Super Contributor
timestamp: August 05, 2021, 12:29:22 am
content: Speaking as a person who has performed countless repairs in a professional environment, I can definitely say that there is nothing unprofessional about just bending the leads. A home brew adapter would be a kludge IMHO and totally unnecessary. On top of that, it introduces several new points of failure so it makes the adapter an actual negative element.

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2021, 02:45:50 am
content: There used to be plastic adapters, about 1 mm thick, with molded holes to neatly force wires from one transistor package type into the hole pattern for another type. I used to have a set of these on a paper backing from the vendor, but they are long gone. Used properly, the transistor would then fit firmly into its new home.

author: Sir Knight
author_rank: Contributor
timestamp: August 05, 2021, 11:38:39 am
content: Quote from: T3sl4co1l on August 04, 2021, 10:57:20 pmThe original prototype did just that, and fit very well, though not snug close to the board the way I'd like.Quote from: T3sl4co1l on August 04, 2021, 10:57:20 pm... and the answer is:Quote from: David Hess on August 04, 2021, 11:21:31 pmThats precisely it. The original design called for a very run-of-the-mill BJT, which was changed to a MOSFET along the way due to some mods. There are other substantial edits to the circuit but this is the major individual part change. When I get my build working I'll be able to figure out some modern parts substitutions but right now I'm just looking to make this work.

author: Sir Knight
author_rank: Contributor
timestamp: August 05, 2021, 11:42:17 am
content: Quote from: EPAIII on August 05, 2021, 12:29:22 amI agree, there should be no home brew stuff in this. After prototyping, everything should look pro, and thats what I strive for. But what I was looking for was:Quote from: TimFox on August 05, 2021, 02:45:50 amThis is exactly what I was looking for. It adapted without adding to the wiring, or doubling the solder joints. If the vendor or product name pops into your memory please let me know!

author: wraper
author_rank: Unknown
timestamp: August 05, 2021, 11:46:00 am
content: Quote from: Sir Knight on August 05, 2021, 11:38:39 amThen why do you bother with minor nuisance which does not affect anything if you just want to get it work?

author: Sir Knight
author_rank: Contributor
timestamp: August 05, 2021, 12:07:23 pm
content: Quote from: wraper on August 05, 2021, 11:46:00 amBecause I can't count on being able to find the precise modern replacement. In the meantime, I bought 20 of these parts and I have a line on many more. So, plan B is find an adapter. Plan C is just redesign the board and have new PCBs made.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2021, 01:43:24 pm
content: Quote from: Sir Knight on August 05, 2021, 11:38:39 amAh, so the next most likely candidate is IPAK or TO-220, which may still require lead forming, or likely doesn't fit at all (IPAK pins are wider than TO-92, and TO-220AB larger still; TO-220 by the way is so old and generic that it can be made with round wire leads, but good luck finding those!).Or an SMT breakout board with header / edge grab style pins, though they might not fit into the pitch required, and that's a lot more work than a direct replacement.Though it might still be cheaper than a TO-39 MOSFET, there's not a lot of those I think, outside of maintenance supply and surplus......Actually, that does really narrow it down, doesn't it, Digi-Key only stocks VP2206N2, 2N6660, 2N6661, VN2210N2, and a few others listed but not stocked by DK themselves, or obsolete. And yeah, literally all of them would be cheaper as a SMT jellybean breakout board, oh well.(Not to impugn your sense of professional style, mind -- I suppose it would be more professional to redesign the thing altogether, than to stuff in a breakout board. The breakout might not be possible/feasible anyway, depending on if and how the TO-39 is heatsinked. And redesign might not be possible for reason of historical value, or various approvals and tracking (e.g. mil/aerospace).)Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2021, 01:52:48 pm
content: About the spacers: the only thing I could find in a quick Google search washttp://www.mecint.co.uk/Catalogue/10The brand name "Transipad" in that catalog page rings a bell (I last used such things over 40 years ago).Googling Transipad came up with thishttp://microdimensional.com/md/mountingpad/asp/mpadmain.aspx

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2021, 02:02:56 pm
content: Wow... I know people gripe about mainstream manufacturers having awful websites, but man... I haven't seen a website so old-fashioned, and so facially unhelpful, in a long time!Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 05, 2021, 03:28:12 pm
content: Yeah, I didn't bother to penetrate the website--I don't know if they are even available now. I was just happy to remember the brand name.What I remember from my youth is a promotional folder (card-stock weight) with a sticky surface that held samples of a wide range of product: mainly simple spacers, but some that adapted the footprint. At the time, the "Transipad" parts were available from Newark and other normal vendors.

author: David Hess
author_rank: Super Contributor
timestamp: August 05, 2021, 03:37:59 pm
content: Quote from: Sir Knight on August 05, 2021, 11:38:39 amMy solution has been to slightly modify the circuit to use a pair or several TO-92 transistors in parallel in place of a single TO-39 part. The modification comes down to adding the minimum amount of emitter ballasting resistance.Even in designs from the 1970s, Tektronix sometimes did this when a single transistor with the right combination of performance and power was not available or perhaps too expensive.Of course this only goes so far with TO-92 parts because their variety has significantly decreased as well.

author: Sir Knight
author_rank: Contributor
timestamp: August 06, 2021, 02:19:53 pm
content: My apologies to everyone for the late reply, but for some reason my post-reply notifications have turned off here.Quote from: T3sl4co1l on August 05, 2021, 01:43:24 pmForgive my rookie brain, but what is an SMT Jellybean breakout board? I found stuff about SMT breakouts but not with the word jellybean in there. The breakout concept did give me an idea though - I had some corrugated plastic sheet here so I cut it up to make a buck for a drop-in board. There are two major areas of modification on my prototype, and I could move all the mod parts to a riser board, then make that board pin/location compatible with the original board. It would make modifications very simple: instead of doing a piece by piece replacement I'd de-solder the original parts, press the mod board into the original, and then flow all the new leads at one try.The parts obsolescence isn't an issue right now, I have sources for a lot of components that typically aren't available. Thats how I came upon the supply of my TO-39 MOSFET.

author: Sir Knight
author_rank: Contributor
timestamp: August 06, 2021, 02:25:14 pm
content: Quote from: TimFox on August 05, 2021, 01:52:48 pmAccording to those docs what I'm looking for is actually a "Type 9 lead conversion pad". The pot of gold has moved closer, thanks to you!

author: Sir Knight
author_rank: Contributor
timestamp: August 06, 2021, 02:28:06 pm
content: Quote from: edavid on August 05, 2021, 03:37:22 pmRight now I don't know enough about this circuit to do that kind of conversion. You could say "just compare the data sheets on the raw devices", but I need to get the prototype working with the vintage parts so I can check various test points and make sure I know what the part is supposed to do in the circuit, not just on paper.There's a few other parts in there that will get replaced, one of them is a device that is so rare I can only find them a couple of pieces at a time, unless I want to risk money on potential off-shore counterfeits. In the meantime I still have a great supply of these TO-39 MOSFETS.

author: Sir Knight
author_rank: Contributor
timestamp: August 06, 2021, 02:42:13 pm
content: Quote from: David Hess on August 05, 2021, 03:37:59 pmI don't know enough about that technique to know if it would change the sound of this amp noticeably. Maybe I could mess around with that down the road when I'm working on parts substitutions, so thank you for the advice.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 06, 2021, 02:55:25 pm
content: Quote from: Sir Knight on August 06, 2021, 02:19:53 pmOh, jellybean goes with the SMT part; it's likely you can find a closer equivalent in SMTs, there are a LOT available. Then just a regular breakout board for it.Tim

author: TimFox
author_rank: Super Contributor
timestamp: August 06, 2021, 03:35:17 pm
content: Quote from: Sir Knight on August 06, 2021, 02:25:14 pmHave you found a retail vendor that carries "Transipads"?

author: Sir Knight
author_rank: Contributor
timestamp: August 06, 2021, 04:55:25 pm
content: Quote from: TimFox on August 06, 2021, 03:35:17 pmI have not. The search continues.

author: fuzzoli
author_rank: Regular Contributor
timestamp: August 07, 2021, 12:55:59 am
content: I found a place in Pennsylvania. Any interest? I know the "owner".

author: David Hess
author_rank: Super Contributor
timestamp: August 07, 2021, 01:28:17 am
content: These days a lead support could be 3D printed.My solution for lead forming usually involves 0.1" punchboard and needle nose pliers. I insert the leads through the punchboard and press down while straightening them on the other side as needed. They make special pliers with a wide smooth jaw (flat nose?) for exactly this sort of thing.

author: Sir Knight
author_rank: Contributor
timestamp: August 07, 2021, 10:11:32 am
content: Quote from: fuzzoli on August 07, 2021, 12:55:59 amQuite the collection you have there. Do any of those do what I need?Quote from: David Hess on August 07, 2021, 01:28:17 amTrue. And if I strike out elsewhere I may do that. A friend of mine has a 3D printer and uses it to make washers and other plastic thingies for his products. I could probably convince him to make a run of parts for me if I provide the file.Quote from: David Hess on August 07, 2021, 01:28:17 amLineman's pliers, perhaps?I'd rather stay away from anything that would require me to break out the pliers. I have less than perfect control of my application of force on occasion, and leads are pretty delicate things. I forget what I'm working with during the heat of the moment, so I'm that guy who overtightens aluminum and brass hardware because "another 1/4 turn? Why not?!"

author: David Hess
author_rank: Super Contributor
timestamp: August 08, 2021, 02:12:00 am
content: Quote from: Sir Knight on August 07, 2021, 10:11:32 amNo, they look like wider needle nose pliers and often have smooth jaws instead of teeth as shown below. They are good for bending and straightening leads and wire. They remind me of tiny sheet metal pliers. Of course common needle nose pliers work just fine also.QuoteWhen the leads gets pushed through the punchboard, then tend to come out at odd angle on the other side. The pliers are just for straightening them so that they can be more easily inserted into the printed circuit board and soldered.

author: WattsThat
author_rank: Frequent Contributor
timestamp: August 08, 2021, 02:55:42 am
content: The flat jawed pliers are sometimes referred to as as Duckbills. Xcelite and others make the usual miniature styles for electronics. Xuron makes some inexpensive flat jaw pliers that are great for lead bending. Their model 450 is nice for fine pitch work.The spacers that fuzzoli posted: The brand name that comes to mind is Bivar. They’re still in business but no longer making spacers for metal can transistors. I’m shocked I tell you, shocked.

author: Shock
author_rank: Super Contributor
timestamp: August 08, 2021, 03:48:40 am
content: If you have space and length, you could drill a hole in the pcb and mount the leg into the through hole backwards.You could use silicon tube or a ring over the transistor can and side mount it. Would look professional and deliberate and you can always insulate the legs if required.You could also lead form and use some silicon tube and or mold epoxy or silicon to make a mount. Lead forming with tools you will barely notice anyway. The trick is to make it look deliberate, so even something like an s bend looks way better than a breakout or adapter board.

author: Sir Knight
author_rank: Contributor
timestamp: August 13, 2021, 06:03:51 pm
content: Quote from: Shock on August 08, 2021, 03:48:40 amAll good ideas. After doing a lot of board modifications I've gotten over my trepidation at drilling PCBs, so I'll start with that and see where it goes. I've found the brass-colored Dewalt drill bits seem to cut through the typical PCB and trace without complication, leaving a perfect hole in many cases.