a monitor IIa project begins

[Coastsider]

Red indicates positive. In addition to red paint or ink on the terminal, you will also find on some drivers in Infinity speakers a red paint or ink dot somewhere in the vicinity of the terminal and sometimes even an office supply type of red stick on dot to indicate polarity.

 

[loquatious]

thanks for clarifying the polarity of the terminals. that dab of red paint seemed kind of tentative, and i didn't know if it could be trusted. here is a progress report:


the serial numbers on my IIa's are 5000342 and 5000343. i'm doubtful infinity made 5 million column IIa's, and i'm wondering if maybe i actually have production units 342 and 343. at any rate, for identification purposes i'll refer to them as 342 or 343. those of you with IIa's, i'd be interested in what serial numbers you have on yours.


this project is going to happen in two phases. this first phase is simply to get them working and presentable. i've ordered caps, but since there are no electrolytic caps in the crossover, i decided i'm going to hold off on recapping until i have done some listening in stock form.


i've been looking at a lot of foam to top off the walshes, and have found one that meets most of the criteria, though still not all. i'll post exhaustive detail on the foam search soon in the big walsh tweeter thread, where it really belongs.

http://audiokarma.org/forums/showthread.php?p=6837229#post6837229

in the meantime, a pic of what i ended up with is attached. i think they look darned good just sitting on the bench.


i've been addressing some cabinet issues. the thirsty old veneer has been sucking up coat after coat of howard's with good result.

the bases on my speakers are crude compared to the rest of the cabinet. they used a frame of 1" X 1.5" particle board covered with thin black vinyl (neither of particularly good quality). the miters are very sloppy (too sloppy to glue), as is the installation (nowhere near square), and the bases individual pieces are held to the cabinet with screws.

speaker 343 had cosmetic damage to the base. the combination of vacuum cleaner dings and damage to the particle board edge and corners from a previous owner "walking" the heavy speakers around made them require attention. my temporary solution is to simply invert the base and rotate it 180º. it presents a better face now. i'm going to add plastic feet like all my other infinity towers of similar size came with. this will make moving them around more forgiving to the fragile particle board, but may decouple the speakers from the floor a bit which could affect the bass output slightly. with the inverted bases, they look ok now, but i think (when i get to phase two of the restoration) i will make duplicate bases out of hard maple, and cover them in black tolex. that will keep the original look but be much more substantial than the originals.

on speaker 342, the walnut piece at the base of the rear grill (with the cut-out for the speaker wire connections) had fallen off. this seems to be a common problem, as i've seen numerous photos of this issue before. the front and back of the cabinets are covered in black vinyl, then these walnut-skinned pieces are glued over the vinyl. the glue just lets go at some point. this piece simply got re-glued with hot glue.


while looking into replacement woofer options, i found some interesting things i thought i'd post. there seem to be more possible woofer replacements out there than i'd expected.

tubed said his originals tested at 3.9 ohms. while doing research, i was looking at pictures of original double-doped woofers, and found examples where the backs of the tar-doped paper cones had 776902 printed on them.

the woofers i sourced from early quantum jr.'s appear to be the same woofer with a different treatment applied. they have manufacturer numbers on the magnets i've never seen before, but seem to indicate late 1978 manufacturing dates. these both read 4.1 ohms on my multimeter. the baskets on these are the same as the double-doped originals, and the magnets appear to be similar as well. both use that same 776902 paper cone, but these early quantum jr. woofers are painted with a transparent stiffening agent instead of tar-type dope, and don't add painted-on mass over the voice coil. a larger 4.5" dustcap is fitted. perhaps the oversized dustcap is adding mass? this paper-coned version of the quantum jr. woofer seems to be capable of the 1" excursion claimed for the double-doped woofer.

the quantum jr. woofer seemed to go through a lot of evolution during it's 78 - '83 production run. the factory brochure shows a double-doped woofer, but i think that may have been a prototype in the photo. i've yet to see a production unit with the tar-doped woofer. the early production units seem to have used the treated paper cone version i've got. this was later supplanted by the 902-0018 gray poly cone iteration, finally there was the 902-1187 clear poly cone version, which is the recommended replacement for the monitor IIa (listed on the HK data sheet).

i'm seeing another 12" infinity woofer that looks just like the gray poly 902-0018, but it has a 490702 part number. ohm-wise it seem to be in the ballpark too, (photos showed it testing around 3.6 ohms), and is said to be sourced from the infinity 5000, reference studio monitor, and others.

i also found an old forum (non-AK) that had specifically mentioned another out-of-production woofer called the swan 305, made by eminence, as a drop-in replacement for a monitor IIa woofer.

thats where i'm at right now. reassembly should happen this week, and giving them their first listen is not far off.




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[Coastsider]

the serial numbers on my IIa's are 5000342 and 5000343. i'm doubtful infinity made 5 million column IIa's, and i'm wondering if maybe i actually have production units 342 and 343. at any rate, for identification purposes i'll refer to them as 342 or 343. those of you with IIa's, i'd be interested in what serial numbers you have on yours.

I have the original sales invoice for my Monitor IIa's from Pacific Stereo in Los Angeles dated 3/15/76 in the amount of $720.00 (before sales tax) and it states that the serial numbers of the speakers are S001743 and S001810. These were obviously recorded inaccurately as a quick check of the speakers shows them to actually be 5001743 and 5001810 matching the numbering convention on your IIa's.

The Monitor IIa's I picked up a couple of years ago that are currently incomplete and in my garage are serial numbers 5002165 and 5002168 and as I suspect that the leading 5 is simply a model identifier, would indicate that at a minimum more than 1000 pair were produced.

 

[tubed]

5005048/5005049 here.

 

[loquatious]

thanks for posting serial numbers. so, that makes it seem pretty conclusive that production of the monitor IIa started with serial number 5000001, and mine are indeed very early production units.

my IIa's came from georgia, where they were originally purchased. i'm kind of surprised the serial numbers are so low and are sequential. i'd have thought early production units would have likely been disseminated in california, where they were built.

i'm always curious how some pairs of speakers manage to have sequential serial numbers, given all the warehousing and moving from place to place they must have gone through before finally going to the consumer. it's kind of amazing to think my IIa's were built on the same day, and have been together ever since.

i'd think a pair with sequential numbers would be pretty rare, yet tubed has some as well, so maybe not. i'd think coastsider's examples would be more typical, with a slight gap in production numbers.

just for comparison, production of the column II (also from the mid 70s) started with serial number 3000001. i got my my column II's from their original owner. they spent their whole life here in socal, and have serial numbers 3012412 and 3012567, 155 units apart. this is enough of a gap that production changes to the crossovers occurred between the two. i know of another column II owner who's original "matched pair" have the old infinity logo on one, and the new logo on the other! kind of disturbing from an owner's point of view, but pretty interesting from a historian's point of view...



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[Coastsider]

my IIa's came from georgia, where they were originally purchased. i'm kind of surprised the serial numbers are so low and are sequential. i'd have thought early production units would have likely been disseminated in california, where they were built.

i'm always curious how some pairs of speakers manage to have sequential serial numbers, given all the warehousing and moving from place to place they must have gone through before finally going to the consumer. it's kind of amazing to think my IIa's were built on the same day, and have been together ever since.

i'd think a pair with sequential numbers would be pretty rare, yet tubed has some as well, so maybe not. i'd think coastsider's examples would be more typical, with a slight gap in production numbers.

My take on this serial number conundrum is that the likelihood of getting a sequentially numbered pair would have been increased in a smaller market with smaller dealers since it was likely that Infinity did fill a dealers order with a block of sequentially numbered speakers. In my case, having got mine at Pacific Stereo in Los Angeles, a large chain of stores in a large market, the likelihood of sequentially numbered speakers remaining together once they hit the chain's distribution center and were broken out to ship to the individual stores would be reduced significantly, especially if the guys loading the trucks were not given any specific instructions beyond just accurately recording the serial numbers off of the boxes as they were loaded. If by some chance sequentially numbered speakers actually managed to make it to a store it would then be up to the local staff to try to keep them together as they went out of the door with customers, but again in a large volume location that might not always happen.

It's just my theory but in 1976 when I bought mine at the Los Angeles store which I am assuming was downtown where I was working at the time, Pacific Stereo, as has been documented in other AK threads, had a lot of stores in Southern California and was moving huge amounts of gear.

 

[loquatious]

quick update:

cabinets are done, drivers are in. phase one is officially complete.

i got to listen to the monitor IIa's for the first time today. just a couple hours in the garage and only pushing them with 65 watts, but i have to say i'm quite happy with what i heard. boatloads of bass. even though i'm running the stand-in woofers, i'll be surprised if the originals can dig much deeper than these 1st generation quantum jr. woofers do. i'll keep looking for originals though. pretty impressive mids and highs as well.

haven't been able to A/B them yet, but my first impression is that the monitor IIa's may trounce my column II's. both models were available concurrently and its hard for me to believe that the monitors were substantially less expensive than the columns, especially given how they sound. of course, you could snort lines off your column II's (they have glass tops), maybe infinity figured that feature was worth the extra dough.

given the missing woofers, and the walshes and cabinets needing attention, i figured just getting these monitors presentable and functioning would be a fairly long term project. never expected i'd be ready to move them into the house less than 30 days from purchase. sometimes things just fall into place. a worthwhile endeavor, and paying off sooner than expected. nothing wrong with that at all. thanks to all who helped make it happen.

i'll update again after i have some hours (weeks) on them, and post pics of them once they're in the house.



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[Coastsider]

Good to hear you are making steady progress with your Monitor IIa's

Both the Column II's and the WTLC (Wave Transmission Line Columns) were strange anomalies in the 70's Infinity line, for the most part very different from any of the other 70's models given their twin woofers, one front and one down firing, and two tweeters, one front and one rear firing with paper cones in the WTLC's and piezo's in the Column II's, and of course the addition of the Walsh tweeter from the Monitor series added to the WTLC's. So both "column" speakers along with the other models of that same period, the 2000II's, 2000AXT's, the 1001/1001A's and some even more obscure models like the POS's make it appear as if Infinity was trying to fill every conceivable niche and price point that the market might demand. I believe that the Monitor IIa's and 2000II's are at the top of the heap given that their driver compliment is almost identical, with the Monitor's tower form factor giving it a small advantage even though I think the 2000II's cone midrange driver is superior to the Monitor's dome, but I would leave it up to anyone else familiar with most or all of the available models of that period to score the rest of them in some sort of order.

All in all an interesting time in the history of Infinity during which they produced some unique products.

 

[curtpenn]

here is a project update.

i've been on a quest for a suitable foam to top off the walsh tweeters. its proving an interesting challenge. a lot of criteria to meet. the original foam was 1/2" thick. the foam needs to be an open cell type, so it is permeable to air. it needs to be flexible enough to conform to a dome shape, yet resilient enough to hold the cone in place. it needs to be open enough to be acoustically transparent, but dense enough to damp the waves at the top of the cone. a lot of variables, but there is a lot of foam out there to choose from. i've been looking at quite a few different specialty foams but have, so far, rejected all of them for one shortcoming or another. i'll keep looking, and when i find one that best fits the bill, i'll pass the info along.

in the meantime, i sourced some 12" woofers to use as placeholders while i search for some doped originals. the stand-ins, which i bought loose, ostensibly came from some quantum jr's., which many say are suitable donors. the tech sheet for the quantum jr. list 902-1187 as the part number for the woofer, the same number listed as a replacement part for the monitor IIa woofer.

as usual, there's an anomaly. they don't have numbers that i've seen before. not the 902-0018 or 902-1187 numbers usually i see associated with quantum jr. woofers. but they do look like the woofers i see in most photos of production quantum jr's.

these have paper cones that are treated with something (clear acrylic?) on the front side of the cone as a stiffening agent, and have big 4.5" dust caps. though cosmetically identical to each other, one magnet has 95141078 stamped on it, the other has 45141278, and both paper cones have 776902 printed on the untreated side.

they do read 4.1ohms on the multimeter, which is pretty close to the 3.9 reading that tubed got from his doped monitor IIa woofers. these look very much like slightly bigger versions of the woofers in my column IIs. certainly they come from the same manufacturer. they fit the openings in the monitor IIa perfectly and the mounting holes line up.

the terminals are not marked + or –, but one is dabbed with red ink. i've seen this before on photos of other infinity drivers. can someone verify whether the red marked terminal would indicate the positive or the negative terminal of the woofer?





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I had acquired some spare backup 12" Watkins for my Quantum 2s some years ago. Just noticed one has a different sized magnet and is stamped 5142777. I have two others stamped 55140576 and 95141078 like the one you mention. Was surprised to see the differences.