Another Qb Refurb Job

Spitfirejoe

Cheap Audio Aficionado
I was moving my Qb's around the other day, and heard a "rattle" inside. It sounded like something came loose on the crossover. Since we are in a Winter Weather Advisory here in Eastern Nebraska, and since I had finished my other indoor project, I decided that was the signal I needed to tear into these and give them a refurbishment. I knew the capacitors should be replaced by now, but hadn't had anything to give me the nudge to get started.

A little history - these were a real financial splurge for me back in 1978. I bought them new, with the bent metal stands, after my wife and I were married. I've always loved the sound of the Qb - very "musical", very well balanced, crystalline high notes and a bottom end that didn't get muddy. I've babied them, but the only modification I ever did was to use a 1 amp fuse on the emit tweeter circuit instead of the 1.25 specified. I've blown the fuses a few times, but never damaged an emit - knock on (vinyl veneered) wood...

So - on to the disassembly;

Drivers came out fine, look good. Tweeter coil came unglued from the crossover panel, in fact everything that wasn't firmly screwed or bolted to the crossover panel was loose. Not surprising after almost 39 years. The plastic covering on the outside of the crossover panel was torn up during it's removal from the cabinet. It seems like it was just clear contact paper or something, as it peeled right off with little effort.

2Qb Disassembly 2.JPG 4Qb Crossover 2.JPG View attachment 6761667Qb Crossover 5.JPG

Another installment follows this one.
 
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I've always wondered why Infinity didn't ""Mirror Image" the Emit tweeters on the Qb. I suppose it was a simple matter of logistics - too much trouble to make matched "sets" of speakers. Anyway - after seeing Elnaldo's Qb work, I decided I could do this task.

I took the front baffle out of the cabinet (more on this in another installment) and then proceeded to cut an inset hole for the Emit. I measured the location of the original and just moved it to the other side. I scored the edges with a sharp utility knife, then went to work with a sharp 1 inch wood chisel. Within a half hour, I had a square hole with the correct dimensions to fit the Emit.

I used three layers of 1/8 inch thick craft wood to partially fill the old hole. Each layer was glued in place and then I poured some extra wood glue around the edges to fill some of the space.

9 Qb Mirror 1.JPG 12 Qb Mirror 4.JPG 13Qb Mirror 5.JPG
Sometime next week, I will fill the remaining space up with wood filler. Then I'll sand the entire front of the baffle and spray it with flat black paint.
 
So, about the cabinets...

I had read everyone's recommendation to re-seal the joints in the cabinet. When I opened mine up, everything seemed tight and well sealed except the front baffle. I did apply a bead of wood glue all around the inside edges of the cabinet. There was one slightly loose area that would rattle a bit if I hit it with a hammer from the inside. I didn't take the time to figure out how the baffle was actually installed, as I should have. I just went ahead and started whacking at it from the inside of the cabinet with the hammer. It started to come loose pretty easily, so I kept going.

Not my finest moment.

The baffle was cut with a tongue all around it's circumference, and this fit into a matching groove in the sides of the cabinet. The groove broke loose all around the cabinet and actually was only being held on by the wood grain vinyl on the outside. Taking the baffle out completely resulted in a couple bad breaks and a tear in the vinyl on two sides - and of course near the top where the damage will be most visible. The other speaker will simply get the bead of wood glue around all the cabinet joints.

Photos posted to show others how to really screw up what should have been a simple job.

15 Qb Cab 1.JPG 16 Qb Cab 2.JPG
17Qb Cab 3.JPG18Qb Cab 4.JPG
The damage actually looks far worse than it is in reality. The baffle should slip back in place pretty easily, and wood glue should repair the breaks. I have long clamps that I can use to apply even pressure all around the edges of the cabinet. With care, I should be able to minimize the visibility of the damage I've done.
 
Crossovers -

I've taken everything off the crossover plate, and have ordered replacement capacitors from Parts Express. I also ordered some better binding posts and a bag of polyfill to add to the interior of the cabinets. I couldn't find film caps to replace the tweeter cap, so just went with Dayton Audio Metallized Polypropylene units that had a 1% tolerance rating. I'm hoping these will be adequate to my purposes - and am willing to listen to anyone who has a different opinion. They weren't so expensive that I wouldn't order something different if advised to do so.

On that note - is there anything else in here I should consider replacing? Please speak up!

Some photos for posterity...

5Qb Crossover 3.JPG 6Qb Crossover 4.JPG
 
The main change there, would be to replace the small electrolytic (12.5uF for the mid-range) with a fresh capacitor. It will be a Nigh-Day difference with the stock capacitor.

The tweeter capacitor is already a film cap, I can't tell you if you will hear any difference replacing it. I've replaced it anyway. The dayton you mention is a film capacitor (polyester and polypropylene film are "film capacitors")

I've replaced the old resistor too. Used those green resistors, mundorf I think.

The coil you mentioned loosen is the mid-range low pass. you can use hot glue to hold it. don't use a bolt or a metallic core (screw), since it's an air core inductor. inserting a metal core will change it's performance in some way. glue as far as you can from the larger woofer inductor.

To re-install the baffles, I suggest to glue some 1/2" X 1/2" wood pieces on the inner frame, so the baffle can rest on them, and hold more glue. That's the way the woodworker who made mine did it. An inner frame all around the cabinet.
 
Ahh.. shows you what I know about capacitors! I understand now that what I bought ARE "film" type.

Good idea on the "inner frame". I can do that easily, and it should let me do a better job replacing that front baffle properly. I have bought some 1 inch wood dowels that will be cut to length and installed between the front and rear of the cabinets to provide some other reinforcement. I think I'm going to glue a 1 x 2 piece of wood across the back where the screws from the metal stands enter the cabinet to reinforce that area. These cabinets are pretty heavy and it bothered me to have three screws through MDF "wood" holding everything up.

I did understand not to run a bolt through the air core inductor coil, but didn't recall from the schematic I downloaded that this was in the mid-range circuit, not the tweeter line. I had planned to hot glue all the loose pieces into place. Will screw the crossover mounting plate back into place with some sealant between it and the cabinet.

Thanks for the information, and for the original inspiration! I really appreciated chef free's information on Qb refurbishments as well.
 
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could i suggest using a heat gun to peel off the wood grain vinyl? you can then prime and paint the cabs, and wind up with a much more modern looking speaker. i did mine a few years ago, and was happy with the result
 
http://elektrotanya.com/PREVIEWS/63463243/23432455/infinity/infinity_qb.pdf_1.png

The mid-range crossover is the 12.5uF small cap and the 1.0mH air core inductor.

My speakers came without the emit attenuation... Just a pot for the mid-range, what I set at mid position, or just a bit more.

Re-finishing the cabinet of course is a plus, but a lot of work I think... There are more modern vinyl finishings, but I don't know how to glue those new self-adhesive vinyl to the particle board.
 
I had that exact same rattle and it motivated me to open my Qb's up for the first time too!

Here's my advise:
Do not change out the yellow film caps, they are part of the Infinity sound.

Clean the pots thoroughly with some DeOxit and lube them with FaderLube.

I added about 30% more stuffing to tame their 100Hz "thump", you might want to play around with that.
 
Thanks for the input everyone. I finished the first application of wood filler today, and did a little sanding. I have some low spots - as I expected I would - so will have another layer to apply. They should come out looking like they were never modified, which is what I'm shooting for.

Shrinkboy - if the damage I've done is too visible when I'm finished, I will probably peel off the vinyl. Thanks for the tip on the heat gun - that should make it easy. If it comes to that, I'll probably have a finish carpenter friend do a complete re-veneer with some nice wood. I just hope it doesn't come to that, as the original vinyl looks really good with the exception of the damage I just caused.

Chef - I've seen you recommend that the yellow film cap not be removed before. I thought I had also seen someone say that they didn't change this one at first, and then later decided it was necessary. My knowledge in this area is severely lacking, so may I ask what is it about these caps that make them special? Do they not degrade with time like the others? I can understand the drying out of oil filled caps, and I guess I could understand that a different type might not do that. It would be great to skip replacing that cap if it isn't necessary, and especially so if it has some effect on the "Infinity Sound" that we love. I'm definately going to add some more stuffing. I don't know how I would "play around" with that. I haven't invested in a meter or anything to measure speaker output. I might consider that sometime.I will also clean and lube the pots. I have to get some fader lube, and have found that cleaning such things without following up with fader lube is ineffective.
 
Those yellow tweeter caps are film caps which have a reputation for lasting for many years, unlike electrolytic caps,like that silver 12.5 uF, which are the kind which dry out. . I changed mine out for a pair of correct value polypropylene caps I had laying around. I immediately regretted it and switched them back two days later.

I just tried different amounts of stuffing until the speakers sounded a little less boomy. All by ear, I don't have any fancy test equipment nor the skill to use it!

Good luck on restoring these fine speakers, they were my first step up into "real" Hi-Fi back in 1978 too! Man, a $400 for a pair of speakers on a $2.65/hr restaurant job was a stretch!
 
I've replaced both capacitors, including the yellow tweeter cap, and did an A-B comparison, and the main change was in the mid-range, vocals specially, very evident. I can't tell the tweeter sounds better, but definetly not worst, they sound amazing with the new caps, I've used Clarity caps PX, the blue ones. I have the original caps laying around for future projects. But technically, the original capacitors should be still fine, they are made of a metallized film, wrapped and sealed, nothing that should "age" in a bad way if moisture don't get into the sealed film. (crackled ends could allow moisture to get into the capacitor, that's not good)

For the 12.5uF, if you see my thread, I've used a large motor run capacitor, 12.5uF 450V, with a dayton 0.01 by-pass (just in case...I neither have fancy equipment to measure if that makes a dfference)

I never added the poly fill you always suggest, I'm a bit lazy to open the cabinets again !! But for sure when I buy some fill for other speaker project, I'll get some for mine.
 
Good idea for making them mirror imaged pairs. I don't know if my wood skills are up to that but something to consider. I was wondering if you couldn't mark it out and use something like a dremel tool to make the hole and put less stress on the cabinet that way. I've got mine resealed but I've been so busy with other home fixes that the crossover update hasn't made it to the top yet. I have some RSb's as well I need to update the crossovers on. Then do a comparison test. The mids on the Qb's without the updated crossover still sound pretty darn good to me as is!
 
Making progress, but I'm slow on this project - trying to make sure I do it "right the first time".

The first crossover is rebuilt and ready to install now. I have some questions, and I'm starting a new topic on that as it seems to be a basic electronics question and not specifically related to this thread. Just trying to figure out why the negative input lead goes to the positive terminal on the mid and tweeter, but the negative on the woofer.

The "mirrored tweeter" part of the project came out nicely. Njoy Music- this didn't take much woodworking skill. I just measured the spot, scored it with a utility knife, and then used light taps with a hammer on a wood chisel to remove the unwanted particle board. I took out a little at a time until the tweeter fit properly.

Some pics...
16 Qb Mirror 7.JPG 18 Qb Mirror 9.JPG 19 Qb Mirror 10.JPG
 
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The cabinets actually have become a lot of work because of my ham-handed disassembly process. I should never have taken the front baffle out. The tweeter relocation could have been done with the baffle in place and would have been a LOT less work. Having it out of the cabinet allowed me to add some bracing, but I damaged the vinyl veneer quite a bit. It came back together nicely, and the damage is both minor and in a "less than visible" area behind the speaker grille cover. If I didn't show it to you, it might go un-noticed. I will always know it's there, however.

The interior edges have all been resealed with a good bead of wood glue. The new bracing was glued and screwed to the cabinet, and the baffle plate was drilled with a counter sink bit to allow the black drywall screws to sit below the surface of the wood. I haven't decided for sure yet, but I think I'm going to fill the counter sunk screw holes for the new brace, then sand them down and repaint. The cabinet would look almost identical to the other one that way.24 Qb Cab 5.JPG 26 Qb Cab 7.JPG 27 Qb Cab 8.JPG

I've cut a 1 inch dowel to length and will install it behind the woofer. The top most screw attaching the woofer is being replaced with a longer one and the dowel has been pre-drilled to accept the screw. It is also going to be screwed in from the back side, with a black drywall screw like I used in the side braces. I'll probably go ahead and counter sink the head, fill the hole, sand, and repaint the back side.
 
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The first crossover has been completed (I think... Please see my new thread on Qb Crossovers)

New 1% Dayton Audio Cabs replaced the 12.7 uf cap. The yellow 6.8 cap stayed installaed. New binding posts were installed to replace the old spring clip models. The pots were opened up, sprayed with DeOxit Cleaner and exercised. After drying, the got blown out with canned air and I hit them with a spritz of DeOxit Fader Lube. Six self tapping screws will hold the crossover in place and I'll use a bead of silicone sealant between the plate and the back of the cabinet. Everything has been securely hot glued in place. All new parts came from Parts Express.29 Qb Crossover Done 2.JPG
30 Qb Crossover Done 3.JPG
 
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Would you mind letting us know what replacement binding posts you used? I've been wanting to add this to my list of things to do when I get a chance to redo the crossovers and I'd like to get everything at once. I hate those spring clip terminals! Thanks!
 
Here's how they are listed: Sure Electronics AA-AA11151 Mini Gold Plated Insulated 5-Way Binding Post Set 2 Red 2 Black. Part Number is 320-3375. Model Number is AA-AA11151

I like these because they give me three options - twisted bare wire through a hole in the post, the U shaped spade lugs, or banana jacks. I've never used banana jacks, but am thinking about trying them.

I had to drill out the main hole just a bit bigger and then the jack slipped right in place. I used some black silicone RTV sealant to seal up the other two small "slits" below the main hole.
 
Thanks for the info! Like Elnaldo I don't like cracking things open a lot so being able to do this all at one time makes a lot more sense. I think I'll see about getting new resistors too while I'm there just in case. I'm anxious to get these up to snuff and back in the rotation.
 
I've always wondered why Infinity didn't ""Mirror Image" the Emit tweeters on the Qb. I suppose it was a simple matter of logistics - too much trouble to make matched "sets" of speakers. Anyway - after seeing Elnaldo's Qb work, I decided I could do this task.

I took the front baffle out of the cabinet (more on this in another installment) and then proceeded to cut an inset hole for the Emit. I measured the location of the original and just moved it to the other side. I scored the edges with a sharp utility knife, then went to work with a sharp 1 inch wood chisel. Within a half hour, I had a square hole with the correct dimensions to fit the Emit.

I used three layers of 1/8 inch thick craft wood to partially fill the old hole. Each layer was glued in place and then I poured some extra wood glue around the edges to fill some of the space.

View attachment 676195 View attachment 676196 View attachment 676197
Sometime next week, I will fill the remaining space up with wood filler. Then I'll sand the entire front of the baffle and spray it with flat black paint.

Could you just flip the front baffle over and then router the edge of the woofer and mid holes to recess the drivers, thus moving the tweet to the other side, or would it make the ledge of the recess too thin?
 
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