My 7C upgrade - new tweeters, caps, resistors, sound damping

jrogers13

RD-0194 tweeters, Audyn Q4 33 and 12 uF caps, 2.5 ohm Mills resistors, poly switch removed. These speakers are from 1985. I think I got them from a yard sale years ago and they have been seldom used since. They sounded pretty good but I decided to mess with them anyway. First the easy tweeter swap to the RD-0194. Big difference noticed on vocals and dialog, as good or better than my new Klipsch center speaker, which I immediately returned (getting new Polk center). Than the caps. Not wanting to spring for sonicaps, and finding limited availability, I went with Audyn from Parts Express (~ $15 for 33uF). Mills 2.5 ohm resistors from Parts Connexion (sold out at PE). Then 1/2 inch sound damping foam from PE. So, now to the soldering. Being a novice, after practicing on a scrap circuit board and taking the board off of the cross-over, I was pretty sure I had a good chance of **** it up. After some googling, I found some easy techniques for soldering new caps and resistors to the old leads. I snipped off the leads at the old caps, made tight pigtails on the new leads by wrapping them around a toothpick, put the pigtail around the old lead and soldered. Made good connections, so easy. Poly switch leads were long, so I cut out the switch and soldered the leads together to make a jumper. Used pigtail method for the resistors too - leads on the old ones were short but adequate. NOT necessarily advising this method, I know it’s a bit of a hack, but it was so easy (no desoldering) and seems to have worked fine for me. Then I lined the whole inside of the speaker boxes with the 1/2 inch adhesive-backed dampening foam. The new caps, resistors and foam improved the clarity of the speakers, but the difference was not as great as the new tweeters. This might raise some discussion…
Maybe next time I’ll do the desoldering after more practice.

jrogers13

The 37 year old electrolytic caps both tested at 39.6 uF after removal and the old 2.0 resistors both measured 2.2 ohms.

heiney9

Welcome to the forum,

How long of a break in have you given them? New parts will take time to settle in. About 100 hours or so and then you should evaluate the new sound.

Good luck
H9

Gardenstater

My 1979 7Bs have a 4.5 Ohm in series with Peerless tweeter and a 2.7 Ohm in parallel. Your 7Cs only had a 2.5 Ohm resistor. Parallel or Series? Some will say that you should increase the series resistor value by a small amount (maybe 0.5Ohm ) to compensate for jumpering the polyswitch.

There are many here who seem to adamantly oppose damping foam in the passive radiator part of the enclosure. You used 1/2" which isn't that thick. I reserve judgement on anything that I haven't tried but you have tried it, so have you experienced any reduction in bass response performance?

Enjoy the mods and future ones

PS: I just saw your post above saying old resistors marked 2.0 Ohms and measured 2.2. Probably were 10% tolerance so at the high end.

PPS: Just saw your 39.6 uF measurements. That's interesting. I have read that electrolytics have a pretty high tolerance. Maybe Polk took the time to try to match them for each speaker build however.

Navy_Goat

jrogers13 wrote: »

RD-0194 tweeters,
Audyn Q4 33 and 12 uF caps
2.5 ohm Mills resistors
poly switch removed.

These speakers are from 1985. I think I got them from a yard sale years ago and they have been seldom used since. They sounded pretty good but I decided to mess with them anyway.

First the easy tweeter swap to the RD-0194. Big difference noticed on vocals and dialog, as good or better than my new Klipsch center speaker, which I immediately returned (getting new Polk center). Than the caps. Not wanting to spring for sonicaps, and finding limited availability, I went with Audyn from Parts Express (~ $15 for 33uF). Mills 2.5 ohm resistors from Parts Connexion (sold out at PE). Then 1/2 inch sound damping foam from PE. Then I lined the whole inside of the speaker boxes with the 1/2 inch adhesive-backed dampening foam.

So, now to the soldering. Being a novice, after practicing on a scrap circuit board and taking the board off of the cross-over, I was pretty sure I had a good chance of **** it up. After some googling, I found some easy techniques for soldering new caps and resistors to the old leads. I snipped off the leads at the old caps, made tight pigtails on the new leads by wrapping them around a toothpick, put the pigtail around the old lead and soldered. Made good connections, so easy. Poly switch leads were long, so I cut out the switch and soldered the leads together to make a jumper. Used pigtail method for the resistors too - leads on the old ones were short but adequate. NOT necessarily advising this method, I know it’s a bit of a hack, but it was so easy (no desoldering) and seems to have worked fine for me.

The new caps, resistors and foam improved the clarity of the speakers, but the difference was not as great as the new tweeters. This might raise some discussion…
Maybe next time I’ll do the desoldering after more practice.

Welcome! It is always great to hear about folks saving older PA speakers. I have heard great things about Monitor 7s, I would like to get my ears on a pair some day.

Regarding de-soldering, it can certainly be difficult. Sounds like you got to a "good enough solution for now, but I have some suggestions for next time. If you didn't have some solder wick check out the link below. Plenty of options:

solder wick

Old solder can be stubborn so some flux would also help. You will need to clean the circuit card up after you apply the flux so puck up some isopropyl alcohol at the same time.

flux for soldering

The last piece of advice I will offer is sometimes I find it helpful to add a little bit of new solder to help the older solder heat up. The trick with using wick to remove solder is A LOT of heat. Fortunately you are not reusing any of the old components so you only have to worry about burning up the PCB which is unlikely. Just keep an eye on the board and watch for discoloration.

Hope this all helps.

jrogers13

The 2.5 ohm was used to replace the 2 ohm to compensate for removing the poly switch. I don’t know if the foam near the radiator helps or hurts but thanks for the comment. I do have two small subwoofers (Pioneer SW8MK2s I found at the Restore for $25 each) that take care of the base the way I like it, no pounding. Sound great together with the 7Cs. Using an old Yamaha HTR 6090 receiver, I think it’s 125 WPC.

F1nut

You lined the entire cabinet with 1/2" damping material? Why the hell would you do something that stupid!?!

jrogers13

Ok, thanks, don’t hold back. What part, if any, would benefit from damping?

jrogers13

I got the idea for lining from this post, which seemed pretty good. It’s not exactly clear from this how much damping was used

https://silverfacestereo.com/2013/08/23/how-far-can-you-go-polk-monitor-7b-rebuild/

F1nut

A 4x4 inch square of Black Hole 5 or a 4x4 inch square of 1.25" thick Sonic Barrier from Parts Express behind the mid-driver only. That's it.

jrogers13

I removed the 1/2-inch dampening in the bottom and behind and around the passive radiator. I could not tell much of a difference between No lining, fully lined or half-lined, maybe because it is only 1/2 inch.
I’ve seen mention of dynamat on the forum. How is that used in these speakers?
I did find hurricane nuts at the hardware store, that will be my next adventure. Fortunately, all the gaskets look great and the boxes seem to be airtight.

Gardenstater

Well speaking as one who has a 7B, and has done the mod, I highly recommend @xschop 's semi rings over hurricane nuts. If you do the hurricane nuts and glue them in, it would be hard to do the semi rings. If you look at his discussions you'll find the information about them. I have done every mod one step at a time so I could savor the effects of each and Dynamat or other (Noico, BallisticShield etc.) is non negotiable in my opinion. There is not a single mod I have done that I am not thrilled with the results.

PS: Yeah not surprised really because 1/2" foam (what density?) will not absorb very much in the way of sub 60 Hz frequencies.

jrogers13

The lining is sonic barrier from parts express, open cell, I didn’t see a density listed. I read about Larry’s rings but not xchops semi rings. I’ll find out about that. And I’ll get some dynamat too. Did you do the MW and the radiator? THANKS!
Amazing how addicting this new hobby is.

Gardenstater

Yep. Upper picture is the MW and the lower pic is the PR. The semi rings are golden in the Monitor 7 in my opinion. The baffle is very wide and unbraced and benefits from stiffening. You'll love the bass slam you get after installation. It was very noticeable improvement. Even better than I got from Dynamat on the baskets.

F1nut

jrogers13 wrote: »

I removed the 1/2-inch dampening in the bottom and behind and around the passive radiator. I could not tell much of a difference between No lining, fully lined or half-lined, maybe because it is only 1/2 inch.
I’ve seen mention of dynamat on the forum. How is that used in these speakers?
I did find hurricane nuts at the hardware store, that will be my next adventure. Fortunately, all the gaskets look great and the boxes seem to be airtight.

Dynamat Xtreme goes on the basket of the mid-driver and passive radiator. All you need is a strip on the spokes and a bit on the flat disc area of the PR. There is no reason/benefit to cover every square inch.

Use cap screws with the hurricane nuts. There's a trick to installing the nuts. After enlarging the original holes slightly place the nut on the backside of the hole, then using a cap screw with a washer start to screw the cap screw into the nut until it pulls the nut into the hole. Do not over torque the cap screw otherwise you'll spin the nut.

Gardenstater

jrogers13 wrote: »

I removed the 1/2-inch dampening in the bottom and behind and around the passive radiator. I could not tell much of a difference between No lining, fully lined or half-lined, maybe because it is only 1/2 inch.
I’ve seen mention of dynamat on the forum. How is that used in these speakers?
I did find hurricane nuts at the hardware store, that will be my next adventure. Fortunately, all the gaskets look great and the boxes seem to be airtight.

So you didn't notice any difference in bass response. What about midrange clarity? No noticeable difference? If not it will be especially interesting to see what you notice with a fairly small amount of BH5 (or similar of similar thickness) directly behind the MW. I used 6" x 6" and got no reduction in bass performance, only improved midrange clarity.

jrogers13

I don’t have a trained ear, my wife tells me I’m deaf. I think I need a set of songs that push different aspects of the speakers. I did listen to the bass after taking out the bottom dampening, and then switched the speakers since it was in stereo. I couldn’t tell any difference but maybe with the right music and volume it would show up. I did order some Dynamat Xtreme, so I’ll do that and see what happens. I really like the speakers and I’m not sure the incremental improvements will be audible to me, so I do it on faith and because it’s fun.
Try the song “Seeker” by Hiromi. It really shows off the speakers, especially bass. I have it on Apple Music.

jrogers13

When you do hurricane nuts do you drill holes for the extra screw holes on the PR, or just the four?

Gardenstater

jrogers13 wrote: »

When you do hurricane nuts do you drill holes for the extra screw holes on the PR, or just the four?

I absolutely would do all 8, for extra stiffening of the baffle.

F1nut

jrogers13 wrote: »

When you do hurricane nuts do you drill holes for the extra screw holes on the PR, or just the four?

Yes, it will couple the PR basket to the baffle better than just 4 screws. It will not stiffen the baffle.