Replacing Monitor 7B capacitors?

astrashe

I have a pair of Monitor 7B speakers, and I think they need new capacitors.

They sounded great when I bought them on eBay in 2014, but now they're almost impossible to listen to. I don't really know that this is the problem, but I believe that the capacitors are bad. I'd like to put them back into good condition.

I'm not great with a soldering iron, and I'm kind of squeamish about doing the repair myself, but after trying to find someone in my area (NYC) who could do the work, I think it's probably the only viable path forward for me. An older thread on this forum contains a schematic:

https://forum.polkaudio.com/discussion/190981/monitor-7b-repairs

I really don't know what I'm doing here, and I have a few questions.

First, people always talk about replacing resistors alongside capacitors. Is it important?

Second, the schematic shows two capacitors, one 12 mfd Mylar, and one 34 mfd electrolyte. The 12 mfd capacitors are readily available, but I can't find 34 mfds. Is it possible to substitute another value?

Third, how sensitive are these circuits to the heat from the soldering iron? Is this a difficult repair to pull off?

pitdogg2

Not difficult at all. If you've never soldered, my suggestion is to find something to practice on or someone who has soldered electronic gear.
Two 17uf get you 34uf.
It makes no sense to leave the old junk sand cast resistors if you plan on working on the XO. Resistors are very cheap in the grand scheme of things. I'd also recommend going to a 10-12 watt resistor instead of the factory 5 watt. It is cheap insurance and cost a few pennies more.

Soldering is not hard but it is very easy to mess up if you're scared.

One thing that happens with these older drivers is the magnets shift and cause major problems. The first thing you need to determine is if that has happened to yours. Gently apply pressure evenly are the center dust cap with 3-4 fingers and push in gently. If you feel resistance stop, it you feel scratchy feeling stop. If you apply more pressure on one side or the other you will feel scratchy, that is why it's very important to apply pressure evenly and push the driver in evenly.

Describe what it is your not liking about what you're hearing now.

F1nut

34uF, https://www.soniccraft.com/product_info.php/gen-sonicap-34-uf-200vdc-p-4638

skrol

You say that they used to sound good but now don't. It could be a few things. Replacing the electrolytic caps is a good start and likely necessary by now. Also:

• Make sure all the drivers are functioning.
• Test to make sure the cabnets are sealed by gently and evenly pressing the PR and hold. The MW should deflect outward and slowly return to its resting position. If it doesn't move or moves back quickly then you like;y have a leak around a driver or a cabinet seam.
• Your XO may have a polyswitch to protect the tweeter instead of a fuse. They are notorous for going bad, increasing in resistance and adding distortion if they have been activated. If your XO's have them then replace them with a 0.5 ohm resistor.