Monitor 5B Crossover Resistor - 2 or 2.7 ohms

Zentish

I'm planning to do a recap of my M5B's, built and purchased in 1985. I pulled the crossover and took some pictures. My speakers have SL2000 tweeters and I figured the crossover should match the schematics here in the forums but they don't quite match - the resistor is 2.7 ohms instead of the 2 ohms specified in the schematics.

I'm considering TL'ing my M5B but want to be really sure with my starting spot.

I've seen several pictures of M5B crossovers with 2.7 ohm instead of 2 ohm resistors. If I re-cap or TL my speakers do I use a 2 ohm or 2.7 ohm resistor?

THANKS!

StantonZ

It depends on whether you have a poly-switch or (external) fuse. IF you have a PS, you should remove it, in which case you can:

1) replace the 2.0 Ohm resister with something <=2.7 Ohm and "short" the PS leads, or
2) keep the 2.0 Ohm resistor in place and REPLACE the PS with something <=0.5 Ohm

If you have an external fuse (and no PS), then you probably had something like a 2.7 Ohm resistor. Clear as mud?

Zentish

Clearer at least!

My M5B
- does not have a polyswitch
- does have a fuse

So if I do the TL modification (replacing the SL2000 with an RDO-198) will I keep the 2.7 ohm resistor or drop it to 2.0 ohms as all the diagrams say?

westmassguy

Something is amiss. If you look at schematic_m5b_orig, check the revision list at the bottom. The Fuse was replaced with a Polyswitch, and the Resistor was changed from 2.7 to 2.0 ohms to compensate. I would measure the inductors, and make sure they match what's supposed to be there, before attempting the TL Mod.

Zentish

westmassguy wrote: »

Something is amiss. If you look at schematic_m5b_orig, check the revision list at the bottom. The Fuse was replaced with a Polyswitch, and the Resistor was changed from 2.7 to 2.0 ohms to compensate. I would measure the inductors, and make sure they match what's supposed to be there, before attempting the TL Mod.

hmmm... I don't have the equipment to measure the inductors - and I don't see any markings on them at all.

I guess my choices would seem to be
1) find a more "normally" spec'd M5B crossover, TL that, and drop it into my enclosure as a replacement along with an RD0-198
2) don't TL at all, replace the existing CAPS and Resistors like for like, upgrade the SL2000 with an RD0-194

I saw a VR3 M5 crossover mod that showed the 2.7 ohm resistor and thought maybe that was one of the "normal" variations of the M5.

westmassguy

Simple LCR Meters are not that expensive, and it's something you should have in addition to a regular multi-meter when working on crossovers. If they were my Monitor 5s, I'd double check the inductor values, and if the matched the SL3000 Mod schematic, I'd do the Mod. The fuse block can be removed or left in place.

StantonZ

Zentish wrote: »

I guess my choices would seem to be
1) find a more "normally" spec'd M5B crossover, TL that, and drop it into my enclosure as a replacement along with an RD0-198
2) don't TL at all, replace the existing CAPS and Resistors like for like, upgrade the SL2000 with an RD0-194

I chose option #2 (last year) and don't regret it; even if you could determine the (current) inductor values, the large one is a PITA to replace--and they don't "degrade" like resistors and (mostly) caps.

westmassguy

I wasn't advocating changing anything at this point, I merely suggested that he measure what he has, which will point him in the right direction for an upgrade. If the Inductors measure correctly for a 5B, than the SL3000/TL Mod can be done. If not, the only route would be the RDO-194-1. The RDO-198-1 is a much better tweeter, and is worth going the extra mile, in my opinion.

skrol

Polk had a tendency to tweak the XO's over the years even for a given model (ie Monitor 5B). As you saw above, some years had fuses, some years had polyswitches. Some of the value differences may have been based on what they had on hand. The Monitor 5B's that I recently sold were from Dec. 1984, SL2000, single fuse, no polyswitch and had 2.5 ohm resistors (which is likely with in the 10% tolerance of the factory production 2.7 ohm resistor).
Stan