Monitor 7Bs Large Inductor measurements

Sumbrada69

I am rebuilding 7B crossovers and measured the new replacement parts and old inductors using a cheap LCR and good DVM. I was able to get values for the large ones (ends disconnected and clipped at ends) but not the small ones still mounted on the circuit board. LCR would not recognize it. It would not recognize the large until I clipped it to the ends. The inductors were measured multiple times before and after recalibration.
The schematic says 1.55 mH (no ohm value?). 1 inductor was 1.91 mH, 1.0 ohm and the other was 1.87 mH 1.4 ohm.
The 4 caps measured (Claritycap CSA 12uF, Jantzen 33uF) measured with a 0.1 uF.
The 4.5 ohm Mills were 4.5 ohm, but the Mills 2.7 ohm (both) measured 2.49 ohms.
Is there that much error in inductance measurement with the cheap LCRs (even though they were close to each other) or am I doing it wrong?
If the values are correct will that much error effect parts interaction?
Will the 2.7 ohm resistors being 2.5 ohm have effect? Thanks

pitdogg2

Most are +/- 10% when new.

Gardenstater

Mills were available in 1% and 5%. Vishay Mills are 1%, 5%, 10%.

Sonicraft and Partsconnexion are selling the 1% ones

https://partsconnexion.com/product_images/downloads/mills_MRA.pdf

https://www.vishay.com/docs/31801/mra.pdf

I wouldn't like having them be 2.49 when supposed to be 2.7, but it is good that they are the same. Could be that it will sound just fine but being the shunt resistor it may effect the crossover point (?) and the voicing.

I'll have to check what I measured for the DCRs (no LCR here) of the inductors on my XO. You want them to be the same and yours are way off, which is a bit unsettling lol.

Edit to add - Bear in mind that the "schematic" isn't an official Polk schematic but just something that some unknown person sketched and put on the forum. It definitely shouldn't be taken as gospel. It didn't even have the correct MW which is definitely MW6500 (not MW6502), with the Peerless.

I seem to remember my inductors, large and small, measuring about 0.7 Ohms. I'll verify later.

Sumbrada69

The resistors are new black Mills 2.7 ohm 1% but measure 2.49 ohms. Do I need to get true? 2.7 ohm replacements? I have had the resistors too long to return.

Gardenstater

Sumbrada69 wrote: »

The resistors are new black Mills 2.7 ohm 1% but measure 2.49 ohms. Do I need to get true? 2.7 ohm replacements? I have had the resistors too long to return.

They were probably mislabeled at the factory. Not something that you should have to suffer for. I'd contact the seller and ask them to work with you because they misrepresented what they sold you, through no fault of their own. Heck, for all we know the seller knew this fact when they unloaded on you.

Edit - Do they definitely say 2R7 on them?

pitdogg2

Black are original Mills resistors, they've not been made since Vishay bought out Mills.

Gardenstater

approx. 0.7 Ohms for the large inductor and about 0.75 Ohms (flips back and forth between 0.7 and 0.8) for the small inductor, both measured on an old cheapo Radio Shack DVM.

Had the meter out anyways to fix my MIC backdoor motion sensitive light. I thought it was a waste of time probably but I got access to the circuit board and saw some white fluffy deposits on the traces side and cleaned off with a toothbrush and then used contact cleaner in the sensitivity switch and the timing switch. Pretty surprised that it works

skrol

It sounds like you are in the ball park measuring the DCR. I measured the inductors in my Monitor 7 Series 2, which are a little different. It has an air core 1.75mH, 16 awg in the network for the mid-woofer. Its DCR measured 0.69 ohms with a Fluke 187. For the tweeter network they used a liminated core 0.22mH 22 awg inductor that measured 0.46 ohms.


I was advised by the designer (ZSP) to replace the laminated with an air core as it was a cost saving measure. I am wondering how important it is to match the DCR. Simulating in PSpice it doesn't seem critical but I'm curious about real-world. It is rather small compared to the driver impedances.

Gardenstater

Looks like the large inductor is 18awg (meas. 0.042") and the small inductor is 22awg (meas. 0.026").

Sumbrada69

The mills resistors are black shiny with silver ends. They say Mills 2.7 ohm 1% Mexico 2204 which I got from Sonic Craft. I can't return, they were purchased months ago. With a DVM the small inductors both read 0.5-6 ohm. Large ~ 1.0 ohm Anybody know what truly is the value of the large inductor with this version of the crossover (7B with peerless, 6500, fuse). If 1.55 mH then either my LCR is way off (very possible) or mine are more than 20% high. If truly high what would this do to the MWs (MW6500) sound?

Gardenstater

Before you said one large inductor was 1.0 ohm and the other was 1.4 ohms? That is a huge difference and they are both way higher than what I measure at 0.7 Ohms. That would attenuate the midwoofers.

Having the large inductance being higher than Polk specs would lower the frequency at which the midwoofer's response drops off and might leave you with a trough in response when combined with the tweeter's response.

Sumbrada69

Remeasuring the lg. inductors got 1 ohm on both. I will try the LCR again tomorrow and see if the inductances are the same. I do not have a a reference inductor but I have another set of 10As that should have the same inductors and I'll test them with the LCR. It could be the LCR. I have another DVM I will try also. Would the inductors still connected to the crossovers effect their values? The 10s crossovers are original unmodded, (being done later) and intact.
IF the values are correct then it sounds like I have a problem.

Gardenstater

Hmm. Make sure when you measure the inductors, that you aren't measuring through the enamel coating on the magnet wire. I measured mine from solder pad to solder pad (or the ring terminal on the binding post for the large one).

This makes me want to get the proper measuring equipment to check inductances. I have a long wish list of stuff though.

Your small inductors are lower DCR than mine which is around 0.75 Ohms. I'm wondering if Polk made XO value changes during the production of the 7B. What is the serial #/ manuf. date of your crossovers?

Sumbrada69

I learned a lesson, if a measurement is questionable retest with a different instrument or 2. I have 2 other DMMs a cheap and middle cost one. With both other DMMSs Lg. Inductors 0.7-8 ohms; Sm. 0.6 ohms. Tested a 10A crossover 0.7 ohms. Retested with LCR Lg., Sm., and 10A inductors. Lg. 1.8 mH (1.55?), Sm. (0.22-3) 0.48 mH. The LCR seems to have a 0.25 ish high mH bias. If subtract 0.25 mH from any or all and the values are what are expected. So I guess no true issue in the end. Something to remember though.
The serial nos. are 43229, 42334. Yes spaced. Bought used actual 2 finishes so probably from 2 different sets. I don't care. Have the right parts, cabinets in excellent shape. Can always re-veneer later.
Thanks for all the help especially Gardenstater.

Sumbrada69

1 more thing, measured the questionable Mills 2.7 ohm resistors with the 2 other DMMs. Both say 2.7 ohms not 2.49 ohms. My "Best" DMM does not like very low resistances. For other tests its great.

F1nut

Vintage Polk speakers never had sequential serial numbers. In fact, it was common for the S/N's to be far apart.

odcics2

My 7B speakers. Sequential numbers. Never say never when dealing with old Polk speakers!

Gardenstater

Sumbrada69 wrote: »

I learned a lesson, if a measurement is questionable retest with a different instrument or 2. I have 2 other DMMs a cheap and middle cost one. With both other DMMSs Lg. Inductors 0.7-8 ohms; Sm. 0.6 ohms. Tested a 10A crossover 0.7 ohms. Retested with LCR Lg., Sm., and 10A inductors. Lg. 1.8 mH (1.55?), Sm. (0.22-3) 0.48 mH. The LCR seems to have a 0.25 ish high mH bias. If subtract 0.25 mH from any or all and the values are what are expected. So I guess no true issue in the end. Something to remember though.
The serial nos. are 43229, 42334. Yes spaced. Bought used actual 2 finishes so probably from 2 different sets. I don't care. Have the right parts, cabinets in excellent shape. Can always re-veneer later.
Thanks for all the help especially Gardenstater.

The XO that the measurements I gave you , were on one of a pair of spare XO's I have and the serial # is 52558, 01/28/83. We still have a definite discrepancy where in this one of mine, the small inductor DCR is slightly more than the large at 0.75 Ohms (large was 0.7 Ohms). The other of the pair is 52557. I get 0.8 large, 0.8 small on that one.

Edit - testing the 1st one now I get 0.7 large, 0.7 small. Not the greatest meter but it does read 00.0 with the leads touching.

My 7Bs are 19726 and 19728. I remember the dealer trying to get two boxes that were close from their stock. He asked me if I needed sequential and I said nah don't bother.

F1nut

odcics2 wrote: »

My 7B speakers. Sequential numbers. Never say never when dealing with old Polk speakers!

That's very cool, but very unusual.

Sumbrada69

George I think our values are about as close as 2 different people, different meters, and different crossovers. 0.1ish ohms is gotta be within our error range. My original values were way off but was my testing equipment. I feel confident now my parts are within spec. Time to finish connecting the crossovers. After doing some research the cheap LCR-LC2s, T7s (mine) are not very accurate with inductors. Thanks for your help.

plastic_avatar

https://youtu.be/nagjPyNMu9Q?si=RivCt4cYRMsKieBx

F1nut

Exceptions to the well known facts.