Refurbishing a Monitor 7A

jimbo1421

Hello all.

Before I tackle rebuilding my SDA-1B crossovers I thought I would refurbish some easier speakers. I have a pair of Monitor 7As and two pairs of Monitor 10s. That ought to give me plenty of practice, polishing my soldering skills and learning the names of the parts. (I wouldn't know a cap from a cap in my ****!)

First up - Monitor 7A. These sounded quite good before I took them apart. I think I might use them for surrounds with the SDAs or in a bedroom or office system.

Before I start, I have some questions, the answers to which will determine how much I spend on the crossovers. I attached some pics of the drivers. Are the tweeters the American Peerless? There are no markings on them except for a number stamp; one is 55142778, the other is almost illegible. Are they replaceable? I ask because while I was desoldering a big lump of solder and wire from one of the terminals a puff of smoke emerged from the face side. I hooked it up and played it again; it sounded about as before, kinda scratchy, but functioning. Both tweeters sounded scratchy when hooked up individually, something I didn't notice when they were all in the cabinet.

Secondly, one tweeter has a white powdery substance on the inside which brushes off. Any ideas about what it is or if it signifies some damage or corrosion?

Thank you for your attention.
Regards,
Jim

jimbo1421

Hello again,

I have attached a photo of one of the MW6500s. Does the label look a bit odd. It looks to me as if some kind of fluid has leaked under the label. Notice the light spots near the center where the label is not completely stuck. Is there fluid inside the driver that can leak? Or is the mottled appearance from the label adhesive, dark where the label is stuck down tight, light where it isn't?

Regards,
Jim

lixuelai

Those look like Peerless with the hole in the middle and the metal plate. Not sure I would have experimented on a Monitor 7 with Peerless. :eek:

AudioGenics

found these comments about "white powdery"....

calcium silicate type material or...

Old steel chassis boxes are frequently zinc plated.
When the plating gets damp, it oxidizes.
you get a white powder that is zinc oxide.

JohnLocke88

Hello. Just curious are you using a soldering "iron" or "gun?"

I am by no means an expert, but I read somewhere that a 'gun' is very high wattage, which can damage electrical components. Maybe what happened to your tweeter?

I really have no idea, but hope this might be of some use.

Those are the peerless tweeters. I have a pair of 7Bs I just got off craigslist. One of the tweeters also has that "raspy" sound with certain recordings... It sounds very similar to the other one why playing music, etc.... but when I use the test tone, it becomes very distinct.

Sorry for the minor hijack, I'm just also trying to figure out what happened to my tweeter too.

jimbo1421

lixuelai wrote: »

Those look like Peerless with the hole in the middle and the metal plate. Not sure I would have experimented on a Monitor 7 with Peerless. :eek:

Did that puff of smoke that I mentioned have anything to do with your judgement?:o

Jim

jimbo1421

JohnLocke88 wrote: »

Hello. Just curious are you using a soldering "iron" or "gun?"

I am by no means an expert, but I read somewhere that a 'gun' is very high wattage, which can damage electrical components. Maybe what happened to your tweeter?

I really have no idea, but hope this might be of some use.

Those are the peerless tweeters. I have a pair of 7Bs I just got off craigslist. One of the tweeters also has that "raspy" sound with certain recordings... It sounds very similar to the other one why playing music, etc.... but when I use the test tone, it becomes very distinct.

Sorry for the minor hijack, I'm just also trying to figure out what happened to my tweeter too.

Hi John,

I used a 45 watt desoldering iron but the lump of solder and old wire was so big that I had to hold the iron on it for a while, too long, I guess. Someone else had been inside the box before me. The leads to the tweeters had been resoldered without removing the old solder and broken off wire. Also, one MW6500 looks like a replacement; the basket is bright steel, not painted black and there is no date on it.

The tweeters didn't sound raspy in the box, just when I hooked them up separately to my receiver with alligator clips. The one that emitted the puff of smoke during desoldering didn't sound any worse after the smoke incident. Perhaps it was just some paint that vaporized, although I see no sign of that.

Are these the American or Danish Peerless tweeters?

Regards,

Jim

JohnLocke88

Someone told me the American's are the ones with the holes, and the Danish are the solid ones. Could have that completely wrong though, I'll check my 7B thread and update.

skrol

My USA Peerless have a dimple.

jimbo1421

JohnLocke88 wrote: »

Someone told me the American's are the ones with the holes, and the Danish are the solid ones. Could have that completely wrong though, I'll check my 7B thread and update.

skrol wrote: »

My USA Peerless have a dimple.

Thanks, guys. They are American then. If they need replacing, either due to age or mishap, are any available?

Jim

kawizx9r

You know I was going to add, you should seal'em up when you puttem back in an enclosure with caulk cord around the edge of the driver basket. Proved to be a much better seal on my sda's and loved the tight bass I got, also utilized high density foam tape (or you can use dynamat) on the basket themselves. Not sure if its an approach youd take on a smaller speaker but it doesnt hurt to try?

jimbo1421

kawizx9r wrote: »

You know I was going to add, you should seal'em up when you puttem back in an enclosure with caulk cord around the edge of the driver basket. Proved to be a much better seal on my sda's and loved the tight bass I got, also utilized high density foam tape (or you can use dynamat) on the basket themselves. Not sure if its an approach youd take on a smaller speaker but it doesnt hurt to try?

I sure will, kawizx9r. I have a bunch of rope caulk and Dynamat Extreme left over from my Monitor 10s. Today I reglued the back of one of the boxes. It had pulled loose out of the rabbet by about 1/8".

Next I will be shopping for replacement capacitors and resistors for the crossovers. Any suggestions for suppliers in the medium price range?

Jim

skrol

Mine also had a Made in USA sticker. See posts #18 and #20 in the following thread:
http://www.polkaudio.com/forums/showthread.php?t=83521&highlight=Monitor

jimbo1421

No Made in USA sticker on mine. No dimple on the shiny plate behind the magnet, either. They do have the hole in the silk dome.

Jim

skrol

For the dimple I was referring to the dome. Some have a hole in the center some have a dimple. Your backs do look a bit different than mine. Do you know what year yours were mfg'd? Mine were 1983.

jimbo1421

1979 according to the only date stamp, which is on one of the MW6500s.

Jim

jimbo1421

Hello again.

I have attached a crossover picture. I cribbed the replacement capacitors from ben62670, the resistors I am not sure about.

The big yellow cap has printed on it:
T.I. 12uF J
100VDC
Mexico

Replacement: Dayton DMPC-12 12uF 250V Polypropylene
http://www.parts-express.com/pe/pshowdetl.cfm?PartNumber=027-430

The smaller black capacitor reads
TSI-7818
PXC34LP50
34uF-50VAC
AC

Replacement: Jantzen 33uF 400V Crosscap
http://www.parts-express.com/pe/pshowdetl.cfm?PartNumber=027-944&DID=7
Is the 33uF an OK replacement for the 34uF?

One resistor reads
7745 TRW PW5
4.7Ω 10%

and I found a Dayton DNR-4.7 4.7 Ohm 10W Precision Audio Grade Resistor
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=004-4.7

The other resistor reads
7745 TRW PW5

and I found Dayton DNR-2.7 2.7 Ohm 10W Precision Audio Grade Resistor
http://www.parts-express.com/pe/showdetl.cfm?Partnumber=004-2.7

I also looked at the Mills resistor selection page (http://www.parts-express.com/mills-crossover-resistors.cfm) but did not see the precise values of mine. I have seen Mills recommended on other threads but I don't know if 2.5Ω and 4.5Ω is close enough.

Are these the right choices? Could they be improved upon for a $20 uptick in the price?

Regards,
Jim

schwarcw

Greetings Jimbo!

Those replacements will work just fine. You may want to try a better cap in the tweeter circuit. A 12 uF Sonicap Gen 11 from Soniccraft is what I would use. I think it's worth the extra money. The exact resistor values are recommended. Good luck!

jimbo1421

Thanks for the confirmation, Carl. I think the Sonicaps run about $20 apiece, right? I know they are well regarded here, but for a first time effort and the chance of **** up (see post #1) I think I will stick with the Daytons. This is a training exercise for me and I can always change them out later.

Regards,
Jim

jimbo1421

I ordered the caps & resistors today and also Dynamatted the PRs and MW6500s. While waiting for the parts I am looking ahead to final assembly. The speaker wires were soldered to the speaker terminals and I am thinking of using spade terminals. Can anyone comment on the relative merits of the two connection methods?

Jim

jimbo1421

Hello again,

The new capacitors and resistors arrived today. Does anyone have any tips for desoldering & removing the old parts without burning anything? Should I try to remove as much of the old solder as possible or leave it to reuse? Some of the puddles of solder are as much as 1/2" across. It would seem too easy to have it run across the gaps in the printed circuit.

Thanks,

Jim

madmike

Use Solder wick to remove the majority of the solder then you end up with a cleaner solder joint. There is a video on how to do this here http://www.ehow.com/video_4435740_use-solder-wick-remove-solder.html

jimbo1421

Thanks for the link, MM. I watched the whole series. Do you prefer the wick to a desoldering iron with the suction bulb?

Jim

hearingimpared

jimbo1421 wrote: »

Thanks for the link, MM. I watched the whole series. Do you prefer the wick to a desoldering iron with the suction bulb?

Jim

I prefer solder suckers or bulbs.

jimbo1421

Since a bulb desoldering iron is what I have, that is what I am using.

There does not appear that there is any in/out or +/- direction on either the caps or resistors. I assume that it does not matter which way they are soldered in, right?

Jim

jimbo1421

Does it matter if the resistors are tight against each other? Will they overheat? I am running out of room on the board and the new parts are much bigger than the old.

Jim

DaveOrAstroDave

Hello there,

Wow, it looks like you're doing exactly what I just did, right down to the parts selection! The only difference is I had to buy a new inductor too. Since you're using 10w resistors, I wouldn't worry so much about heat.. they're made to handle twice the power, so they shouldn't heat up as much to begin with, plus the added surface area means they'll dissipate heat faster.

Also, yes, you are correct about the capacitors and resistors not having a polarity that you have to respect. Either way should work fine.

I didn't think to dynamat my pair of 7A's, but I did use Moretite caulking cord to seal the drivers to the cabinets, as recommended in a few other threads. You might want to give it a try

As for future upgrades, I was thinking of splitting the crossover network and add a second pair of terminals so the speakers can be bi-wired, but I'm skeptical about what kind of difference it would make...

Anyway, good luck with the refurbishing project! As for me, I've still got one more speaker to go.

nikolas812

jimbo1421 wrote: »

Since a bulb desoldering iron is what I have, that is what I am using.

There does not appear that there is any in/out or +/- direction on either the caps or resistors. I assume that it does not matter which way they are soldered in, right?

Jim

jimbo1421 wrote: »

Does it matter if the resistors are tight against each other? Will they overheat? I am running out of room on the board and the new parts are much bigger than the old.

Jim

1. Right. There non directional. Doesn't matter which way there oriented. Some caps do but there usually labeled +/- if so..

2. I don't see why not. I think you would be fine. I think most people like to use Hot glue to hold everything together so nothing vibrates. I sometimes like to use a good double sided tape in some applications..


Nik




Nik

skrol

You can actually use Moretite for vibration reduction on the PR and MW baskets. It dampens out ringing very well. I don't know how well it will stay in place over a long duration though.

nikolas812

DaveOrAstroDave wrote: »

As for future upgrades, I was thinking of splitting the crossover network and add a second pair of terminals so the speakers can be bi-wired, but I'm skeptical about what kind of difference it would make...

Huh. I never thought of this.. Bi-wiring may not make the biggest difference. But you would have the ability to Bi-amp and that opens up all kinds of things. Just one that comes to mind is a Tube amp on the highs and a SS amp on the Lows..



Nik

jimbo1421

Hey Nik & Dave,

Thanks for your comments. Since it is difficult to get the components to sit as tightly on the board as the smaller originals I'll get a hot glue gun tomorrow and stick them down that way.

I am still thinking of adding spade terminals to the speaker connections rather than resoldering them. When I desoldered the tweeter connections the caulk or glue that holds the leads in to the groove on the face bubbled, so I am reluctant to solder them back. Is there any reason not to use spade terminals?

Thanks again,

Jim