Help please. Rebuilding Monito Audio MA5 crossovers as 1st DIY project

[getaliife]

Through finding this site I'm getting into building my own stereo kit, first project is to rebuild the crossovers for my first speakers, 1977 Monitor Audio MA5s, currently gone all mushy, due , I believe from research here, to worn out capacitors. Removing the crossovers I can see that there is heat damage on the board from the resistors so I intend to replace all caps and resitors. MA do not have a circuit diagram available so I've looed up the values and would need your help to conform that these look correct and also where to source. The inductors look fine. I have no expereice in these matters and am relying on O level physics from 40 years ago!!!!!

High pass filter,
in series, one resistor, marked ERG 74ER 3R3m is this 3.3 Ohms.
two identical capacitors, spilt with the inductor to ground, orange, orange green white brown, are these 3,300,000pF +-10%, 100v?

Low pass filter
resistor to ground, 56R ( I think, badly faded) 56ohms?
cap to ground -yellow,violet green.white, brown , 4,700,000pF, + 10%, 100v?

resistor in parallel to inductor, 33R, 33Ohms?

three further questions
what should be the wattage for the resistors?
where can I buy this stuff?
what do the pF numbers traslate to in today's capacitors

Any help very gratefully received. Thanks in anticipation

[MCSmith]

I PMed you.

Lift the white wire from the network board to get the tweeter out of the circuit, then check the tweeter's voice coil's DCR

I think you will find that it is nearly a dead short; mine read .5 ohms

Sorry to say it, but you (and me) need new tweeters

Marshall

[system7]

Welcome to the Monitor Audio restoration club!

These MA5's look fun:



I'd say those are a typical 70's KEF/Chartwell/BBC clone with a 8" bextrene or polypropylene bass and what looks like an Isofon (KK8?) plastic tweeter possibly with a modified metal front plate. All quite fixable. Morel do a CAT-298 94mm or a 104mm that might do it at wilmslow audio. But you might just have corroded connectors.
Morel Classic CAT298

Worth fixing, for sure. You'll doubtless have a 3rd. order tweeter crossover, simpler than this KEF 104 design. Maybe second order bass crossover. The bass resistor values aren't making much sense to me. Try and draw the circuit and they fall into place. 5-10W Wirewound resistors available at Maplin along with pricey (but worth it) audio grade polypropylene capacitors. Laters...

Oh wait...old post...LOL

[strawberry]

Get 10 Watt resistors. You can keep the set up for the woofer but switch out the stuff for the tweeter. Go with 5.6 uf, inductor, 22 uf, 3.3 ohm, tweeter. Try switching the plus and minus on the tweeter and see what sounds the best. Another fix for the tweeter is to skip the 22 uf, replace with wire. See what sounds the best. This is a crossover at 3 kHz for both drivers. Look at ebay for new parts.

[system7]

I used to have those Isophon tweeters in my Chartwell PM400 units, strawberry. Fried them at parties too!



Turns out Monitor Audio used genuine KEF B200 units in the early days. Not sure which SP numbers though.

[sreten]

Hi,

They look like B200's to me, which one who knows. Isophons as well.

rgds, sreten.

[getaliife]

GUys
thaks forthe interst. Thanks to marshalls sugestions I have now confirmed that the tweeters are blown. My sopeakers are as pictured above but with w non-MA badged tweter and the B200 SP1014 mid/bass. I've also found a look up table for crossover design and can only deduce that i worgly read the coulor coding on the original speakers.

[system7]

Somebody asked me how you go about replacing these blown Isofon units with something similar.

I also quite like the look of the SEAS Prestige 27TFFC in this case, being quite similar to a 104mm KEF T27 overall which was the weapon of choice in the old days with KEF B200 units.
Seas Prestige 27TFFC H881

It's 91dB efficient at an Re of 4.8 ohms. I'd think you could just slot it right in. But if it's too loud, or if the crossover is misbehaving due to the different impedance, you would simply pad it with a couple of 10W wirewound resistors.

Suppose the isofon was actually Re 7 ohms on a multimeter. You'd simply add 2.2 ohm wirewound in series. With an efficiently loud replacement you can also do a proper L-pad with a shunt resistance of say 10 ohms and a series resistance of say, 3 ohms. This gets close to the old Acoustic Research attenuator we know and love. It seems to have some nice sonic qualities.

The math isn't too hard and explored here:
L-Pad (Driver Attenuation Circuit) Designer / Calculator

It's worth knowing that a crossover with a missing or fried tweeter presents a very nasty resonant load to an amplifier, so don't test at high levels till it's all working right.

[MCSmith]

Originally Posted by system7

Somebody asked me how you go about replacing these blown Isofon units with something similar.

I also quite like the look of the SEAS Prestige 27TFFC in this case, being quite similar to a 104mm KEF T27 overall which was the weapon of choice in the old days with KEF B200 units.
Seas Prestige 27TFFC H881

It's 91dB efficient at an Re of 4.8 ohms. I'd think you could just slot it right in. But if it's too loud, or if the crossover is misbehaving due to the different impedance, you would simply pad it with a couple of 10W wirewound resistors.

Suppose the isofon was actually Re 7 ohms on a multimeter. You'd simply add 2.2 ohm wirewound in series. With an efficiently loud replacement you can also do a proper L-pad with a shunt resistance of say 10 ohms and a series resistance of say, 4 ohms. This gets close to the old Acoustic Research attenuator we know and love. It seems to have some nice sonic qualities.

The math isn't too hard and explored here:
L-Pad (Driver Attenuation Circuit) Designer / Calculator

It's worth knowing that a crossover with a missing or fried tweeter presents a very nasty resonant load to an amplifier, so don't test at high levels till it's all working right.

I have never done this sort of substitution. I understand the addition of the resistor to bring the DCR in line with the old DCR of the Isophon being replaced.
But I have to ask to be absolutely certain I am doing this correctly, the 2.2 ohm resistor is simply inserted, in series with the wire from the cross over "feeding" the tweeter's terminal?
Is this correct?
I understand that we want to bring the new tweeter's DCR in line with that of the old, but what I don't understand is the other comment regarding "padding" with 10 watt resistors.

Are we talking about the same thing here as well, or something else; "you would simply pad it with a couple of 10W wirewound resistors"

Is this statement simply going on the assumption that we are doing this change over in both boxes? Again, simply bringing the new replacement's DCR in line with that of the old Isophon?
(which we will by the way) or something else entirely?

Pardon my sluggishness, and thank you!

Marshall

[system7]

You are doing two separate things really. The series resistor of (maybe) 2.2 ohms is just to present the designed isofon load to the crossover. This will keep the frequency response flattish. And yes, you just put it between the crossover output and the input terminal of the tweeter. The SEAS is a 6 ohm unit in general parlance, but 4.8 ohms voice coil resistance is your starting point in L-pad design.

The attenuator L-pad consisting of two resistors can incorporate the 2.2 ohm load correction, but with different values can also attenuate the SEAS tweeter. The 91dB SEAS is likely more efficient or louder than the isofon which was possibly 88dB. It's a bit suck it and see. 3dB difference is twice as loud.

Most tweeters are set far too loud IMO. You have plenty of scope to reduce their level to something you can live with. FWIW, I would always replace BOTH tweeters together.

You're not going to blow up anything just slotting the replacement in with no modifications, but we all like to tweak things, eh?