Restoring / Modifying Goodmans Magnum K2s Advice Needed

[david83]

Hi all

I’ve recently been given some Goodman’s Magnum K2’s and they are in quite poor condition as is to be expected for something dug out the back of a garage.

Overall cosmetic condition is poor, and I have been advised the crossover would probably benefit from a recap. The mids have been damaged then repaired with PVA and lack brightness - or are they just like that anyway?

The tweeters look a bit shabby but appear to be ok and the woofers also appear to be ok and cosmetically are perfect probably due to the plastic wrapping on their paper cones.

Obviously it would be nice to be able to restore these to a workable state just because they’ll stay out of landfill, but I am at a crossroads. I have no problem with the cosmetic side of things, mending / filling varnishing the housing etc, recovering the grills, replacing seals that sort of thing, but I need some input and advice on the drivers and crossover unit.

My questions are:

1. What capacitors do I need to replace these with? Someone has said Alcaps? I can solder but I need to guidance on what to buy - doesn’t look like I can see the numbers written on these without desolating.. does anyone happen to know what these caps are? Here is a pic of it: https://i.postimg.cc/qqD778F7/94643-B11-A49-D-4-BCD-AE58-B791-D40-C0-EF9.jpg

2. The mids are probably going to need replacing… is it worth trying to source K2 mids or is there something else that would go nicely in there? I can find many threads of peeps who have loved these units and an equal amount that have hated them. I’m not dead set on it being original spec unless the mids are actually good? Any suggestions for replacements and pros/cons would be appreciated

3. Same for tweeters, worth replacing? Suggestions? Pros/cons? I have heard the original tweeters are probably more worth saving than the mids, but I’m open to any and all advice. They certainly sound crystal clear to me, but if I’m dipping into non-purist with the mids, I’m open to suggestions for the tweeters too.

If I am to replace the mids and tweeters, it has to be something that will come through those woofers. It might just be that they are damaged, but they are definitely drowned by those woofers as they stand.

Cheers, Dave

 

[Galu]

Answer to question 1.

Undoing the metal clamp should allow sufficient access to the capacitors to read their values without immediately having to unsolder them.

I would replace them with bi-polar electrolytics like these ones available from Wilmslow Audio:

Mundorf Electrolytic ECap Capacitors

 

[Galu]

I've attached your crossover photo here for ease of reference, as well as an image of the loudspeakers for those unfamiliar with the model and drivers.

 

[Galu]

And here's some, probably unhelpful, specs of the Magnum K2:

Impedance: 4 - 8 ohm
Max. RMS power: 50W
Frequency Range: 30 - 22,000Hz
Retail Price: £46.20 (in 1971)
Sensitivity: 2W produces 96dB in the average domestic listening room.

 

[david83]

Hi Galu. Thank you for your help.

I have undone the clamp as you suggest and gently turned them to see what is printed on them. See attached photos. The 2 red ones appear to be the same. So they are 10uF and 5uF so I will need these: 10 fd ECap100 Electrolytic Capacitor and for the 5’s I guess the closest thing is 4.7 here: 4 7 fd ECap70 Electrolytic Capacitor not sure if this is acceptable or do I need a 5? How Will dropping it to a 4.7 change the sound produced?

One thing I am unsure about is what the 3 “doughnut shaped” parts are? They look to be some kind of coil, one for each driver. Do these also need replacing? One of the things I wanted to do was increase the internal wiring. Something thicker and copper, but looks like I’ll have to solder to each of these coils too.

 

[Galu]

4.7 uF is just fine and will make no audible difference (other than being fresher than the old 5uF!).

The coils do not need replacing as they do not deteriorate over time like electrolytic capacitors.

There's no need to go overboard with the thickness of the internal wiring.

 

[david83]

Ok that’s great thanks. Are these caps the same thing? https://cpc.farnell.com/visaton/5427/mkt-a-foil-capacitor-4-7-uf/dp/CA08511?st=Electrolytic bipolar

Ie, am I looking for just bipolar electrolytic? The only reason I ask is because I need some other stuff from CPC and I would get free delivery if I could bump up the order a bit.

I am still not sure if I should try and acquire the original mids or if I should replace them. I was hoping someone who has a pair of these and had swapped out the mids could recommend something.

 

[Galu]

You can order bipolar electrolytic capacitors from cpc if you like.

It is extremely difficult to suggest replacement mids without knowing the specifications of the originals.

You say the mids are "probably going to need replacing". Have you tested them and are they in working order?

I would see what effect the capacitor replacement has before considering changing out working drivers.

EDIT: I see you said the tweeters are "crystal clear".

You also say that the mids and tweeters "are definitely drowned by those woofers as they stand". This may not be the case after the capacitor replacement.

 

[david83]

Yeah I’m sorry that does come across as contradictory.

The mids do produce sound, but they are lost in something with bass, even without they seem dull. I assumed this was the result of the PVA.. you think it’s more likely to be the caps? The tweeters are crystal clear but I still feel the output is not balanced - so they are ‘just acceptable’. Maybe the first point is the caps as you suggest. I’ll get those ordered today.

 

[Galu]

Sorry David, although I re-read your description, I failed to note your comment/question about the mids - "The mids have been damaged then repaired with PVA and lack brightness - or are they just like that anyway?".

What do you mean by "repaired with PVA"? Perhaps a photo may help.

Severely deteriorated electrolytic capacitors would cause a lack of brightness.

 

[david83]

One mid looks like it had a tear. The other was probably detaching around the edges. Here’s some pics, and the tweeters too.

 

[Galu]

The drivers are certainly not in pristine condition!

The mended tear is unlikely to have made the mid sound dull.

If the surround on the other mid is not completely detached, it is an easy job to glue down the detached sections.

I would advise you to change the caps then take it from there.

 

[david83]

It looks like it’s been done previously. Hence the PVA on both the tear and the surround. As you say they aren’t pristine.

I’ll get the caps ordered and post back when I know how that’s affected things

 

[Galu]

If it does come down to replacing the mids, it is worth contacting Wilmslow Audio who may be able to advise you regarding suitable replacements.

Speaker Repair, Speaker Kits, Loudspeaker Repair, Kits and Drive Units, Subwoofer Amplifier Kit

[SIZE=+0]

We are able to supply alternative replacement drive units for a wide range of loudspeakers... There are hundreds of ... loudspeakers that we have replacements for, just contact us for help and advice.

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[ticknpop]

Get Solen polypropelenes, they’ll never go bad and cost only a little more than bipolar electrolytics that will age out. - and they sound better

 

[Galu]

Buy polypropylene film capacitors by all means, but I caution you that they will be physically larger than the electrolytics, so check their dimensions carefully if you want the replacement capacitors to fit into the existing retaining bracket.

Here are the ones available from your preferred supplier:

https://cpc.farnell.com/c/electronic-electrical-components/capacitors/audio-capacitors

EDIT: The 10uF polyprop is not currently in stock at cpc - due beginning of August.

 

[lohk]

Please listen to the individual speaker chassis solo, with and carefully without crossover. If they show some sort of distortion caused by the repairs, junk the speaker and do not invest into new parts, it would be a waste of money. Proper repair of speaker cones or coils is not an easy task.

 

[david83]

Thanks all for the info.

I have already ordered the electrolytic caps but have found some polypropylene caps on Ali Express that look well priced. I can see from the photos in the reviews the difference in size. Audiophiler Mkp Capacitor MKP 250V MKP 400V MKP 630V 10UF/400V 0.1UF 0.22UF 0.33UF 6.8UF 7.5UF 8.2UF AEAK|Capacitors| - AliExpress

Thanks for the warning lohk, I will check the mids and I am happy to recap the crossover.. I assumed the mids would be duff, hence wanting to replace. I am trying to keep things out of landfill primarily and I do this as a hobby but I’m always grateful for forewarnings.

Does anyone know what the coating on the tweeters is? Can this be purchased / reapplied?

 

[Galu]

Does anyone know what the coating on the tweeters is? Can this be purchased / reapplied?

The coating will be proprietary and probably best left as is. If the tweeters sound as "crystal clear" as you say, I would not interfere with them.

However, if you are brave enough to experiment, Anita's Tacky PVA Glue would be non-destructive to the original coating/impregnation of the dome.

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This product is also used to reattach foam and rubber driver surrounds.

 

[david83]

Food for thought, thank you. Watch this space.