Marshall gold-back 12" speaker testing

[Severino]

Hi.
I recently decided to repair and old 1x12" combo which I had from an old friend that left it on the workshop as dead.
Let me put some history on what I did with the amp.
The amp was dead, I removed the speaker for switching in another repair on a peavey combo.
When I wired this speaker It didn't sound well, It did sound but really bad, I can't put it into words, I switched the speaker for a scorpion one (again from a dead peavey) and then the sound was ok.
This was years ago, and after succesfully repaired a dead peavey bandit I decided to give a try to the mashall.
As expected, blown fuses and the output transistors j-something labeled, were shorted. I have ordeder a pair of tip142/147 that are the usual substitutes.
Anyway that's not (yet) the issue.
As I said, this speaker didn't sound well. My first test has been measuring the DC resistance of the speaker. It's labeled 8ohms, when I've checked speaker DC resistance on 8ohms usually the reading was 5.6ohms but I get an stange 7.3 ohms reading.
Which is the suspected failure on a speaker when the readings are higher than expected?
Thanks.

[Enzo]

Forget the readings, the resistance check tells you ROUGHLY the impedance of the speaker. It also tells you if the voice coils is open or not. DOn;t read anything else into it.

Speakers fail in many ways, remember, they are mechanical systems. If an amp failed and sent DC through the speaker a while, the voice coil could have warped from the heat. That makes it rub inside the magnet gap. The wire wound around the form that makes up the voice coil can come unglued from the form, and that too can rub. The cone material can lose its structural integrity, so it can flap when it moves. SOmeone could have bent the frame at some point or warped it by overtightening the bolts, that can make things rub or move funny. Your magnet can be cracked, causing misalignment. The pole piecce up the center could have shifted. The spider or the surround could have come unglued or have torn.

Speakers can also go intermittant, maybe they work OK until a certain volume - ie cone movement - and they then cut out. That can be due to a failing connection between the tinsel wires and the voice coil. It can also be - and probably more likely - just a failed tinsel wire.

Hold a good speaker by the frame up in front of your face pointing away. Ball up your fist and whack the back of the magnet. Thje good speaker ought to make a low "TUM" sound or just a dull thud. If the speaker makes any sort of loose sound or a little flap as you hit it, then the speaker needs reconing. Try that, take a 12" you have that is OK, and thump the magnet, now do the same to your bad Marshall speaker. I bet it makes a flapping sound.

[52 Bill]

Hold a good speaker by the frame up in front of your face pointing away. Ball up your fist and whack the back of the magnet. Thje good speaker ought to make a low "TUM" sound or just a dull thud. If the speaker makes any sort of loose sound or a little flap as you hit it, then the speaker needs reconing.

I thought that I was the only one who knew this trick. My dad taught me that one a million years ago.

[Enzo]

But I thought you knew, I am a million years old.

[Enzo]

Pretty good trick though, isn't it?

[daz]

But I thought you knew, I am a million years old.

LOL ! Funny guy Enzo. But seriously, you do seem like you could be that old if one were to judge you by your knowledge of electronics. Thats a compliment if it's not quite obvious enough.

[Severino]

I didn't know that!

Forget the readings, the resistance check tells you ROUGHLY the impedance of the speaker. It also tells you if the voice coils is open or not. DOn;t read anything else into it.

OK.

If an amp failed and sent DC through the speaker a while, the voice coil could have warped from the heat.

If the output transistors shorted It's very likely that DC was on speakers for a while until the main fuse, well, fused.

That makes it rub inside the magnet gap. The wire wound around the form that makes up the voice coil can come unglued from the form, and that too can rub. The cone material can lose its structural integrity, so it can flap when it moves. SOmeone could have bent the frame at some point or warped it by overtightening the bolts, that can make things rub or move funny. Your magnet can be cracked, causing misalignment. The pole piecce up the center could have shifted. The spider or the surround could have come unglued or have torn.

Visually the outer membrane is intact and tapping on the cone does give a thump noise.

Speakers can also go intermittant, maybe they work OK until a certain volume - ie cone movement - and they then cut out. That can be due to a failing connection between the tinsel wires and the voice coil. It can also be - and probably more likely - just a failed tinsel wire.

I will clean the speaker and inspect the wires. But no intermittant, the sound was bad at any volume If I recall correctly.

Hold a good speaker by the frame up in front of your face pointing away. Ball up your fist and whack the back of the magnet. Thje good speaker ought to make a low "TUM" sound or just a dull thud. If the speaker makes any sort of loose sound or a little flap as you hit it, then the speaker needs reconing. Try that, take a 12" you have that is OK, and thump the magnet, now do the same to your bad Marshall speaker. I bet it makes a flapping sound.

Tested and YES, It makes a spring-like sound or flapping, as if something is loose. The working speaker does not do that. I didn't know this way of testing, really useful.

As for reconing the speaker, I saw on youtube a guy doing that, but a membrane replacement is almost as expensive as a cheap celestion "hot 100". 40€ a replacement cone and 45€ a new speaker. Unless there's something than can be done beyond reconing, this speaker is dead. I will clean it up and check the wires just in case..

Thanks, really useful information, I will post later pictures when It's clean, but makes a springy noise when "hammered" on the magnet.

[Enzo]

If a magnet is cracked or the pole piece shifted, there is mostly no saving a speaker. Voice coil problems are solved by reconing, so that is the option. But certainly reconing might not be economically wise. No point in spending $40 reconing a $40 speaker.

[J M Fahey]

Quote Originally Posted by Enzo View Post
Hold a good speaker by the frame up in front of your face pointing away. Ball up your fist and whack the back of the magnet. Thje good speaker ought to make a low "TUM" sound or just a dull thud. If the speaker makes any sort of loose sound or a little flap as you hit it, then the speaker needs reconing.
I thought that I was the only one who knew this trick. My dad taught me that one a million years ago.

I think I beat you both
My Dad was a Doctor, and he taught me the "midle finger tap" , which is commonly used to test for excess liquid in the lungs, bronchitis inflammation, etc.
Perfect for thumping the cone , with no need to hit the frame, which might give out "other" mechanical noises, which might confus the issue.

By the way, he also taught me to practice injections using oranges, and lots of other wonderful things, but I guess we are moving away from Electronics.

[Steve Conner]

As for reconing the speaker, I saw on youtube a guy doing that, but a membrane replacement is almost as expensive as a cheap celestion "hot 100". 40€ a replacement cone and 45€ a new speaker. Unless there's something than can be done beyond reconing, this speaker is dead.

Marshall Gold Backs are the cheap speakers they use in their low-end Valvestate cabinets. The Celestion Hot 100 should sound just as good if not better.

[J M Fahey]

Yes.
They used to be cheap line Eminences (Not even what we know as Legends, which are quite acceptable) but I guess they later went for some even cheaper and obscure Asian OEM maker.

I *do* repair them , but I'm in Argentina where it's worth it because:

1) I spent less than $10 in parts.
2) I charged $50 which in "buying power" is equivalent to U$100 in USA.
3) a Celestion, any of them , costs between U$100 and u$150 and
d) Goldback or not, it carries the Magic Marshall logo printed on its back, and is perceived here as equivalent to a good Celestion, so guys who "listen with their eyes" find them "very good" ...... "otherwise Jim Marshall wouldn't use them", would he?

e) truth is , they are not bad in a general line, but lack treble big way, which is not *that* bad in the buzzy amps it's often mounted in (as in Valvestates).
By the way, Celestion Hot 100 also have no highs , and are quite similar.
Both have heavy 2" Voice coils, which explain that sound.

EDIT: charging €40 for parts is highway robbery