MAGNATONE M10 - 9-11 KHz Squeak when pushed into distortion

[Helmholtz]

Ok, I'm hearing some HF artefact now.

Adjust to the frequency where the effect is strongest and scope the amp's output.

BTW; volume setting is not directly related to output power.

[g1]

Is that still with the same speaker? It does sound like a bit of cone or cabinet buzz to me.

[TelRay]

Ok, I'm hearing some HF artefact now. Adjust to the frequency where the effect is strongest and scope the amp's output. BTW; volume setting is not directly related to output power.

yes, I guess that with the pure sine wave it's easy to identify something that is out of place. but i think if i had done that from the beginning somebody would have said: "that's a guitar amp. do you hear that when you plug your guitar in?" hahahahaha
so, i followed your advice and found the most noticeable effect at 670 Hz. this time i left the speakers hooked (no dummy load connected) and used the best ear protection i have around the house
i do not see anything on the scope
higher volume, moving frequencies around... just the sine wave, nothing unexpected. which takes me to g1's questions.

Is that still with the same speaker? It does sound like a bit of cone or cabinet buzz to me.

yep, same speaker
i've had this amp for about 2 years now and it never left the house (thanks to the pandemic) and was not abused in any way. it did not show this "effect" until recently and that's why i did not want to believe it was "speaker related"
unless there is any scientific approach i guess the next step would be taking the speaker out of the enclosure and test again.

thank you guys

[TelRay]

finally got that speaker out after looking for cabinet buzz and there are micro cracks on the cone. this is not a foam ring type suspension but a "one piece paper cone".



i'm trying to get some info about the speaker and it seems to be an 80L5N-1 8" 16ohm 14oz. AlNiCo Oxford speaker
the manufacturer and date code reads 465 (OXFORD) 435 (week 35 1964?)

so... how to repair this thing?

is there an "easy fix"? or should i take it to a professional.


thanks!

[trobbins]

I've used an acid-free PVA type glue that dries clear and flexible, won’t yellow, and is meant for dense paper (Helmer) - you can use tissue paper or old speaker cone or Medical Micropore - or even Japanese Kozo type paper.

[TelRay]

thanks trobbins!
if i understood correctly you apply the material (tissue, micropore, kozo) over the crack as a patch and glue it with PVA.
in this case should i attempt to fix the small cracks or go straight into preventively have the whole circumference reinforced? (like a complete ring patch)
and when you say “tissue”, is that like… a kleenex? i think it makes sense as a patch well glued to the cone. just want to make sure i’m thinking of the right materials
thx!

EDIT: found an interesting post on the forum “repair a hole in a speaker cone. PVA or CA?”
this is "golden": "The sCrew driver holes were put in your speakers to add fuzz" hahahahaha

[trobbins]

The material needs to be flexible but also robust in 'stretch' - so a strong 'klennex' (multiply) may be ok. Another option is a swatch of thin dress or shirt material, or even pantyhose. But it sort of does depend on the 'hole' or crack and whether you are just helping edges to but up again, or are adding in a 'missing bit'. If its a heavy duty guitar cone then weight is not too much of a concern, but vintage general purpose speaker cones can be very thin and hence light weight, and so the weight of the material and the amount of applied pva needs some thought. If it's likely to keep cracking around the whole perimeter then it may well be worthwhile applying a lightweight 'ring', noting that adding mass really close to cone perimeter is in a zone which doesn't add to 'moving mass', but similarly may be near to where the ripple spring compliance is.

[TelRay]

thank you!
i've found that backlight is a great tool to find smaller incipient (not so obvious to find) cracks. So... 11 points to reinforce




about the PVA glue being "acid free"

I've found that Elmer's Glue All is a bit acidic (ph5) while Brodart Bind Art is neutral (pH 7 according to the safety datasheet) or Lineco Neutral pH (as the name says)

let me know if there's a preference

[trobbins]

The only preference is what is on a shelf near you. I went to a nearby large linen/haberdashery store and got Helmer product - below link.

[TelRay]

wait for a couple of days now…

[TelRay]

i mounted the speaker yesterday (after adding more glue to the back of the cone and let dry for another 24 hr) and it passed both the sine tone sweep and guitar test with flying colors