I just purchased a pair of 57’s that need refurbishing and I have some questions, I want to proactively thank people for answering what must be boring rehashes of tired old questions:
Symptoms:
Muffled and quiet sound on both speakers and dustcovers that are at the least very loose or possibly ripped.
My first plan: To rebuild the EHTs and replace the dust covers.
Questions:
1) What are the symptoms of treble panels that have previously arced?
2) What are the symptoms of a bass panel diaphragm that has ripped?
3) Can I inspect the diaphragm without splitting the stators?
4) If I have to split the stators to inspect the diaphragms will I likely damage the diaphragms in the process?
5) If I can split the stators and maintain the integrity of the diaphragms can I simply recoat the diaphragm on one side?
6) Is the goal to keep the original diaphragms no matter what or is it suggested to simply do a complete rebuild?
7) What is the best way to generate a frequency sweep to measure the resonant frequency of a rebuilt panel?
Symptoms:
Muffled and quiet sound on both speakers and dustcovers that are at the least very loose or possibly ripped.
My first plan: To rebuild the EHTs and replace the dust covers.
Questions:
1) What are the symptoms of treble panels that have previously arced?
2) What are the symptoms of a bass panel diaphragm that has ripped?
3) Can I inspect the diaphragm without splitting the stators?
4) If I have to split the stators to inspect the diaphragms will I likely damage the diaphragms in the process?
5) If I can split the stators and maintain the integrity of the diaphragms can I simply recoat the diaphragm on one side?
6) Is the goal to keep the original diaphragms no matter what or is it suggested to simply do a complete rebuild?
7) What is the best way to generate a frequency sweep to measure the resonant frequency of a rebuilt panel?
Mild arcs are hard to diagnose. Major arcs tend to radically change the high frequency response. I check by measuring the panel quasi-enchoic response with a cheap measurement mic and a soundcard. IF you measure both speakers and they have very similar responses, chances are your panels are ok. IT's very unlikely that one or two damaged panels would be damaged exactly the same.
No output from that panel and maybe some sizzling noises.
You can look through the bass panel with a flashlight on the back side. You should be able to see what's left of the milky coating and the saran diaphragm. IT's easier to see if the dust cover is removed, but you can look through the back side and illuminate through the grey paint on the front.
Yes, almost certainly.
Yes, but you won't be able to keep the diaphragm intact.
I do a complete rebuild, but I've been rebuilding quads for over 20 years. There's not much worth salvaging in a panel that is over 35 years old.
I would use a function generator, you can get them for cheap on ebay. Or you can use a soundcard and some software. Keep in mind that the resonant frequency will drop as the panel breaks in.
Good luck with the project. I was in the same boat in the mid 90's and that led to my little sideline gig.
Sheldon
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