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JBL SA600 Vintage Amplifier Review

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amirm

amirm

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Here is the offending screw:
JBL Loose grounding screw.jpg
 
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amirm

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Wow... Does it seem like a screw that could have come loose on it's own or would it have to be loosened by a screwdriver by hand etc?
Definitely looks like it can come lose by itself. It goes from loose to not with smallest turn of the screw (not enough threads per inch). And it is awkward at that because you have to have a small screw driver to grab it inside. I am sure it was borderline tight and came loose in shipping. If I didn't know what to look for, you would not know that it is loose. I moved the wires around and realize that the washer for the grounding wire would slide around under the nut.
 

Doodski

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Definitely looks like it can come lose by itself. It goes from loose to not with smallest turn of the screw (not enough threads per inch). And it is awkward at that because you have to have a small screw driver to grab it inside. I am sure it was borderline tight and came loose in shipping. If I didn't know what to look for, you would not know that it is loose. I moved the wires around and realize that the washer for the grounding wire would slide around under the nut.
Funniest places that manufacturer faults can pop up and show themselves... lol. :D
 

DonR

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A big issue with modern TVs is the grounding screws of the circuit boards coming loose. The numerous heating/cooling cycles of the TV cause the screws to slowly work themselves loose.
 

EJ3

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Yes. Or as I suggested to the owner, to put some nail polish on it to keep it from turning loose.
An ancient 'trick' used by experienced folks in many of the tech school/trade school taught industries that is not 'officially' taught but is a word of mouth (or self thought) innovation that works for a temporary(permanent?) 'emergency' repair, many times while waiting for the super glue.
 
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An ancient 'trick' used by experienced folks in many of the tech school/trade school taught industries that is not 'officially' taught but is a word of mouth (or self thought) innovation that works for a temporary(permanent?) 'emergency' repair, many times while waiting for the super glue.
Yeh, I learned that from my older brother decades ago. Nice way to use up your wife's finger nail polish that she is not using. :)

We also used to use clear ones on case screws to detect if the customer had opened the unit! :)
 

EJ3

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Yeh, I learned that from my older brother decades ago. Nice way to use up your wife's finger nail polish that she is not using. :)

We also used to use clear ones on case screws to detect if the customer had opened the unit! :)
YEP! It also seemed to work well on "red bug" (chiggers) bites.
Kind of like putting lacquer thinner on your poison ivy itches: the annoyance of what you just did is higher than the itching.
 

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Found and fixed the problem! There is an internal RCA jack that connects the power amp to pre-amp. There is a grounding screw for the shield. It had come lose. Tightened it and here are the results:

View attachment 228310

This is power amp only although before the fix was nearly as bad as the full amp I tested in the review. I will put it back together and re-run all the tests.
Great to see. I am immensely looking forward to the re-run.
 

mhardy6647

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Dab of Loctite?

PS I am really glad the source of the poor performance was tracked down! The originally measured performance was abysmal... on par, and maybe even worse (!) than an unrestored example would manifest!
 

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Dab of Loctite?

PS I am really glad the source of the poor performance was tracked down! The originally measured performance was abysmal... on par, and maybe even worse (!) than an unrestored example would manifest!
Use blue removable threadlocker. Not cyanoacrylate.
 

Labjr

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Yeh, I learned that from my older brother decades ago. Nice way to use up your wife's finger nail polish that she is not using. :)

We also used to use clear ones on case screws to detect if the customer had opened the unit! :)
I service music and audio equipment. Just the other day I had a customer with a recording studio messaging me because he was trying to tune the oscillators of an old Oberheim analog synthesizer by ear. Another engineer told him to put nail polish on all the internal pots and coils. Bad idea. Not like someone broke in and opened up his synthesizer while he was sleeping. The oscillators drift for various reasons. Parts age, moisture, other components drift out of tolerance etc. And it really needs to be done with test equipment. Everyone is trying to fix everything themselves these days. They expect free technical info and don't value anyone else's time and experience.
 

GXAlan

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Wow!! Beats a lot of modern Arcam’s :)
 

GXAlan

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Fixed, the SINAD is 71.5 for amp alone…

So in 1966, it was doing a better job than today’s

1) Arcam AVR10 and JBL SDR-35 in digital mode
2) NAD T758
3) Anthem MRX520
4) Topping PA3
5) Optoma NuForce
6) Aragon 2004 MkII
7) Sunfire Cinema Grand
8) Audio Research D300
9) Lyngdorf TDAI-3400
10) Sonos Amp

And of course tube amps. I don’t think any tube amp tested here has beaten one of the very first silicon solid state amps.
 
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pma

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Found and fixed the problem! There is an internal RCA jack that connects the power amp to pre-amp. There is a grounding screw for the shield. It had come lose. Tightened it and here are the results:

View attachment 228310

This is power amp only although before the fix was nearly as bad as the full amp I tested in the review. I will put it back together and re-run all the tests.

I am glad that you have found and fixed the issue, Amir. The measured result now corresponds quite well with the simulated result, below is the simulated distortion

Locanthi_distortion.png


Also the PSU hum/buzz now corresponds with simulated PSR of the amp. Great job, @amirm !
 
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