Threshold S/500 issue and circuit request. - diyAudio
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Old 23rd April 2014, 02:31 PM   #1
fogf1 is offline fogf1  United States
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Default Threshold S/500 issue and circuit request.

Good day all,
I have a S/500, s/n: D87090454, that has an issue. Specifically, the middle of the right side heat sink gets much hotter than the same section on the left side. I saw there were schematics for the S/550 posted and I'm wondering how different the two units are. The one I have is an 'Optical bias' unit. I have to check the offset to see if there is a glaring difference. Going to open it up and test the outputs and resistors to see if they are all ok. Anyone have any other suggestions?
Thank you for any suggestions.
D
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Old 23rd April 2014, 02:44 PM   #2
Zen Mod is offline Zen Mod  Serbia
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test and post pictures

check voltages across all emiter resistors , and we'll know more
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Old 23rd April 2014, 02:49 PM   #3
fogf1 is offline fogf1  United States
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Will do! Thanks for the suggestion.
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Old 24th April 2014, 02:21 PM   #4
fogf1 is offline fogf1  United States
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Ok, here's what I have so far, and I promise pictures will follow.
Output offset L: 2mV
Output offset R: 19.2mV

Voltage drop across L emitter resistors: all in the range of 72-80mV
Voltage drop across R emitter resistors: all in the range of 126-146mV

It looks like the outputs are being driven hotter across the board so I'm now looking at the small input circuit board as the source of my troubles.

Diagram anyone? Or suggestions?
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Old 24th April 2014, 02:57 PM   #5
Zen Mod is offline Zen Mod  Serbia
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if they're all inside that range , then electrically everything is OK , and temp difference must have culprit in mechanical domain*

key word is middle in your post #1

thermal goo is Dodo;
that means that you need to dismantle all output transistors , clean thoroughly (degrease also ! ) their cases and T Al bar of old thermal goo , same goes for mica thermal pads ; if some are suspicious/cracked , put new instead , but choose/buy ones of tinier variety
then put new (tiny!!) layer on both sides of (all ) mica thermal pad(s) and mount transistors back

considering physical layout of amp , it's best to start dismantling T bar from heatsink , then perform cleaning and re-gooing of transistors one by one , which will prevent possible mixup of their position

that's step 1 , pretty important ;


*step 2 is main reason for uneven temp. at heatsinks - most certainly uneven thermal contact (physical pressure , as measure of goo condition and screws tightening (between T Al bar and heatsink) )
you can cure that easily - for step 1 - you already unscrewed all bolts holding together T Al bar to heatsink ; clean old thermal goo from both sides ( T bar and heatsink) , degrease everything ; put thin layer of new thermal goo ; put T bar back , tighten all screws evenly.

Iq difference between channels (judging by voltage across emiter resistors) can be explained by time factor , nothing else , and it's easily settable ;plenty of schematics and info for that , floating in forum

however - if you think that's too much for you , feel free to send that bad amp to OPLDF

Attached Images
File Type: jpg S500.jpg (48.3 KB, 180 views)
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Last edited by Zen Mod; 24th April 2014 at 03:04 PM.
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Old 24th April 2014, 03:32 PM   #6
Zen Mod is offline Zen Mod  Serbia
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regarding schematic , I believe you'll find something here :

Threshold S/550

Nelson Pass related | Zen Mod Blog
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Old 24th April 2014, 11:55 PM   #7
fogf1 is offline fogf1  United States
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Thank you for the info on where to begin.
There is a single trim pot on the small input / bias circuit board. Is that an input trim or is that for bias?
Here are some shots of my amp.

Threshold S 500 Front.jpg

Threshold S 500 Top Front.jpg

Threshold S 500 Rear.jpg

Threshold S 500 Inside.jpg

Threshold S 500 Input Bias Board.jpg
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Old 25th April 2014, 12:10 AM   #8
Zen Mod is offline Zen Mod  Serbia
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am I blind , or I can't see where pot is .... ?



however - it must be for Iq , what else .....

easy to test - one DVM across output terminals , second DVM across one emiter resistor , fiddle with it slightly and you'll see what is happening

but - I advise to do that after entire procedure explained in my previous post

maybe Pa will chime in with more info , in a meantime
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Old 25th April 2014, 01:11 AM   #9
fogf1 is offline fogf1  United States
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It's a small square white trim pot in the back left, behind the green cap on the input/bias board.
After I clean and lightly re-goo I'll fiddle about with it.
Thanks.
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Old 1st July 2014, 05:35 PM   #10
fogf1 is offline fogf1  United States
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Hi all,
I know it's been a long time that I've checked in.
I reseated and resoldered all the output transistors on my S/500.
New development. It's popping Negative Rail fuses on the side I worked on.
All the transistors test good. All the resistors test good.
I'm at my wits end because I can't seem to locate the problem and I'm chasing my tail.
Anyone near NJ that works on these things?
I'm throwing in the towel.
I've fixed a lot of stuff over the years but this one has me bested. Maybe I'm just getting old.
Thanks for all the help.
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