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Old 22nd March 2015, 01:37 AM   #1
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Default Threshold 4000 repair. Need some help.

A friend gave me a Threshold 4000 that was in need of help. Both channels seem to be shorted. Here is the worst one.

Click the image to open in full size.
Jon at vintageamprepair.net help me with the output sub. Here it is back together. There is still something not right. See the voltages in the picture at 120volts input. The 1k resistors number 3 & 4 get hot pretty quick.
Click the image to open in full size.

What should I check next?



This schematic shows the differences I have seen on my board so far. I moved it here from post 17 so I can update it.




Click the image to open in full size.
400



Click the image to open in full size.Volts30 with working no outputs

Click the image to open in full size.diode fixed

Click the image to open in full size.Volts30 with rebuilt

Last edited by tonebells; 13th May 2015 at 01:14 AM.
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Old 22nd March 2015, 10:05 AM   #2
Zen Mod is offline Zen Mod  Serbia
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I see you already replaced all tantalums
whatever , my usual procedure with amps that burned , is to check every resistor in situ; if I have some doubts in measurement , desoldering and lifting one end of resistor is enough to clear doubt .
same applies for diodes

also - fastest way is to , one by one, desolder each semiconductor , test it and put it back or replace , if bad .
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Old 22nd March 2015, 02:25 PM   #3
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I was hoping to find a shortcut as there are a lot of transistors on there. I guess I will get to work.

Tony
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Old 22nd March 2015, 02:52 PM   #4
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shortcuts through mileage ; no other way
though , in case of that ,you wouldn't ask for advices , maybe just for schematic
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Old 22nd March 2015, 07:35 PM   #5
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I have this schematic.
Click the image to open in full size.
I checked everything on the board and all test OK. The bias trimmer is set to 1.5K ohm is that a good starting point.
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Old 22nd March 2015, 10:05 PM   #6
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recheck everything

bias value isn't critical to offset , which is too high

if you have same offset on both channels , you have same mistake/culprit on both
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Old 23rd March 2015, 10:37 AM   #7
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Do you know what the normal current draw of this amp is with no load?
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Old 23rd March 2015, 12:51 PM   #8
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it really depends how much is biased

I believe it can be anything between 20 to 80W/channel , plus general losses ..........
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Old 23rd March 2015, 01:39 PM   #9
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Quote:
Originally Posted by tonebells View Post
A friend gave me a Threshold 4000 that was in need of help. Both channels seem to be shorted. Here is the worst one.


Jon at vintageamprepair.net help me with the output sub. Here it is back together. There is still something not right. See the voltages in the picture at 120volts input. The 1k resistors number 3 & 4 get hot pretty quick.
Click the image to open in full size.

What should I check next?
The voltage at the joint of resistors No:3 and 4 must be same as above (49V) at the positive side.

No power supply schematics ?
Where is that joint connected to ?

+23V must be 0 Volts (assuming it is connected tothe output through fuse).

Last edited by nec3; 23rd March 2015 at 01:52 PM.
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Old 23rd March 2015, 01:48 PM   #10
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yeah , it must be the same - that's voltage reference for upper and lower cascodes

see at sch 1K and 1K5 (though - it would be good to recheck their position , it looks suspicious on pic ...... 1K5 inside , 1K outer - to rails)
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