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Old 10th February 2016, 08:40 PM   #1
Ripcord is offline Ripcord  United States
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Default Threshold 400A Cascode

Need some advise. I was given a Threshold 400A Cascode from my buddy, to keep. The problem, it blows the rail fuses as soon as you turn it on. I popped the top off and just visually looked around. Looked very clean, no sign of any blown or distressed parts. I've built a few solid state and tube power amps, power supply's and such, pretty basic stuff, but am not a savvy bench tech. I own a variac, dim bulb tester and a scope, (with limited knowledge on it). I assume there's no "do this and there's your problem" answer but I hate to just let it sit there. I want to use it! What do you think?
Attached Images
File Type: jpg 400A Back Smaller.jpg (260.0 KB, 153 views)
File Type: jpg 400A Front Smaller.jpg (268.1 KB, 155 views)
File Type: jpg 400A Inside Smaller.jpg (325.6 KB, 156 views)
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Old 10th February 2016, 09:04 PM   #2
Zen Mod is offline Zen Mod  Serbia
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- how to repair an amp ?

- simple , take it and repair it .......

-best way to catch a wabbit ?

-simple , put some salt on his tail .........



I'm always going with PSU check first (shorties , blown diodes,shorted caps) , then output side (one channel , then other) - checking outputs for shorts then conduction (diode test)
that would need dismantling either emiter/base resistors or transistors itself from circuit

then front end - first for shorties then , if needed , dismantling each semiconductor from pcb and checking it with diode/megaohm test and functionality test (if needed)

so , see things are simple ....... even if I didn't mention setting/testing procedure afterwards

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Old 10th February 2016, 09:36 PM   #3
nec3 is offline nec3  Turkey
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With rail fuses removed, check power supply output positive and negative voltages.
With no power, check if any short at the + and - rails of circuitry to the ground.

This thread may be useful :

Threshold 4000 repair. Need some help.
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Old 11th February 2016, 03:25 AM   #4
Ripcord is offline Ripcord  United States
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Ok ZM, I took your advise. Got some salt, went outside...... probably easier to work on the amp instead I'll start with the PSU. And nec3, thanks for the tips. Question, would it be ok to desolder the four wires for the VU meters so I could take the front off for easier access?
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Old 11th February 2016, 08:02 AM   #5
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Take pictures, many pictures before desoldering and dismantling. This is a very useful tips - trust me! ;-)
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Old 11th February 2016, 07:55 PM   #6
Ripcord is offline Ripcord  United States
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Ok, so I decided to start taking it apart and yes, I have taken a ton of pictures so far. Anyway, here is what I have found so far.
1. One of the output transistors has been replaced so it has been worked on before. ( I circled a second one on the right because I'm not sure if it's original or not.)
2. One of the 0R68 emitter resistors looks puffy
Posting pictures so you can have a look.
Attached Images
File Type: jpg 400A Output Transistors.jpg (340.9 KB, 95 views)
File Type: jpg 0R68 Emitter Resistors.jpg (285.6 KB, 93 views)
File Type: jpg 0R68 Emitter Resistors Close.jpg (273.2 KB, 69 views)
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Old 11th February 2016, 08:17 PM   #7
Zen Mod is offline Zen Mod  Serbia
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just those with CLASSAsomething are looking as genuine (as from factory) outputs

resistor - it certainly looks poofed
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Old 11th February 2016, 08:21 PM   #8
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I would hav changed all output-transistors (eqvivalent is recomanded in another thread). Change resistors too!
And all electrolytic caps.

Careflly check all solderings, meashure for unwanted shorts and use variac to start amp :-)
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Old 11th February 2016, 08:57 PM   #9
Zen Mod is offline Zen Mod  Serbia
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unfortunately , yes

those transistors need to be matched per channel

N one matched group
P one matched group

it would be too ambitious to try matching so many of them N to P
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Old 11th February 2016, 09:05 PM   #10
Ripcord is offline Ripcord  United States
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To recap, I need to replace all output transistors with matched N's and matched P's, replace all electrolytics and 0R68 resistors and reflow all solder joints. Yikes. Ok, not saying I can't do that but will have to think on it for a bit. Thanks you guys for all the advice.
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