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Old 16th June 2011, 10:47 PM   #1
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Default Fried one channel of a DCB1

Some week ago I've probably fried the 2SK170s of the left channel of my DCB1 Mezmerize with static while I was tuning the Lightspeed volume control...

The right channel works fine while the left is dead so I suppose that the PS section is OK.

Now, after 20 days, I've a nice packet with 4 matched 2SK170s but before mounting them I would like to do some prelimary tests on the circuit... what should I check?

Thanks in advance
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Old 17th June 2011, 10:27 PM   #2
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Did the original work before it died?

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Old 17th June 2011, 10:31 PM   #3
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Its unlikely static blew up your transistor if it is in circuit.

A good test is to route the VAS output back into the ltp and see if the driver is working OK without any output transistors in.
Make sure the VAS is working right with bias working correctly and no significant DC output.
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Old 17th June 2011, 10:32 PM   #4
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Quote:
Originally Posted by Nelson Pass View Post
Did the original work before it died?

Sure it did!

And it was great..
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Old 17th June 2011, 10:43 PM   #5
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Hi Nigel,

Quote:
Originally Posted by nigelwright7557 View Post
Its unlikely static blew up your transistor if it is in circuit.
I've seen a spark... from the screwdriver to one of the LDRs, after that one channel of both the LS and the DCB1 is mute...

What else can be?

Quote:
Originally Posted by nigelwright7557 View Post
A good test is to route the VAS output back into the ltp and see if the driver is working OK without any output transistors in.
Make sure the VAS is working right with bias working correctly and no significant DC output.
Sorry but I don't know the terms.. what are VAS and LTP?

From what I can see (and if I understand it correctly...) from the attached schematic there's a single transistor (well also a second one but it's used as a current source, or not?)...
Attached Images
File Type: gif DCB1 buffer.gif (4.8 KB, 126 views)
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Last edited by ClaveFremen; 17th June 2011 at 10:46 PM.
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Old 17th June 2011, 10:47 PM   #6
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Quote:
Originally Posted by ClaveFremen View Post
Hi Nigel,



I've seen a spark... from the screwdriver to one of the LDRs, after that one channel of both the LS and the DCB1 is mute...

What else can be?



Sorry but I don't know the terms.. what are VAS and LTP?

From what I can see (and if I understand it correctly...) from the attached schematic there's a single transistor (well also a second one but it's used as a current source, or not?)...
I see now its a class A. Not a lot to go wrong.
You need to check the transistors for shorts.
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Old 17th June 2011, 10:52 PM   #7
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Quote:
Originally Posted by nigelwright7557 View Post
I see now its a class A. Not a lot to go wrong.
You need to check the transistors for shorts.
Thanks Nigel.

Since these transistors are matched I'm going simply to replace them all four.

It's safe to assume that since one channel works the shared PS works fine?

I should check also resistors?
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Old 17th June 2011, 10:57 PM   #8
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Quote:
Originally Posted by ClaveFremen View Post
Thanks Nigel.

Since these transistors are matched I'm going simply to replace them all four.

It's safe to assume that since one channel works the shared PS works fine?

I should check also resistors?
If they both work off the same bridge rectifier and smoothing capacitors then yes.

Yes check the resistors too.
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Old 18th June 2011, 12:37 AM   #9
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Quote:
Originally Posted by ClaveFremen View Post
Since these transistors are matched I'm going simply to replace them all four.

It's safe to assume that since one channel works the shared PS works fine?

I should check also resistors?
Just keep the screwdriver away from it.

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Old 18th June 2011, 09:55 AM   #10
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Quote:
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Just keep the screwdriver away from it.

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