Fried one channel of a DCB1

ClaveFremen · 16th June 2011, 10:47 PM

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

Nelson Pass · 17th June 2011, 10:27 PM

Did the original work before it died?

nigelwright7557 · 17th June 2011, 10:31 PM

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.

ClaveFremen · 17th June 2011, 10:32 PM

Quote:

Originally Posted by Nelson Pass

Did the original work before it died?

Sure it did!

And it was great..

ClaveFremen · 10:46 PM

Hi Nigel,

Quote:

Originally Posted by nigelwright7557

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

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?)...

nigelwright7557 · 17th June 2011, 10:47 PM

Quote:

Originally Posted by ClaveFremen

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.

ClaveFremen · 17th June 2011, 10:52 PM

Quote:

Originally Posted by nigelwright7557

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?

nigelwright7557 · 17th June 2011, 10:57 PM

Quote:

Originally Posted by ClaveFremen

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.

Nelson Pass · 18th June 2011, 12:37 AM

Quote:

Originally Posted by ClaveFremen

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.

ClaveFremen · 18th June 2011, 09:55 AM

Quote:

Originally Posted by Nelson Pass

Just keep the screwdriver away from it.

16th June 2011, 10:47 PM	#1
ClaveFremen diyAudio Member Join Date: May 2005 Location: Milan, Italy	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 __________________ Dario ClaveFremen "Bailando Salsa en el Sietch"

17th June 2011, 10:31 PM	#3
nigelwright7557 diyAudio Member Join Date: Apr 2008 Location: Carlisle, England	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. __________________ http://www.murtonpikesystems.co.uk PCBCAD40 pcb design software.

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17th June 2011, 10:27 PM	#2
Nelson Pass The one and only Join Date: Mar 2001	Did the original work before it died?

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