Fried one channel of a DCB1

[ClaveFremen]

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]

Did the original work before it died?

 

[nigelwright7557]

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]

Did the original work before it died?

Sure it did!

And it was great..

 

[ClaveFremen]

Hi Nigel,

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?

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]

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]

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]

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]

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]

Just keep the screwdriver away from it.