Diodes for marshall vs150h TSB

[emux2]

Hello,

This is me first post here but I want to start out by thanking Enzo for sending me the TSB and schematics for my VS150h. I am trying to locate the diodes for doing the TSB. It calls out BZX55B5V6 and all I can find is BZX55C5V6. I looked at the data sheets and the major difference is that the "B" has a Zz@Iz resistance of <40 and the "C" has a Zz@Iz resistance of 25. Would it me a one for one substitute or would I need to add a few more "c"'s to equal the resistance of the "B"? Or am I just being too critical?

[Enzo]

It might be more informative if you specified what those parts do in the circuit. Not everyone here has the drawing, and without studying mine I have no idea what diodes they are.

Aha! I got lucky. You mean the protection zeners on the input jack?

They are just 5.6v zeners wired back to back. They are in shunt across the input. SO any time the signal at the input exceeds about 6v either way, one of them will turn on and limit the spike to that level. Whether that shunt turns out to be 25 ohms or 40 ohms is of little consequence. AS far as the excess voltage is concerned it is a short circuit. Thinking about it, I guess the 25 ohm version you found is 15 ohms "better."

And if those zeners are what you refer to, are you sure they are not already on the input jack little board? is there a little British flag sticker on the rear panel?

[emux2]

Yes the diodes are to shunt the input. The back of the amp has the British flag but there are no diodes on the input jack. I was told this had happened before and was fixed. Apparently the shop did not has the TSB. Thanks again for your help.

[Enzo]

They are not right on the jack, they are on the small circuit board the jack is on. To update the amp, they just swapped the little board complete with the updated one. It is not something extra soldered onto the old board, it is a complete new board designed to include the parts. That is what the flag sticker is supposed to represent.

This bulletin solves a certain oproblem, but does not prevent parts failing for other reasons.. But it is simply two zeners in series, so even if it turns out redundant, no harm. And if I am wrong about your sticker, then you are covering yourself.

[emux2]

I pulled it apart today and found that the input board matched the picture on the service bulletin so it didn't look like the diode mode was there or incorporated into the board. I was told that the first time this happened that it was fixed under warranty and it looks like they just swapped the preamp board. So I put the diodes on and changed the chip out and it nows works like a champ. Thanks again for all of your help. I most likely would not have been able to trouble shoot it down to the chip called out in the service bulletin. Now I have a fully functioning amp for the cost of two diodes and an IC chip.