Marantz 2325 2SA747 - low breakdown voltage

[ocdecio]

I am working on the amp boards for a 2325 and checking the outputs I found one of them (A747) testing a breakdown voltage of 167V, whereas all others test 247V or so.

Is that a sign that I should replace the whole set? Would replacing just the A747 with another pulled from a working unit be OK?

 

[rBuckner]

Well, it won't see that much voltage, but I'd wonder why it's different. Did you use a curve tracer to see that? Just curious. What is the gain on an Atlas tester or equivalent? It makes me nervous when outputs get near a reading of 15.

 

[ocdecio]

Hi Randy, here's the "bad" and a "normal" transistor. This is the tester I use.

Gain on the first transistor is 38 which isn't bad.

 

[runningdog]

Gain on the first transistor is 38

Are you measuring gain on the DY294, or an Atlas Peak, or?

Here's some results for a pair of 2SA747's from a 2325 amplifier:

 

[ocdecio]

Are you measuring gain on the DY294, or an Atlas Peak, or?

Here's some results for a pair of 2SA747's from a 2325 amplifier:
View attachment 2897555
View attachment 2897556

Nice, thank you!

Measured gain from an Atlas equivalent. The DUOYI gave it like 15 or so but I go by my regular meter when it comes to gain.
So you'd think 167 should be OK?

 

[runningdog]

So you'd think 167 should be OK?

I do not know the answer your original question. As a percentage it is a lot lower than the other one. Is the gain similar between the two?

 

[ocdecio]

I do not know the answer your original question. As a percentage it is a lot lower than the other one. Is the gain similar between the two?

Yes gain is similar. I just found it odd to have a single transistor in the 160V range all others in 220+V

 

[ecluser]

All these measurements largely exceed the Vceo rating (-120V) of the 2SA747. The maximum voltage these transistors will see in the 2325 is 120V. Furthermore, in this circuit there is a base resistor of 47 Ohms , so the important parameter to measure is the Vcer (with R=47). At this low base resistance, Vcer is always larger than Vceo. In other words, don't worry.

 

[ocdecio]

All these measurements largely exceed the Vceo rating (-120V) of the 2SA747. The maximum voltage these transistors will see in the 2325 is 120V. Furthermore, in this circuit there is a base resistor of 47 Ohms , so the important parameter to measure is the Vcer (with R=47). At this low base resistance, Vcer is always larger than Vceo. In other words, don't worry.

Thank you! I won't worry about it then, and put it back with the others. This is great information so we don't replace these (excellent) outputs unnecessarily.