Headroom / distorsion issue on BC315 (AMEK BC3)

[MrSedor]

If I have to change them, maybe I can try BC550? The 2SD786 seems to be discontinued...

[Matt Syson]

Hi
Unlikely to be the transistors. A simple DC voltage check would show if they are OK or not as the voltages on the base collector and emitter should be the same for both. If both transistors were 'dead' it probably wouldn't work at all.
Matt S

[MrSedor]

Hi and thanks for your reply!
I checked the transistor of 2 bad modules and 1 good and it's the same:
DC voltage between Emitter and Base: 14,40V ; Collector and Base: 13,40V.
So this is not a transistor problem I guess...

Do you think it could be a power rail issue on the module?

[MrSedor]

So,

I checked the CMRR, but there were no such difference between a good and a bad module.

I checked the voltages of the 2 AOP presents in the preamp section without any bad result:
U1: between 4 and 8 : 17,42V
U2: between 4 and 7 : 14,76V
This result is almost the same with every module.

But after that, I realized that the bad distortion on faulty modules didn't depend on the module gain. I tried adding 10dB more on the preamp, with the same (not more) distortion.

So I believe now there is a problem before the input amp.
I think I ve got to check if there is the same issue using the mic input of the module (input amp is the same for both line and mic in).

I'll post the schematic of this part of the module as soon I can!


All the best, and thanks for your advices!

[MrSedor]

And one more thing: I tried to send unbalanced ( cold wire cut) signals to the modules:
on a good one: lot of distortion
on a bad one : lot of distortion but 40dB lower.

...


Could it be a balancing problem?

[MrSedor]

And here's the schematic:

[MrSedor]

BC315 - HostingPics.net - H

[MrSedor]

Hi,

I just founded some zener diodes shorted (with the multimeter). Depending on the track, it s CR9, CR8 or both. Is it normal or could it be the problem?

According to the previous schematic, these diodes are just after the transformer (mine is not exactly a transformer, but the TLA version), and before the first transistors stages.

So, maybe there's the way to go?

All the best,

Rémi

[Matt Syson]

Hi
The diodes should not show a 'short circuit'.
This could be the problem.
They MUST be replaced by the same type zener diodes to protect the input from the phantom power.
Matt S

[MrSedor]

Hi,

Thanks, I m gonna buy some BZX55C22 diodes to replace the bad ones.

This console was used about 15/20 years in a radio studio, maybe it comes from a bad use of the phantom power.

But now I use the console just for mixing throught line inputs, so I don't use phantom power. Could it still cause this distortion issue?

Anyway I'll change the zeners soon.

Many thanks!

[MrSedor]

Hi again!

From an AES convention Paper, it appears that sending phantom power (via the mic input) to a line output device can damage the back to back protection zener diodes in the preamp (which are CR7, CR8, CR9,CR10, according to the BC315 schematic).

They wrote it results a distorsion "greater than 0,3%" at 1khz 1Vrms for 0db gain, which is the sound I can hear while I'm sending audio throught the bad modules.

My console was before used in a school radio studio, so I think this phantom mistake probably happened in the past.

The TLA preamp is design to amplify both line and mic with the same circuit, so my distorsion problem may come from this damage.

What do you think?

[MrSedor]

Hi,

I think I begin to understand, please correct me if I'm wrong:

On a bad module, CR9 is shorted in one way, so it becomes like a "regular" diode.
So the back to back zener diodes are now back to back zener diode / regular diode.

When signal comes from the cold and goes to the transistor Q1, the negative part of the signal pass until -22V, which is ok at this point of the circuit. But the positive part of the signal is shorted to ground and doesn't come to Q1. It results a very distorted signal at Q1 output, and then distortion when cold and hot signal are merged to be unbalanced at U2 output.

Is that right?

Thanks,

Rémi

[MrSedor]

Hi again,

Yesterday I changed diodes on 2 bad modules, and it was that!
Now it is very very good sound! Much more transparent and detailed.
In order to be sure, I ll change all the protecting diodes today...

Many thanks

Rémi

[mjrippe]

Hi Rémi, It looks like you solved this pretty well on your own! Thanks for sharing what you found so that others will have this knowledge in the future :-)