Dead Left Channel Troubleshooting

[sarakisof]

I have a phono mixer console here with a dead Left master channel. The mixer uses many socketed opamps on pre and amp section and gets two transfo balanced XLR outputs.
Replaced almost every opamp that had to do with that channel with no luck.
Got the scheme from the company. You cannot find online. Blurred but still readable.



After backwards signal tracing starting from the XLR out and going all the way back, I'm finally close to something.
The signal gets disappeared just after this resistor. I get signal at IC1's pin7 but after that resistor it gets lost, just like I'm showing below.

Green=signal, red=no signal.


Before start changing everything around the Q1 Nfet BC264 transistor, could you help which component could be the most culprit? I'm guessing the BC264, that trimmer resistor there and the two e-caps? Replaced the first upper e-cap but didn't change the issue. Then I'll replace the Q1.

Any thoughts?

[PRR]

> replace the Q1.

Just snip 2 legs.

That whole business with the transistors must be a "mute". If stuff goes bad, it may stay muted forever.

[sarakisof]

That whole business with the transistors must be a "mute". If stuff goes bad, it may stay muted forever.

I don't get 100% what you're saying here Mr. PRR.

Took out the nfet transistor. Black probe on Drain, red on Source. Diode mode. Got continuity between D and S.

[antonis]

Took out the nfet transistor.

What happens with transistor out..??
(from signal continuity viewpoint..)

[GibsonGM]

Not sure if this helps, but if it's stereo and there is another channel that's the same and working, comparing the voltages between the 2 can sometimes pinpoint what's going on. But maybe you've already isolated the offending section so it may not be worth much.

[sarakisof]

 

Took out the nfet transistor.

What happens with transistor out..??
(from signal continuity viewpoint..)

Will check later when return home Αντώνη.
Meanwhile I'm having hard time sourcing a BC264 locally.
I cannot even find BF244's damn every old component becomes harder and harder to source nowadays.

Any other European N Channel FET equivalent that could work here?

[antonis]

I cannot even find BF244's

Did you ask your neighbour Στάθη..??
https://www.stathisnet.gr/transistor-vf244a

[sarakisof]

I cannot even find BF244's

Did you ask your neighbour Στάθη..??
https://www.stathisnet.gr/transistor-vf244a

Indeed. "Αναμένεται σύντομα" if you look carefully and they won't stock it again as they told me.
Do you have any BC264/BF244/5s laying around?

Can actually BF244/5s be used safely instead of BC264 here?

[antonis]

I think (at least, for particular circuit) J111/112/113 should do the job..
https://datasheetspdf.com/pdf-file/496899/Philips/BC264/1
https://www.acdcshop.gr/content/J111.pdf

[sarakisof]

 

I think (at least, for particular circuit) J111/112/113 should do the job..
https://datasheetspdf.com/pdf-file/496899/Philips/BC264/1
https://www.acdcshop.gr/content/J111.pdf

I'll order from your namesake from Kalamaria, will add some other beauties too so to combine shipping.   

[sarakisof]

Took out the nfet transistor.

What happens with transistor out..??
(from signal continuity viewpoint..)

Will check later when return home Αντώνη.

Boom 💥 Left channel is alive!

That whole business with the transistors must be a "mute". If stuff goes bad, it may stay muted forever.

I don't get 100% what you're saying here Mr. PRR.

I get it now. 😁

For the record, I don't think i should leave the transistor out so I'll go on ordering a nos one as stated above.

PS. "To be continued.." I have to sort out an issue concerning the 4th fader pre channel before the unit gets in 100% working condition. I'm getting a popping noise but i think i got it, just left it for the end. If I'm in trouble I'll be back.

Thanks guys, it's amazing how many things you can do with a diy signal tracer and a 1khz sine from YT when in luck of oscilloscope.

Cheers.

[PRR]

The transistor can be good, but getting a bad signal from defective capacitor, resistor, or joint. Close inspection. Compare voltages with a good channel in all modes.

The mute is probably not "essential". Factory designers add pennies to keep equipment "polite", no bangs or thunks at power-up etc. For personal use, as long as you know "Klank OK", you don't need mutes (unless it is spitting speaker cones across the stage).

[sarakisof]

I posted above:

I have to sort out an issue concerning the 4th fader pre channel before the unit gets in 100% working condition. I'm getting a popping noise but i think i got it, just left it for the end. If I'm in trouble I'll be back.

This "clocking/metronome" pop noise gets inserted just where the red arrows are showing.



Just after the Level pot. Pot measures fine. Noise is the same for L-R channels (like it's in Mono). When i turn this Level pot the noise gets pitched, repeated more quickly and its volume doesn't get affected. I recorded it so you can hear by yourselves. Notice how its pitch increases as i turn the pot.

https://we.tl/t-BkHxIwbfn3

Any thoughts guys?

EDIT: Issue fixed. Some e-caps , replaced both IC s in that channel. Machine 100% in working order.