Mezmerize DCB1 Building Thread

Well a BC517 is installed, so must either be faulty, or does the slightly low 11.3V output of the currently fitted 7812 affect this voltage too?

Have ordered the following spares anyway.
 

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Salas

diyAudio Chief Moderator
Paid Member
2002-10-08 11:31 am
Athens-Greece
but having to take the board out (which is a bit of a pain in my build) I'm going to replace those two as well as the regulator to be thorough.
Not to put the board back after replacing those semis just on hope all relay routes work, I have an idea. While the LM7812 will be out, you could feed external +12V/0 DC to the "note" jumper and to a ground point with crocodile leads. From a battery, some small psu etc. To can debug the whole relays system without the rest of preamp operating. Giving you freedom. No need of sinking the Mosfets & full access to the solder side.
 

Salas

diyAudio Chief Moderator
Paid Member
2002-10-08 11:31 am
Athens-Greece
Well a BC517 is installed, so must either be faulty, or does the slightly low 11.3V output of the currently fitted 7812 affect this voltage too?

Have ordered the following spares anyway.
Don't know, possibly. I have not met 11.3V output from a working LM7812. Such chip regs have an output voltage tolerance spec but not allowing that low.
Where 11.3V is to be expected is across the relay's coil (same as across the fly-back diode) because 0.7V VCE is stolen by the driving Darlington. 12V relays have 9V must operate threshold though. BC550CTA is what we usually had for the other transistor. You can also get those instead of CBU if stocked, not to wait. Whatever is stocked. Only difference is in how their leads are formed.
 
Not to put the board back after replacing those semis just on hope all relay routes work, I have an idea. While the LM7812 will be out, you could feed external +12V/0 DC to the "note" jumper and to a ground point with crocodile leads. From a battery, some small psu etc. To can debug the whole relays system without the rest of preamp operating. Giving you freedom. No need of sinking the Mosfets & full access to the solder side.

Great idea, thanks! 🙂 I’ve got a 12V psu from an old laptop so could use that for the testing phase befre putting the board back.
 
Don't know, possibly. I have not met 11.3V output from a working LM7812. Such chip regs have an output voltage tolerance spec but not allowing that low.
Where 11.3V is to be expected is across the relay's coil (same as across the fly-back diode) because 0.7V VCE is stolen by the driving Darlington. 12V relays have 9V must operate threshold though. BC550CTA is what we usually had for the other transistor. You can also get those instead of CBU if stocked, not to wait. Whatever is stocked. Only difference is in how their leads are formed.
I’ll start by just replacing the reg, as that 11.3V output is outside the usual 2% tolerance spec. I cannot see how it might have been damaged, so perhaps it was one that got through QC testing when it should have been rejected. It’s an ST brand which are usually reliable.
 
Is that output measured with the red probe on pin 3 and black probe on pin 2 GND?

*Pull it out but don't replace it before the 12V external PSU aided debugging. Not to have a bridge back to the rest of preamp during that.

Black probe was attached to the mounting tab of the reg, which I believe is attached to pin2 internally.

Yes I’ll defintely do this. Just had notification my order has shipped, so hoping I can get going again by midweek.
 

Salas

diyAudio Chief Moderator
Paid Member
2002-10-08 11:31 am
Athens-Greece
Thank you.
I was wondering if I save for cheaper transformer or expensive Japanese made.
A reasonable good quality transformer is enough for DCB1 because its line level and no gain. Not very susceptible to a hum field. An R-Core is very nice but not necessary to it for instance. But it would have made a big mains noise interference difference in a phono stage if installed in one same box.
Electricity in Japan
 
Is that output measured with the red probe on pin 3 and black probe on pin 2 GND?

*Pull it out but don't replace it before the 12V external PSU aided debugging. Not to have a bridge back to the rest of preamp during that.

Hi Salas.

I rigged up a 12V supply, using the power LED ground, and the NOTE area. I stupidly didn't remove the 12V reg first though (and you told me to above!). My fault for doing this whilst tired :/ The LED string on one side lit up. Will I have damaged anything by turning it on briefly like this with any bridge back effects?
 

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I wouldn't think you damaged something in the preamp's main circuits with just 12V. You probably energized the plus side shunt reg for a moment. Maybe the 7812 chip reg itself could have sustained a hit because reverse biased. But it is the suspicious one to replace anyway.

Thanks, that's good to know :) So with the reg. now removed, do I make the same measurements again around the BC550/517 delay area? I noticed the output relay clicking, but again no relay clicks from rotating the input selector switch.