Yamaha EMX512SC mixer-amp SMPS query

[1984ARC]

Can anyone please help me with a query on versions of the EMX512SC SMPS? While repairing one of these (H variant, for UK) I was baffled by some voltage readings with reference to calculations that I had made from some resistor values on the schematic - until I realised that the unit I am working on has two resistors that are different values to the schematic.

These are R443 & R445, in the bootstrap circuit around Q414. In the schematic (from the Service Manual) they are 15K & 10K respectively, but in the unit I am repairing they are both 33K. This is quite a significant change, so I am thinking there must be some good reason for it (perhaps reliability, or the range of the mains supply voltage properly catered for). These resistor values are not indicated on the schematic as dependent on the model variant. (So far, I haven't found any other differences.)

What I would really like to know is which are the "improved" set of values, the 33K & 33K that I have, or the 15K & 10K in the service manual schematic? (I have the one that's downloadable from here and several other places) - I don't know how to determine which is more up-to-date, the service manual or the unit I have.

I would like to know because I would want to update the unit I am working on, if the service manual resistor values are the ones that are more up-to-date - mostly just to improve the unit.

[Also, rather more academically, because I have calculated that, on the unit I have (with 33K's), then at switch-on with a 245V ac supply, until Q414 emitter voltage starts to rise (slowed by R440 and C423) there will be just over 22V from gate to emitter of Q414, but its absolute maximum Vge is 20V. I am wondering if this may damage Q414, after many switch-ons. (A possible mitigation is the slowed ramp-up, at switch-on, of the voltage that feeds Q414 gate, caused by the initial volt-drop across the main soft-start resistors. But the precise effect of that is a bit uncertain - not least when you consider the case of a switch-off and then on again after a few seconds, during which C423 might have substantially discharged but the main reservoir capacitors still hold up, which means hardly any volt-drop across the soft-start resistors when switched back on, so no - or reduced - mitigation from that.)]

Any assistance or comment very much appreciated. Many thanks.

[Enzo]

NO NO NO. FIX first, update later.

This is a circuit, not a collection of parts, and you might "update" those parts only to find much later than that change also required some other change for it all to work, and we'd be halfway between the versions.

This thing presumably worked before, as it sits, so the values you see in it are correct for it. As you suggested, there could be many reasons for the difference, and for vastly different purposes, so rather than guess, leave it alone. As to improving the unit, one needs to know in what manner it might be improved. If these are the startup resistors, either it starts up reliably or it doesn't.

Welcome to the forum. Good form would be to provide a link to the schematic. I don;t have it myself, can you post a link?

[doctor]

File is about 19MB:
YAMAHA EMX512SC-EMX312SC Service Manual free download, schematics, eeprom, repair info for electronics

[1984ARC]

Thanks, yes.

Thanks, Enzo, for your helpful comments. I often think it would be great to get into the head of the designer of some circuits to see what they were thinking - or, in this case, why changes were apparently made later (perhaps by a different person...) But that's not going to happen, so I do see what you mean by your advice.

Sometimes there are later addendums to service manuals, along the lines of "when repairing a unit, implement these updates". I guess those were the lines I was thinking along, but I do agree that in general it's unsafe to assume such an action is the right thing to do without that official instruction. And I certainly wasn't implying any advice to others to take such a course in this or any other case - it could be a dangerous thing to do without the knowledge and understanding of everything necessary in the particular circumstances concerned.

And thanks, doctor, for beating me to it with the link to the service manual (for info, part of the PSU schematic occurs part way through it, but the whole schematic is right at the end - the PSU section is on the last page).

[Jazz P Bass]

How about firing off an email to Yamaha in Japan?

Include the serial number.

You never know.