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Onkyo TX 38 Memory Loss

Picked up this TX 38 for $5.00 as not working. Got it home, after cleaning out the dead bugs shorting out some of the traces and clogging the POTS. Got it to turn on play ok for a bit Then the right channel got distorted. There’s several leaky caps that are getting replaced but another problem is it doesn’t hold any of the presets when turned back on. The internet says it’s a bad .047F, 5v cap at C712. Ok easy enough. Out with the old, in with the new. BUT there’s NOTHING but an empty socket , no cap, looks like there’s never been one sitting at C712 on this board. With the amount of crud that was on this board I doubt if anybody opened the case just to snag a cap. There’s no traces of desoldering there either, unless they were a real pro.
Am i missing something here? Is the internet, and the schematic, wrong about a cap being there? If I do drop a cap there will this fix the memory problem?



Recaptcha's picture

Re: Onkyo TX 38 Memory Loss

It is possible that there is a revision difference between your board, the internet, and the schematic. Manufacturers will change their circuit designs slightly to lower cost, and to improve design. Capacitors are designed to filter, and it is highly possible that enough filtering was already present, so Onkyo just removed one. Excess can be an issue at the beginning of production. Mainly, it is important to identify what the capacitor does, if it is on the sound circuit, or on the power circuit, or something else.

I've repaired many PCBs with loads of missing components, and never seen a problem, but you have two sources telling you there should be a can sitting on that board. If it were me, I would replace. Let me know how it goes.

Re: Onkyo TX 38 Memory Loss

That capacitor is not for filtering; it holds a charge that powers the microprocessor when the tuner is turned off.

Recaptcha's picture

Re: Onkyo TX 38 Memory Loss

Really, well that's an interesting twist. That would explain why it isn't working, now wouldn't it?

WOW, I just looked the cap up on google, and that is definitely a mini battery like capacitor. Notice how much it resembles a coin cell.

There actually called Supercapacitors BTW.

Re: Onkyo TX 38 Memory Loss

I'm not looking on throwing a lot of money getting this one functioning. Picked it up mostly to improve my diagnostic/repair skills. Found the .1F supercap for a reasonable price on eBay. It ships from China. Won't get here until the new year. The other caps will be here tomorrow. Hopefully it was a board re-design that resulted in the empty slot. If so there's a chance one of these caps will cure the TX's memory problems.
I'm also working on another TX model from about the same era. Completely different board design with no memory problems.

Re: Onkyo TX 38 Memory Loss

I can't imagine it being a "redesign" as a tuner with no memory would be a PITA; you would have to manually tune every time you turned it on. That circuit is really only two components: a resistor to limit current and the capacitor. If the new capacitor does not charge up I would look at the resistor; as said before, it will take hours to achieve a full charge. Of course, observe proper polarity.

Re: Onkyo TX 38 Memory Loss

Could have been left out at the factory; stranger things have happened. I like to use .1 Farad; they hold memory longer but take longer to charge up--just leave the tuner on for several hours the first time. When installed, check the voltage across the cap; should be just at 5 volts after charging. They usually have a large value resistor in series to prevent loading, so don't be surprised if it takes a long time.

Re: Onkyo TX 38 Memory Loss

Thanks. Ordered a .1F cap. I’ll let you know how it goes.
On a similar note; what’s up with the Onkyo TX series? I see these for cheap usually in miserable condition almost every time I go looking.

Recaptcha's picture

Re: Onkyo TX 38 Memory Loss

Onkyo TX is a famous line of receivers, but only the woodgrains are really famous. Any Onkyo TX, is nothing to sneeze at. As far as condition, if it is made out of material that shows age and wear easily, that would explain condition issues.