Aiwa MX-Z3300M receiver repair, no display, capacitors in series

[Chungalin]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

Sorry, all display-related repairs I've made on this model (and others alike) were successful with just replacing those 2 caps. Are you sure you haven't damaged any tracks during replacement?

[makissat]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

Both capacitors are 47 mfd. I replaced the C16 capacitor instead of a 33 mfd capacitor,because i didn't have a similar one, with a 44mfd capacitor.

[Longbow]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

Well this is an interesting circuit in any case. I've never seen a doubler hooked up like this, so the question is whether the designer just needed a quick and ugly 70 volts, or whether there is another reason for using this weird configuration using 2 electrolytics in series.

Our typical supply for the VFD isn't regulated either, like the one shown here. It seems like there must be another reason for part failure every few years. One or both diodes in the circuit being faulty might stress the caps...? Another strange thing is that the 2 caps are different values, but not that different. Well, I give the originality award to whomever was behind it. You know how the EE programs say that practical experience can be left to on-the-job training.

It would be fun to look at some waveforms at various points in this circuit. The latest post also might hold some clues to the real culprit, since replacing the 2 series caps did not solve his problem.

[Longbow]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

have already changed the capacitors C15 and C16 but the display still doesn't work

You could start by listing the voltages that you measure at various points in the doubler. That might help the rest of us figure out why the unit is built like this.

[makissat]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

At last I managed to fix the machine ... After a general check of the main panel I found no problems ! So I moved on to the display panel and there I found two burned resistors and a burned electrolytic capacitor! I replaced them and the screen is working properly now ! Here is a photo of the display board


https://postimg.org/image/6wrhyawxz/


Has anyone found a way to overcome once and for all the 4-year main board capacitor problem? At the C15 and C16 capacitors?

[Chungalin]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

Has anyone found a way to overcome once and for all the 4-year main board capacitor problem? At the C15 and C16 capacitors?

Have you read the 3rd post of this thread?

[R_J]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

I just checked a mx-z9300 schematic and the two caps used in that model are: C15, 47uf/25v & C16 100uf/25v. they also changed C17 from 47uf/63v to 47uf/100v

[redwire]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

I did a LT SPice run, to confirm the voices in my head.

It's a lop-sided voltage doubler. No need for a bi-polar part as the capacitors are not supposed to fully discharge between cycles.

The problem is the ripple current is very high ~220mA peak which is too much for most vanilla electrolytics this size.

Although you need ~20uF part, Akai used the two larger 33uF and 47uF parts in series, as larger value caps can take more ripple current. I could not find a 22uF 50V rated to >220mA 60Hz ripple anywhere.
Adding a 22R 1W series resistor lowers ripple to 140mA.

I'd use two Panasonic EEU series, 47uF 50V rated 240mA 60Hz ripple and put a 22R 1W resistor in series with C15/C16.

I used ESR 0.7 ohms for 47uF and 1.5 ohms for the 33uF in the simulation. Lower ESR makes ripple current worse.

[Longbow]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

Well, I hardly think this kind of technical fanfare addresses the issue at all. It has already been determined what this circuit is and how it works. Spice models, in my personal opinion, belong in the classroom, (and are especially good at L/C filter design). The main questions were why there are different values for the 2 capacitors, and next what else besides the main doubler capacitor failure might be responsible for the continuing failure mode.

This circuit differs from the more typical doubler in that it uses electrolytics instead of mylars (for example). The leakage current in a 2-electrolytic circuit is going to affect the operation, by placing unpredictable voltages across each capacitor. The voltages are unpredictable, because the leakage current in the 2 unequal capacitors is different and unknown. That current will also change as the capacitors age. These unknowns might relate to part failure, or maybe not.

Congratulations to makissat for scoping out problems on the display board. Unexpected increases in display board current draw definitely are an important possibility to explain the above periodic failures. I would personally be more satisfied with his solution, and suggest that Chunglin check those display parts to see if they are failing as well. Please post results!

The only other reason for unequal doubler capacitors that I can think of at the moment would be a space problem on the pc board. Perhaps they just did not have room for 2 large equal capacitors - nothing more technical than that.

[Chungalin]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

The lack of space theory doesn’t sustain. This is a rather big receiver (43cm wide old style, I think) so caps could have been larger.

Remember that last repair (one year ago) I put 100V caps instead of 50V, so now timing will be different. Also, remember that I always talk about work repairs, not my stuff, and it’s up to the customer to bring me back the receiver if he wants to repair it again. This is not a controlled experiment, unfortunately.

Anyway, from an electronic workshop point of view ($$$), it’s a "dream" repair, because fails again in a long enough time that customer doesn’t complain, the diagnosis time is almost ZERO, and it always works again with 2 cap replacement (except makissat...).

Please understand me, I try to make repairs that last as long as possible, otherwise I wouldn’t have opened this thread, just replace the caps with same values and wait the boomerang to come back in a few years.

[redwire]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

It's not their rated voltage, it's not their capacitance value, it's not them being polarized, it's not a PCB size problem.

Like I said, the problem is high ripple-current killing the capacitors.

I would say that identifies the issue that has people puzzled and guessing as to why the caps don't last. It also explains why 33uF+47uF was used, for those who understand.

[Stormforce]

Re: Aiwa MX-Z3300M receiver repair, no display, capacitors in series

Hey guys sorry to jump on the thread I have the MX-Z9400m, the display has suddenly stopped. I have noticed when powering on the amp the red LED on the volume control flashes 4/5 times I persume giving a fault code?

Would this issue be as described in the thread?