Dead Roland AX synth - Capacitor problem?

[Crystaleyes]

Got a question regarding how best to identify what the problem is on the main pcb of this Roland synth.

Symptoms are, that it turns only for a second... All the lights and display flash, then it turns off.

I had thought it could be a shorted cap so removed the main pcb and tested all the caps for a short to ground using a resistence tester, however none were.

Next I pulled out the Chinese T7 component tester and found 7 caps which are connected in parallel, all reading 36pF (when these caps are rated between 470 and 10uF). None of the other caps on the board measure less than 10uF.

The caps are all the aluminium can smd type, but my question is why am I getting these readings?
Obviously, if one cap in a parallel setup is shorted, then they will all read as shorted, however I'm not getting a 'shorted' reading in ohms and actually, the T7 always reads around 36pF if the 'Test' button is pushed with no device connected, so if it was only on one cap, that cap could be open.

Some pictures below.
The four together in the second pic are included in the seven.

The owner had wanted to just replace the board, but none are easily found online.

Obviously, I could just recap it, but I want to understand why I'm getting these readings, and whether the symptoms are even typical of bad caps?

[stj]

Re: Dead Roland AX synth - Capacitor problem?

calibrate your tester - short the 3 terminals and turn it on

you cant test caps in-circuit, people who say you can are full of shit.
maybe 30-40 years ago, but not on modern stuff.
the tester is effected by other parts - it has a high enough voltage to turn transistors and diodes on!

[Crystaleyes]

Re: Dead Roland AX synth - Capacitor problem?

Hi fella.

The tester was calibrated a couple of days ago but can do it again.

As for testing in circuit, I am aware of that, but as you know these tincan smd caps are not always these easiest to remove without damaging them or the pcb, hence why I was looking for a short. Kinda hoping for an easy solution.

None of the cap values are out of the ordinary, so it could easily be done to remove the caps in question one at a time, and replace them, then see if that resolves things?
Obviously, it may be that there is some other problem, and I don't really wanna devote loads of time chasing my tail on this synth.
That said, if I could be sure there was a cap fault, I would happily replace them all.

[redwire]

Re: Dead Roland AX synth - Capacitor problem?

Roland uses high quality caps, I can see the two solid polymer ones too. Not sure why there are two through-hole Panasonics sticking up there.
This leads me to think the problem is not the caps. It's not cheap chinese stuff.

See if any small IC's are shorted and getting hot, check the power supply voltages.

[Crystaleyes]

Re: Dead Roland AX synth - Capacitor problem?

In fact if I check the voltage across those caps in question they all measure 5v, so none of them are shorted.

There is one regulator which I can see that is for the 3.3v line and that measures fine, then there are two LM2576's which both have stable voltages and are not heating up.
The only components which do evaporate IPA faster are two JRC 2374's so I'll check out their datasheet and see if they can be verified easily or not.

One thing that does stand out is that on the opposite end of the board from the 9v input, there are nine caps with no voltage across them, they are not shorted.

It's a shame the Roland service notes aren't easily available.