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SOLVED: MB 6582 power problems


urtzurd
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Hi,

First of all, I want to thank the Midibox community for such great projects and support!

In the last months I've been working building my first MB 6582 and unfortunately I had some strange issues after having it working for some weeks. I'd like to ask for your advise to do a postmortem analysis on what went wrong (more on the postmortem thing later). I decided to go for option B for the power supply, as I want to use both revisions of the SID chips.

When I finished the main PCB I tested all the voltages and everything was fine. Then I tested loading the ICs and with the SID chips and it worked successfully. I was very happy and continued working on the control surface, doing some tests with some new SID chips I got from ebay. To power the MB 6582 I was using an old C64 brick. So far everything was fine.

Then I decided that I should get some other power supply as these old bricks can die any moment, so I contacted Ray Carlsen and ordered one of his power supplies (great power supplies and great guy). Everything was working OK for a couple of weeks, but after that the new power supply stopped working with the MB 6582: the 9V and 12V rails read very low voltages (around 2-3V). Strangely the old C64 power brick continued working properly. This made me think the problem could be on the new power supply, and after many tests I decided to replace the 9V AC transformer myself, always with the help and advise of Carl. Everything was the same. One day, using the old C64 brick the SIDs were not working and I measured the 9V and 12V rails and they were low voltage too... If I switched the MB 6582 off and on again, most of the times it would work again properly.

After that, it was clear that the problem was on the MB 6582, so I started measuring voltages in many places and discovered that the input voltage of the first voltage regulator was around 18V when using the new power supply, and 16V when using the old C64 brick! This was a very bad sign, but it also explained why the one power supply would work and the other won't: the regulator was operating under too much stress and this 2V difference would make it shutdown. Now being on the right track, I decided to replace the electrolytic capacitors and discovered that one had some dark marks on its side.

When firing it again I started measuring voltages and thinks looked better, but I accidentally slipped one of the probes and made a shortcut in the voltage regulator pins :-( This blew the fuse in the new power supply, but the 5V part is also protected with a relay, so the power supply survived. Unfortunately I noticed that the other 2200uF electrolytic had dark marks on it. Then I decided to replace all the components on the power circuit, from the bridge rectifier to the capacitors that are after the voltage regulator. Unfortunately my desoldering skills are not very good and when I finished I created a new problem.

Now the voltage regulator has a low input around 4V. The bridge rectifier reads close to 9V on its output, but the negative pin relative to common reads -4V. I checked for AC voltage between the outputs of the voltage regulator and it reads around 4.5V AC :-(

Then I replaced the bridge rectifier again, and that made quite obvious that I had damaged the PCB contacts, as now I have some exposed copper areas around the pins of the bridge. So it's quite obvious that I have created a short in the PCB (this is why I said postmortem) and I should build a new one once the PCBs are available at the midibox shop.

My first diagnosis was that it could be that I placed one of the big electrolytics reversed, but I have a picture of the board when it was working OK and they seem to be in the right position (see attached picture). I'll build a new board from scratch, but I'm still wondering what went wrong and how to avoid it happening again. Any advise or checks would be more than welcome.

Thanks a lot and sorry for the long post. Cheers!

 

2016-03-26 23.27.16 copy.jpg

Edited by urtzurd
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  • 10 months later...

I'm updating this post in case someone experiences a similar problem.

After some months I got new PCBs and built another MB6582 main board. To my surprise, it seemed that I was having the very same problem as with the previous one. So, I started to do some checks with a cheap DIY oscilloscope and discovered that I was getting some AC power from the ground line. The reason: the replacement power supply I got from Ray had the ground wired to earth, and my place has very dirty earth! I guess that's a common practise in the US, but here in the UK the wiring in some old flats is terrible and you get tons of noise and leaks from your neighbours.

I guess that this "moving" ground caused the filter electrolytics after the bridge rectifier to go bad after some time, as they were getting some voltage on the negative lead, ouch!

As the replacement power supply was built in a metal enclosure and I needed to have both rails floating, I decided to build my own PSU based on the "dual power supply" that can be found in the forums. I feel safer with a plastic enclosure :wink:

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15 hours ago, ChinMuzik said:

Hey glad to see you figured the issue. 

How were you able to get the PT-10 case closed with the headers that join the boards? Those look very high.

Check out the gallery images that @urtzurd posted (on main page of forum). You can see that the female connectors are pretty short, especially compared with the more common DuPont-style.

I said it in the gallery comments, but it bears repeating: these are some of the cleanest MB-6582 internals I've seen. Looks damn great.

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I used these ones: https://m.reichelt.de/Molex-Vielfachsteckverbinder/MOLEX-22012085/3/index.html?ACTION=3&GROUPID=7981&ARTICLE=185655&OFFSET=16&&SID=96WMbx06wQATMAAGj1-PA8d388a3a2ac60bea71117c221d597d2c&LANGUAGE=EN

But mind you that the fit is very tight and the ribbon cables are tricky to get properly folded in order to close the case. It's a good idea to use shorter ribbon cables than I did. When I noticed I had already soldered them on the other side and it's quite a PITA to redo them.

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@ChinMuzik, you may also want to check out this thread on the topic. @jjonas used some very tiny headers that seem to work well, although apparently the crimping tool for them is expensive. It also seems that the cheaper and more common DuPont-style will work (tightly) on the CS board, just not the base board.

Personally, the more I thought about it (and the more I thought about how I use my other DIY instruments, especially my sammichSID), the more I realized that I wasn't likely to be opening and closing the case very often once it's all built. With that in mind, I just soldered a whole bunch of individual wires, cut and stripped to the same length (got the idea from Altitude) . It was time-consuming, but probably not worse than crimping 60 wires would be. This way, it's perhaps slightly more prone to failure than using connectors (which I think is well-mitigated if I rarely open the case again), but one wire failing is trivially easy to replace.

Edited by jaytee
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I used JST XH crimp terminals. The "official" JST crimp tool is indeed expensive (200-300€ or more), but after writing in the thread jaytee referred I did find a tool that works with the JST XH terminals, and cost only £20 (from a UK seller). The tool is called "SN-01BM".

Naturally it's not as good as the official JST crimp tool; for example the crimps get a bit stuck in the jaws after crimping, and you have to use a bit of force to pull them out, while with the official JST tool they just fall out. Likewise the crimped terminals' fit into the housings is not as perfect as with the official tool. But in my estimate the cheap too is good enough.

JST XH crimp terminals and housings are available on eBay.

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