Roland S-760: A question about replacing crystals

[Ciderfeks]

My S-760 sampler has got 3 crystals on the main board that are badly corroded and look like they're about to fall apart (it looks as though Roland covered them in a glue that has caused them to corrode over the years).

The crystal values are 18.432 MHz, 16.9344 MHz and 12.288 MHz and they are all of the two-pin 2x6 mm cylindrical variety. In looking to replace them, I cannot find those values in that 2x6 mm package, only in the Hc-49s and Hc-49u package types. So my question is, is it ok to replace one crystal package type with another, so long as the value is the same; is it just the physical package that's different? Are there any other factors I should be aware of?

Many thanks for any pointers!

[Circuitbenders]

what do you mean by "two-pin 2x6 mm cylindrical variety"?

Are you sure they are corroded? That seems a little unlikely.

I don't see why HC49 ones wouldn't work, but i'd be surprised if they actually need replacing.

[Ciderfeks]

Hi Paul, thanks for your quick response!

Looking at them again they're actually 3x8 mm cylindrical, not 2x6 mm - my mistake. They look like the typical "watch" kind.

Hopefully these pics will show you what I mean:

How they should look (you can see the glue I mentioned):

www.dropbox.com/s/1yc1vzeeb73grp6/xtal_01.jpg?dl=0

How the corroded ones look:

https://www.dropbox.com/s/aisayj501nckhkd/xtal_02.jpg?dl=0

[Ciderfeks]

Sorry for links - couldn't get the insert image to work - now I can!

So, how they should look:


How the corroded ones look:


Thanks!

[Circuitbenders]

Wow, i've never seen anything like that before. Are you sure the actual crystals are corroded and its not just the glue and some kind of weird residue? Have you tried cleaning it off?
Looking at that, i'd be very wary of that cap to the right. You might want to check if its leaking, or has that fishy small of electrolyte. Otherwise its very weird that the effect is so localised.

I would just go replacing them if they're working, and i'd definitely try cleaning off the crap first in case its just on the surface, but as i say, i don't see why the HC49 ones wouldn't work.

[Ciderfeks]

Hi Paul - thanks for your reply, much appreciated.

I had wondered if it was just surface corrosion but hadn't taken the plunge in trying to clean them up since they looked so fragile and, at the moment at least, everything seems to be working as it should (I only noticed this by chance when I opened the unit up to replace the LCD display).

I'll double-check that cap but I hadn't noticed any sign of leakage. Given that there are quite a lot of these crystals all over the main board and for some reason these three are the only ones that are covered in glue and the only ones that are displaying this issue I'm pretty sure its the glue that's caused the problem - it wouldn't be the first time Roland used a glue that went dodgy over the years!

Cheers for the advice 

[Ciderfeks]

Ok so I have now successfully repaired the S-760 crystal problem. Since I couldn't find anything about this issue online - or that much about S-760 repairs in general (anyone still using the '760??) I thought I'd post my findings here in case anyone else runs into this problem (I am fairly convinced that the damage to the crystals was caused by a glue that Roland coated them in, so others having this problem might not be as unlikely as sounds! ).

Short version:
These three crystals (X5/X6/X8 on the main board) have a direct effect over two vital things in the S-760 - sampling frequencies and overall sound output. If/when they die you will either lose sampling at various frequencies and/or you will lose audio output altogether. The originals components are hard to find, particularly the 16.934 MHz, but you can replace them with HC49 crystals of the same value. I used crystals that matched the data sheet of the originals as closely as possible - including the load capacitance - and hopefully, everything now seems to be working great.

Longer version if you're interested:
The original crystals are Epsom CA-301 series crystals. I couldn't find anywhere that had all three values in stock. Digikey and Mouser each sell the 12.288 MHz and the 18.432 MHz respectively but with minimum delivery charges you're looking at paying around £13 for each crystal. I couldn't find anywhere that still has the 16.9344 MHz in stock so, since I was going to have to substitute that one anyway, I thought I may as well try substituting them all.

I read online that the load capacitance of the crystals is important in some oscillator circuits but not in others, depending on whether they are in parallel or series. Since my understanding of basic electronics is not good enough to tell the difference by looking at a schematic, I figured if I could match the load capacitance of the originals that would be a good belt 'n braces approach. All three originals have a load capacitance of 18pF so that's what I looked for in my replacements, as well as matching the frequency values.

I am still none the wiser as to whether load capacitance matters in this case but for the benefit of others that can tell the difference, here is the schematic for the three crystal oscillator circuits and the associated IC - note that the attendant caps and resistors are all SMD's mounted to the solder side of the main board underneath the crystals.




The HC49 package is a similar size to the originals but a completely different orientation, so a bit of fiddling is required to get them to fit. There are a lot of exposed test points on the S-760 main board, so I put insulating tube on the folded over legs of the HC49's - a bit of insulating tape on the board underneath the crystals might have been a good idea too, but I managed without.

First one in:



All in:



Hope this info might help someone out - thanks to Paul for initial advice!