OB-8 filter taking its time?

[syntheticsolutions]

I have an OB-8 in beautiful condition that recently had a full and extensive service from a well known professional tech in the uk.

When first turned on, 7 voices function perfectly well however one of the voices has the cutoff frequency set significantly higher than the others. The envelope or modulation amount is irrelevant to the mark up. The temptemental voice does not completely bypass the filter, it is just set higher than the other voices.

Now for the strange bit...

After around 20 minutes to half an hour, the odd voice simply settles down to the same cutoff calibration as the other voices rendering the whole synth fully functional.

Does anyone know why this would happen and should I send it back to the chop shop or just get used to turning her on half an hour before using?

[Z]

I owned and used an OB-8 quite a bit in the early/mid 90s and own 2 OB-8s right now that I don't use much, but I've never experienced such issues with the filters with the synths I've had/have. I had Switched On service one that I'm selling that had 2 voices with different issues.

Sounds like your OB-8 needs to see the doctor or deal with letting it warm up a bit before using it.

[Bitexion]

Does the OB-8 have separate boards for each voice? Or just voice chips? If so, there is an issue with that chip/board at least..

[madtheory]

Send it back to the tech already!

[syntheticsolutions]

Does the OB-8 have separate boards for each voice? Or just voice chips? If so, there is an issue with that chip/board at least..

I am not sure, never had to open her up. I know the problem is only with one voice. I suppose I just want to know if the issues are likely to get worse as a result of not getting it fixed. I can deal with waiting for it to warm up when using it but would rather send it to the repair shop if the synth is likely to be surcome to damage as a result of neglecting the issue

[madtheory]

Seems to me that the guy didn't finish the job you've already paid him for. It might be something simple, an oversight, like he forgot a trimmer or a connector. There's no sense in having to wait 20 mins for a synth of this calibre. And I think the tech would agree, if he is as well regarded as you say he is. I know it's a pain in the a*s to have to bring it back but it's peace of mind in the long run I think.

[rhino]

never had one, so take the old man with a grain of salt:

Take it back to the tech. Mistakes get made, things get missed, Give him(her) a chance to make it right.
If you choose not to do that, then:

The VCF chips should be in sockets. Determine which voice is acting up and swap filter chips with the next one.
If the new voice is slow, then chances are the chip has become temperature sensitive. Let it warm up, or get a new one. If all voices sudenly are OK, the sockets are dirty and/or corroded (or even a cold solder joint)

From the schematic, the VCF cut-off CV has 2 TLo81 op-amps in it's line (one on either side of the LAG analog switch. These could be failing.

[Shalebridge]

The VCF chips should be in sockets. Determine which voice is acting up and swap filter chips with the next one.
If the new voice is slow, then chances are the chip has become temperature sensitive. Let it warm up, or get a new one. If all voices sudenly are OK, the sockets are dirty and/or corroded (or even a cold solder joint)

From my experience with these, the temperature sensitivity is the culprit most of the time. I routinely find one or two that need more warm-up time to even out. Happens with the oscillators too. A new CEM3320 is around $60, and the one you have seems to still work properly after the warm-up. I would stick with the longer wait.

[cornutt]

It might be worth going ahead and acquiring a spare while you can still get one. My expeirence with semiconductors that get this thermal is that they fail not too long after.

[synthparts]

If you do want a new CEM3320 I have some... I would probably agree that you should take it back to the tech...

[themilford]

I feel like maybe it's time to start a Kickstarter to have all those CEMs newly manufactured. At some point all of these old machines are gonna need them and all the old stock will be dried up or only faulty ones will be left.