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Service manual errors / KAS question

2013-12-15 by <david@...>

Hi all!

Hope everyone is doing great. I'm still working on a major CS80 overhaul and was getting ready to put in a couple hundred 25-turn trimmers. The first ones I went to replace were the 8 for the output buffers on the KAS (right edge of schematic here - http://therogoffs.com/cs80/manuals/CS80_Service_Manual/04%20-%20KAS%20schematic.jpg). The schematic and board layout and big, fold-out overall schematic all show these as 100k. However, all the trimmers on the board are actually 10k. I don't have another CS80 sitting around so I was hoping someone could verify whether the documentation is wrong or this board has the wrong parts!

While I'm at it, I thought I'd list other errors in the service manual.

The first are on the KAS board layout ( http://therogoffs.com/cs80/manuals/CS80_Service_Manual/05%20-%20KAS%20Circuit%20Board%20&%20Wiring.jpg ).

The pins outs for the big, 40-pin KAS and DAC chips are labeled backwards. They should be like all DIP/SOIC parts with pin 1 on the upper-left pin (with the notch indicating the top) and then counter-clockwise around to pin 40. This could really mess someone up trying to do any repairs or modifications who's not experienced with electronics (and who probably, therefore, shouldn't be in there!).

I've been working on fixing these and will be uploading my corrected versions soon. Please let me know any others you know of and also about the 10k/100k trimmers on the KAS.

DAvid

Re: [yamahacs80] Service manual errors / KAS question

2013-12-15 by David Rogoff

More I just found:

VR5 on the KAS is actually 5k, not 2k.

This one is troubling since it’s a time-constant: all the 0.01µF caps that feed the 3.3MΩ resistors into the output buffer S/H are actually 0.033µF - the same as the sample caps. I’m not 100% sure exactly when these paths are for. They basically a low-pass filter (Tc = 0.01e-6 * 3.3e6 = 33m sec for values in docs and 109mS for the actual value on the board) across the sample caps. I’d guess this is to remove any glitching when the analog switches are switching. However, I don’t know how bad the glitching can be and if the time-constant was purposely increased to fix an problem or if it was just simpler for the assembly-line person to just make the whole row of caps the same 0.033µF parts from the bin!

Ideas? Comments? What’s on your KAS board (a rejected Capitol One catch-phrase…)?

David

RE: Service manual errors / KAS question

2013-12-16 by <galaxiesmerge@...>

Wish I could help but I'm clueless: I do have some of the original CS80 documents lying around the house so I will take a look and find out: if there is a difference, I will FedEx them to you. Give me about a day or so to see if I can locate them here. Cheers!

Re: [yamahacs80] Service manual errors / KAS question

2013-12-16 by David Rogoff

On Dec 15, 2013, at 7:47 PM, galaxiesmerge@... wrote:

Wish I could help but I'm clueless: I do have some of the original CS80 documents lying around the house so I will take a look and find out: if there is a difference, I will FedEx them to you. Give me about a day or so to see if I can locate them here. Cheers!

Thanks. I’ve never known of difference versions of the CS80 service manual, but I guess it’s possible. The silkscreen on the board matches the manual too - and not the actual part that was on the board. I’m looking forward to someone having the time and ability to take a look inside their CS80 and let me know what they see. This is why it’s always good to be working on a couple of them at a time and I had to go and actually finish the other one that was here and give it back to its owner. How silly of me!

David

RE: Service manual errors / KAS question

2013-12-16 by <joachim.milson@...>

Concerning the 0.01µF caps:
This part of the circuit is to create the glissando from the portamento: the low-pass filter just smooth the steps of the glissando to produce the portamento. It is explained on pages 24-25 of the CS-Series servicing guide.
Depending on the Portamento Short-Long slider position, two values of the resistor are used, whereas at least in theory, the resistor value should vary continuously with the slider. So resistors are already approximations and certainly a precise value of the capacitor is not that important.

The change of the value of the caps was probably a last minute improvement. If the portamento sounds good at any position of the Short-Long slider, the value of the capacitor is certainly correct.

Re: [yamahacs80] RE: Service manual errors / KAS question

2013-12-16 by David Rogoff

On Dec 16, 2013, at 2:47 AM, <joachim.milson@...> <joachim.milson@...> wrote:

Concerning the 0.01µF caps: This part of the circuit is to create the glissando from the portamento: the low-pass filter just smooth the steps of the glissando to produce the portamento. It is explained on pages 24-25 of the CS-Series servicing guide.
Depending on the Portamento Short-Long slider position, two values of the resistor are used, whereas at least in theory, the resistor value should vary continuously with the slider. So resistors are already approximations and certainly a precise value of the capacitor is not that important.
The change of the value of the caps was probably a last minute improvement. If the portamento sounds good at any position of the Short-Long slider, the value of the capacitor is certainly correct.
D-oh! Thanks for the reminder Joachim. I knew that but had a mental block. Still not sure whether it’s this particular CS80 or the docs.

To remind myself - and for the amusement of others - the glide/portamento is based on the glissando setting. Basically, the DAC chip gets the portamento clock (PC) which varies with the slider. This tells it how fast to transition, in semitone steps, from one note value to the next. To turn this into a smooth glide, as Joachim wrote, the individual semitone steps are smoothed out with a simple resistor/capacitor circuit that’s switched in using analog switches. As he wrote, this RC time constant should really change with the glide time. It doesn’t and Yamaha cheated by setting it to a “good” fixed value.

However, they are doing something a little better - and kind-of weird. I never looked at this part of the logic very closely, but if you check the big, fold-out schematic ( http://therogoffs.com/cs80/manuals/CS80_Service_Manual/cs80_foldout_schematic.jpg ), they are doing some odd stuff with comparators and logic gates to basically turn the port/gliss switch value and the port time slider voltage into a 2-bit digital code that controls the 2 analog gates for each voice and can therefore switch the smoothing time between 4 values, including basically instantaneously for glissando and 3 different RC time constants based on the portamento time slider.

So, the smoothing time doesn’t constantly vary with portamento time - which would be the best case. But it does switch between 3 values to at least roughly track with the portamento time. This doesn’t answer my question and actually makes the situation more complex :)

David

RE: Service manual errors / KAS question

2013-12-18 by <joachim.milson@...>

I've just posted three close-up photos of my own KAS:
http://groups.yahoo.com/neo/groups/yamahacs80/photos/albums/519437225

(these components are very likely original)
They all confirm the differences between values printed on the boards and the actual ones.

(...) and can therefore switch the smoothing time between 4 values, including basically instantaneously for glissando and 3 different RC time constants based on the portamento time slider. (...)

Well, I think there's no fourth value, only these three: 3.3M, 411k (=3.3M || 470k) and 0k, as in the table p 25 of the servicing guide.

RE: Service manual errors / KAS question

2013-12-18 by <david@...>

Joachim - thanks so much - great pics!

I had already ordered some 10k trimmers that should arrive today.

More updates to my corrected version of the service manual! I'll really be on the lookout as I go through the other boards - especially anything analog like the M-cards and the OE/PRA/KBC.

David

RE: Service manual errors / KAS question

2013-12-18 by <david@...>

Oh - and you're of course correct about only 3 used values. The 4th would be 0k in parallel with 3.3M and 470k, which is still zero (give or take the 100Ω - 200Ω on resistance of the 4016 switches).

David

RE: Service manual errors / KAS question

2013-12-30 by <david@...>

Hi all.

Happy Holidays to all!

I just uploaded some more pics/docs to the folder http://groups.yahoo.com/neo/groups/yamahacs80/photos/albums/519437225

These are my corrected versions of the CS80 service manual with the correct parts and values. I couldn't upload the ODG version of the PCB because of Yahoo's 10MB file limit. I'll put it up on my site eventually but at least posted the JPG for now.

Now I'm soldering in hundreds of replacement 25-turn trimmers on all 16 voice cards. It's going to take a bit of time!

David