ChromaTalk Archives: December 2001
- Field Change Notices
- New troubles on broken Chroma (7 messages)
- Digital 5v supply (2)
- Owner of the repeat Chroma on Ebay (3)
Chris Ryan  · Sat, 1 Dec 2001 10:21:55 -0800
I've posted the second set of Chroma Technical Service Bulletins. The eight "Field Change Notices" cover a variety of topics and include schematics.
- FCN2-001: To prevent Dual Channel Boards from drifting beyond tuning range after warm up.
- FCN2-002: To reduce noise on the 2MHz clock line thereby improving cassette load operations.
- FCN2-003: To adjust balance in +5V digital current limit circuit, preventing power supply from blowing fuse.
- FCN2-004: To provide computer access to end of conversion from A/D converter.
- FCN2-005: To insure the Dual Channel Boards tune properly during auto tune, and to prevent Dual Channel Boards from drifting beyond normal tuning range.
- FCN3-006: Power supply update modifications.
- FCN3-007: To eliminate intermittent operation of the XOFULL Flop, possibly causing external interface communications problems.
- FCN3-008: To delay the write Synthesizer Address Latch Strobe (WRSYNA), thereby preventing the previously latched address from falsely strobing the channels.
Let me know if there is enough detail in the scans of the schematics; not being an adept I can't be sure.
Sandro's troubles were first discussed in the November thread The long story of my broken Chroma.
Sandro Traversi  · Sat, 1 Dec 2001 23:42:51 +0100
- I got a Chroma
- Putting Dual Channel Boards on slots # 0-2-3-4-5-6-7 it show no sound no noise just a very tiny whistle very very low in volume.
Set Split 50 and Autotune do had no effect. Set Split 31 too, but changing P33 (waveshape) on values 3-2
It would be good to create a "scratch" patch (... said me Dave).
Done - what I tryied was on a scratch patch (except P33 testing) .
- If I put a DCB in slot #1 on boot it starts with a double whistling by free oscillators (but Set Split 29 shut them up!)
- So it's something wrong on slot # 1
- 5) Then I had a big suspect on Z13a on Channel M board, the only one IC dedicated to give Strobes to DCB # 1
Ok, I replaced it - no effects - everything as before - same sympthoms. Just to report many bootings with fuse blowing (500mA fuse), but that has been always a feature of my Chroma.
Looking well at Channel Board schema I noted also that IC Z8 gives a distinctive out to Channel #1. So I replaced also Z8 (a 4556 too).
Then, re-booting I hoped to have good news, but I had to be particularly unlucky indeed.
Now at boot, often alternated with fuse blowing bootings, Chroma simply doesn't start! 8-<
It lights on all leds, but no Data Readout no Parameter Display . Obviously it doesn't run Autotune and switch on panel cannot be managed as CPU it's locked (I guess).
Once switched on , I also measured voltages (on Channel M/B) and it seems everything ok I measured also +5v on I/O board - it gives +4.99v and +4,92v around CPU circuit (same voltages yet measured before).
Hinted by an old ChromaTalk # , I also measured voltage on blue wire on molex PSU connector , that closest to the keyboard (it should be the reset supply ???). It gives a +5,01 v. Is it correct ? As I now suspect about reset circuit ... Or I should even suspect about PSU , in order of the many fuses blowed on re-booting ?
I'm on a loss right now.What to do ? I hope new hints from Chroma friends.
David Clarke [21030085++] · Tue, 04 Dec 2001 21:56:19 -0500
Based on your earlier description I had thought that all slots (except #1) worked OK - but I'm not so sure of that now. Rather than trying to debug a system full of cards, it may be easier to only pick a card or two.
- If you boot the system with only a card in Slot 0, does the Display show Err 01234567 or Err 1234567 ? (i.e., does the since card pass the power-on test)?
- Can you get any single card to pass the power-on test? If you can - I'd recommend that you use that card (and that slot) for your further tests.
Once you can get to the point of the system passing tests on one board, you can then try to set up the scratch patch on it and see if it will produce any sound.
Also, if you only have 1 board in the system then it will be easier to see what it is doing (since the output of the boards are actually connected together).
If you can get access to an oscilloscope, you can watch the output on the "SUM 0" bus during a tune request. You should see the board attempt a low frequency, and then a high frequency.
If you don't see changes, it would probably be easier to start at the input of the board and follow the signal until it disappears.
Start by looking at pin 3 on Z19 or Z20 on the channel motherboard. This should be the output from the DAC. During a tune request you should see this change rapidly - if you don't, start back towards the DAC. If you do, confirm that the output of the pitch buffers (Z21/Z23) also change.
If the pitch control voltage is changing, go to the voice board itself and look at the output of the VCOs (Z10). If you can't see a changing signal there (during a tune), go no further and look at the control/setup for the VCO. If you're OK to there, continue on through the VCF, etc.
By continuing on in this way you should be able to see where the problem is occuring.
Sandro Traversi  · Wed, 5 Dec 2001 12:00:53 +0100
Power troubles on broken Chroma
Ok Dr. Dave,
your advices are very useful , although my actual problem I think is not about Dual Channel Boards.
At the present, at booting , often alternated with fuse blowing bootings, Chroma simply doesn't start!
IT DOESN'T PASS THE POWER-ON TEST.
At the boot, it lights on all leds, but nothing appears on Data Readout nor Parameter Display . It doesn't run Autotune and switch on panel cannot be managed as CPU it's locked (I guess).
All this happens either with 0 or 1 or more DCB inserted.
I also measured voltages (on Channel M/B) and it seems everything ok. I measured also +5v on I/O board - it gives +4.99v and +4,92v around CPU circuit (same voltages yet measured before).
Hinted by an old ChromaTalk # , I also measured voltage on blue wire on molex PSU connector , that closest to the keyboard (reset ) and it gives a +5,01 v.
Is it correct ? I have to suspect about reset circuit ? Or I should even suspect about PSU , in order of the many fuses blowed on re-booting ?
Jerry Leonard [21030100++] · Wed, 05 Dec 2001 09:59:04 -0800
This one looks familiar to me. Mine had the same problem and the cause was the reset circuit / power supply. I did the MAP4000 thing and built the reset circuit (the schematic is attached) and the problem was fixed. I'm attaching two files of the reset circuit, a JPEG and a Powerpoint file that is easier to read. They're both the same schematic. Good Luck, hope this helps.
This circuit was developed by an electronic technician using the original Rhodes Chroma service manual schematics to emulate the original power fail circuitry. The technician designed this circuit to work with the MAP4000 power supply, after obtaining and studying the MAP4000 schematic. The part numbers are listed next to the appropriate symbol and can be obtained at any electronic components store. Get the least % tolerance in resistors. They're the best.
The PF, Reset, and +5DC labels are where the Chroma wires hook up.
Sandro Traversi  · Fri, 14 Dec 2001 14:02:16 +0100
Many thanks J L
I'll take it as a X-mas gift :-).
BTW I just resolved some problems with my Chroma. Power on bad test and leds frozen on boot were caused by a bad setted connection betw I/O and CPU boards.
So just a good insert has resolved what I (prematurely) charged to the reset circuit.
Now I stand at this point.
- On boot , Chroma passes power on test , blinks all leds one time and display message "E01234567";
- Listening at max volume setted, without touching keyboard, you can hear a very tiny but percetible sound as free vcos running at fixed frequency;
- Autotune or Set Split 50 do have no effect on sounding and it still displays "E01234567";
- in Set Split 31 mode that sounding stops - also if I do Set Split 29 (Osc Muting) - and if changing Parameter 33 on values 3 or 2 you can hear a noise sound;
- touching the keys, I note some lightings on DataReadout displays and sometimes happens that a specific key mutes the P or W noise , and even touching chords keys it repeat that muting effetc to that specific key (I seems to me as the keyboard switch works)
- I usually measure Voltages both on Channel M board and I/O board and they seem pretty OK - even +5 digital is = +5.05v , as I also followed setting and trimmering procedures showed in Service Manual (how precious ! ;-> )
So by now , here's my conclusions :
- I think Power Supply is OK - although I'm using a 1A fuse in place of 500mA as too many blowing fuses...
- as showed above I also think keyboard board and KEYB SCAN work;
- CPU works well as I can select every Program or Parameter and every value scale is correctly showen on display;
- I got 7 Dual Channel Board working (until March 2001) - I do think they're good and very improbable they're all suddenly broken;
- Audio / EQ board has no problems;
- then I noted that in SS31 mode I do get effective changing about that Parameters controlled by Data Latches or Strobes (as P33 or HP/LP param.), while no effect from those Parameters controlled by DAC and SH Address -
So now I do suspect about something to fix or in the Channel Mother board (SHA circuit) or about the DAC.
BTW, advises are anytime welcome.
Jerry Leonard [21030100++] · Fri, 14 Dec 2001 17:23:39 -0800
Your present Chroma is exactly the problem I had with my Chroma. I was lucky enough to acquire another I/O board and that's how I got around the problem. So I do know for sure that your problem is definitely on the I/O board. (That really narrows it down.......HA) I still want to fix the other bad I/O board, but didn't get to the root of what exactly was wrong. It's been so long since I worked on it, at least a year probably closer to two. What I seem to recall though, is that I was getting it narrowed down to the circuit (on the I/O) that interfaced with the computer board. It's been so long, I'm sorry I can't be any more specific.
But, if you make progress, please let me know what it is, because my Chroma problem is exactly what yours does.
I'll go back over the service manual schematics and see if I can remember more about what I did.
Don Tillman · Fri, 14 Dec 2001 20:33:46 -0800
This one looks familiar to me. Mine had the same problem and the cause was the reset circuit / power supply. I did the MAP4000 thing and built the reset circuit (the schematic is attached) and the problem was fixed. Good Luck, hope this helps.
This circuit was developed by an electronic technician using the original Rhodes Chroma service manual schematics to emulate the original power fail circuitry. The technician designed this circuit to work with the MAP4000 power supply, after obtaining and studying the MAP4000 schematic.
I should point out that I think this is a really bad idea.
The circuit here is exactly the "Reset" circuit from the Chroma Service Manual. But it is being used completely out of context.
This is not going to be reliable. For instance, there's no guarantee that the power fail line current won't fry the 2N3904 base-emitter junction.
Nev · Tue, 4 Dec 2001 14:02:26 -0500
Hi all, finally got myself a fixer upper Chroma, (thanks Chris). Initial checkout shows the 5v digital line to be low at about 1.84V. Analog supply is fine at 5.05v as are the 12v lines
Upon power up all lights flash once and then just the sound of the tapper buzzing.
Haven't had much time beyond this to spend on the machine yet but if anybody has suggestion as to what to check on the digital voltage side I would appreciate it.
Thanks, here's hoping I can get this baby going again!
David Clarke [21030085++] · Tue, 04 Dec 2001 21:29:42 -0500
I'm not sure if you mean that the 5v line actually looks like 1.84v, or if it is low by 1.84 v (i.e., approx. 3.2v). Either way, that's certainly way too low for any of the digital electronics to really come alive.
As a very first check I'd attempt to selectively disconnect one (then the other) power connectors carrying 5v dig. to the boards. If the 5v rail suddenly jumps back up, then you might have something on one of the boards (CPU/IO) board pulling the rail down.
More likely something is sick in the 5v dig. portion of the power supply. There really isn't too much there in terms of components, so I'd recommend just starting at the beginning and seeing where the voltage disappears. Check to see what is avail. after the rectifier diodes (CR1/CR2). Then check to see what sort of voltage you see at the top of CR4, a 33v zener. CR3 is a 12v zener.
The go on to check out the voltage on top of the big cap (C3, the one with the little circuit board screwed to the top of it).
If everything is OK to this point, it strait to the 5v dig. regular section (and there's only 3 devices in there - so it shouldn't be too bad hunting down the prob.)
David Clarke [21030085++] · Sun, 30 Dec 2001 11:40:20 -0500
Anybody know if the German Chroma that has appeared off and on at Ebay (priced at EUR 2000) is listed in the site's Chroma registry?
... just curious.
Sandro Traversi  · Mon, 31 Dec 2001 13:24:41 +0100
I found an other Chroma on Ebay.de starting from Eur1250 (a good price).
I mailed both sellers to know serial number and to join Chroma Registry.
... just a little bit more curious ...
Sandro Traversi  · Mon, 31 Dec 2001 17:03:43 +0100
Ulrich has promptly answered ...
am already registered there look for "ulrich behrenbeck" there you should be able to find my serial number too  regards