The Crumar GDS Restoration Blog...

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Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Fri Oct 03, 2014 9:19 pm

Well to all those who might have thought I got lucky... hmmm...

I decided to post my experiences as this is actually quite a typical insight into the life of a vintage synth engineer...

The last two days have been somewhat of a challenge to say the least.

The very same anomaly has happened to me now on three major restorations (all gear from this era) ie. after a few days of run time following an extended period in storage curious faults started to develop...

In the case of both Will's Polymoog and the Chroma a depressing amount of 4000 series multiplexers, op amps and logic gates almost on a day by day basis suddenly decided enough was enough... only after several more days of graft did both instruments finally settle down and the failures stopped happening.

Well, depressingly, the GDS suddenly decided it was going to turn itself into a Xmas tree displaying seemingly random patterns instead of informative LED status on the console panel after a few days of fault free operation..

After many hours of poking around with a DSO (my efforts greatly accelerated after receiving the original schematics so a huge thank you Stoney) I made a breakthrough....

This wasn’t before I’d swapped the LED driver, LED drive buffers, decimal point buffer and memory register all now in turned pin sockets with no luck...

On looking at the signals on the ICM7218 LED driver IC in the keyboard console more closely and cross referencing what I was seeing with the datasheet it suddenly occurred to me that the crazy patterns of LEDs I was seeing were due to the chip being in “decode” mode rather than “direct” mode. This resulted in 0 being interpreted literally as the 6 segments required to display a zero on a 7 segment display rather than all panel LEDS off so hence in LED test what should have been a single walking LED (or segment) was in fact all 64 LEDs on the front panel counting down in hieroglyphics!

Like so many 8-bit peripheral chips the data and control bytes are multiplexed on the same bus and bit D5 is used when in control byte mode to select “direct” mode but should return to segment data when in data mode...

Trouble was that I was seeing bit D5 permanently asserted..

I traced the stuck bit from the driver IC to the buffer in front of it – the same – so I buzzed out the huge 25 way cable to the IMS8000 Computer (which is literally half the size of a washing machine!) – no probs – I then found the same stuck bit on the LED memory register on the S100 card. I then scoped D5 on the bus side of the LED register and my oh my what a mess compared to the other bits on the bus! What I first noticed is that D5 within the data bus on the keyboard circuit S100 card is very busy but the signal never drops below around 1 volt or so – all the other bits swing nicely rail to rail. The edges are very poorly defined too as if there is no current available..

On looking at the circuit the only thing left is the 8T97 bus transceiver to the S100 bus itself.

I have some of these on order now and am really hoping this fixes the problem. Its either this or something on bus within the card is clagging the data line and never letting it fall to zero. If it turns out to be the transceiver then I’m very tempted to replace all of them in turned pin sockets as I ran into similar issues with the Chroma on all the voice cards and didnt want to take any chances in the end.

Incidentally, I proved my theory about the mode pin by temporarily pulling the D5 output pin from the LED register socket and shorting the line to ground. Sure enough the panel LEDs came up albeit with 8 permanently lit as expected with the data line missing (1 segment per 8 numerals) but otherwise returned to normal operation.

Phew!!

Other than this the GDS ran for over 4 hours straight today without issue.

I ordered $400 (!!) of capacitors today for the beast for a full recap (the huge PSU caps are ferociously expensive!) All of the caps on the custom Crumar boards are now replaced – got about 50 or so now to replace in the stock computer).

I did find some curious pin holes in some of the orange dipped tants I pulled out. I have had so much trouble with tants over the years that I replaced them all with low ESR electrolytics with a 100N across them. Also need to replace all of the 4000 series MUX in the keyboard console as they are notorious for causing weird failures at that age (often resulting in strange leakage currents between the control lines and the switched signals – many a nightmare debug session caused by that including the repair from h**l on a Juno-106 with the weirdest of faults once!)

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Re: The Crumar GDS Restoration Blog...

Post by madtheory » Fri Oct 03, 2014 11:39 pm

Fab diagnosis Hideaway Studios.
MiK wrote:For the effort - the main part is understanding the original design in depth to be able to create the necessary algorithms at all. But if i'm done with it, i will open source it if there are no legal reasons against it. But after this time, i don't think so, as i don't copy the hardware and if there were patents on the way this thing works, they should be expired since a while.
According to Mark Vail's Vintage Synth book, Hal Alles published the design to release it to the public domain, so MTI (was that the right name?) and Crumar could use it. Apparently Bell Labs at the time didn't have a mechanism to sell the work so it was the only way to make it available outside of Bell Labs.

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Re: The Crumar GDS Restoration Blog...

Post by jbfairlight » Sat Oct 04, 2014 7:44 am

This is great news, two Crumar GDS found and in restoration ! Good work Dan ! I'm really jealous :thumbleft:

Technically it is very interesting for DKI Synergy !
For the historical level and also technically.
It can help us be finally completed the development of a user interface to change the parameters of sound.

Image
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Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Sat Oct 04, 2014 10:04 am

jbfairlight wrote:This is great news, two Crumar GDS found and in restoration ! Good work Dan ! I'm really jealous :thumbleft:

Technically it is very interesting for DKI Synergy !
For the historical level and also technically.
It can help us be finally completed the development of a user interface to change the parameters of sound.
Hi JB.. good to see you here.

Many thanks for one of the best pictures of the GDS I've seen yet. She was certainly looking much happier for herself in those days!... the beast is somewhat battle scarred now and if I'm honest I'm really torn as to whether things like the snapped off faux wood trim on the computer and the badly gouged veneer on the keyboard should be refinished or left as a nod to its past life.

This is made more difficult a decision because, as you can see here, there clearly was a time when she was taken care of and maybe this is the era she needs to be returned to.

I am replacing the missing trim plate on the front of the keyboard because it looks terrible without it.

For the rest, I'd actually like to throw this one open and hear other people's opinions on this one...

Do I...

1. Give the GDS a very deep clean but otherwise leave the tatty cosmetics as they are?
2. Tone down the effects of the cosmetic damage but otherwise leave it original?
3. Replace the wood veneer on the end caps and restore the faux wood computer side panels?
Last edited by HideawayStudio on Mon Oct 06, 2014 4:09 pm, edited 1 time in total.

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Re: The Crumar GDS Restoration Blog...

Post by MiK » Sat Oct 04, 2014 10:23 am

HideawayStudio wrote:1. Give the GDS a very deep clean but otherwise leave the tatty cosmetics as they are?
This is what i usually do. Your machine has a history and i think it's worth keeping the patina it has.

Sure, replacing the trim plate makes sense. I also replaced all slider caps because some were missing.

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Re: The Crumar GDS Restoration Blog...

Post by jbfairlight » Sat Oct 04, 2014 11:02 am

For me, it must refurbish like the original.

I think in past Klaus would probably prefer to have a machine in very good condition.
You know I have repaired machines for some musicians, and often prefer a machine like new :mrgreen:

Another photo for you :D

Image

and other with Max Mathews :
Image


and the vidéo :

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Re: The Crumar GDS Restoration Blog...

Post by jbfairlight » Sat Oct 04, 2014 12:23 pm

I just look at the photo of the GDS oscillator board that you posted and it seems that this is the same as in the Synergy II :ugeek:

GDS oscillators board :
Image

Synergy II oscillators board :
Image
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Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Sat Oct 04, 2014 1:29 pm

jbfairlight wrote:I just look at the photo of the GDS oscillator board that you posted and it seems that this is the same as in the Synergy II :ugeek:
Yes already made this clear.. they are identical - almost - the digital OSC board in the GDS is branded Crumar on the silkscreen and the Synergy Digital Keyboards.

Interestingly there is marked in pen on the GDS pcb /6 - not sure if this is mod or sub-revision 6 or actually board number 6. Often boards are marked with hand written running number in small batches so its possible its literally board number 6.

A quick before and after of part of my exercise to banish those pesky orange dipped epoxy tantalum capacitors from the GDS once and for all. In this case its what would have been an extremely expensive 16-bit DAC in 1979 (not to mention a very nice output transformer!)

Image

Also... removing the pot encoder board from the keyboard console revealed some more hidden firecrackers that were replaced with high quality metal film caps.

Image

Interesting seeing those pics of the GDS in the live setup (many thanks) - I've just has a closer look at the computer unit and sure enough the wood grain patterns and the two small gouges in the top left hand corner of the left hand cheek line up exactly with the beast in the workshop :geek:

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Re: The Crumar GDS Restoration Blog...

Post by SynthFred » Sat Oct 04, 2014 9:07 pm

This is a great project. Thank you for sharing it here. I am one of the original Synergy users. My vote for the GDS restoration is to fabricate nice brand new wood panels for the computer and the keyboard, but to have the originals cleaned up and kept available for replacement as needed for special purposes such as photographs or performances.
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Re: The Crumar GDS Restoration Blog...

Post by ppg_wavecomputer » Sun Oct 05, 2014 12:03 am

jbfairlight wrote: [...] and other with Max Mathews :
Image
[...]
Has anybody noticed the damage in the sidecheek of Max' machine?

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Re: The Crumar GDS Restoration Blog...

Post by dlmorley » Mon Oct 06, 2014 10:55 am

What a fantastic thread. I first got a Synergy 25 years ago and then found a II+. Still have the II+ but it is being, hopefully, repaired. They sound fantastic to my ears, and a GDSA would be one of my dreams, but I doubt it will ever happen. In any case, fantastic to see that you are renovating yours and I will follow the thread with great interest!
David

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Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Mon Oct 06, 2014 2:50 pm

dlmorley wrote:What a fantastic thread. I first got a Synergy 25 years ago and then found a II+. Still have the II+ but it is being, hopefully, repaired. They sound fantastic to my ears, and a GDSA would be one of my dreams, but I doubt it will ever happen. In any case, fantastic to see that you are renovating yours and I will follow the thread with great interest!
David
Hi David,

Good to have another Synergy owner on board - hope yours is returned to the land of the living soon. You can take great delight in knowing that even in the Synergy SynHCS manual the Synergy II+ is described as a GDS as this is what they were trying to promote the II+ as and it is true enough to say the two instruments sound very similar. That said, the software is somewhat different and the voice patch files sadly not directly compatible with one another (although I do wonder if a patch conversion utility was developed at DK to essentially port the GDS factory library to the Synergy (I will ask Stoney about this.)

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Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Mon Oct 06, 2014 3:27 pm

I have -FINALLY- tracked down the root cause of the pesky stuck bit that was causing hieroglyphic patterns all over the front panel LEDs of the GDS Keyboard Console!...

The replacement 8T97 bus transceivers turned up today and I carefully removed what I thought was the rogue IC and fitted a new one in a turned pin socket (U19)...

Oh dear!

Same as before...

So back to the circuit diagram....

It is worth noting at this point that the circuit diagrams Stoney sent me are the original hand drawn schematics and I'm told the guy who drew them up was also involved with the design of the original Bell Labs Synth - the Alles Machine.

Hal Alles in front of his "Blue Monster" at Bell Labs...
Image

The GDS is by no means a distant relative of this $100,000 300lb monster and I'm told some of the circuits have been carried over directly. It is interesting, for example, that the Alles Machine has many more sliders than even the GDS on its front panel but the circuit that decodes them in the GDS actually uses 64 ADC channels to read sliders, pots and switches - so the switches are basically read like two position pots. Interestingly, I've seen the opposite done in the past. Some of the Roland PG series programmers use sliders as mode switches.

A small section of the wonderfully hand drawn GDS schematic...
Image

After much poking around I found that only 1 IC is connected to the D5 pin on the bus transceiver but what I did discover is this data line jumps over to the adjacent Keyboard Circuit Board via a 50 way IDC ribbon cable. As I started tracing where on earth this data line was ending up I noticed that a via that jumps the track to the other side of the board was sitting in a pool of ancient solder flux directly next to the pad of decoupling capacitor sitting on the 5 Volt rail. I have experienced some very curious faults over the years caused by old solder flux which can sometimes become slightly conductive with age. This effect can sometimes, for example, cause problems with the unusual charge pump oscillators in the Rhodes Chroma.

Old solder flux can often be seen as brown patches around PCB pads...
Image

Being suspicious about this I removed the card and cleaned away the old flux with isopropyl alcohol and reassembled everything.

I ran the KPTEST utility and sure enough a single LED walked all the way down all 64 front panel LEDs. :D

So I then decided to remove the card again and clean the whole pcb which was covered in old flux.

Ooops!

This time on running KPTEST the LEDs were fine but a sea of ADC errors were flagging up like the sliders had been possessed!...

Some more frantic poking around and then noticed that when I wiggled the ribbon cable between the two keyboard control circuit cards the errors momentarily went away...

Before I had even first powered the GDS up following my exercise to scoop everything out of the large IMS8000 computer chassis the connector that plugs into the Digital Oscillator Board broke away from its ribbon cable. This can happen with any IDC connector but the ribbon connectors in the GDS seem to be rather too prone to this so I had fitted a new connector.

It turns out that the small ribbon linking the two keyboard cards had started to suffer the same fate.

Image

I quicky crimped up a new 50 way ribbon (same size as old SCSI 1 cables) and magically all the errors went away...

Phew!

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Re: The Crumar GDS Restoration Blog...

Post by Walter Ego » Mon Oct 06, 2014 7:50 pm

HideawayStudio wrote:As I started tracing where on earth this data line was ending up I noticed that a via that jumps the track to the other side of the board was sitting in a pool of ancient solder flux directly next to the pad of decoupling capacitor sitting on the 5 Volt rail. I have experienced some very curious faults over the years caused by old solder flux which can sometimes become slightly conductive with age. This effect can sometimes, for example, cause problems with the unusual charge pump oscillators in the Rhodes Chroma.

Old solder flux can often be seen as brown patches around PCB pads...
Thanks for your fascinating thread. I had a Synergy II+ briefly, but that's a little beside my point. This part triggered a memory of my Akai AX-80 sitting waiting to be repaired. From what I remember, much of the PCBs under the LCDs that display parameter info have this gold stuff all over. I'm going to go home and look under the hood again today and do a little alcohol test.

Again, sorry if that detracts from the GDS project, but this is useful info!

Good luck with the ongoing project. What a fantastic work of restoration.
Walter Ego
seamonkey wrote:I nominate this for STUPIDEST THREAD ever in the history of the internez. ;)

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Re: The Crumar GDS Restoration Blog...

Post by jbfairlight » Mon Oct 06, 2014 10:16 pm

Laurie Spiegel Playing the Bell Labs Digital Synthesizer, better known as the Alles Machine or Alice, was an experimental additive synthesizer designed by Harold G. Alles and Douglas Bayer at Bell Labs in 1977-78.

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