The Crumar GDS Restoration Blog...

Discussions about anything analog, digital, MIDI, synth technology, techniques, theories and more.
Post Reply
User avatar
ppg_wavecomputer
Expert Member
Expert Member
Posts: 1046
Joined: Wed Feb 06, 2013 3:37 pm
Gear: more keys than hands (and feet)
Band: ['ramp]
Location: teutoburg forest, eastern westphalia, germany
Contact:

Re: The Crumar GDS Restoration Blog...

Post by ppg_wavecomputer » Tue Oct 07, 2014 8:54 am

At that time, being a synthesiser player was still interesting.

Stephen
"Like the light from distant stars, Stephen Parsick's music has existed for some time, but is only now reaching us on Earth." Chuck van Zyl

https://doombientmusic.bandcamp.com/

https://ramp1.bandcamp.com/

https://stephenparsick.bandcamp.com/

User avatar
HideawayStudio
Supporting Member!
Supporting Member!
Posts: 1390
Joined: Fri Jan 09, 2009 10:41 pm
Real name: Dani Wilson
Gear: 163 tubes in a large wooden box!
Band: Shortwave
Location: UK
Contact:

Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Fri Oct 10, 2014 1:08 am

GDS PSU Finally Recapped...

Now armed with a costly set of replacement reservoir caps for the rather scary 35 year old originals in the large linear PSU in the IMS8000 computer I set about removing the PSU chassis and removing the old ones.

These capacitors are not exactly dainty - they bolt to the pcb with studs and add up to nearly 200,000uF or 0.2 Farad ie. easily enough to vaporize the end of your screwdriver with a blinding flash if accidentally shorted out!

The transformer is pretty brutal as well with the secondary windings for the 8 volt rail (all the 5 volt rails on the S100 bus cards are locally regulated) being as thick as a 4 inch nail!

Image

Removal and fitment turned out to be a pleasantly satisfying exercise as the replacements were an exact match with the terminal post holes on the pcb and the capacitor clamps on the chassis.

Image

So all was back together ready for a successful test run this afternoon...

You don't really get a feel for the scale here - the PSU chassis is over half a meter long!

Image

User avatar
StepLogik
Expert Member
Expert Member
Posts: 1492
Joined: Fri Dec 29, 2006 4:16 am
Gear: Gauntlets of Ogre Power, Vorpal Sword, Chain Mail +3, Ring of Invisibility, Staff of the Magi, Boots of Speed
Location: Boston, MA

Re: The Crumar GDS Restoration Blog...

Post by StepLogik » Fri Oct 10, 2014 1:14 am

There really is nothing more satisfying than a PS rebuild! That looks really sharp!

User avatar
HideawayStudio
Supporting Member!
Supporting Member!
Posts: 1390
Joined: Fri Jan 09, 2009 10:41 pm
Real name: Dani Wilson
Gear: 163 tubes in a large wooden box!
Band: Shortwave
Location: UK
Contact:

Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Sun Oct 12, 2014 11:01 am

Arghh... I'm now being pestered by a loose connection somewhere on one of the two keyboard-circuit S100 bus cards in the IMS8000 computer. On cold startup the front panel controls are going haywire but a quick wiggle on the card and everything starts working normally again.

Intermittent faults are often a bit tricky to track down... fortunately the KPTEST program can be run continuously whilst poking around in the beast which flags up errors from the console scanning hardware in real time.

Often the strategic light use of the end of a hard handled screwdriver whilst powered up can be a great means of finding such loose connections!

Almost certainly a dry joint as very few IC sockets are used in the GDS or possibly a badly contacting edge connector on the backplane.

Oh joy!

On the cosmetic front....

This poor old beast has clearly had a tough life on tour!

A cabinet maker has visited and had a good look at the rather forlorn looking keyboard console with an aim to rebonding a number of serious cracks in the plywood substrate that are weakening the structure and replacing at least some of the seriously badly peeling wood veneer. There has been a real botch job done on the front cross member which needs to be machined down, several cracked screw holes drilled and doweled and a replacement strip of wood inlaid and bonded in place before the new metal front trim piece can be fitted.

User avatar
Dr. Phibes
Active Member
Active Member
Posts: 525
Joined: Wed Mar 23, 2011 5:12 am
Location: UK

Re: The Crumar GDS Restoration Blog...

Post by Dr. Phibes » Sun Oct 12, 2014 3:05 pm

Why does it need such a big bastard power supply?

User avatar
HideawayStudio
Supporting Member!
Supporting Member!
Posts: 1390
Joined: Fri Jan 09, 2009 10:41 pm
Real name: Dani Wilson
Gear: 163 tubes in a large wooden box!
Band: Shortwave
Location: UK
Contact:

Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Sun Oct 12, 2014 5:26 pm

Dr. Phibes wrote:Why does it need such a big bastard power supply?
It's really old school ie. linear not switched mode. The equivalent modern SMPSU would be literally 10 times smaller and 5 times lighter.

The GDS is awash with TTL devices which are really power hungry en mass. The current on the local 5 volt rails is probably in the region of 10 amps but each board uses linear regulators from the main the 8 volt rail which are very inefficient so even more power is required from the transformer which is also providing the power for two enormous 8" floppy drives... all in all, not exactly dainty...

How things have changed! :biggrin:

User avatar
ppg_wavecomputer
Expert Member
Expert Member
Posts: 1046
Joined: Wed Feb 06, 2013 3:37 pm
Gear: more keys than hands (and feet)
Band: ['ramp]
Location: teutoburg forest, eastern westphalia, germany
Contact:

Re: The Crumar GDS Restoration Blog...

Post by ppg_wavecomputer » Sun Oct 12, 2014 6:18 pm

I am imagining KS powering it up in Hambühren-Oldau (which is a tiny village)... when the GDS was booting, all other houses in the neighbourhood were temporarily cut from electricity in a Homer Simpson sort of way.

Stephen
"Like the light from distant stars, Stephen Parsick's music has existed for some time, but is only now reaching us on Earth." Chuck van Zyl

https://doombientmusic.bandcamp.com/

https://ramp1.bandcamp.com/

https://stephenparsick.bandcamp.com/

User avatar
tim gueguen
Senior Member
Senior Member
Posts: 795
Joined: Thu Jun 08, 2006 2:31 am
Location: the Canadian Prairies
Contact:

Re: The Crumar GDS Restoration Blog...

Post by tim gueguen » Mon Oct 13, 2014 12:33 am

Any RF interference problems, either with nearby equipment of yours, or any nearby neighbours? This strikes me as a design with plenty of potential for putting out hash.
Keys: Realistic Concertmate 500, Korg K25, Korg Micro X

Guits: '86 Fender Japan '50s Reissue Strat, '80 Aria Pro II TS-300 Thor Sound

User avatar
HideawayStudio
Supporting Member!
Supporting Member!
Posts: 1390
Joined: Fri Jan 09, 2009 10:41 pm
Real name: Dani Wilson
Gear: 163 tubes in a large wooden box!
Band: Shortwave
Location: UK
Contact:

Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Mon Oct 13, 2014 8:26 am

tim gueguen wrote:Any RF interference problems, either with nearby equipment of yours, or any nearby neighbours? This strikes me as a design with plenty of potential for putting out hash.
Actually the GDS is remarkably quiet on the electrical noise front and is surprisingly noise free on the audio side of things too (much more quiet than some digital synths I have!) as everything is within a metal chassis, the PSU is linear and a separate 16-bit D/A board with a Burr Brown converter and a huge output transformer driving a since balanced output which I'm sure in part gives both the GDS and Synergy a certain warmth and character.

Speaking of sound. The Synergy sampled beautifully and the hope is at least some of the patches I chose for my Synergenesis project can be fed back into the GDS as both instruments share the identical high speed digital oscillator array:

Last edited by HideawayStudio on Thu Oct 16, 2014 10:23 am, edited 1 time in total.

User avatar
HideawayStudio
Supporting Member!
Supporting Member!
Posts: 1390
Joined: Fri Jan 09, 2009 10:41 pm
Real name: Dani Wilson
Gear: 163 tubes in a large wooden box!
Band: Shortwave
Location: UK
Contact:

Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Thu Oct 16, 2014 10:15 am

Now on the last pcb to recap today which is the floppy disk controller S100 bus card in the (huge!) IMS8000 computer.

Will post some pictures later..

Then I've got to turn my attention back to reviving the GDS's dreaded ADDS Serial Terminal which is proving to be a bit of a nightmare as tracking down another one for spares has been almost impossible (I say almost as I've located three now but their owners are very reluctant to part with them)

I now have a potential route to return this beast to working order using a donor monitor chassis and suitably similar picture tube to the original but it will need some experimentation first.

Just a reminder that my initial play test following the first successful power up of the GDS in a very long time is now hosted on SoundCloud. The experience was somewhat reminiscent of the recording I made when I first powered the Novachord up in that I was very much distracted by the thought of something potentially going bang in a big way so please excuse my somewhat random improvs...


User avatar
HideawayStudio
Supporting Member!
Supporting Member!
Posts: 1390
Joined: Fri Jan 09, 2009 10:41 pm
Real name: Dani Wilson
Gear: 163 tubes in a large wooden box!
Band: Shortwave
Location: UK
Contact:

Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Sat Oct 18, 2014 9:52 pm

Back to The Serial Terminal from h**l!....

Having spent quite some time recapping most of the S100 cards in the IMS8000 computer I turned my attention back to the dreaded ADDS Regent 20 terminal.

When I started on this restoration I had in my mind all sorts of difficult scenarios resurrecting the 35 year old computer, digital oscillator board and keyboard console but to date the unpleasant surprise has been the serial terminal which has posed a string of issues that need resolving.

The biggest problem has centered around the Matsushita monitor chassis and picture tube. For reasons never likely to be known the glass pip on the end of the CRT was cracked off. Whether this was the result of an accident or whether it was deliberately let down in a workshop is subject to pure speculation.

As time as gone by I am starting to wonder if its in fact the latter as since making this discovery I have also noticed that the diode that rectifies one of the LOPT secondaries feeding around 70 volts to the top side of the video transistor has been removed and not replaced.

As a separate task I decided to source a monitor with similar scan rates and resolution as a potential donor replacement chassis for the original. It turned out that the old GT65 monochrome monitor for the CPC464 personal computer was very similar so I located one and purchased it.

On arrival I managed to feed the appropriate signals from the main processor board in the terminal to the monitor. After a bit of tweaking sure enough up came a stable picture but to my dismay the picture was far too wide and some 20% of the end of the lines were beyond the edge of the screen. Sadly this monitor has no width control and all manner of tweaking including the B+ rail had insufficient effect.

Image

All was not lost...

What this proved was that the picture tube in the GT65 is good and has the same pinout, neck width and mounting positions as the failed CRT in the terminal. What it also proves is that the terminal electronics are working perfectly...

But if that wasn't enough...

To my frustration, having got a picture out of the terminal I quickly discovered at least 5 keys didn't work on its keyboard!!

I removed the keyboard and unscrewed the pcb assembly from the keytops only to be cruelly reminded of a past experience with a keyboard of the same design. The keyboard turns out to be a Cherry "Solid State" capacitive foam and foil type mechanism which are notorious for giving trouble if the foil pads have started to corrode.

Image

Unfortunately it seems a report of beer being fed to the monster may not be so far from the truth!... sure enough all 5 failed keys were in a cluster with a curious sticky tidemark around them.

Fortunately this form of keyboard is modular so I quickly decided to remove the faulty keys and swap them for a number of lesser used keys having swapped over their key tops.

Image

So at least I have all of the letters and numbers available again.

It seems this form of keyboard was not in manufacture for a very long time and I'm really struggling to find a specific model of keyboard from the era that could possibly be used as a donor for replacement key contacts.

So the next task is to remove the CRT from the GT65 (having remembered to discharge it first!!) and fit it in the metal chassis of the original monitor. With the help of another Regent 20 owner I then need to determine exactly what type of diode has been removed, fit it, and carefully attempt to power the terminal monitor up.

A slightly nicer task was recapping the Z80 processor S100 bus card which turned out very neatly:

Image

And another successful task...

At 35 years old the contents of the IPL ROM (Initial Program Loader) on the I/O card was seriously living on borrowed time. The problem was that most modern EPROM programmers won't touch 2708 ROMS as they need several supply rails to read and write. I was really struggling to find a suitable programmer when I finally stumbled across the curiously retro Andromeda Research EPROM+ (AR-32a) that supports these chips as well as most of the fusable link Bi Polar PROMs on the digital oscillator board.

Image

What I like about this programmer is the fact that the software is bootable and will run on just about any knackered old PC as long as it has a serial port. No drivers or anything are required - just stick the CD in and go... and they really mean it.. I grabbed a 15 year old laptop and within 5 minutes the system was up and running!

Only another 10 minutes later I had read the contents of the original IPL ROM (which is almost impossible to obtain!) and burn it onto another 2708 which booted and ran perfectly in the IMS8000... phew!!

User avatar
synthparts
Senior Member
Senior Member
Posts: 779
Joined: Sun Oct 09, 2011 12:05 pm
Location: SF Bay Area, CA
Contact:

Re: The Crumar GDS Restoration Blog...

Post by synthparts » Sun Oct 19, 2014 4:04 am

I would suggest converting anything that uses 2708 ROMs over to 2716s or 2732s. You can then usually eliminate the need for the -5V and +12V power supplies. I've done this on synths like the Rev 1/2 Prophet-5 and OB-X...

Also switching to CMOS chips is a good idea to reduce the load on power supply...
Vintage Synth Parts - http://www.synthparts.com

User avatar
Henfield
Junior Member
Junior Member
Posts: 122
Joined: Wed Aug 29, 2007 12:19 am
Gear: MU Modular in a Moog 1P style cabinet, Moog Minitaur, and Moogerfooger rack.
Location: Detroit, MI

Re: The Crumar GDS Restoration Blog...

Post by Henfield » Sun Oct 19, 2014 6:20 am

This whole thread (or blog) is incredibly interesting! Even for me, a non-electrical person who is all thumbs with a slobbering iron. It is great to know that there are dedicated people out there to perform this type of resurrection, and a whole group of kindred souls that are willing to help with expertise, parts, and experience!

Keep moving forward............

User avatar
ppg_wavecomputer
Expert Member
Expert Member
Posts: 1046
Joined: Wed Feb 06, 2013 3:37 pm
Gear: more keys than hands (and feet)
Band: ['ramp]
Location: teutoburg forest, eastern westphalia, germany
Contact:

Re: The Crumar GDS Restoration Blog...

Post by ppg_wavecomputer » Sun Oct 19, 2014 10:50 am

HideawayStudio wrote: [...] To my frustration, having got a picture out of the terminal I quickly discovered at least 5 keys didn't work on its keyboard!! [...]

Unfortunately it seems a report of beer being fed to the monster may not be so far from the truth!... sure enough all 5 failed keys were in a cluster with a curious sticky tidemark around them.[...]
If anybody needed further evidence of provenance, here you go -- Klaus' and Rainer's favourite beverage of that era!

Stephen
"Like the light from distant stars, Stephen Parsick's music has existed for some time, but is only now reaching us on Earth." Chuck van Zyl

https://doombientmusic.bandcamp.com/

https://ramp1.bandcamp.com/

https://stephenparsick.bandcamp.com/

User avatar
HideawayStudio
Supporting Member!
Supporting Member!
Posts: 1390
Joined: Fri Jan 09, 2009 10:41 pm
Real name: Dani Wilson
Gear: 163 tubes in a large wooden box!
Band: Shortwave
Location: UK
Contact:

Re: The Crumar GDS Restoration Blog...

Post by HideawayStudio » Fri Oct 24, 2014 5:58 pm

THE ORIGINAL GDS ADDS TERMINAL IS FINALLY ALIVE.. ALIVE!!

After a very very long story and almost reaching the point of wanting to top either myself the ADDS terminal or both I had a breakthrough....

Image

The other day I removed the known good “donor” tube from the GT85 and fitted it into the ADDS monitor chassis.

To my delight it mechanically fitted perfectly.

Image

I then replaced a curiously missing diode on the pcb with a fast recovery diode that seemed correct for the specific task it was doing.

I then carefully powered it up on a current limited PSU.

A mega bright and out of focus raster appeared with horrible flyback lines, no picture and a rolling frame.

I had a closer look and to my surprise found that the vertical hold trim pot was also missing!

What the heck was the last guy working on this thing doing??

So I replaced that and sure enough the frame locked but still a crazy bright mess on the screen.

So I started changing some poorly looking caps with no real difference.

After two days of fiddling round I was starting to get pretty down.

I have been very fortunate to find someone in the US who used to service ADDS terminals and really knows his stuff otherwise I think I’d have lost it by now.

It was he who suggested I’d be ok using a donor CRT from a different model altogether.

On this news we both went back to the drawing board and managed to find a spec sheet for the tube I’d found and compared it with the original.

He noticed that most of the specifications were very close bar one voltage – the B1 grid which is used to control the brightness...

So today I spent quite some time trying to determine how I could bring the monitor into spec with its new tube and after much poking around I managed to trace the G1 grid CRT pin all the way back to its biasing circuit.

This has all been made much more difficult because there just aren’t any schematics available for this monster.

Having traced the biasing circuitry back into the digital board I managed to temporarily substitute a resistor with a pot and literally dial in the correct amount of bias whilst the monitor was running.

Sure enough all the flyback lines disappeared and the brightness settled down leaving a nice picture behind it!!

So I read off the values of the pot legs and fitted a resistor network its place to the same values.

Hey presto... a working monitor!

Image

It needs more soak testing and a touch of alignment but to say I’m relieved is a huge understatement.

This thing has been a nightmare to resurrect and this morning I almost gave up – thank heck I didn’t!

Image

I have a potential source of replacement foil disks for the failed keys... a task for another day.

So, for the first time the GDS is under control from its original serial terminal and it somehow feels much more fitting than using an old laptop running HyperTerm!
Last edited by HideawayStudio on Mon Oct 27, 2014 6:01 pm, edited 1 time in total.

Post Reply