ARP Quadra service notes

In a Moog Mood? Here's a forum for discussion of general Moog topics.
Technician Larry
Posts: 76
Joined: Tue Mar 01, 2011 5:07 pm

ARP Quadra service notes

Post by Technician Larry » Mon Mar 19, 2012 12:24 pm

In 1978 I attended factory training for ARP instruments. Back in those days ARP, SCI and Moog invested in training service technicians. In this case it was an all expenses paid trip to Denver for a week of classroom sessions. The Quadra was introduced that year and we were issued a preliminary set of schematics - 30 loose sheets of 11 X 17 schematics only. Over the next 34 years I worked on a total of about 3 of these instruments, and none of those repairs required any deep understanding of what made this design unique. I have one in the shop now that turned out to be quite a challenge, and I hope this report will save the next tech who goes there some time and aggravation.
***********
The ARP Quadra was a unique and innovative state of the art design for a very brief period in electronic music history. The schematics I have are dated 1978 - back when the use of microprocessors was in the infancy stages of circuit design. Oberheim and SCI were already on the verge of releasing their new microprocessor designs which would make the Quadra obsolete almost before it was released. One apparent consequence of these changing times was that ARP never published a service manual for the Quadra. If they ever did, I did not get my copy. 34 years later I am stuck fixing Quadras without the benefit of any circuit descriptions, completed block diagrams, error corrected schematics or PCB layout sheets. Fortunately the PCB artwork was pretty complete with key component designations silk screened on the component side and etched into the solder side of the boards.

The combination of old school analog circuits interfaced with primitive micro processor control created some confusion in the absence of any circuit descriptions. I have not seen any design quite like this on any other keyboard. The documentation I am writing here will serve as a circuit description for a complex gating arrangement dreamed up by ARP engineers. A design that faked me out of my sanity for a while.

When the Quadra being discussed came in for service it seemed to be mostly functional. Externally it appeared to be in excellent cosmetic condition except for the four sliders with sheared off handles. There were some straight forward logic problems causing some switching errors, the after touch was not responding hardly at all, the key contacts were in atrocious condition, and the gating was somewhat messed up. Putting the keyboard mode switch in the multi-trigger position made it appear that it mostly worked, but in single trigger mode not much worked as it should.

The switching logic was an easy fix. After touch was a matter of mechanical adjustment - again an easy fix. The key contacts and buss bar alignment was about as ugly as I've seen. After observing that each key switch functions as a single pole DOUBLE throw switch, and that the key up closure is as important as the key down closure, contact adjustment was pretty straight forward. Each key switch performs 2 functions. A digital signal is sent to the microprocessor and an analog gate circuit initiates the Envelop Generators and biases on the waveforms for strings and poly synth on the voicing boards. The processor responds to the digital signal and generates the CV for the bass and lead voices. It also produces trigger pulses when the keyboard control is set to multiple trigger mode or when the arpeggiator is on. The gate signals however originate from analog circuits on the Keyboard Electronics and Lower Voicing pcbs.

This keyboard has three separate gate buses designated lower Octave 1 Buss, 2nd Octave Buss, and Upper Buss. As I was probing around looking for the correct gate signals and reasons why they were not present, I discovered evidence that a previous technician had been chasing the same problems. I found related components had been replaced and also that the Octave 1 and 2nd Octave buses were bridged with a blob of solder. With enthusiasm I removed this "mod" thinking that now it will start to make more sense. Unfortunately, removing the bridge only led to more confusion. The circuit behavior became more unpredictable and confusing than before lifting the bridge. Now we get to the interesting part.

Image

Each key switch (SPDT - single pole double throw) pole is connected to a CMOS non inverting buffer - a CD4050 located on the Keyboard Electronics pcb. These are hex buffer arrays so there is one chip for every 6 notes. Pin 1 (Vdd) of the buffer IC is connected to ground and pin 8 (Vss) is connected to -12V VIA a current sense circuit (see photo 1). R88 supplies voltage to buffer IC pin 8 with NO KEYS DEPRESSED. The key switch poles are at ground with key up, the buffer output is at 0.0 Volts, and there should be NO current load generated by the buffer IC. R88 will read 12.0 V on both sides. When a key is depressed the buffer drives a key voltage to the Voicing boards and then loads the Vss supply. The load creates a voltage drop across R88 which then forward biases CR 50. CR50 sources all necessary current to the buffer ICs and the voltage on the buss remains constant at a diode drop above -12V supply. Z 24D compares this voltage with a reference set by R87 and R89 and generates the Gate signal as long as the buss sees a load from any key depression connected to that bus.

Image

The Gate circuits for 2nd Octave and Upper buses are located on the Lower Voicing board. These two comparator circuits are configured to output the opposite polarity from the Octave 1 circuit as some additional logic is employed before the Gate signal is generated. Other than that they function the same as the Octave 1 buss. 2nd Octave buss connects to Z3 and Z4 pin 8s. Upper buss is connected to Z5 thru Z11 pin 8s. Both of these buss comparator circuits were not behaving predictably. Strangely enough they sometimes worked when either a scope probe or meter leads were connected to one or the other comparator input, but sometimes they functioned briefly depending on some parameter I could not identify. I did find a "wrong" value resistor @ R162. The schematic designates 2.2K ohms but R162 was 2.2M ohms. I suspected a schematic error since the 2n Octave buss has 2.2M in this slot, but of course, not knowing what I know now, I had to check it out. That circuit does not even come close to working with 2.2K @ R162.

Image

What I found was a 200 to 300mV drop across R147 (in the case of the Upper buss circuit) with no key depressed, and a similar but different condition on the 2nd Octave comparator. While all 12 of the CD4050 hex non inverting buffers were fully functional, 2 of them - one on each of the buses, were causing a current load on the buffer lines with no key depressed. There is no easy way to measure the current draw on each IC. To prove this I had three choices. #1: I could remove each IC one at a time. This board is no fun to work on and very time consuming to do a neat job of this, so forget that. #2: DIP (Dual Inline Package) pins can be carefully clipped such that they can be reliably reconnected with solder if you know how to do it. It requires good quality small cutters with the appropriate shaped nose. Mine have been misplaced and I use my backup cutters which don't do such a neat job. I opted for #3: Make a surgical incision on each pin 8 trace to isolate those ICs from the buss one at a time. This was the obvious choice dictated by the pcb layout making it easy to do so. The idea is to make one clean cut at an angle so one side of the trace can be lifted slightly creating a certain open circuit.
Image
After the lifted trace is pressed back down there should be no gap and solder will flow over the cut assuring a reliable connection. If you damage the trace leaving a gap it is a good idea to bridge the gap with some wire.

Image

The 2nd Octave buss has only Z3 and Z4. I cut the pin 8 trace to Z3 and presto - no more load on the bus. The Upper buss has 8 CD4050s. I was not so lucky and had to cut 5 traces before the errant load disappeared. Reconnecting them one at a time I tested each one to make sure there was only one CD4050 with this ailment. After removing these "rogue" ICs, the gate comparators worked reliably and I gained confidence in the design. Pretty innovative as long as you can count on CD4050s not drawing any current when connected in this way. Looking at the data sheet I don't necessarily see any guarantees.

All of this probing and testing made it necessary to remove the Keyboard Electronics board from the keyboard assembly multiple times. The connectors are supposed to be held snug to the Keyboard Electronics pcb by means of flimsy little plastic arms that clip over the edge of the fab. These are not adequate, and the pressure of inserting the pins when re-seating the pcb to the keyboard assembly causes these connectors to get pushed back stressing out the solder tabs and becoming miss aligned with the mating pins. Some hot melt glue was an easy and effective remedy.

Image
Last edited by Technician Larry on Mon Mar 19, 2012 3:27 pm, edited 2 times in total.

User avatar
Voltor07
Posts: 5194
Joined: Sun May 25, 2008 3:04 am
Location: Waukegan, IL USA
Contact:

Re: ARP Quadra service notes

Post by Voltor07 » Mon Mar 19, 2012 2:57 pm

Wow...what a mess! :shock:
Sub 37 #000068, Minitaur, CP-251, MF-102&103, EHX #1 Echo, EHX Space Drums/Crash Pads, QSC GX-3, Miracle Pianos, Walking Stick ribbon controller, Synthutron.com, Lowrey Teenie Genie.

User avatar
MC
Posts: 2888
Joined: Tue Aug 12, 2003 2:20 pm
Location: Secluded Tranquil Country

Re: ARP Quadra service notes

Post by MC » Mon Mar 19, 2012 5:35 pm

The Quadra was not one of ARP's high notes.

They shoehorned an Omni 2, Odyssey and Little Brother (for the bass synth) into a behemoth and it didn't even have proper patch storage/recall it only highlighted the panel sliders that needed manual setting. The polysynth was just a TOS string synth not a proper voice assignment polyphonic system. Broken keys and/or slidepots were common on used Quadras. It was a rush job and the lack of a service manual showed it.

Quickly outclassed by the P5 and OB-X. It was a David Friend concept (the top guy also responsible for the ill-fated Avatar) and was the impetus for the board to demand his resignation. Al Pearlman resumed control and started the Chroma synth but ARP was liquidated before it got to production. Sad story really.
Gear list: '04 Saturn Ion, John Deere X300 tractor, ganged set of seven reel mowers for 3 acres of lawn, herd of sheep for backup lawn mowers, two tiger cats for mouse population control Oh you meant MUSIC gear Oops I hit the 255 character limi

User avatar
Kevin Lightner
Posts: 1587
Joined: Sat Jan 15, 2005 5:20 pm
Location: Wrightwood

Re: ARP Quadra service notes

Post by Kevin Lightner » Mon Mar 19, 2012 6:28 pm

Good post. :)

Arp did publish a service manual though.
I had an original I bought from MDS, but loaned it to someone in 1985 and never saw it again.
Here's what I have for it now:

http://www.synthfool.com/docs/Arp/ArpQuadra/
Better to be king for a night than schmuck for a lifetime. - R. Pupkin

User avatar
Kevin Lightner
Posts: 1587
Joined: Sat Jan 15, 2005 5:20 pm
Location: Wrightwood

Re: ARP Quadra service notes

Post by Kevin Lightner » Mon Mar 19, 2012 6:34 pm

Btw, you can avoid cutting traces to locate a bad cmos chip if you use an IC tester.
I have no idea where to find another one like I use, but it has proven invaluable over the past 20 years.
Here's a pic and some info on the one I use. It was only $179 dollars back in 1990:

http://analogsynthblog.blogspot.com/200 ... ool-2.html

Also it's possible to locate a shorted IC by using an ohm meter set to very low ohms and tracing the path of most resistance.


PS: I just found one of these being sold for $50 here:
http://www.maxipub.com/electro/specs/instruments.htm
Better to be king for a night than schmuck for a lifetime. - R. Pupkin

Rob Smith
Posts: 180
Joined: Mon Mar 15, 2004 10:56 am
Location: Huntington, NY

Re: ARP Quadra service notes

Post by Rob Smith » Mon Mar 19, 2012 7:50 pm

That keyboard was part of Tony Banks rig for a very long time. Duke (1980) thru Invisible touch (1986). It was also used live during that time period also. My friend Chip Weinberg was one of Robert Perlmans go to guys in Manhattan in the 70's and 80's. I will ask him if he has the manual for the Quadra. I will try to talk to him tomorrow.
Last edited by Rob Smith on Tue Mar 20, 2012 4:29 am, edited 1 time in total.

Technician Larry
Posts: 76
Joined: Tue Mar 01, 2011 5:07 pm

Re: ARP Quadra service notes

Post by Technician Larry » Mon Mar 19, 2012 11:04 pm

Wow - thanks guys. I did not even look outside my own files for a service manual as I knew I should have one if they published it. Perhaps I should look further in my archives. I stopped looking when I came to my file folder from ARP school with the schematics. Those schematics were all I ever needed in the past. When ARP announced they were closing the doors I had about $500 worth of pending warranty claims that I was waiting to get paid for. Most of that was for replacing those pathetic membrane switch keyboards in the electric piano. So I know I was definitely on their mailing list. So, thanks Kevin. I just opened that pdf from your site.
http://www.synthfool.com/docs/Arp/ArpQuadra/
Scanning the content I see that section 3.5.3 contains one paragraph explaining the Gate Logic. It completely skips over the two sentences that would have really helped me out on this job, so I'm probably time ahead by not even looking.

On the production line at Tektronix back in the early 70s we had two pieces of test equipment that could have helped avoid cutting those traces. One was an ohm meter that allowed setting a reference and then "zoomed in" to a very high resolution to measure slight differences between measurements. That would be handy to have, but this is the first time I might have ever needed it in decades. The other was made in house by a guy in my group. It had a probe with a directional coil that sensed current flow in a trace. You need these tools on the production line where you might possibly find internal layer fab shorts. That is mostly what the current sniffer was used for. You might also find solder bridges, wrong value components, components with insertion errors, etc - all of which we don't expect to find in the field after the unit presumably passed all QC and functioned correctly at one time. An IC failure in the field is usually located by looking at voltages. It is rare that all the voltages in and out were correct, but the failed CMOS ICs shut down the circuit anyway. It is so easy to get fooled.

ARP
Posts: 170
Joined: Fri Sep 30, 2005 5:14 pm
Location: USA

Re: ARP Quadra service notes

Post by ARP » Tue Mar 27, 2012 10:39 pm

Thanks for posting this ! i've owned a Quadra for the past 25 years and love it. Although the Quadra has gotten a bad rap over years for it's quirky design and it's membrane switch issues, it is a great sounding analog machine and a blast to play.
"Although they heard the music..they didn't understand the tune"

Jonathan G
Posts: 25
Joined: Sat Mar 19, 2011 11:54 pm

Re: ARP Quadra service notes

Post by Jonathan G » Sat Feb 16, 2013 2:00 am

Is there any trick to getting the aftertouch to work better? I'm only getting it to change the pitch by about 1/2 of a semitone. I've already tried adjusting R1 on the touch sensor board, and am not sure where to go from here.

Jonathan G
Posts: 25
Joined: Sat Mar 19, 2011 11:54 pm

Re: ARP Quadra service notes

Post by Jonathan G » Sun Feb 17, 2013 2:14 pm

I managed to remove and clean the touch sensor mechanism, and am measuring 0 - 300kOhms across it, so it appears to be working. Now looking at the Keyboard Electronics Board, I'm thinking the area toward the bottom right corner may be where the problem is. There is a funky multi-colored tantalum capacitor in the circuit, just after a TI MC1458P op amp, that may be causing problems. It looks OK to the eye, but there appears be some corrosion on the board nearby. The only problem is that it has some very odd color codes on it that are not in line with any color charts I've found online. It's brown-green-light green, with a blue modifier dot. Odd. Maybe it's just a 1979 thing.

Jonathan G
Posts: 25
Joined: Sat Mar 19, 2011 11:54 pm

Re: ARP Quadra service notes

Post by Jonathan G » Sun Feb 17, 2013 2:42 pm

The cap in question is in the top left corner of the pic. Assuming the blue smudge down the side is the dot, or modifier, doesn't make sense, at least according to this chart: http://www.marvac.com/fun/tantalum_capacitor_codes.aspx

Image

moogsaurus
Posts: 56
Joined: Fri Nov 16, 2012 8:01 am

Re: ARP Quadra service notes

Post by moogsaurus » Mon Feb 18, 2013 9:12 am

Excellent post
The usual blurb on the Quadra is; "its just an AXXE/Omni thrown together" never worked on one-never even seen one in the flesh - rare bird! Most of the ARPs I did were the portable ones,but using ARPs theyre pretty good at what they do,and I'd imagine the Quadra would be no exception!
Great to read some indepth true info,rather than the usual tired rehash stuff!

synthaxe
Posts: 85
Joined: Wed Sep 19, 2012 5:10 pm
Location: Central Illinois

Re: ARP Quadra service notes

Post by synthaxe » Mon Feb 18, 2013 1:54 pm

One of my "dream" synths.
Little Phatty Stage II, Minitaur, MF-105, MF-103, MF-102, MF-101, Mesa/Boogie Mark IV, EVH 5150III 50 Watt, EVM12L Speakers, ESP Eclipse-II FR, USA Charvel So-Cal, Squier Vintage Modified Surf Stratocaster, Alesis MidiVerb 4, and tons of Electro Harmonix.

lukeA
Posts: 8
Joined: Fri Mar 01, 2013 4:03 am

Re: ARP Quadra service notes

Post by lukeA » Fri Mar 01, 2013 10:37 am

A very interesting read indeed... especially as I am about to begin a Quadra restoration of my own!!!

Mine has many issues- only the lead section works and is the arpeggiator is stuck on, couple of keys don't work, and a couple of the membrane switches don't work.

For this reason I am considering just going through and replacing all the logic chips, possibly all the caps while I'm at it. I figure after this would be a good place to start trouble shooting!

Would anyone have any sources for a new front panel if I can't fix the membrane switches?

Thanks,

Luke

User avatar
Kevin Lightner
Posts: 1587
Joined: Sat Jan 15, 2005 5:20 pm
Location: Wrightwood

Re: ARP Quadra service notes

Post by Kevin Lightner » Sat Mar 02, 2013 4:57 pm

lukeA wrote: Mine has many issues- only the lead section works and is the arpeggiator is stuck on, couple of keys don't work, and a couple of the membrane switches don't work.

For this reason I am considering just going through and replacing all the logic chips, possibly all the caps while I'm at it. I figure after this would be a good place to start trouble shooting!
With all due respect, I'd do the opposite.
I'd suggest first checking / servicing the membrane switches.
The Quadra executes sequential routines, ie: check the keyboard, check the panel, send data to DAC, etc.
If a membrane switch is shorted, it will read and execute that function and move on.
On the next cycle it will repeat this, always doing the same thing because the membrane switch is still shorted.

From what you said, much of the logic is already working.
The CPU, keyboard scanning, memory, DAC, etc.
Best to assume it all works and first test something already known bad like the panel.
You could replace a bunch of ICs (and possible introduce a new problem) and still have the original problems.
The voices out could be dividers, but are better left until the instrument is under full control.

Finding another Quadra panel might be difficult.
The membrane might be repaired, but are very difficult.
It's almost impossible to solder to the internal conductors.
Most of their problems are from tarnished pads or from being warped or melted.
The latter can cause continuous switch closures and is very difficult to repair.
The connector pieces can also get worn out or cracked and even careful use of silver paint is difficult.
Some people have just abandoned them altogether and drilled holes for buttons.

So I'd first check to see if any membrane switches are in a state of continuous closure.
(replace the batteries also. ;-)
No use doing more until the panel is good.
Better to be king for a night than schmuck for a lifetime. - R. Pupkin

Post Reply