ARP 2600 upgrades and service



			UPGRADES / SERVICE FOR THE ARP 2600

The Blue Marvin

This is a photo of the very rare, blue version of the ARP 2600 with the matching ultra rare 3604 keyboard. There are approximately 20 of these beautiful machines known to be in existence according to our estimates. I love the look of the metal cabinet 2600's, but Alan Pearlman didn't speak highly of the enclosure when I first mentioned the old metal cabinet 2600s to him when we first met. "We had an aircraft fuselage engineer design the cabinet for the very first 2600's, what a nightmare! It was odd looking, very difficult to service, and the corners tended to crack. I took the front panel (the front panel layout was designed by Alan himself) got rid of everything else, and came up with the tolex covered shuke type cabinet". This was explained to me by Alan R. Pearlman as he drove me around town and to lunch at a local Japanese restaurant in his black Saab. The shuke is the inside angled tolex section that the front panel is bolted to in a standard 2600P/2601. Over the years, it's obvious that Alan's cabinet solution made the machine much more roadworthy and serviceable. But you just can't help appreciating the "Mod" appearance of the first 2600's. Alan also talked quite a bit about Dennis Colin and described him as a genius, He was the electronic engineer who along with Alan, designed many of the 2500 and 2600s critical audio processing circuits.

The 2600 uses microchip technology in all of its gain stages! The first 2600's, like the one pictured above, used hi-fi chips (Teledyne) with a high slew rate. Although these chips sound great, they were very unreliable and prone to latch up. Early on in 1972, plagued by failures in the field, ARP was forced to change these chips to a much more reliable type (National Semiconductor LM-301 or SL-19986) The problem is that they slew the wave shape producing a muffled sound. The LM-301 is classified as a general purpose amplifier, and is considered inadequate for audio by present standards. Nearly all the standard production gray-face 2600's use the LM301. This is one of the reasons why the legendary Blue Marvins sound better! If your 2600 doesn't look like the one in the photo above, and odds are it doesn't, your 2600 is not using the original circuitry and does not sound the way it was originally intended to sound by the ARP engineers.

The C.M.S. Audio path upgrade restores your 2600 to the original performance specs. If you're going to use your 2600 in a modern digital/analog studio, we strongly recommend the CMS Audio Path Upgrade. If you want to keep your 2600 totally original for collector reasons, CMS offers full restoration and repair services back to ARP factory spec.




	This is a photo of the very first portable ARP 2600. The cabinet was designed and built by Alan Pearlman himself. We recently fully restored this unique unit here at CMS for a customer. This is the actual machine mentioned by Alan Pearlman in the above text that he used to work out the design of the 2600P. It may seem oddly familiar to some people. It was instantly recognizable to us as we unpacked it. This is the exact machine ARP used for many early promotional photos. It was used for photos in the instruction and service manuals and the 2600 promotional poster. If you look closely, you will notice some notable differences compared to a standard 2600P: The keyboard uses the ARP 2500 SP module for the keyboard front panel. The tolex covered shuke that covers the back of the keyboard is triangular shaped and does not level off on top. The color of the console panel is a unique and beautiful light blue gray. The console front panel is mounted further forward than a standard issue 2600 making the front panel flush with the edge of the shuke. The lid clasps are in different locations. This unit also features a tolex covered removable rear hatch for easy service. There is only one exactly like this in the entire world.

UPDATED : 04/26/2023

STAGE ONE UPGRADES / MODIFICATIONS FOR THE ARP 2600

These upgrades are internal audio improvements to the circuitry of the 2600.

Listed in order of popularity:

POWER SUPPLY REBUILD - (ALL MODELS) - 189.00

The power supply in your 2600 is about 30 to 40 years old. Certain components in the ARP 2600 power supply have a shelf life / working life of 20 years maximum. If your power supply has not been serviced in the last 20 years, chances are your power supply is not up to spec. This can result in excessive noise, ripple, and instability. Your power supply is removed. All critical components are professionally replaced. Stability and load are tested using factory specs. Rebuilt ARP power supplies are available from CMS but we require the old power supply in exchange.

AUDIO PATH UPGRADE PACKAGE - (ALL MODELS) - 399.00

This mod replaces all gain stages in the 2600 console with audiophile grade components for crystal-clear highs and a deep low end, while still maintaining the distinctive character of the 2600. Upgraded sections include: lag processor, voltage processors, reverb stages, pre amp, VCF mixer, output mixer, output buffers, etc. We also fix the annoying VCA thump. Noise in the reverb section is reduced by 35%. Filter headroom is increased by a factor of 2! Stability and sound improvement guaranteed. Later model 2600s (almost all wood / tolex 2600s) use inferior sounding gain stages that severely impede the true sound of your 2600. Earlier 2600's (Blue Marvins and Gray Meanies) use "high slew rate" chips that sound much clearer. Restore your 2600 back to the specs that the original ARP engineers intended! Don't be fooled by imitations from "noob techs" or DIY attempts. If the Audio Path Upgrade is not performed correctly, it can become unstable and lead to high frequency self oscillation of all the amplifier sections.

17 INCH REVERB TANK - (ALL MODELS) - 99.00

Ever wonder why later model 2600s have such a lame reverb? Later model 2600s use smaller reverb tanks with a shorter delay time than earlier 2600s made by ARP. The only reason I can find for this change is economic. Our brand new reverb tanks use 3 springs, counter wound and placed in parallel for a rich, full sound. This mod restores your reverb sound back to the original "Blue Marvin / Gray Meanie" configuration.

MOOG STYLE FILTER MODULE - 4012cx - (ALL MODELS) - 399.00

The 4012 filter was used from 1971 (blue Marvin) to the middle of 1976. If your 2600 is a gray face, It probably utilizes the 4012 module. The 4012 is a ladder type filter using discrete transistors. The original 4012 made by ARP is prone to instability. This causes 2 types of problems when the filter is wide open:

1. Unwanted high frequency self oscillation when using resonance. This can cause serious problems in the recording studio. The high frequency sine wave can wreak havoc with A to D converters and soak up serious headroom.

2. Significant gain reduction of the signal when the resonance is off. This can rob the 2600 of it's classic powerful punch! Reduction in gain at high frequency settings can reach 50%.

The 4012 replacement filter by CMS is guaranteed stable yet uses the exact circuit that ARP used. Better quality components and personal attention to craftsmanship yield a filter of superior sonic characteristics! See the Sub-module section for more info.

VCA MODULE UPGRADE - (4019) - (LATER MODELS) - 125.00

If your VCA module is un-encapsulated, we can give it crystal clear highs and a deep low end. Also provides lower distortion and a faster slew-rate. If you have an encapsulated ARP VCA, this upgrade is not available, but improved replacement modules are available, see sub-modules page, model 4019cx.

FILTER MODULE UPGRADE - (4072) - (LATER MODELS) - 99.00

All later ARP filters (after 1975) have a cutoff frequency limit of 11 kHz. This mod upgrades the cutoff frequency range from 11 kHz to 28 kHz. We also upgrade the filter output amp and control summing amp. This is a necessity for all un-encapsulated filters. A cost effective alternative to a 4012 module.



	The rare ARP 2600C restored by CMS.

	STAGE TWO UPGRADES / MODIFICATIONS FOR THE ARP 2600 These upgrades / modifications are more extensive and time consuming than the above. They usually involve partial to complete disassembly, front panel machining, re-assembly etc. NEW - SECOND FILTER WITH SWITCH- 225.00 So you like the switch-able filter feature on re-issue 2600s but you don't like the sound. You are not the only one who feels this way. Because of the surface mount technology and other methods used in reissue 2600s, the sound cannot be improved. We have come up with a universal second filter adapter board that installs in the real ARP 2600 that permits you to have the best of both worlds. Customers that have the 4072 can now keep their original second generation filter and also have the first generation 4012CX installed, and yes, the reverse is also true. A toggle switch on the front panel engages the first version (4012) or the second version (4072) filter. This new mod from CMS features no changes to the original signal path of the 2600. Price shown above does not include the cost of the second filter sub-module. Listed in order of popularity: SYNC SWITCHES - 199.00 Two independent high quality miniature toggle switches that syncs VCO-1 to VCO-3, and VCO-2 to VCO-3 This setup enables dual sync (two separate harmonics) if desired. Stability guaranteed, unlike the factory version. Explanation: Some people think this mod is used to simply synchronize 2 VCOs together. Not so. That is pointless since they end up sounding like 1 VCO. This mod is used to obtain the classic "ARP sync sound" found on some other ARP synthesizers. Basic patch: 2 VCOs are synchronized together. Modulation is applied to the slave oscillator (usually ADSR). The output of the slave VCO is fed to the filter etc. to obtain the sync sound. Sounds best with a square wave. The effect appears on all the waveform outputs of the slave VCO. You must have at least 1 un-encapsulated VCO to install this mod. ENVELOPE GENERATOR RANGE TIME EXPANDER SWITCHES - 69.00 each Miniature high quality toggle switches that provide 2 additional time ranges for the attack, decay and release time of the ADSR generator and the attack and decay time of the AR generator. ADSR: The ADSR is quick in a stock 2600. The 3 position switch we install gives you a x1(factory speed), a x2 and a x3 speed, providing much longer times previously unavailable on a 2600. AR: Conversely, the AR generator in a 2600 is quite sluggish in certain situations. In addition to the normal X1 mode, the high quality 3 position switch provides a new high speed mode for sharper attacks, as well as a X2 mode for longer attack and release times. AR ENVELOPE GENERATOR UPGRADE - 139.00 This mod is a critical upgrade of the AR circuit in the 2600. The AR in the 2600 suffers from a sluggish response time and a poor input impedance. In addition, the output voltage can change depending on the gate voltage used to trigger it. It can also put a drain on your gate supply especially if you are feeding that gate to multiple destinations. The gate input is permanently linked to the ADSR input so you can't independently trigger the AR from another source like a VCO etc. This CMS mod fixes all of those issues but still maintains the original core circuitry of the 2600 AR. Comes complete with a separate high quality gate input jack that conveniently separates the inputs of both envelope generators for independent operation. 1/4 INCH LINE LEVEL JACKS - 99.00 Professionally installed 1/4 inch Switchcraft jacks at line level (designed to interface with your studio mixer etc.) on both main outputs, VCA output, and pre amp input. 4 jacks total. VARIABLE VCF KEYBOARD TRACKING - 75.00 The filter tracking on a 2600 is fixed at 1v per octave. This is quite limiting especially if you are shooting for natural textures. This mod provides a high quality miniature rotary potentiometer that adjusts the filter tracking ratio from 0 to 1v/octave. Now features a high quality 2500 control knob. NEW - REVERB DRIVE CONTROL - 75.00 This mod provides a high quality miniature rotary potentiometer that adjusts the reverb driver gain from 0 to overdrive. Now features a high quality 2500 control knob. POWER SUPPLY UPGRADE - (ALL MODELS) - 379.00 The power supply in your 2600 is about 30 to 40 years old. This upgrade features a professionally installed, high power, highly stable, military spec power supply. This power supply features a high speed regulation circuit that reduces cross-talk between sub-modules in your 2600 system. The overall sound quality and oscillator purity are noticeably improved. Warning: These power supplies must be installed by a professional. Incorrect installation of a power supply upgrade can lead to system instability.


*STAGE THREE UPGRADES / MODIFICATIONS FOR THE ARP 2600* These major upgrades / modifications are very extensive and time consuming. These mods are designed for users that are seeking to professionally "Hot Rod" their ARP 2600 to the max. They require complete disassembly, front panel machining, rewiring, and the power supply upgrade. Listed in order of popularity: VCO OCTAVE SWITCHES - 115.00 each This variable version octave switch has a position toggle and a potentiometer to dial in any desired interval over a 2 octave range. This upgrade uses the same high quality miniature toggle switches, military spec potentiometers that were used on the ARP 2500 system including a high quality 2500 style knob. LED INDICATORS - 59.00 each Miniature LED indicators providing visual feedback on both E.G. outputs (red), S/H clock (green) and envelope follower (yellow). Requires the power supply upgrade. DUAL LFO - 389.00 We install 2 LFOs below the voltage processors. They have triangle and square outputs and also have LED indicators included. This mod, as all our others for the ARP 2600, features high quality Switchcraft jacks, military spec potentiometers and ARP 2500 style knobs. Requires the power supply upgrade. COMPARATOR / DISTORTION - 199.00 This can be used to square up a waveform. It also makes a good distortion device according to John McEntire from Tortoise. Includes LED indicator. 24Db DUAL MODE FILTER - MODEL 2621- no longer available - see our euro-module 1047 The one key element missing in a 2600 is a high-pass filter with resonance and associated coupling circuitry. The CMS 2620 is a dual mode, 24Db (Low-pass/High-pass), discrete, transistor ladder filter from the CMS 9000 series modules, Installed in the left speaker section. This original CMS design features an innovative circuit that extracts a high-pass response from a 24BD discrete ladder filter circuit producing a unique sound and beautiful resonance quite different from the existing low-pass filter in the 2600. In addition, the volume of the filter output does not drop off drastically as the resonance is increased as all other ladder filters do. This also makes the low-pass output quite useful as an alternative sounding 24db low-pass filter. Complete with anodized face plate, high quality controls, 1/8 inch Switchcraft normalized patching. Special link switch enables the coarse frequency control knob of the 2627 to control both filters for a truly remarkable sounding band-pass function (24Db x 24Db). The resonance on the high-pass function is also very unique and distinctive. Requires the power supply upgrade. Now comes with voltage controlled resonance. VCO / 4 CHANNEL MIXER - MODEL 2627 - no longer available - see our Euro Rack section - module 1004 Installed in the right speaker section, this VCO utilizes the same exact circuitry used in the other VCO's in your 2600 to assure exact tracking and pitch stability characteristics. All four waveforms are available as well as pulse width modulation. Low frequency mode and normalized patching are also provided. The mixer section serves as a high quality audio mixer with the inputs either normalized to all 4 outs of VCO-4 or externally patched. The mixer can also be used as a voltage summer or inverter. This major upgrade uses high quality Switchcraft jacks, military spec potentiometers and ARP 2500 style knobs. Requires the CMS power supply upgrade.

MODIFICATIONS FOR THE ARP 3620 KEYBOARD

INTERVAL SWITCH - ALL MODELS - $49.00

The switch provides a easy and fast method of locking the pitch interval of the second voice without the use of a foot switch. Useful for instantly setting up precise intervals or octaves of selected VCOs.

LFO FREE RUN SWITCH - $59.00
Most people do not know this, and it is probably because it was never mentioned in the ARP 3620 owners manual but the 3620 LFO is re-triggered or restarted when you hit a key on the 3620 keyboard. This can sometimes be annoying especially when using a slow LFO to modulate something like a filter for example. You can actually disable this function by setting the trigger mode to "SINGLE", however the LFO will always restart in the "MULTIPLE" trigger mode. When this mod switch is in the "free run" position, playing the keyboard will not restart the LFO under any circumstances.




	A fully modified 2601 Many of our customers prefer to have their machine in original condition. We offer full restoration services that restore your 2600 to original ARP factory specifications. Call or email Phil at CMS to schedule a free estimate on your ARP 2600. Every 2600 restoration varies widely depending on condition, road use, storage conditions, poor servicing, substandard aftermarket sub-modules and moisture damage. Estimates are not available by phone or email.


	ARP 2600 RESOURCES Running a 2600 from MIDI - highly recommended 2600 Identification chart 2600 Packing Tips - Shipping Procedure

e-mail: filter@discretesynthesizers.com

phone: 912-727-2195