ARP Quartet

ARP Quartet
ARP Quartet

The ARP Quartet is not a real ARP. It was made and sold in Italy by SIEL as the SIEL Orchestra, and then ARP bought the rights to distribute in the US and rebranded it as the Quartet. It is a string synth with four sections of which two can be selected at a time, which are stacked, not split into two regions of the keyboard as is the case with some string synths.

Being “not a real ARP,” it does not have all the pitfalls that ARPs have: tantalum capacitors, crappy prototype-style PCBs, etc. It is the first synth I have ever worked on in which every IC is socketed! That’s amazing. I give it big points for that.

Service Tips: this has the kind of keyboard where the key contacts are made out of springy things like wound guitar strings (without a core though). Do not put DeOxit on these contacts. I’m not sure why, but they hate DeOxit. It doesn’t damage them, but it makes them work less well. It is much better to clean them with acetone. For the buss bar, I polish off the tarnish with regular household metal polish cream.

This one had a bad TDA1008 divider IC in a failure state in which all keys of that note played all octaves of the note– I hadn’t seen a TDA1008 do that before.

Work done: full replacement of all electrolytic capacitors (130+ capacitors!). Cleaned buss bar and key contacts, cleaned sliders and switches and lubricated sliders. Replaced one diode to fix envelope problem with one note, and one TDA1008 (discussed above).

9 thoughts on “ARP Quartet”

  1. love my siel or400 which is, i believe, the second version of the siel orchestra and was rebranded by sequential circuits as the prelude.

  2. Hello,

    I’m having a couple similar issues with mine:
    1) one key is not working (a lower “E”), it sounds like the envelope crapped out on it (it started to drop the note and eventually the key was just dead). All the other “E” keys do work. If I need to replace a capacitor- which type do I order for this synth? How did you know yours was due to a diode issue? which diode parts would I need to order to replace mine as well?
    2) a few of the patches on this synth give out a wierd bell-like scratchy ring on the attack of the key. I’ve read that this could be a divider chip issue if it happens for all notes up the octave on the synth. When replacing the TDA1008, is it a matter of swapping in a new one? or is there any sort of extra steps?

    1. Hi Art,
      In answer to your first question– after opening up the front panel of the synth, if you unscrew the keyboard from the bottom you can flip the entire keyboard up and you will find a circuit board actually screwed into the bottom of the keyboard. On this board there are groups of diodes, capacitors and resistors for every note. The groupings are fairly clear. Count up from the bottom the number of these groupings that corresponds with how many notes up from the bottom your bad note is. The capacitors are labeled with their values and ratings but as I recall the diodes may not be– but they are 1n4148. If you don’t really know how to troubleshoot this, just replace the capacitors first and if that doesn’t help, the diode… and add solder to and reflow all the pads/joints in that area.
      As for the TDA1008s. They are actually on the same board as the stuff I mentioned above, but a bad one wouldn’t affect all the notes in one octave… it would affect all the octaves of one note! (same words, different order). They are socketed so you can just pull the bad one out of the socket and put in a new one, but it sounds like that might not be your problem. I would try cleaning your key contacts for the notes making the noise.

      1. Hello,

        I’ve been slowly trying to work on this synth and I’ve diagnosed my issues to a faulty capacitor.

        Problem is – I don’t know what type to order for a replacement as the info on this electrolytic cap is vague.

        I assume every Arp Quartet is going to be built the same specs, perhaps you can share what type of cylindrical shaped caps are right underneath the key bus bar? they’re paired as positives (top row right across the whole board) and right underneath is a negative row.

        My negative rows are showing “63V, temp range of -40C to +85C”, but no capacitance uf ratings?
        My positive row of caps is showing “40V 4.7uf” but no temp rating!


        1. The other row of capacitors under the keyboard are all 1 uf.
          Generally the voltage and temperature ratings don’t have to match exactly for capacitors that will only be subjected to these kinds of low voltages, and you can always just get something on the high side if you’re not sure.
          These only ever see 15V or even less (12V? I don’t remember for sure), so you could go with a rating as low as 25 V for either of these, but I usually stock mostly 35V-rated electrolytics for basic synth (non-power supply) circuits, so that’s what I used.
          85 degrees should be a safe bet for anything other than a power supply capacitor as well, for which I use 105 degree ones.

  3. Hi

    I’m slowly reviving a Quartet with many snapped sliders – do you know the maker of the slider pots in the Orchestra/Quartet? I’ve established that Crumar mainly used Piher but I’m in the dark as to which brand Siel were using.


    1. Sorry, I don’t! a lot of these old sliders are impossible to find now I’m afraid– sometimes you have to come up with a creative way to repair the one you have!

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