paia 2700 lots of issues... hum and more
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Author:  patrickvf [ Fri Oct 04, 2013 11:22 am ]
Post subject:  paia 2700 lots of issues... hum and more

Hello, I have received a Paia 2700/2720 yesterday, and for the most part it works fine (and I'm able to make a half decend phaser from the bandpass and inverter modules :D however, there's a big damper - first off when I switch on the Paia and I have the headphones/line connected to VCA out, there's a VERY noticeable hum present. It's not present on all modules, but mainly on the VCA. inverter outputs and the individual VCO outputs (and I don't think the CV pins are supposed to hum either when not connected?) Furthermore, when I connect the keyboard trigger to the envelope generator, there's a faint, but noticeable, static beep bleeding through into the signal path. This is irritating! Is there any way to fix the issues without having to get an 5 year degree in Electronics? I did some solder work before, I just need someone to point out what to solder where. Well until then, I'll use it as an oversized phaser, lol.. wait.. there's still the hum! :cry:

PS: Some of the modules (VCO, EG and BPF) seem to have modifications because they have extra switches that I don't see in images of the original 2700/20 modules. Also, the keyboard tracks when and however it pleases, and turning up the portamento takes down the pitch? Is that supposed to happen? Anyways it's no biggie because I can use my ultra rare Multivox MX-880 for some keyboard action, HA!

Author:  patrickvf [ Mon Oct 07, 2013 7:26 pm ]
Post subject:  Re: paia 2700 lots of issues... hum and more


No help? Nothing? Hello?

PS: The VCA output is also extremely low. The inverter module seems to give it a pretty good boost, but the hum gets boosted up as well. So.. any suggestions are highly appreciated.

Author:  PAiA-Scott [ Tue Oct 08, 2013 10:28 am ]
Post subject:  testing a 2720 series modular

When testing for audio output from your 2720 series modular, the audio outputs should be connected to an amplifier, or a mixer or other device with a line-level input and output connected for headphones.

Headphones need more power than is available from the module outputs. Without the extra power, the audio signal would be weaker than it would be into a more sensitive input.

A weaker than usual signal would cause hum to be more apparent.

Hum can result if a ground connection is missing between the power supply and a module, or, from the modular system to the external audio-amp/phones/speaker.

It can also be more than is usual if the power supply is over-loaded.

There were only two 2720 modular systems, one with a button keyboard and one with a "real" keyboard

Both had a power supply and a complement of modules for a basic monophonic voice.

Is yours one of these types, or, a collection of modules in a 4700 series or user-provided case?

Can you post a photo?

I don't think the Multivox keyboard you have is suited for controlling the modules in the system. PAiA 2720 and 4700 series modulars have linear response VCOs which require an exponentially scaled pitch control voltage -- doubles/halves going up/down successive octaves.

FatMan is a solution, and a modern mono' synth' voice too. It could be layered with the sound from the 2720 system, connected through a simple mixer or two input amplifier/speaker, or further processed though the filters and amplifiers in the system.

It has the needed exp. Pitch CV and gate-triggering derived from MIDI -- and a control voltage proportional to the velocity byte contained in the MIDI Note Message.

Author:  patrickvf [ Tue Oct 08, 2013 1:44 pm ]
Post subject:  Re: paia 2700 lots of issues... hum and more

Thank you very much for your reply and advice.

The synth has all of the modules that you mentioned plus an inverter/buffer, envelope follower and a sine/pwm module. I'm not sure if any of these are 2700 io 4700 type modules but all modules have the same amount of rust on them, so I assume that they might be of the same type. It came to my attention that the CV/Trig outs of the keyboard is like the original 2700, not the 2720, so I guess I have one of first models? I'm not able to take and post any pictures of it right now, but I'm going to follow up with that asap.

I did hook up to the mixer straight from the VCA (or patch panel, inverter/buffer) to begin with and had the phones hooked to the mixer (I didn't clarify that earlier), but the output is very low and I have to turn up the mixer channel all the way. The hum is still there, but not on all modules, mainly on the VCO (along with the signal), VCA, and the noise out of the control oscillator. I have also noticed that the hum coming from the VCA seems to have a higher pitch? Could there be a separate issue with the VCA?

There's something up with the PWM dial on the VCO too - the span is very narrow, and the PWM effect sounds very distorted, more like fuzz or feedback, and gets much louder at it's peak.

As for controlling my synth with the sequencer - it provides output for both HZ/V and 1V/Oct standards, but haven't even tried that yet. But if that fails, I still have my EHX pedal sequencer...

PS: I have come to the conclusion that I want to try and fix the keyboard as well. There is a key that doesn't trigger/hold pitch and I kind of know what's going on there, but also the portamento/glide is kind of funky too - when I turn up the value, the pitch drops dramatically to a point where the VCO stops working, and the scale is completely out of whack, so.. any good advice for that is highly appreciated as well.

Greetings, Patrick

Author:  patrickvf [ Thu Oct 10, 2013 4:42 pm ]
Post subject:  Re: paia 2700 lots of issues... hum and more

Ok, here are some shots of the synth and the modules that are modified. Enjoy! PS: The top and back of the synth have a funky fabric backing of dancing skeletons on a black background 8-)

PS: Any ideas what the extra switches on the bandpass filter are supposed to do? The ones by the input and output don't seem to do anything but I hear a light pop sound when moving the one switch at the top, and that's all it does. Perhaps it's a failed mod?

Author:  PAiA-Scott [ Mon Oct 21, 2013 10:14 am ]
Post subject:  Re: paia 2700 lots of issues... hum and more

Thanks for the links to photos of the system.

It is a 2720R -- built from kits back around 1976 - 1982.

Didn't the seller of this offer any insight or documentation as to the modifications?

Only a handful of the kit assembly manuals for the all of the modules in this system package are scanned and posted here on PAiA Talk, but the diy project is detailed in this Radio Electronics magazine article by John Simonton: ... rticle.pdf

The first couple of pages include details about the 2720-7 Power Supply module, including a schematic. It might benefit from new electrolytic capacitors (1000uF/25V) as these can loose their ability to filter/smooth power supply ripple with age. More than normal ripple voltage on one or more of the -9, +9, and +18 volt DC power supply outputs might be contributing to hum in the audio outputs. Each of the modules will have electrolytic filtering capacitors too. New ones here, and perhaps even ones with more capacitance, might help minimize ripple and prevent hum from appearing in the audio outputs.

The 2720-8 manuals can be referenced for the keyboard electronics in the 2720R:

The glide-retrofit add-on was simply an op-amp circuit to buffer a resistor-capacitor, RC, filter circuit which worked to slow the change from one keyboard pitch control voltage to the next. The eight pin DIP op-amp on the circuit board might be a 741 type instead of a better suited, high input impedance type, and resulting in the drop in pitch when advancing the glide control. Later versions of the 2720-9 Glide Retro-fit kit included a 308 type op-amp to minimize the load on the RC circuit.

Had you noticed this 2720 series system overview posted in the manuals section?

Author:  patrickvf [ Wed Oct 30, 2013 8:04 pm ]
Post subject:  Re: paia 2700 lots of issues... hum and more

No, he didn't tell me anything about the mods, he only stated that the paia produced sound and it had not been further tested. He must be one of them storage raiders who sells the contents found in abandoned storage facilities on ebay or something. I found out that there was a loose wire touching the metal plate of one of the modified modules. I soldered it where I thought it looked like it belonged (to no effect of the functionality of the module itself) and the hum is not as bad anymore, but it's still there. I have also noticed that the hum seems to cause a kind of distortion if the bpf goes below a certain level but other than that it sounds great! I was messing with VCF fm and patching stuff through the inverter which gave the filter all kinds of new characteristics. Any idea what could be the issue with the PWM of the VCO? I noticed that some outs are internally interconnected with the Sine/PWM module? I'm gonna disconnect some of the extra wires and see what happens.
About the portamento - would a TL081/82 do the trick? I have plenty of these sitting around somewhere. What about an alternative way to fix the issue, perhaps with transistor and some r/c components?

PS: I have an opportunity for winning another paia with mixed modules on ebay. Can't wait to consolidate the two into one huge modular :D

PPS: I neglected to mention that when I try to play legato the tuning is non-existent, as in the pitch just increases very, very slightly. That also doesn't change with the portamento engaged. Any suggestions what I should look for to get that fixed?

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