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Model 700 Programmer - Restoration
 Overview
The Sequential Circuits Model 700 Programmer is a rather useful way of adding 64 patch memories to an analog synthesizer. In November 2002 I bought one from the USA that need some repair work. The front panel had corroded quite
badly and it was externally in a poor condition. So I decided to do a full electronic restoration and rebuild it into a 5U 19" panel so it could sit with the AM Modular cabinets. The picture on the
right shows the Model 700 as it arrived from the USA.
A detailed description of the Model 700 is here: more »
In April 2003 I acquired a second Programmer 700 (a Mark 1), this required a lot less work to
get it fully operational. It will be recased and restored.. more »
Restoration Part 1 The first step was to see if the Model 700 would power on, and to do a
smoke test! A quick internal visual inspection showed no burn outs, so I powered it on and was greeted by a set of LED's that worked perfectly but no control voltages or memories.
Plus the tale tell smell of burning - a tantalum capacitor was on its way out!
A detailed inspection revealed:
- The +5V rail was okay - the regulator was replaced as a precaution
- The +15/-15V rails were dead - both regulators were replaced
- The comparator reference voltage had failed - thanks to the blown tantalum capacitor. I replaced this with an electrolytic and replaced the scarred 47 ohm resistor on the
inbound connection to the LM723 regulator chip - which I also replaced as a precaution.
- The lithium battery was dead - so it was replaced with a Varta 3V lithium 2/3AA
- The transformer was in very good condition - so it was left alone.
- Some IC's had previously been replaced, as there were some new IC sockets. I added sockets when replacing chips, as Dave Smith had only put sockets in for the SRAM
memory chips.
An order for new regulator chips was sent out, and whilst I waited for the parts to arrive I
replaced the tantalum capacitors with electrolytic capacitors, and completed the following upgrades:
- The electrolytic power regulator capacitors were replaced with high temperature versions - they are smaller too!
- All ceramic disk capacitors were replaced with new dipped versions
- The rusty old pots were replaced with Bourns sealed conductive versions
- New E-mu control knobs in solid aluminium replaced the beaten up originals
- New slide switches replaced the worn out Switchcraft switches which had started to have a redundant centre position, as well as lots of surface rust.
Restoration Part 2
With the new power regulators in place and the rails working again, it was possible to power up the 700 and see what else had failed. Quite a list!:
- Voltage 1 had gone permanently -12V, this was fixed by replacing IC4 (LM348 Op Amp)
- Voltage 2 was fine.
- Voltage 3 was correct but modulated at about 20Hz, once again replacing a LM348 Op Amp at IC3 sorted the problem.
- Envelope 1 was at 0V - the SSM2050 and SSM2020 checked out okay, so it was clear that IC3 was the problem. When replaced the envelop worked perfectly.
- Envelope 2 was fine, although the release time was over 30 seconds! So I will add a trimming control, as per the SSM2050 application notes - so that both envelopes can
be trimmed to the same times.
- The patch buttons didn't work consistently, so I replaced almost all of the CMOS logic
around the scan and clock logic, using new IC sockets. The key problem turned out to be IC14 - it needed an exact replacement MC14163. Near equivalents gave erratic responses.
- The 12 position rotary switches have been replaced with new plastic ones. The original shafts were damaged when I removed the control knobs, and they were rather clunky.
April 28th 2003 (5 months after purchase!) and everything was working perfectly again. I may
replace the set of matched resistors that determine the control voltage quantisation, with 0.1% precision resistors - along with a new LM393.
Blue LED's
One of the nicest visual upgrades on old analog gear is to replace the rather dull red LED's with some new blue LED's. The 700 uses eight 3mm LED's within the front panel
switches that select patches. The old ones are easily desoldered and removed. The cap of the tactile switch can be easily prised off for access. New blue LED's are easy to solder back
in, but the associated resistors need to be replaced with new ones, to ensure the blue LED's are driven at the right brightness and voltage. The new LED's are so efficient that a 1k8
resistor is a suitable replacement for the original 150 ohm ones. Lower values result in too bright a blue light!
Blue LED 7 Segment Display The "blue" theme continues. After considerable searching I
finally found a dual 7 segment display in blue from Digikey in the UK. The original display is a high intensity red display, a Fairchild MAN6740 with 420ucd luminosity and a forward voltage
of 2V per segment. The replacement blue display is a LiteOn LTD-5523AB, with 4300ucd luminosity and 3.8V forward voltage. It requires the seven LED resistors to be changed from
150 ohms to 22 ohms, to ensure the correct display intensity. The red arcylic filter has to be replaced with a neutral one.
New Panel And finally a nice new 3mm aluminium front panel, 5U high, 19" across in
aluminium with black lettering. This replicates the old panel exactly, so that the PCB's can be mounted directly to the panel. The rear jack sockets for inputs and outputs have been moved
to the right hand side and are 3.5mm jacks (my studio analog synth standard). The new front panel was designed duirng May - July 2003, with many checks against a paper print out of
the front panel. The panel was finally ordered on 8th August.
 Restoration - Again
The second Model 700 I bought in April 2003 required a lot less work, I've added a lithium battery and replaced all the decoupling and PSU capacitors. It is a Mark 1 model, which means its based around lots of CA3080's, with no
SSM chips, very different to the Mark 2.
I have kept the original red LED's and 7 segment display, but rehoused it into a 19" rack case and added a 240V power supply. The potentiometers are well shot, so they were
replaced with high quality Spectrol units (there is not enough space for the deeper Bourns 91A). The control knobs are being replaced as well. They stand proud of the front panel by
1/4" thanks to the factory not cutting the pot shafts down to size!
Overall the unit worked very well as bought, which is a good job as it cost £300. |
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