A&H Spectrum Mixer Refurb 2

Overview The A&H Spectrum finally arrived on 20th January 2020, very well packed and at 59 Kg! After some serious unpacking the refurb can now begin, with the PSU being the first to be looked at, so that I can power on the Master/Group section initially then add the Channel sections.

PSU Refurb The power supply is an Allen & Heath RPS3 from 1991 (serial number 7) and is a 3A linear power supply based on tried and trusted LM723 chips. Whilst the metal chassis has some rust patches rusty and the PCB is old, it looks in ok shape, although the 48V rail power cap is either bulging or has a dome! After carefully checking the power supply components I switched it on; +16.3V, -0.8V and +48V. So looks like we need to replace a component!

The original reservoir capacitors are 10,000 uF 35V with 85 degree temperature rating. I want to fit large capacitor values at 105 degree rating, but I am limited to a 30mm diameter as that is the size of the mounting. I located 15,000 uF 35V 105 degree capacitors made by EPCOS and fitted them. This will reduce the ripple voltage into the regulators down to 100 mV.

The LM723 chips have date codes from 1988, so these were replaced and I added IC sockets, then all the smaller power capacitors and the original MJ3001 power transistors and bridge rectifiers were replaced. One of the bridge rectifiers had failed, hence no -15V rail. Remarkably the 723 has become obsolete, so it is was a good time to buy some before they get expensive.

RPS3B and Heat The power supply has been upgraded to the B version, which outputs 4A rather than 3A, but no change was made to the thin 4A 16/0.2 wiring, and as a result all the wiring to the rectifiers has been heat damaged and become brittle and broken, which explains the -15V rail failure. I replaced all the wires into the regulator PCB with thick 32/0.2 which is rated at 10A, this provides a healthy safety margin. I also checked the current limiting resistors and they are not damaged. The refurbished power supply powered on successfully and the voltages were trimmed to 16.00 volts.

The original transformer is a traditional open frame model at 225VA. It provides two secondary circuits at 21.5V for the 16V rails and one at 50V for the 48V phantom power. I am keen to switch in a toridial transformer for the main rails and it has to be 225/250VA which means a diameter over 100 mm. I will have to mount this on the inside of the front panel, as that is the only space big enough. Alternatively I could just buy a cheap secondhand RPS11 which has 5A rails and a toridial transformer – lets see what eBay offers!

Fader Cleaning The high quality Alps K-series 100 mm faders are in generally good condition but they needed a deep clean as the action was not smooth. I started with the Group and Master section and took them apart, cleaned them with IPA, then Dexoit Fader cleaner, and finally put some fader lube on the lower brass rail. Each slider was cleaned so that it would slide from one end to the other on gravity alone, once apart. Sometimes steel wool was needed to remove corrosion on the brass rail.

I may buy two new Alps faders for the Master section but lets see how well the cleaning has worked. At £15-20 each I don’t want to replace all 26. I will clean the Channel faders as part of the Channel refurb once I see what issues there are in each of the channel cards.

Fader Knobs The black slider caps are nice and chunky but well worn with the white line disappearing, however I do have all 26 in place! I will have to keep them as modern caps are too short and stand proud of the panel or it means dropping the fader down by 4mm which is really not possible as there is no room in the chassis.

Panel & Knob Cleaning Whilst I waited for parts to arrive to get the RPS3 working, I cleaned the front panels with surface cleaner and deep cleaned every knob and switch cap (after removing them). This was a very long, but worthwhile task. A small number of knobs were broken or missing, as were some caps. I ordered 8 new knobs which are slightly taller and not faded! I put them in the AUX sends section, thereby releasing some original knobs and minimizing the colour change from new to old.

The brown caps for PAN are not available, so I ordered 30 light green caps and a couple of red and blue caps were also needed but I am hoping these colours match the originals. The caps are slightly smaller and need a little glue to remain in place.

Power On and Testing Once the PSU was working I plugged it into the console and powered up – all ok, no smoke! Testing uncovered a number of issues:

• Right Master Out no signal (solved)
• LED bar graph display 1 – 8 non functional
• Group 2 and 8 input from bus failed
• Channel 5 intermittent

The rest of the circuits seem to work ok, the switches are noisy but this reduces with use. The noise floor is good but some channels seem to distort easily and there are different levels in the Groups.

Group Channel Refurb The eight Group PCB’s are in good condition but not all working at the same volume. The plan is to upgrade all the electrolytic capacitors and make further changes depending on the outcome of the Channel 1 upgrade. Care is needed with these 30 year old PCB’s, and I use a vacuum desoldering gun to minimize the risk of tracks lifting.

Master Section The master section has a right channel failure, and there are no schematics available for this section, so I am using the Sabre master channel which is not quite the same design. The right channel failure seems to be a simple plug connection that righted itself on re-cabling. The main upgrade in the master section will be to replace the balanced output drivers (NE5532 with trimmers) which live on a separate PCB attached into the Master PCB, with THAT 1646’s on a new PCB.

Input Channel Recap

Input Channel Refurb Channel 1 had a broken shaft on the PAN pot, so I had to pull out the PCB to swap this with a new one, which was easy. At the same time I replaced the electrolytics with high quality Panasonic capacitors (Audio and Power versions), and removed the signal path TL072’s (IC 3-6). I soldered in IC sockets and initially tried a set of TLE2072 Op Amps from a AMEK Rembrandt channel, as these are fast low offset but with only a 14% increase in power current consumption.

I added 100nF decoupling capacitors to every Op Amp, underneath the PCB, as there is only one 15nF for the whole board!

This is an experiment to see if upgrading actually provides any benefits, as the cost per channel is around £15-30 depending on which Op Amp I use to upgrade. The change to the TLE2072 provided a clearer sound and the EQ was improved. The standard EQ uses Mylar capacitors, whilst high end consoles used better capacitor types. The PCB space is tight, so polypropylene capacitors are too large, so I am trying a set of PPS capacitors.

Input PCB

Input Conn There is a small PCB for each input channel that holds the jack sockets and XLR, it has a dual TL072 Op Amp to enable the -10dB and +4dB gain switching on the Tape Input. I have upgraded the Op Amp to a TLE2072. There is also space on the PCB for a SSM2142 Balanced Output chip, and its clearly marked on the silk screen.

This addition provides a balanced output for the Channel Output, which can be switched from Bus Out to Direct Channel Out from the front panel. I soldered in an IC socket and added a SSM2142 for the first 8 channels, as these are used as recording inputs to the Focusrite Octopre. The two 22R resistors need to be removed to ensure the Channel output signal goes into the SSM chip rather than straight to the jack socket.

Tracking Setup The Spectrum is used for tracking up to 32 synths/samplers into Cubase using eight Groups. Cubase is used as a 16 track analog style recorder with analog tape plugin available on every track. A key principle is to be constrained to 16 tracks and not to add more. The channel EQ and the two main FX processors can be used on tracking. I have constrained my use of Cubase plugins to just a TC2290 delay and the Softube Tape, no other EQ or effects are used.

16 synths come in via the Channel line inputs and another 16 via the Group line inputs. The first 8 channel outputs go into Cubase via a Focusrite Octopre which has compression available. I can switch these from Group to Direct Out, if I want to record the first 8 synths directly into Cubase, and the Spectrum outputs are balanced,

Mix Down Setup The Spectrum is also used for the final mix of a song, with 16 outputs from Cubase going into the unbalanced Tape inputs on each channel or they can be brought into the Groups. By leaving them on the channels I can record 16 synths at mix down from the Group line inputs. The stereo master of the desk can be recorded onto DAT and/or into Cubase. All four hardware FX processors can be used in mix down.