Restoring a Vintage Allen and Heath S6-2 Production MixerFollow article
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Replacing capacitors and making a remote On Air light for a classic production mixer.
The Allen and Heath S6-2 was a broadcast production mixer made by the British audio electronics company, Allen and Heath, in the mid to late 1970s. It has a very nice 70s look to it, with a black facia, a bit of textured black vinyl trim, wooden side cheeks and two nice old style VU meters – definitely worth spending some time and effort to get it up and sounding great again.
Our example came complete with its original brochure and schematics, with PDF versions also available to download.
A couple of tickets stapled onto the inside of the brochure dated 1978 give an indication of the age.
To help restore this mixer to its former glory I am going to replace all the old electrolytic capacitors - well over 100 of them!.
The main PCB was removed from the case and as it is too large to fit on our regular board holder, I made a jig to hold it using some 2020 aluminium extrusion and a multi-angle vicewhich allowed me to adjust the position of the board to get at the capacitors.
To make life a bit easier I had also got my hands on a de-soldering station.
I obtained the 4 different value capacitors that needed replacing - 1 uF, 10 uF , 47 uF and 100 uF . As I have noticed with previous recapping projects, modern capacitors of higher values tend to be quite a bit smaller than their older counterparts, so their legs need to be bent to fit the old footprint on the PCB.
The original components had varying voltages in each value but I opted to make life easier by using a single voltage, making sure it was at least as high as the ones I was replacing.
I was grateful to see that the board is clearly marked with both the value and the polarity of the components. Also, the old capacitors are light blue in colour and the new ones are dark brown, so as my work progresses it is easy to see at a glance which ones have been replaced. I decided to divide the board into three sections, replacing one value at a time in each section.
Having the board more or less vertical in the jig meant I could apply the de-solder gun to the solder pads on its underside, whilst gently pulling the capacitor from the other side until it came away. I was wary of damaging the traces on the board, de-soldering any of the other components or disturbing the obvious re-working that had taken place over the years, so I started with the temperature on the de-soldering station set at a conservative 200 C and gradually increased it until I found the minimum effective temperature at 270 C; bearing in mind that on a board of this vintage I was dealing with leaded solder that has a lower melting point than modern unleaded.
Locating the components to be replaced on the underside of the board was made easier by shining a light through it – as it is fibreglass and semi-transparent, the correct solder pads could be relatively easily identified.
The de-soldering station worked really well and it ended up taking just over a day to complete the re-capping.
Cleaning the faders
The fader potentiometers had accumulated quite a bit of dirt over the years – the sloughed off fibres of the beards and jumpers of numerous sound engineers - in fact, compacted lint trapped at the ends was preventing the wipers reaching the ends of their travel, so they needed a good clean.
I pulled out all the dirt that I could by using a hooked piece of wire and some fine-nosed tweezers. I then applied some switch cleaning lubricantto clean and lubricate the contacts.
I temporarily extended the wires that connected the board to the switches and power connector that were still fixed in the case of the mixer – this allowed me to check out the board with it lying next to the case rather than re-assembling it and then needing to take it apart if I found any faults.
I used the sine wave generator on my USB oscilloscopeand then its 2 channels to compare the input and output signal on each of the mixer channels. Everything looked and sounded OK, apart from one of the “Gramophone” inputs that needed re-soldering.
Power supply re-capping
Once I was happy the mixer was working as it should, I turned my attention to re-capping the power supply, which also includes the relays for the remote control of gramophone and tape decks that can be switched from buttons on the mixer, and an “On Air” indicator connected to Mic 1.
Thankfully only three 2200uFcapacitors this time and it would appear one had already been replaced at some point – but I replaced it again anyway.
While I had the power supply case open I also replaced the voltage regulator’s heat sink mountand the fuse holder which appeared a little the worse for wear.
Fitting the “On Air” light
Also in the power supply case, I found the cable strain relief grommets for connecting the relay switches. I decided to make myself a simple On Air warning light, using some laser cut acrylic and adhesive strips of LED lights. I connected the lights to a separate 12V desktop power supply, with the relay in the mixer’s power supply unit providing control. Now when I enable the Mic 1 input, my external On-Air indicator lights up – not that the cat takes any notice!
Putting it to good use
Back when Yorkshire hosted a stage of the Tour de France I helped set up a local community radio station broadcasting on FM called Recycle Radio. As lockdown took hold earlier this year, Recycle Radio was revived as an Internet station and I was asked to contribute a music show – Music From The Red Tin. My first attempt at recording the show was somewhat plagued with technical problems – mainly not getting the music and mic levels right. The mixer, a “must for up and coming DJ’s” according to the brochure, would be a great way of improving things.
I successfully used the mixer to record an hour-long show, taking the audio out from the Red Tin into the mixer’s tape input and using a field recorder set to record and pause in lieu of a microphone, going into the mic input. The auto fade feature that mutes the signal from the tape input when the mic inputs are being used proved invaluable, as can be heard in the video above.