Project: DSE K5408 Two-Channel Audio Camcorder Mixer Kit

A while back, I stumbled across a number of old Dick Smith kits being sold at the CCARC Wyong Field Day. I promised the seller that I would build the kits (eventually) and they might even make it onto the blog. Seeing that I’ve just about cleared my cache of cheap Chinese kits from eBay, it’s probably a good time to take a trip down memory lane.

The Kit

Ahh, this brings back memories! The familiar black, yellow and white Dick Smith Electronics logo, with the head in the middle for obvious comic effect. Regardless, the categories of products that DSE used to sell are listed below – there were even some shops with this signage outside.

DSE was a major purveyor of electronic kits. For the most part, low-cost kits generally sat inside plastic bags stapled to cardboard hang tags, on display on a rack. But the more comprehensive and premium-level kits that were of the quality that your friends couldn’t believe you built it (or didn’t buy it) were all sold inside cardboard boxes which would be piled on shelves. The edge of the box would be covered in a duotone coloured label, in this case, sky-blue indicating an “audio” kit.

Mamy of these kits were not Dick Smith’s own circuits, but instead, those of the major electronics magazines at the time (including Silicon Chip which is still with us today and Electronics Australia). They would just make it easy for you – by providing you the correct PCB and all the components necessary to build a finished item inside the one box.

This latter fact becomes much more obvious when we look at what is supplied – including front panel label, hardware, case, battery holder and IC sockets. It really puts some of the modern kits to shame, which leave you with just a “module”, leaving you to supply your own power supply, enclosure, input/output connectors, etc.

The big difference, of course, was price. From memory, many of these kits were AU$20+ in the late 1990s to early 2000s, with some reaching close to AU$100 if I recall correctly. It didn’t help that they were Assembled in Australia – the labour cost would have definitely added up, but at least it kept jobs in Australia. These were such expensive kits that when I was a child, I could only afford one, the FM Minimitter. It also happened to be the first kit I built on my own – hence my fondness. For the real beginners, there was always the Funway-series and Discovery-series kits which were lower-priced but were pretty much modules without much in the way of enclosures or other hardware.

Unpacking the box, all the promised components are neatly arranged including a pile of documentation which has slightly yellowed over the years, the case label, a full retail jiffy/zippy box, a bag of hardware, a bag of components and the PCB.

When it came to kits, DSE was not joking around. They cared very much for the constructor and to reduce the chances of kits being “damaged” during construction or failing and requiring too much troubleshooting, so they took the time to include a “Guide to Kit Construction” with every one of these kits. This was a small booklet that teaches the basics of component identification, population of PCBs, soldering, wiring and has some reference tables inside as well. This goes hand-in-hand with the red notice that reminds kit constructors that while the kit components are warranted, there is no warranty for your labour or any troubleshooting service included. It gives the overly-ambitious kit builder a way-out – return the kit without constructing it for a refund.

They also cared enough to do some market research – kind of like the registration cards of the 1990s and the online surveys of today – to find out what the kit-building audience had in the way of experience, how their kits compare to the competition, what areas of interests kit-builders had and what improvements could be had. Market research in this way could be returned free of charge.

However, there was also a quality control card with the kit serial number pre-stamped on it. This one allows you to record issues with the kit and note things like damaged or missing components which might be replaced (if they’re feeling nice). I’ve never once had to fill in this card – which is a good thing. The kits were meticulously packed and had just the right number of components – many times they would opt for better components (e.g. 1% tolerance resistors) when they weren’t critical as well. But it’s interesting to note that despite going to the same address, you’ll have to pay to send this one back!

As this circuit was from Silicon Chip March 1993 edition, the kit must have dated from after this time. However, it may be fairly close to that in age, judging from the clues left behind in this second edition assembly manual. As Silicon Chip are still around, I won’t be reproducing the assembly manual here to respect their copyrights.

However, I’d have to say that the information provided in the kit is second to none. Part of the reason is that this is based on a magazine article – so it includes the requisite background information, a teaser as to why you might want to build it and a list of features. From there, they normally launch right into circuit operational theory – which normally covers everything from a block-diagram level right down to the more intricate design features. Then, there are sections focusing on construction as many of these kits may have some tricky parts which require pointers or need to be assembled in a strict order. Finally, there are sections covering the testing/alignment and some troubleshooting tips. It really is a completely educational and informative experience to read these assembly manuals and is part of the reason I was so fond of reading Silicon Chip and Electronics Australia as a kid. The problem with having just a magazine was the difficulty in sourcing parts and making a proper PCB back in the days – either you went to town with a dalo pen or toner transfer and etched it at home, or for the more complex kits, you’d have to call up RCS Electronics and mail order a few boards and they wouldn’t be cheap either.

That being said, we can definitely feel the 90s in the title alone – camcorders were very much a 90s thing, as is this particular design which is a two-channel mixer but is actually mono, as most camcorders and VCRs of the era did not have Hi-Fi stereo capability as yet (or most consumers of the era couldn’t afford such units).

An interesting reminder of the age of the kit is the list of store locations and phone numbers on the last page of the instructions. Sydney area numbers without the preceding 9 really is a nostalgia trip – but also the rural areas with 6-digit numbers and “008” free call numbers.

As was common of the boards of the time, single sided tin-plated PCBs were the norm. This kept down costs, which was important given the small number of boards expected to be produced. As a result, installation of jumper wires was practically standard in many kits. There is also no silkscreening or solder resist – so the risk of solder bridges and solder flowing too far due to over-application of heat is very much real.

At least the PCB is fairly well made, pre-drilled with various sized holes with good alignment and pads of various shapes to optimise soldering success rates.

As mentioned earlier, it comes with a full retail jiffy/zippy box package. This includes the plastic top and bottom, an alternative aluminium panel and screws to close the box. This is a rather “normal” kind of box, of the sort from Reptron in Hong Kong with ribs on the sides to hold PCBs – some Funway kits exploited this method of mounting. But this proves to be a hindrance for side-mounting of certain things – so it’s good to have a file, chisel and nippers. In fact, the need to actually mount the project in a case introduces constructors to hardware-skills which they might not otherwise have needed to use – things like drilling, filing, reaming are often necessary. It’s also a way for DSE to sell basic hand-tools to their constructors – clever! The aluminium plate is often surplus, but as it is very “bendable”, it actually makes a great scrap to have around for a bit of McGuyver-ing.

The supplied components are normally exactly in the quantity as necessary to complete the kit – the main exceptions are usually for wires and the provided solder. Yep – they provide solder for you, so often all you need is an iron, side-cutters and various hand-tools depending on the particular kit (pliers, screwdrivers, drill with assorted drill bits, file and reamer are recommended).

Construction

It would seem that such a kit is rather straightforward to put together compared to some of the more modern kits, but once you start putting one together, you start realising just how much more convenient modern kits are.

For one, the lack of silkscreening makes it necessary to refer to the instructions during construction. Due to limitations in printing at the time, it seems they prefer to have schematics with only the symbol name (R5 for example), to which you have to cross-reference a bill-of-materials list to find the value (R5 – 10kohms), then you can cross-reference the colour chart to find the colour code (10kohm – brown-black-orange-gold). This made kit construction take a little longer than I’m used to.

Another thing that caught me off-guard was the design of the PCB which includes much more generous spacing for resistors than I’m accustomed to. Modern PCBs generally space them as closely together as possible – whereas these were designed to be more constructor-friendly rather than cost-optimised, as a result, I was bending the leads a bit too closely every time. Some other designs featured variable-spacing for resistors mainly just to avoid the need to put separate jumper wires, so that can catch the inexperienced constructor out.

Finally, it seems that time has taken its toll on the kit as well, as component legs and the PCB itself began to develop layers of oxide, making soldering much more difficult than I remember. As a result, I used my own Multicore solder that had a much more active, albeit unsightly rosin flux.

Constructed was completed successfully, including the rather odd PCB-pins which are used to make external wiring connections to the PCB. These standalone pins are quite difficult to fit as they like to “fall out” all on their own during soldering and were quite an annoyance in my youth too. I used to just solder wires directly to the PCB as I do now, or even drill-out some of the holes slightly to get the wires to go through just so I could avoid the pins. I guess it’s good that I’m reliving that memory now as well.

While the PCB was completed in a rather straightforward manner, the soldering wasn’t the prettiest job. Another thing it reminded me of is the use of rather annoying 9V batteries which are now less common than they were in the past. Many kits used 9V batteries as they might have used 5V TTL ICs, married to a 78L05 linear regulator that would have had a 2V drop-out voltage, thus requiring at least 7V input to provide a regulated 5V output. Rather than insist on at least five AA or AAA cells, it was easier just to specify a 9V cell, although these had stupidly low capacities and were not the cheapest to buy.

At the previous stage, it would have been tempting to consider the kit almost done – but in fact, you’re probably only about half-way, as the amount of work required to fit-out the case and wire the project is not insignificant. The case label needs to be applied, holes need to be drilled and reamed to size, components need to be fitted and wires soldered. Due to my reluctance to cut the wires (as I feared they would become too short), they instead remained rather long and messy.

At last, the board is mounted into the case, with the battery installed, which is another key annoyance with many of the kits which feature batteries inside the jiffy box which is closed with screws. As a result, you would have to unscrew the front cover to replace the battery – in the process, flexing the wires and perhaps provoking one to snap somewhere necessitating a repair. Aside from that, there was every chance you could chew up a screw head. Not my favourite design – it was a wonder they didn’t get a custom case with a battery compartment for their kits … I think that would have been much nicer.

Regardless, the finished product doesn’t look half bad. Of course, my lack of handy-working skills show, as I scarred the label in a few places through careless application of pliers in tightening the locking nuts of the potentiometers. The age also shows – the knobs themselves aren’t dirty, it seems they have a sort of greyed/browned dirty appearance because of something from the plastic that might be leeching out.

As noted by the manual, the input RCA jacks are best mounted towards the top of the case to allow the PCB with its wiring and the battery to be cleared. This is always a point of danger – once you start drilling into a case, there’s no going back. I suspect they expected a proportion of kit sales would result in follow-up sales as people who made mistakes would come back to get a fresh case to try again.

The PCB is held onto the case with screws through the rear – the screws themselves were countersunk type, but I forgot to put in the countersinking, so they stand slightly proud of the rear. This is no great issue, as clear rubber feet were provided – but the age of them resulted in them turning to goo almost immediately! Nothing lasts forever!

Testing

Now that the unit has been constructed, I wanted to know if it worked and if it was any good. My instinct would be to think that anything I could build with my hands would not be any good – after all, the basic Funway, Discovery or Jaycar Short Circuits kits were never really about performance. As I have a Rohde & Schwarz RTM3004 with the Bode Plot Analysis option, this would be an ideal device to test the gain and frequency response characteristics of the unit.

With the mixer on maximum gain, the unit’s frequency response doesn’t look bad at all. The gain appears to be 10dB or thereabouts, with only a fall of 0.55dB at 24kHz compared to 1kHz, but the bass seems to have some rolloff. The unit drops to 0dB gain by about 200kHz and has a phase inversion by 900kHz or so, and as such should not oscillate. That’s more than I had imagined.

Zooming into a closer scale, we can see the bass frequencies between 10Hz and 100Hz can be about 5dB down. Phase deviation at the extremes of the audible frequencies are present but limited to about 21 degrees at the upper frequencies.

The next thing I wondered was the channel-to-channel matching. Surprisingly, that wasn’t half bad despite having two separate potentiometers. The difference in absolute gain and phase was well below the accuracy specifications of the RTM3004.

Reducing down to unity gain, would there be any improvement in frequency response? The answer is, no … because it seems that is mainly limited by the capacitor coupling/decoupling.

The 3dB point was measured to be 13.7Hz to 63.7kHz, which is more than enough for audio.

Again, channel 2 was practically a carbon-copy. I tried to measure the no-input noise and I couldn’t – the noise from the oscilloscope’s 10:1 probes appeared to be even more than that generated by the kit. This is no doubt because of the use of a TL064 op-amp, which is a very quiet JFET input opamp. Amazing performance.

I guess back then, getting an audio mixer with a linear potentiometer slider was probably expensive enough that building a kit with rotary knobs would be cheaper.

Conclusion

Dick Smith Electronics really was a pioneer in kits and the care and attention to their kits really shows. The higher end boxed-kits were based on competent and interesting designs from the leading electronics magazines of the time, saving constructors the hassle of sourcing PCBs and components individually, while providing a potential cost-saving, customisation ability and satisfaction of building it yourself. The components supplied were generally of high quality, exact in number with comprehensive documentation to ensure a successful, educational and informative experience. They even cared enough to conduct market research though a mail-return questionnaire.

Compared to kits of today, the boxed kits from DSE were much more comprehensive, intended to produce a final product that might pass as being “bought” in that era. The inclusion of a case and the requirement for working with hand-tools to finish the project is something that is missing from many modern low-cost kits which leave you with just a bare module. However, it was also a stroke of genius, as any construction mistakes would have meant that repeat sales as a real possibility. Compared to the PCBs of today, the boards of the era were very primitive, being single sided, tin-plated without solder-masking or solder resist. As a result, construction took a little longer and there was a real possibility of solder bridging that wouldn’t occur on modern designs. Attention to their PCB pad placement and design generally helps avoid such issues.

It was a nice experience to go back in time and attempt a kit that I might have seen on the shelves as a kid but could never afford (or perhaps had no use for). Despite this, the use of the 9V battery and monaural configuration doesn’t make it particularly useful nowadays – but it was worth constructing for the experience of doing so.

Luckily for me, I managed to pick up a few other kits, so I can enjoy this trip down memory lane for a little longer … before I return back to modern-life.

About lui_gough

I'm a bit of a nut for electronics, computing, photography, radio, satellite and other technical hobbies. Click for more about me!
This entry was posted in Audio, Electronics and tagged , , . Bookmark the permalink.

Error: Comment is Missing!