Yesterday I fetched this classic piece of gear out of my cellar.

I am guilty. The cellar is wet and mouldy and a hostile environment for electronic equipment, but I closed my eyes and left poor JX vegetating there for the past ten years. A wedding musician gave it to me after it was worn and broken from heavy use. I remember that I tried to repair it but wasn’t successful in the end.

Dear JX, I promise to make up for your sufferings. To be continued.

Last year a friend of mine called me for assistance with his „new“ speaker set that he had just acquired second-hand from an unknown guy. In fact, the friend had placed an ad to sell his portable keyboard and then somebody called him and offered a speaker set & powered mixer in exchange that the guy would even bring in for a smooth transaction. They did the deal. A few days later the friend was taken aback by the, um, strange sounding satellite speakers and I agreed – we heard sizzling highs on the left and sizzling highs on the right but no mids anywhere, mids totally absent.

The Solton Micro-Sat 500 set consists of two passive SP104 satellites (100W RMS/200W Peak, 4Ohms) and a passive SP515D subwoofer (500W RMS/800W Peak, 2x4Ohms) with integrated crossover and it sold for 2350,- Deutschmarks (approx. 1150,- Euro) list price in the late 1990s. Solton recommended their 10 channel powered mixer ACS10/500 to be bundled with the Micro-Sat 500, and this is what my friend got instead of cash.

After removing the grills I saw two shiny 8″ mid-range chassis and a ¾“ tweeter per satellite. Would you think that one of these 8 inchers was dead? Maybe. Would you imagine that all four were dead? Not really. The tweeter is labeled Craaft F1 and reminds me of a popular Foster-Culver / Fostex driver that I think to have seen before on Dynacord, Hughes & Kettner, LEM and other speakers. This is a good one, probably the most expensive part of the SP104.

Using wood screws instead of drive-in-nuts for grills and speakers is not a good decision. And what about the prototype-style crossover with parts glued on a wood plate instead of PCB mounting? Would be a real fun to repair this thing. And, by the way, the 8 inchers look like bottom-of-the-line steel chassis („organ speakers“), yet adequate but with a slight cheapo appeal.

At the beginning I wouldn’t have believed that all four mid-range chassis are dead, but in fact they were. So I took out the cutter and became a speaker investigator. Three chassis had a broken coil wire right at that point where the wire is guided out from coil to cone and where coil and cone and suspension are glued together. Usually, at this critical joint, the wire is coated with laquer or glue to prevent it from mechanical stress due to vibrations. It seems that this protection has failed in the course of time and then the wire got thinner and thinner until it burned out. My assumption would be that this is a defect of fabrication.

The fourth chassis obviously has been grilled by overload; think of a musician on stage who tries to get audible mid-range sound from the only one left of once four mid-range speakers.

When I contacted Solton they quickly responded with the address of a service technician who sent me four genuine replacement chassis for about 120,- Euro including VAT and shipping, which is a very fair price in my opinion. The replacement chassis had labels on their back revealing that these are TVM (Tesla) ARO 6608 speakers from Czech Republic. Using Google Cache I found out that they once retailed at 229,- Kc per piece (approx. 8,- Euro as to date of this article).

The Voltcraft Power Monitor Pro is a digital instrument that measures voltage, current, frequency and phase angle on a single phase mains operated device. The measurements are supplemented with intelligent features to instantly display power and energy consumption, trace minimum and maximum values or to clock operating times.

Again this is a cross marked „out-of-the-plastic-box“ unit and it lied around under my desk for at least a year until I remembered its existence a few days ago after having done my taxes. Tax declarations are anything but a favourite pastime, you know, and faulty measurement devices are a perfect compensation in my opinion.

The Power Monitor seemed to work properly as the display showed the default screen on power-up. However, it just always displayed 0 volts and 0 amperes and did not measure anything except from the 50Hz mains frequency. So I opened the plug unit first to see what was going on.

The plug unit contains a small PCB with a shunt wire, a 1µF MKP capacitor and a 100Ω resistor. The shunt wire is connected in series to the load under test and allows measuring the current. The series connection of capacitor and resistor and panel unit electronics is connected in parallel with the mains supply. This circuitry is a cheap and resonable alternative to using a mains transformer in case you both have only a small load to supply and do not need galvanic isolation. No defective parts in here.

Opened up the panel unit and checked the cable connections from panel to plug. One may think that broken cores should be too easy to be suspected but on the other hand there is nothing more obvious to be checked next.

Besides mostly SMD parts on the panel unit there are a few conventional through-hole components that, in combination with the already mentioned capacitor and resistor in the plug unit, form the voltage supply and stabilization circuit. In fact, the Power Monitor requires a symmetric supply voltage of +/-5V. This is achieved with the positive and negative half cycles running each through a 1N4001 rectifier diode in series with a BZX55C15 Z-diode to limit the peak input voltages on the following two 78L05 and 79L05 voltage regulators.

There is a rule of thumb saying that a malfunctioning power supply is the cause of about 90 percent of all failures. This rule prove true again when I found the Z-diode in the negative leg to show a voltage drop of 86mV in both directions. Replaced it with a standard ZPD15 and also replaced the 79L05 voltage regulator although, to be honest, I did not check if this was really necessary. Now the Power Monitor is fully operational again and meanwhile monitoring my fridge.

The Voltcraft Cable Checker comes in a set of 9V battery powered transmitter and receiver and allows you to check and identify up to 16 cores in a cable at a maximum resistance of 25kΩ. A possible usage scenario is testing cables with indistinct colour code, preferably on a wide distance across rooms or buildings and with only one person. Just connect the terminals 1~16 and circuit ground of the transmitter on one end and the receiver will indicate the number of each core on the other end.

Of course, the Cable Checker is another episode of these cross marked out-of-the-plastic-box units and again it was at least a little bit faulty when I took it home. Hey, don’t you like faulty toys?

The receiver worked fine and so did the transmitter, except for channel 6 which was dead. After opening the box I saw three CMOS 4049 UBCN driving the terminals without additional protection. So, this is why the manual says that „applied voltages in excess of 7 V DC or AC will damage the transmitter and it is therefore crucial to make sure that the cable under test is dead“.

Even with respect to the price the Cable Checker is a cheapo and it will die instantly if you forget to think before connecting it. As thinking is sometimes a difficult business, I decided to install three IC sockets for the drivers. The PWM signals generated by the transmitter are shown on the scope for channels 1 (top) and 8 (bottom) at 5ms/DIV.

The receiver is based on an 8031 compatible Atmel 89C1051 microcontroller with 1K bytes internal flash memory. It directly drives a standard 3½ digit LCD. This box is the most expensive part of the Cable Checker and its only input is well protected with a resistor and two diodes.

The Voltcraft Accu-Trainer AT1 is a charging device for AA and AAA NiMH and NiCd cells. It comes with an integrated switching power supply for direct mains connection instead of using a wall wart. The AT1 and it’s relatives AT2, AT3 and AT8 are popular chargers, at least in Germany, with the reputation of treating accus nicely and not frying them.

A few month ago I got this piece at our local Conrad Electronic store in Hannover. They have these plastic boxes located near the cashpoints where you can find faulty or B-stock pieces at reduced prices. This stuff is definitely worth a look as very often you will get properly working units with only a few cosmetic drawbacks. BTW: You know that you got an „out-of-the-plastic-box“ unit if there is a big cross scribed somewhere on the housing with a screwdriver or knife. Keep an eye on it, maybe this is your next eBay snip.

Well, this time it was not cosmetic but a real fault. So I disassembled the AT1 and put it on the desk with a breadboard as isolator and powered up the isolating transformer to go for error search. There was neither smoke nor smell nor any other sign of life, nothing obvious. The switching power supply did not look as if this job would be fun, anyway.

Switched off and checked the semiconductors with ohmmeter and diode tester, checked resistors and capacitors without result. To check the capacitors in-circuit I used a new toy, the Elektor In-Circuit Kondensator-Tester and found one capacitor with ESR value 1Ω to be possibly critical, two others had ESR value 0Ω which is optimal.

Powered it up again and started measuring voltages. This is always a bit fiddly with mains on the board, even behind an isolating transformer. Beneath the controller PIC16C72A there was a three legged something hidden under a shrink hose that looks like a voltage regulator. On supposed input there were 14 volts, on output there was nothing and the output is connected to Vcc of the controller, hmm. Cut the shrink hose to have a look at the labeling.

Found an LM340T5 which is better known as 7805 voltage regulator. This is an easy one; you may be out of toilet paper but you will never miss a 7805 in your spare parts inventory. After replacing I did a quick check using a dead AA accu sticked with tape between the contacts and… yup!

Pulled over a new shrink hose, reassembled the AT1 and that’s it. If you own a dead AT1 maybe this post could be helpful. Although integrated in a very small housing and with internal power supply the device gets hand-hot normally which is a good perspective for long life and reliability.