Canton Subwoofer Dead on Arrival

 The Canton family of speakers are very popular and good value for money for things like a home cinema. 

This one came into the bench dead on arrival. It simply made no noise at all. So my amplifier was hooked up to it to reproduce the issue. This is always a good idea, as it rules out a possible issue with the customers amplifier and allows me to ensure the fix has actually worked.

Once opened, the circuit board was immediately visible. A search on the internet showed there were various mods of the Canton PCB out there and finding the exact one would be a challenge. I finally settled on this one, as it matched my version of what was infront of me most.

I used my vintage Heathkit signal tracer to follow the input signal and see where it was being lost.

Why? Well, this is a set I had purchased in USA years ago and had refurbished and overhauled and brought back to life. So it was fun to get a chance to use it again. It is an old tube operated amplifier, that allows and engineer to trace a signal using the probe and to work out on which component, the  signal stops and goes no further. In this case, the signal was being lost at the cross-over potentiometer.

A good strong signal was going in but absolutely nothing as coming out. A test with my meter showed open circuit no matter where the potentionmeter was positioned. So any easy fix is to join the output of Op-Amp IC1A to the input of IC1B.

This is what the potentiometer looks like.

There is only one seller on ebay currently selling this item used, removed from an older device. However, the cross-over setting is rarely used anyway and the potentiometer can be bypassed. Should the user wish a full repair later, we can look into that option. So for now the R-3 to R2 legs of the potentiometer were bridged using solder.

Once the subwoofer was re-assembled, it was tested using a 40 Hz signal from my signal generator.

The subwoofer was back to life and operating as normal. Time to screw everything back together and return it to it's owner.

Icom IC-251E Dead On Arrival

 The IC-251E is a fairly good 2m CW / SSB and FM transciever made around 1980-1983. The British Mutek company sold a high-performance front-end board for this radio (Design by Ian White, G3SEK). This was because the front end tended to be slightly deaf. It is a double conversion, super hetrodyne design with intermediate frequencies of 10.7 MHz and 455 KHz. Interestingly, the 10.7 MHz is available on the back on the device and can be viewed on a scope.

This little set was given to my local club by an OM who had retired from the hobby. He said it was in perfect working order. It lay for a few years and then it was decided to dust it off and use it as a loaner set to a new OM who had just passed his entry level certification. So that they could immediately get enjoyment out of the hobby on the local repeater or in USB. So it was plugged in and tried out. Instantly, the set went POP and magic smoke came out. The set was dead. So the club passed the set to me for repair.

I first verified the problem, but by this stage, the radio just did nothing. However, plugging it into 12v showed the set worked perfectly. So obviously the problem was confined to the power supply and most probably the electrolytic capacitors. So the set was opened. Getting to the power supply was a big job. It involves removing everything. Rear frame, both circuit boards, pulling out the power supply and disengaging multiple plugin connectors.

Before removing a screw, pictures of the connectors were taken. All of these must be removed, so it is important they all go back in the same places. Many of the connectors have the same size and pin numbers, so it is very easy to plug the wrong connector into the wrong slot on the re-build.

Once that was documented, the tear down could begin. Everything must come apart and I mean everything!

What we are left with, is the power supply unit at the rear of the case.

From the circuit diagram, we can see that the power supply is quite simple. However, the two huge electrolytic capacitors were the first suspects for trouble in an older radio that has sit some time before being turned on again. The start up should always be done using a current limited variac to avoid potentially devistating damage to the circuitry.

As can be seen from the picture above, the electrolytics are 200 uF at 220v. They were desoldered and tested on a home made ESR meter and proved to both be bad. 

New ones were procured and the size difference is remarkable.

As the Nichicons are much smaller, longer legs must be left to fit the differing hole size in the PCB. Owing to the fact, that the tear down was so labor intensive, the new capacitors were tested on my ESR tester to ensure they were indeed good, before soldering them in and putting everything back together.

Before re-assembly, I always sort the screws that I removed. This helps me work out which screws go where. This is especially helpful if you have had to wait a long time for parts and the memory of taking everything apart is a distant one.

Finally, things are coming back together and the radio can be tested before finally installing all the covers and panels.

The radio was tested on the tester and found to be within spec. This radio is now ready to go to it's new home. I hope the new OM will have fun with it.