Introduction

Sony STR-7055 frontSony STR-7055 vingate stereo amp

Welcome back Dear Readers!

(Old) hardware needs care. This is true for every type of hardware, let it be a computer, guitar, air conditioner, truck, train or in this case a stereo amplifier/receiver. I’m a proud owner of a vintage Sony STR-7055, which was produced between 1972 and 1976, and apperantly according to a Sep. 1974 ad in Texas Monthly, it did cost $395 at the time, and it still looks and sounds gorgeous to this day, more than 40 years after manufacturing. Quality and production time aside, it is not unexpected for old equipment such as this to produce some minor (though somewhat annoying) issues from time to time, and this is exactly what happened to mine not long ago. When such problems arise, one must remember, that in most cases these problems can be fixed without ever leaving the house, as no special knowledge is required to fix them, but this is not universal. There might very well be cases, where simple tools found in an average home are not sufficient and the help of a professional is required. In this post I will lay out some basic procedures, which would allow any interested person to find out if they truly need to look for a professional’s help or they are simply facing some common, easily solved issues.

Sony STR-7055 front

Problems with home audio equipment can come in different forms, but luckily for everyone, they rarely appear in highly disguised forms. In most cases, we are facing very well noticeable symptoms, such as cracking noise, lower audio output, failing illumination, etc, which already start to tell us where the source of trouble might be. Before one can start to look for a solution though, it is key to be able to find the true source of the problem(s), which is easily done by the process of elimination in most cases. Also, some issues sometimes start to show signs before they actually hit you full time, signs that previously seemed unimportant.

The problem

In my case the problem arose quite suddenly, literally overnight. The left channel speakers started to sound considerably quieter than the ones on the right hand side. The first thing that pops into one’s mind is the potentiometer, but other components, such as the input selector, stereo mode selector, balance, bass or treble adjuster, input or loudspeaker wires could equally cause such symptoms. In more serious situations a malfunctioning amplification circuitry or a power supply problem at the transistor can also cause this, but let’s not run too much ahead. So why did I single out these components? The answer is rather easy in this case. If you can still hear both sides (albeit differently), then the audio signal gets all the way through the entire amp circuit, so one only needs to find components which affect/handle left and right channels separately.

Finding the source of the problem

Now the task is to exclude most of the candidates by the process of elimination. The easiest things to check first are the switches and knobs (potentiometers), so one just basically needs to switch and turn them quite a few times and listen if there is any change. A momentary change is what we are looking for usually. If we have that, then we have a good candidate for the misbehaving component. The reason behind all this is, that the slight mechanical movement that we are applying during switching might just clear off enough oxide layer built up over time to allow current to flow “properly” for a few moments or more. This happens because oxidation affects circuits and connections involving small voltages most.

So, as described in the previous paragraph, I went through all the switches and knobs, and finally I found a rather good candidate: the potentiometer of the volume knob. This is one of the most used controls on a stereo amplifier, and as such it was no surprise that this came out the be the component that needs a bit of care. One must not get overly optimistic at this point, as potentiometers tend to be affected not only by oxidation, but dust and true wear as well. By true wear I mean the resistive strip upon which the wiper slides being scrubbed off over the years, or a situation when the continuity of the strip simply breaks at some point.

The oxidation/dust problems are easily solved (see paragraphs below), but the “true wear” classified ones warrant a replacement, as there are no satisfactory means to fix those. Remember, if there is a need for a replacement, then both right and left channel potentiometers need to be replaced simultaneously, as it is highly unlikely that the new and old pots will have the same conductive characteristics. Any mismatch will result in unbalanced output, or worse, uneven volume adjustment between channels, which latter cannot be circumvented by adjusting balance settings. It is probable that these were some of the main issues that also contributed to the widespread replacement of pot.meters by digital volume controls in fairly modern stereos. One might argue though here, that digital volume control introduced its own issues, like an unwanted direct connection between volume and dynamic range, and as such no problems were eliminated just were replaced by different ones.

Cleaning contaminated parts

Anyway, it was time to clean the misbehaving pots, but to make everything sure, I cleaned all the other switches and pots as well. As you can see on the image below, there are quite a few things to clean: a sliding switch per function (input), mode (stereo, reverse, etc.), filter (hi,lo, both) and speakers; and 2 pots per volume, balance and tone (bass and treble separate), totalling to 4 switches and 8 pots.

STR-7055 Cover RemovedSome of the moving parts that need cleaning

The electrical connections of the switches are easily reachable (see: arrows in the inset of the image below) fortunately,

STR-7055 SelectorsThe green arrows in the inset show the line of contact surfaces that need some care

but the pots are quite firmly packaged, and placed into some hard to get to places.

STR-7055 Volume PotsA train of potentiometers. These are the easily accessible ones

STR-7055 Tone PotsThese potentiometers are not so easily accessed on the other hand

This makes getting cleaning liquid inside them rather challenging. The arrows on the next image show the only available small holes where the liquid can be sprayed in. The other option is the bottom of the pots, which is fairly open, but that is by far not always accessible. Actually, I could manage to spray cleaning liquid through these bottom openings for only 2 pots out of all 8. This closed type of build and arrangement is not necessarily a bad thing though, as dust will have an equally hard time getting into these components, which means less frequent cleaning.

STR-7055 Tone Pots Close UpThese small portholes are the only way to spray some cleaning liquid in. The bottom big holes are totally unaccessible

Some advice

And since we are talking about cleaning liquid and cleaning of electronics in general, it is also a good time to give some helpful advice in this field. The first most common mistake one can do is not to remove accumulated dust before actually applying any sort of liquid form cleaner. This is an essential step, as cleaning liquid will only displace dust, but not remove it. An inexpensive spray can of compressed air or a small compressor from any local DIY store will do the trick to avoid such problems.

The second mistake one can do is to use the wrong type of cleaning liquid, namely WD40. WD40 is a great product and can be used for indeed many things, but except for a few cases, it is not suitable for cleaning electronic devices. WD40 was primarily developed to remove moisture and to protect against rust, and as such, unless some water based liquid (tea, coffee, juice, etc) got into our electronic devices, we want to use something else. Obviously, it is not possible to give a one-size-fits-all recommendation for good cleaning liquids, but I found the Kontakt Chemie series (Kontakt 60, Kontakt WL and Kontakt61) products rather usable. The first is a deoxidizer, the second is a spray-wash and the third is a lubricant and moisture-repellent, which are exactly what one needs.

Kontakt series (60,WL,61)A good set of cleaning liquids for decontaminating electronics.

Outcome

After doing all necessary work and let the liquids evaporate a bit, I finally tested out the amp, and it sounded wonderfully again. For now, it was an easy fix, but could have been it worse? Easily. For instance the pots could have worn off completely, as described earlier, or maybe any of the resistors or capacitors could have shown real sign of age. In these cases a much longer fixing procedure would’ve been needed, where it could’ve involved taking out individual printed circuit boards, measuring components if they conform to the described ratings (Ohms, Farads, Volts), soldering and more. Frankly, it is hard to decide in such situations, whether one should spend the time on the fixing and learn something new, or should one just simply take the equipment to a repair shop and be done with it lot faster. For me, nowadays it would be the latter, as I have very little free time, but if I had time, I would definitely take route number one.

I hope you enjoyed this little piece, so see you in one of my other posts.