Repair-Service and Modify
I have years of experience repairing and rebuilding classic audio equipment. I will service the electronics we sell, and can repair-service and modify most audio equipment. I do lack a few tools necessary to align stereo tuners, something I plan to remedy.
If you have older audio equipment, much of it can be improved and made comparable with modern equipment. The most common failure is caused by electrolytic capacitors. They do not age well. Electrolytic capacitors will fail after a certain amount of time, determined by heat load, the amount of AC current being passed, the composition of the electrolyte and the time it sits without being used. When they do fail, they can cause traumatic damage: ruined transformers and proprietary integrated circuits (the latter is much worse than the former). The first Technics SP-10-mkii I serviced ran backwards because the supply voltage was off by only 1/2 VDC. It took only a couple minutes to open the power supply and find capacitors that were in the process of going bad.
There are various signs of capacitor failure, including heating (hot to the touch), reduced power output, excessive hum and noise, obvious power transformer hum, oozing electrolyte and bulging of the capacitor. With some tube electronics, capacitors are surprisingly long lasting, including some McIntosh equipment, like the MC240, but they are failing too.
The cost for capacitors in transistor equipment is minimal, unless you are going for extremely high end versions, some of which won’t fit the circuit board due to their larger size. The high voltage capacitors for tube electronics are larger and more expensive than low voltage transistor equipment. I can replace capacitors, resistors, transformers, circuit boards, input jacks, output jacks, then test and listen. Some exceptions are stereo FM tuners, which I can rebuild, but I do not have the highly specialized stereo test and alignment equipment. Also, many newer digital circuits are surface mounted and have extremely small footprints. These compact integrated circuits are very difficult to replace, and most are also difficult to source.
Also, to some extent, I can reduce the feedback of many classic amplifiers, and also improve stability, with improved power supply decoupling. Further, the performance of classic long-tailed pair phase splitters can be radically improved with a constant current sink. The gain and linearity can be improved with constant current sources. It does depend on how much space is available.
To a lesser extent, resistors do fail, but mostly carbon comp resistors. Carbon comp resistors change resistance as they age. They also change resistance with heat and applied voltage. They are hydrophilic and while the moisture can be baked out of “new-old-stock” resistors, I recommend against baking your amplifier. Finally, they are the noisiest resistor type. For a similar sound, you can mix carbon film, wirewound and tantalum resistors. The tonal quality will be similar, but much cleaner/quieter. Avoid metal oxide and cheap metal film resistors in the signal path. For the power supply, always use wirewound, followed by metal oxide if you can’t find the correct wirewound value.
Below are some rebuilds, fixes and modifications, and couple I wish I had never worked on, starting with the complex and mediocre sounding Acrosound Stereo 20-20.
Acrosound Stereo 20-20
Whatever you think of the stock Dynaco ST35 circuit, it’s infinitely better than this one. Beyond that, Acrosound elected to mount large metal multisection can capacitors on the PC boards. Not only that, but the capacitor voltage rating was so low that the surge voltage from the rectifiers would dramatically exceed the maximum rated voltage on start-up, and would stay above the rated voltage when the amp is being used with today’s wall power (which is higher than what they had in the ’50s).
The circuit designer used voltage splitting tricks, with a multitude of resistors, instead of a decent circuit. A single 12AX7 cannot act as an input tube, phase splitter and driver. If it were a triode-pentode like the 6GH8, things would be different. The heat buildup was horrible and the boards were cooked. The capacitors were dead or leaking. One multisection capacitor can was bulging out, showing it was really dead. The footprint of the amplifier is tiny, with not enough room inside the chassis, or above. The carbon comp resistors were well over 15% off their nominal values. The tube socket contacts were eaten up with corrosion and heat stress. Everything about this amplifier was bad. In then end, it looks great and Acrosound is a great name, but it is terribly difficult to rebuild and the circuit is handicapped. That being said, if you have one that you want to rebuild, I can do it for you, and the materials I used will improve the sound. Just don’t expect the kind of sound you would expect of the legendary Acrosound name. Your money is better spent on an ST35
The first thing to do was remove the old can capacitors, while not destroying them. The cans were mounted directly to the PCB, which is extremely unusual. It was impossible to find an exact replacement with the same tabs, so the only course of action was to pull the cans, clip this tabs, then reinstall for aesthetic reasons.
You can see burned places on the boards from the closely spaced resistors. I raised them up a little to give them breathing room. You can see that one of the multi-section capacitors is bulged out at the end (the one for the right channel). This amp was completely dead, but should last for many years with uprated parts.
By Phillip Holmes