No announcement yet.

Trying to repair old amp


Forum Top GA Ad Widget

  • Filter
  • Time
  • Show
Clear All
new posts

  • Trying to repair old amp

    I have a very old amplifier. From the late '70's I guess. Here's the description from the amp: Modular Component Systems, 3845 Stereo Integrated Amplifier, MCS Series, Model #683-3845-8901, distributed by JCPenney, etc. There are problems with the sound. The sound problems are the same whether the amp is connected to my tuner or cd player. Here are the symptoms. All of these symptoms happen sporadically: the left speaker will have sound but not the right, the right will have sound but not the left, when I increase the volume when one only speaker is playing both will play, the sound fades in and out, the sound cuts off, the speaker sound will shut off when the sound from a radio station pauses (e.g. a person pauses in their speech), both speakers have static, one speaker will shut off but not the other. These problems are identical whether the amp is connected to speakers or headphones. An electronics repairman friend of mine replaced the capacitors I believe, but that made no difference. I tried spraying the volume knob stem with contact cleaner and letting the contact cleaner run down into the rheostat. It didn't work. I sprayed the rheostat a couple times, turned the knob several times, tested the amp, and it had the same problems. I then sprayed the rheostat stem several times, turned the knob many times and let the amp sit for a couple days, and it's the same as before I sprayed it. I'd like to try to diagnose and repair it myself without taking the amp to a shop out of both curiosity and cost savings. I have no understanding of electronics. What should I try next? Should I just replace the rheostat? Are rheostats available for an amp this old, and are they expensive? Is it worth trying to repair it? How durable are amps? Should they last decades? Both the equalizer and tuner that I bought used with the amp still function well. The amp, tuner and equalizer look like they came as a set. I bought the set a couple years ago. Does anyone know approximately how old the amp is and where I could get a catalogue or more info on any component? The tuner is: MCS Series, FCC Data Model no. 3701, etc., and the equalizer is: ADL stereo frequency equalizer, Sound Shaper Two MKIII, Model SS-2, ADC Professional Product Group. How good are these components? Thanks for any replies.

  • #2
    is this a tube system? or is it a hybrid tubes and transistors? scratchiness almost always points to the input controls. could be the volume control, balance, tone controls {the bass uses capacitors to filter the high frequencies out} if the controls are film type rheostats {volume controls| I'd replace them. OHMITE manufactures volume controls. look in newark electronics supply for replacements. IRC is another manufacturer. for tube type sets, the sound gets taken off the plate line and is fed to the control grid of the next tube. finally the output goes to an audio output transformer.


    • #3
      The odds of finding a “rheostat” in your amplifier is only slightly higher than your odds of actually resolving the problems in the amp by indiscriminately changing out components, slim to none.

      A true rheostat is both a variable resistor and variable inductor comprised of a coil of resistance wire generally formed over a powdered carbon coil form. It may be formed in a straight line with a slider type movable tap but more commonly they are made by winding the coil on a toroidal coil form in a circular configuration, then attaching a movable tap to a rotating shaft to allow a rotating control knob.

      I got my first interest in electronics when I was a freshman in high school in 1962. On December 24th of that year my family moved into a large 4br Victorian house that had a full basement, and in one room of that basement there was a large electronics workbench and literally stacks upon stacks of AM radio receivers, record players of all sorts, both monaural and stereo from little suitcase size portables to huge stereo consoles, a couple TV’s and a few odds and ends of electronics test equipment, all of which didn’t work at the time. My dad told me to toss all that junk out in the trash but I convinced him that I wanted it to experiment with. At the time I was in the same boat as you are, absolutely no knowledge of electronics but thoroughly convinced that I could figure it out on my own. In the next 3 years before going into the Air Force I managed to reduce that entire pile of electronic devices down to a well stocked supply of components save for one VTVM (Vacuum Tube Volt Meter) and one Vacuum Tube Tester, both of which I managed to repair quite by accident. In over 40 years of cannibalizing circuits I have only come across about 8 or 10 true rheostats, and only then in very high tech military surplus electronics.

      While in the Air Force stationed in Germany I had opportunity to call home a few times for free by means of the MAARS Program (Military Affiliated Amateur Radio Service). The MAARS radio program is a world wide network of ham radio operators who volunteer their time and equipment so that military personnel stationed all over the world can call home by radio to a ham radio station near their home, then the ham operator places a call from his station to the servicemen’s house and connects them via a radio/telephone patch. That may not seem like much of an accomplishment in our world of cell phones and satellite communications but I can assure you that in the 60’s when I was making $140 a month take home in the military and a long distance call from Germany to Ohio was $7.20 a minute it seemed like a miracle of technology.

      When I got out of the Air Force in 1973 I once again returned to my hobby of electronics experimenting, only this time I began by getting a copy of the Amateur Radio Handbook. Within a little less than 2 years of self-study I finally managed to pass the written exam and the code test to obtain my Novice Class Amateur Radio License. With that license I was now legally permitted to talk to the world via amateur radio but the real upside was that I was also now permitted to design and build my own radio equipment, providing of course that I stayed within my legally assigned frequency bandwidth and I only talked by means of CW (continuous wave-commonly called Morse code) and I only used a crystal controlled transmitter not to exceed 75watts peak envelope power
      I currently hold a General Class Amateur Radio License which allows me to still transmit CW (my personal favorite mode of operation) as well as voice by means of AM (Amplitude Modulation) FM (Frequency Modulation), USB (Upper Sideband), LSB(Lower Sideband), SSTV(slow scan Television) and digital modes to and from any point on this known world via HF (High Frequency), VHF (Very High Frequency), UHF (Ultra High Frequency) & Microwave as well as a couple points outside the world, such as EME (Earth, Moon, Earth) which is a technique where VHF, UHF signals are bounced off the moon to come down on the other side of the world, or perhaps link up with one of the 20 or more amateur radio satellites now orbiting the earth and if that is not enough to peak your interest, I personally have talked to the space shuttle three times while it was in orbit and on one occasion I logged into the International Space Station. Point of fact, Amateur Radio remains on the cutting edge of high tech electronic communications because Amateur Radio is the only FCC (Federal Communications Commission) licensed service that permits experimenting and much of the high tech communications that we use today were first designed and prototyped through ham radio, by example, even though cell phones still remain a fairly new concept in the civilian world, in ham radio we have been using the exact same techniques of hand held portables, automated repeater towers and telephone auto patch since the late 1950’s on our VHF and UHF bands and wireless internet is a variation of Packet radio which has been used by hams since the 70’s. When a natural disaster hits it often takes days or even weeks for the commercial telephone systems or civilian government agencies to get their communications systems up and running but for hams it is merely a matter of reaching in your shirt pocket for your handheld or starting the generator and walking to the radio shack, the car, boat, or airplane. In fact, many Red Cross, Civil Defense and local government agencies rely heavily upon “RACES” program (Radio Amateur Civil Emergency Service) to provide the necessary ground zero communications until they can restore their own systems.


      Fundamentally speaking an amplifier is one of the basic building blocks of electronics.
      Under the broad heading of amplifier we can further refine it down to RF amps, (Radio Frequency amps), IF amps (Intermediate Frequency amps) which are used as internal stages in a complex circuit and AF amps (Audio Frequency amps). From this you can see that even high-end audio amplifiers are on the low end of the technology scale. If you have ever had the experience of getting a microphone too close to a speaker you know only too well the high-pitched squeal that results. That is because some of the output signal from the speaker was fed back into the amp and the amp was instantly converted to an oscillator, which is the basic root component of a transmitter. In fact, to build an RF oscillator (Radio transmitter) we begin with an amp circuit then install a feedback network that has frequency control mechanism. (See attached illustration) and to build a simple radio receiver we begin with a resonant antenna and a tuner Pi network to select a specific frequency, then feed that into an amplifier.

      If the scratchy noise in your amplifier only occurs when the volume or balance control knobs are being turned, then there is a high probability that the control is the problem, but based upon the other symptoms you describe it is far more likely that the root of the problem is either stray voltage in the circuit, an insufficient input signal to overcome the amplifier grid threshold or an improper impedance match on your speaker or headset output. The only way to identify this type of problem would be to use a signal injector to feed the amp a known signal, and then trace the signal through each stage with an oscilloscope. Any attempt to correct the problem by just arbitrarily changing components such as when your friend changed capacitors is far more likely to introduce additional problems than to meet any success in restoring your amp.

      The very first thing you should do is try to locate a schematic for your amp. In most instances there is a copy of the schematic glued to the inside of the circuit service access panel. If you can locate a schematic, then scan it and post it we may be able to provide you some additional information about your amp.


      • #4
        Thanks for the informative reply. The scratchiness, etc. occur not only when I turn the volume and balance controls. There is no schematic anywhere on the inside or outside of the amp or the amp cover. If I want to go further with diagnosing the problem that means I would have to purchase a signal injector and an oscilloscope, correct? Are they expensive? I wouldn't mind purchasing them if they aren't too expensive and if they would be useful in diagnosing other audio equipment problems I may have in the future. Are they versatile tools? Would I be able to use them on equipment other than an amp? Thanks for any replies.


        • #5
          You Claim You Have No Electronic Troubleshooting Experience Or Electronics Exposure. Then How Do You Know If What You Are Doing Is Correct And If The Readings You Are Getting Are Within Specifications Of A Manufacturer? Your Test Equipment Is Only As Good As The Operator That Uses It. An Analog Meter [vom] Reads Resistance, Volts, Amps Raw Power. A Digital Reads The Same But With More "finesse" Do You Know The Difference Between Rms Power And Peak To Peak Power? An Oscilloscope Will Show You Frequency, Voltage, Currect, Inductive Reactance Etc. But If You Don't Know What You Are Viewing Its Useless.