I have a high-stress job in the Information Technology field. At lunch I trade my dress shoes for walking shoes and walk my employer's campus, and several other industrial parks in the area. I use walking as a way to reduce stress, clear my mind, and prepare for tasks that need to be completed by the end of the working day.
Recently I noticed a dumpster at one of the industrial parks, and maintenance workers cleaning out a properly, readying it for the next lessee. The dumpster was only shoulder height, the style used to haul away construction waste. I glanced in the top of the dumpster as I strolled by and noticed a compete stereo system strewn of top of old lumber. The stereo system consisted of the following components:
Fisher CA-273 Stereo Amplifier
Fisher EQ-273 Graphic Stereo Equalizer
Fisher FM-273 AM/FM Stereo Synthesizer Tuner
Fisher AD-813 CD Player
I could tell that the components of this stereo system had not seen the light of day in many years. I made several walking strips between the dumpster and my car until all the components of my "dumpster" stereo were safely deposited in the trunk.
Although I have recovered the above stereo components, this blog is going to cover the repair of the CA-273 Stereo Amplifier only.
About the Fisher CA-273 Stereo Amplifier
The Fisher CA-273 Stereo Amplifier was first introduced in 1983. It was intended to be used with FM-273 Tuner, CR-273 Cassette Deck, EQ-273 Graphic Equalizer, and MT-273 Turntable. The CA-273 Stereo Amplifier had impressive specs for the time. It could provide 100 Watts RMS power to the speaker with .09% Total Harmonic Distortion. Its audio bandwidth was between 20Hz to 20kHz. On the back it included Phono, Tuner, CD, Aux and Tape inputs. In addition, the CA-273 has two switched and one unswitched outlet so the you could provide power to other stereo components. The front of the CA-273 included input Function Selector, 3-band Tone Control, Power Level Indicators, Balance, and Volume controls.
Step 1 Initial Cleaning
My dumpster find CA-273 was covered in dust and dirt. In addition, it had been recently rained on.
I sprayed the CA-273's exterior with Windex then cleaned with paper towels. I buy Windex in "Refillable" 1 Gallon jugs as I use so much of it around the house. I then transfer Windex from the refillable jugs to generic spray bottles.
I use Windex moistened Q-Tips to clean hard to reach spots, like between the RCA jacks.
A moistened Magic Eraser sponge works great for cleaning dirty AC cords.
Look at all of the dirt Magic Eraser removed from the AC cord.
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A fine tip permanent marker works great on concealing any scraps or gouges in the top cover's paint.
I used a store brand protectant, an equivalent to Armor All, to bring out a nice sheen to the exterior plastic and metal painted parts of the CA-273.
I then used a lint free cloth to buff the exterior of the enclosure and to remove any lint deposited from the paper towels during cleaning.
Step 2 Initial Testing
Once I cleaned all of the Fisher components found in the dumpster, I connected them together via RCA cables, added speakers and an FM antenna then powered everything up. All components powered up but I noticed I was missing the left audio channel in its entirety. In addition, the CD player would not detect an audio disc, but this is material for another RadioBoatAnchor blog. In order to determine if the issue was with the CA-273 Stereo Amplifier or an external component, I switched the left and right audio cables between the FM-273 Tuner and the CA-273 Stereo Amplifier. There was still no sound from the left audio channel, indicating an issue with the CA-273 Stereo Amplifier.
Step 3 Disassembly
Disassembly was required in order to diagnose the issue with left stereo channel. The first step in the disassembly process is to remove the top metal cover. It is held in place by two screws on both sides. See areas circled in the picture below:
In addition, there is one screw, circled below, that holds the top metal cover down in the back. This screw must also be removed.
You can now access the component side of the main printed circuit board.
Four screws hold the bottom metal cover in place. See areas circled in the picture below:
You can access the foil side of the main printed circuit board, Darlington Power Packs, and the power transistors used to regulate the DC power supply, once the bottom metal cover has been removed.
Step 4 Troubleshooting
Once the top and bottom covers of the CA-273 were removed, I could start my troubleshooting. I was unable to find a free schematic on the Internet for the CA-273 so I was left to my own devices as it seemed silly to spend $25.00 on a Sam's Photofact Service manual when the amplifier was "free".
The first thing I did was check the fuses, circled in the picture below. I do a continuity check of each fuse as I have been fooled by doing visual checks before.
In my case all of the fuses checked good. Next, I swapped the Darlington Power Packs, circled in red below. The Darlington Power Packs are the semiconductor components that directly drive the speakers. I would know if a Darlington Power Pack is defective after the swap as the stereo channel problem would have migrated from the left to right channel.
The Darlington Power Pack swap is relatively easy. Two screws hold each in place. A desolder tool quickly sucks away any molten solder from the pads.
Swapping the Darlington Power Packs did not cause the issue to migrate from the left to the right channel, thereby, I must deduce that a Darlington Power Pack was not the issue. Do NOT try to swap the transistors circled below. They are part of the regulator circuit for the positive and negative DC power supplies are are NOT interchangeable.
I came up with a great idea of using the CA-273 as a Signal Tracer, in order to determine where the fault lies. I set the CA-273 on its side and powered it on. I then connected one terminal of my home brew audio generator to the CA-273 chassis and the other to the a multi-meter test lead.
I set the volume control of the CA-273 to about 75% of its travel. I then placed the tip of the multi-meter test lead, connected to my audio generator, in close proximity to components on the main printed circuit board. I would hear a tone from the audio generator, out of the left or right speaker connected to the CA-273, if their was not an issue with that circuit stage. Do NOT touch the multi-meter lead to an actual lead of a component or you may hear the audio tone at close to 100 Watts RMS!
The main printed circuit board in the CA-273 is roughly divided into three sections. The Left Audio Channel (circled in red), the Right Audio Channel (circled in yellow), and the DC Power Supply (circled in blue).
I had determined that nothing was wrong with the main printed circuit board. Circled below are connections between the main printed circuit board and printed circuit board with the volume control. The connection circled in red is for the left audio channel while the connection circled in yellow is for the right audio. I could hear the audio generator's signal from either channel if I brought the multi-meter lead close to them. The printed circuit board with the volume control feeds audio signals into the main printed circuit board.
When looking at the front of the CA-273, you will see a vertically mounted printed circuit board toward the right-hand side of the chassis. This printed circuit board handles Function selections, switching between different inputs, such as Phono, Tape, Tuner, and CD Player. The connector circled in red below connects this printed circuit board to the printed circuit board with the volume control. When I hovered my multi-meter lead over this connector. I could hear the signal generator's audio tone out of the right speaker but not the left. This indicated that the issue is on the printed circuit board with the volume control.
Knowing that mechanical controls are often a problem, especially with equipment that has been dormant for a long time, I decided to fool with the volume control to see if that was the issue. I adjusted the volume control slider to about a quarter length of its travel. I then connected my audio generator to the left channel Tuner input and played with the volume control slider knob. I found that the left channel would work if I twisted the volume control slider knob slightly. This indicated that the volume control slider carbon track for the left channel was dirty or oxidized and needed to be cleaned.
Step 5 Front Panel Removal
As I already had the CA-273 apart, I decided to disassemble the front panel. This allowed me to better concentrate the contact cleaner in the areas that needed to be cleaned. I was also afraid that the dripping contact cleaner residue could dissolve surrounding plastic parts. I also decided to clean the innards of the Tone and Balance controls as well.
In order to remove the front panel, you must first disconnect the wiring between the audio input printed circuit board and the front panel. In addition, you must disconnect the wiring between the main printed circuit board and the front panel. See areas circled in the picture below.
The bezel attaches to the front panel with three screws on the bottom and at the inner lip at the top. The Function Selector switch is part of the front bezel so I had to pull the wiring through the opening in the front panel in order to free. I also had to remove the silver knobs from the three Tone Control sliders.
Behind the front bezel is a metal panel where the volume control printed circuit board and power level indicator are installed. In order to free the metal panel you must remove several screws. There is a screw on each side, circled in the picture below:
In addition, you must remove several screws on the bottom to free the metal panel. Their locations are circled below.
Screws holding the power switch, and the printed circuit board containing the Speaker and Tone switches, need to be removed. The clip holding the headphone jack needs to be removed as well. In addition, the cream colored plastic pieces on the Speaker and Tone switches must be removed in order to free the printed circuit board from the front panel.
Here is a picture of the CA-273 front panel.
Here is a picture of the CA-273 with the front panel removed.
Step 6 Front Panel Slider Control Cleaning
Through my experience, I have learned that slider potentiometer controls don't age well. Due to their open construction, they are more prone to problems with dust and dirt than enclosed rotary ones. The next step is to free the volume control printed circuit board from the metal panel.
I removed the silver knob on the slider Volume control. The black plastic mount must also be removed, it is held in place with one screw. I also performed these same steps for Balance, the slider control to the immediate left.
I removed ten screws that held the volume printed circuit board to the front panel. You must also remove the Tone control indicators from the slider switches. See areas circled in the picture below:
Finally, time to clean the Volume, Balance, and Tone slider controls. I sprayed a small amount of contact cleaner in each slider's slot then worked them through their complete motion several times.
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Step 7 Testing
It is always a good idea to perform testing before buttoning everything up. I re-attached the volume printed circuit board to the front panel. The front panel was then fastened to the chassis and all wiring between the front panel, main, and audio input printed circuit boards were connected. The Function Selector switch is built into the front bezel. I had to connect the wiring between the front bezel and the audio input printed circuit boards.
I plugged the CA-273 into an outlet then powered it on. I then set the Function selector to Tuner, the Balance slider centered, and the Volume slider to about 1/8 of its travel. I then introduced and audio signal from my audio generator to first the left then right RCA Tuner Input jacks.
SUCCESS! I was able to hear the tone first from the left then the right speaker, the issue had been resolved.
Step 9 Detailing
I like to polish the chrome knobs before assembly. This is the perfect time to do it as the knobs have been removed from their control. Meguiar's Motorcycle All Metal Polish works great for polishing and removing oxidation from chrome coated plastic knobs, like the ones used in the CA-273.
Here is a picture of the CA-273's chrome knobs after being polished.
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Step 10 Assembly
Time to button the CA-273 up. I followed Step 5 Front Panel Removal, in reverse order, to assemble the front panel and attach to the CA-273 chassis. Then following Step 3 Disassembly, in reverse order, to install the top and bottom metal covers.
Step 11 Final Testing
Once the CA-273 was put back together it was time for a final test. Once again, I connected the CA-273 Stereo Amplifier to the FM-273 Tuner, EQ-273 Graphic Equalizer, and AD-318 CD Player via RCA cables. I then added speakers and an FM antenna then finally powered everything up. All components powered up and my left audio channel was once again working!
Bringing the Fisher CA-273 Stereo Amplifier back from the grave was a most satisfying endeavor. It is one less thing destined for a landfill. I have yet to service the AD-318 CD Player, it will be the focus of another RadioBoatAnchor blog. The remaining Fisher components grace my basement workshop and provide background music while I toil away on the next restore project.
Who Writes This Blog?
John is an IT professional from Cleveland, OH who enjoys amateur radio, ham radio, metal detecting,
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