I just picked up a Pioneer A-77X in decent shape for free from work. We dug it out of the back and it wasn't playing, so I was told I could have it. I found a service manual and have thoroughly cleaned it, which has restored sound to the beast. The controls were the culprit, as usual- dirty as hell with the line signal passing through miles of cheap buttons and spaghetti wire before making it to the voltage amplification stage. Why does nobody put all of these controls in the feedback loop? 😡
Honestly I don't think this thing sounds very good either, kind of sounds how it looks: big complex and unsure of itself. Pioneer was never one of my favorite manufacturers and even though I love the styling of this era they just didn't pull it off in my book. Early digital gear is very much my thing but this amp is not. Very powerful, and I hear things in songs I haven't heard before but the bass is sloppy, along with the rest of the sonic presentation. My N55ES eats it for breakfast as far as tonality is concerned.
Anyhow, the right channel is considerably louder than the left. I was suspicious of the controls again so I had my friend solder an RCA input directly to the voltage amplifier board, bypassing even volume control. The right channel is still louder! There appears to be no bias adjustment in this amp. How should I go about determining where the signal is picking up volume on the right channel, or losing it on the left?
Some pictures:
How I found her, disgusting...
All cleaned up
Direct input
Honestly I don't think this thing sounds very good either, kind of sounds how it looks: big complex and unsure of itself. Pioneer was never one of my favorite manufacturers and even though I love the styling of this era they just didn't pull it off in my book. Early digital gear is very much my thing but this amp is not. Very powerful, and I hear things in songs I haven't heard before but the bass is sloppy, along with the rest of the sonic presentation. My N55ES eats it for breakfast as far as tonality is concerned.
Anyhow, the right channel is considerably louder than the left. I was suspicious of the controls again so I had my friend solder an RCA input directly to the voltage amplifier board, bypassing even volume control. The right channel is still louder! There appears to be no bias adjustment in this amp. How should I go about determining where the signal is picking up volume on the right channel, or losing it on the left?
Some pictures:
How I found her, disgusting...
All cleaned up
Direct input
Can't be an awful lot more than bad contact in the output protection relay (or mechanical speaker selector switch if it has one).
BTW, feedback doesn't help one bit with preamp contact issues. In the days of fuses in the amplifier outputs some manufacturers would split feedback to before and after the fuse, however.
BTW, feedback doesn't help one bit with preamp contact issues. In the days of fuses in the amplifier outputs some manufacturers would split feedback to before and after the fuse, however.
Hmmm... I used to be the proud owner of an A80. Quite similar physically. Totally different electrically.
The tone controls in your amp appear to be within the feedback loop of the power amp but switch selectable. As you say, loads of wiring and switches.
Edit... it is the feedback factor that sets the gain of the amp... which is why I mentioned it 🙂
The tone controls in your amp appear to be within the feedback loop of the power amp but switch selectable. As you say, loads of wiring and switches.
Edit... it is the feedback factor that sets the gain of the amp... which is why I mentioned it 🙂
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Boy-oh, the feedback loop does make its way through the front panel then! Even after cleaning of everything with air, a brush, and elbow grease some of the contacts were still questionable. Even Deoxit failed to do the trick this time, besides bringing back sound to the left channel. Guess I will have to tear the whole control board down to the nittle grittle and maybe wire in a bypass for my direct input so I can still use the volume control with it. I was also experiencing some 60Hz hum in this unit when playing with the cover off. It got significantly worse if you touched the wires going from the front panel to the voltage amp whilst ungrounded. In my usual experimental spirit I pulled out my normal trick for finding where to put shielding and placed some metallic duct tape (the REAL duct, some call it aircraft tape) in various spots around the unit. Of course it made a big difference when fitted right over this wire, but I wanted to find the source of the interference. I imagined right off that it would be those big noisy EI transformers and boy was I right. They pegged a laser power meter from 3ft away, a task my 55ES transformer cannot achieve with the meter slap on top of it. So today I set about making a shield for them that would not interfere with their proper cooling. Here is what I came up with, pictures just for fun...
Not bad for using just a dremel and a regular vise with some hunks of aluminum in the jaws as a bending brake I don't think
Not bad for using just a dremel and a regular vise with some hunks of aluminum in the jaws as a bending brake I don't think
Well it appears resistance to ground and signal is the same from the output of the input board (confusing statement) to the voltage amp input. The problem must lie elsewhere in the circuit, and this test doesn't rule out the input board or output/voltage amplification stage.
These Pioneers (well my A80) majored on low noise as being one of the main selling points. Have to say it was one of its better features. Super quiet mechanically and electrically.
Oh no no! Tone controls in the feedback loop are nasty! Remove them as fast as you can and solder in place the usual R(C) network.
I did it in the Sansui Aux-201.
I did it in the Sansui Aux-201.
I agree that tone controls in the feedback loop are not ideal from a bandwidth POV and may not play nice with bad contact and dried-out electrolytics, but I would recommend leaving baby and bathwater where they are - it did work fine when the unit was new after all. Converting active tone controls to passive and stll having them work properly isn't entirely trivial either.
I'm a bit surprised to see this relatively expensive unit use this approach. It's more typical of low-end receivers and such, where presumably one big power amp for everything was cheaper for a given performance level than separate pre + power.
But anyway, the output relays (there's one pair output pair) are the very first thing that should be cleaned or replaced. They're on the power supply assembly, apparently (RY1 and RY2) - if in doubt, trace back the output connections.
I'm a bit surprised to see this relatively expensive unit use this approach. It's more typical of low-end receivers and such, where presumably one big power amp for everything was cheaper for a given performance level than separate pre + power.
But anyway, the output relays (there's one pair output pair) are the very first thing that should be cleaned or replaced. They're on the power supply assembly, apparently (RY1 and RY2) - if in doubt, trace back the output connections.
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This unit is quiet with the cover on. It claims a line S/N ratio better than 100dB. I think it was either 108dB or 118dB in the specs I saw posted somewhere. I was only getting the noise cover off, and wanted to be able to work on it without a constant hum in the left speaker and a volume-graduated one in the right speaker. Now there is only a very slight hum on either channel at maximum gain. You can also hear a spec of dust land on the chassis at max gain 😛
I would like to add one input that omits the tone controls. Doing this whilst keeping the big beautiful alps volume pot that came with this amp functioning for both configurations is easier said than done.
I figured bypassing these controls wouldn't be perfectly straight forward. I have done so with limited success with my direct input, but have not been using it because I need to add some resistors in series with it since it bypasses the volume pot entirely. I am also not sure I have the best impedance match putting signal right to the amp board. I have been taking output signal directly from the wire wrap pins on the output boards, the relays had to be whacked with a screwdriver to get them to open when I first found this thing and I don't have spares. I will have to order some. Omron are the good ones right? I wish the originals worked well, they are nice solid Matsushita units that have a very loud mechanical clunk to them. Maybe I can restore these too.
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The SNR figure sounds about right. This was the A80 I once owned, bought at the time on strength of its specs... I've learned a lot since then 😀
Pioneer A-80 / Integrated Amplifiers - Pioneer / Vintage Audio
Pioneer A-80 / Integrated Amplifiers - Pioneer / Vintage Audio
Manufacturers definitely started spec chasing around the digital age. I am obsessed with gear from this era. Earlier stuff is cool and the quality of materials and workmanship is top notch but the designs we had available in the 80s/90s with ICs getting so cheap were incredible.
I was reading a thread on another forum where a member was also having trouble with his A-77X, although with noise in one channel. He was advised to check zener diodes 1-4 on the board labeled by the service manual "gwh-171" as well as two (C3 and C5) 100uf 25v electrolytic caps. Would this be well advised in my situation? It cured his issue. The diodes are only 22v half-watt parts- seems a little on the light side.
Could anyone advise me on how best to remove these connectors? I'm at my wits end. I get the locking mechanism free but the wires hang on for dear life and breaking these would mean a lot of labor replacing them with something else.
Good thing I have these photos here since I hurriedly yanked the connectors and destroyed the contacts without even marking the connections for polarity as I normally do. The construction quality of this "Elite" amplifier is less than impressive thus far. The design is wonderfully simple and the boards are quality as well as the parts, but I cannot forgive them for using this darned cable.
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Well despite the burn marks on the board around the diodes and nearby resistors they all read "830-835" on the diode setting on my cheap meter. Whatever the heck that means. Some other zeners on the board read in the 600s. Again, I have no idea what sort of value this is displaying or if these should be replaced.
Capacitors C5 and C6 are not even the same kind! Original solder job on both too, nobody has replaced this. Some quality control, Pioneer! I have removed this board to take it in to work on Monday so I can get some answers.
Sounds like it was a known issue and so its probably worth replacing them. Zeners, and any series regulator transistors often run very hot and suffer problems long term... caps get caught in the crossfire with the heat and fail.
Your DVM on the diode range is displaying millivolts as a small test current is passed through the device under test. Any reading on a zener like that is just the forward bias voltage, you can't check the zener breakdown voltage because its far higher than the meter test voltage.
I figured that this meter would once again prove to be useless. Could these be replaced with higher voltage parts without issue or should I replace them with what was there and solder them a centimeter off the board or so?
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