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Old 30th December 2011, 01:20 AM   #1
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Default Help for upgrading Pioneer receiver

I have stereo audio/video receiver named Pioneer VSX-D506S. I am intending to apply some mods on it for better stereo sound. Could anyone help me please?

Here is the receiver service manual: Pioneer-VSX-456-VSX-D506S.pdf
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Old 13th June 2012, 08:51 AM   #2
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Anyone could help?
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Old 13th June 2012, 09:03 PM   #3
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"Better stereo sound" is pretty vague. Do you feel it to be lacking in any area in particular? It looks like a fairly sanely (if cheaply) constructed piece of kit to me overall. What's your skill level?

What one could do is things like:
* Make sure that output relay contacts are squeaky clean (they often suffer from contact issues from tarnishing of the contacts since many are not sealed well, e.g. Takamisawa brand). Those relays can often be considered expendable parts and, when matching replacements are easily found, are best replaced when giving trouble. Degradation of damping factor due to bad contact is one of the major causes of degraded sound quality in old age. Parts in question would be RY201..RY203 (plus RY204 for the headphone jack).
* Add a low-impedance headphone output (make/buy a NJM4556-based cMoy set up as a buffer and tap off between pre and power amp with a ~1k resistor; suitable supply voltages and ground are available within the unit).
* Bypass tone controls.
* Bypass Dolby related circuitry (which is no more than a FET switch in stereo mode).
* Add pre-outs.

I did notice a few oddities.

* While the FM frontend would allow for RF AGC and there is a connection from microprocessor to radio board for activating an RF attenuator, the radio board disappointingly does not make use of this. Instead, voltage at G2 of the preamp FET is set for a compromise gain using resistor R6201. This simple frontend (little more than portable-level, really) probably won't be too happy in very strong-signal areas, and it may not make for a sensitivity champ in weak-signal ones. (Sorry - tuner geek.)
* Gain setting resistor values for some opamps running at line level are questionably low, degrading performance by needlessly loading the opamp's outputs where noise is not too critical. In particular, that's IC1901 in the tone control (330 + 330 ohms where 680/680 or 1k/1k would do fine) and the preamplifier for the tone control, IC409 (330 + 750 ohms where 1k + 2k2 would do fine, this opamp's noise is swamped by the preceding buffer IC403 anyway).
* The unit appears overrated in terms of output power. At least there is no way of getting 100 W into 8 ohms from an amplifier with an EF2 output stage whose supply rails are supposed to be at +/- 39 V at idle. ~80 watts, maximum. Maybe this is related to the 6/8 ohm switching?
* While good-quality parts are used, power amplifier topology is basic (see e.g. Self) and not to the highest standard of reproduction. Not unusually for surround receivers, quiescent current is non-adjustable (you should try checking it), and in order to stabilize it, 0.33 ohm emitter resistors were chosen for the output transistors rather than the usual 0.22 ohms. This thing won't do too well into anything but "real" 6..8 ohm (and up) loudspeakers. Driver emitter resistors are also connected to the feedback line, which is not as well-suited for real-life loads. Something like an A-503R was somewhat more refined already. Still, I wouldn't ditch the power amps until I've seen some measurements on a dummy load.

What I'd tackle first:
1. Output relays, as they are such a common trouble spot.
2. The aforementioned gain setting resistors. Assuming you can get to them and have some matching metal film resistors at hand, this should be a breeze.

The unit also contains quite a few electrolytic capacitors, which may also need to be taken care of at some point. I'm not sure how well they generally hold up in what I'd estimate to be hot and humid climate.
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Old 21st November 2012, 01:21 AM   #4
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Thanks sgrossklass for your help
Actually I'm just an amateur and would be appreciated if you could give me very detailed instructions. I intend to use this receiver for only stereo music and don't care about 5.1 channel or dolby prologic. And that is my purpose for modification.
For the time being, I've just replaced ic409 and ic403 with OPA2107 and some caps on the audio signal route with good film ones of the same value. The sound is better now.
Could you instruct me in detail how to bypass Dolby related circuitry and tone controls and add pre-outs?
Which good relays could you suggest me to use in replace of the old ones?
Where are emitter resistors?
Do you have any suggestions for the power sector upgrade?

Thanks in advance
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Old 21st November 2012, 01:43 AM   #5
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Some pictures for your reference
Click the image to open in full size.
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Old 14th December 2012, 01:44 AM   #6
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no more help? Please
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Old 14th December 2012, 06:44 PM   #7
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Did you change the op amp resistors on tone controls like he said? The output of the op amp is the one labeled "O" on the datasheet. Part datasheets can be downloaded from datasheetacatalog.com. From the top on DIP package, pin one is the dot on the upper left and you count clockwise on the top (label) side.
I use metal film resistors now, myself.
Did you clean the relay contacts with a file? (not sandpaper, leaves non-conductive grit).
There are two dozen electrolytic caps in there, how old are they? The ones with the minus in balls on a stripe pointing at one lead. Over twenty years old, time for a replacement. Do two at a time and check that it works again, amateurs like you and I make a lot of bad solder joints.If you make it worse with something you did, doing 2 caps at a time focusses your eyes right on what mistake you just made.
I use e-capacitors rated >3000 hour service life, this eliminates the low cost **** with bad ESR ratings, usually, also.
Personally, I don't hyperventilate about tone controls, I can tell by listening when the response is best. Go to more concerts and train your ears to what acoustic instruments should sound like, then set the tone controls for most lifelike sound. I've never had realistic sounding speakers until the latest pair, which were $600 a pair used.
The preamp out stuff should probably come from the tone control output op amp, with that driving another unity gain op amp to keep from loading down what is already there with cables. I like Datek 12-617b boards for little op amp projects. I drill #43 holes in existing PWB's next to the power trace when I want to tap off the power for somewhere else. Stick the wire in it from the top and bend over and solder to the trace. I use a hand crank stanley yankee drill for this fine work, usually.
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Last edited by indianajo; 14th December 2012 at 06:54 PM.
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Old 16th December 2012, 08:29 AM   #8
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Not yet, I am still waiting for the resistors from Partsconnexion. However, I have cleaned the relay contacts and resoldered them, also cleaned RCA jacks (maybe consider their replacement later). In the tone control section, I replaced njm4558D opamp with opa2134pa, upc4570c opamp with opa2107pa, 4.7uf/50v caps (C1911, C1912, C1915-C1918) with elna silmic II, 2.2uf/50v caps (C1921, C1922) with elna cerafine. I also replaced the pairs of caps before and after IC409 opamp with mundorf e-cap plain caps, which was a bit hard job due to the lack of room. The result is far from my expectation. Sound is more detailed, the bass is deeply punched and the treble is sparkling. I checked the speaker cable connected from the power amp section (J103 and J104) to the speaker terminals (the green and orange ones in the first photo) and found that was made of silver plated copper, so I decided to leave it original.
There are plenty of electrolytic caps, but i take care only of those in the input and power amp sections which are directly related to the stereo sound. All small power caps were replaced with Sanyo oscon and Panasonic FC.
Regarding the two big black power caps (8200uf/71v elna type), should i bypass them with smaller caps and which value for the best?
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Old 16th December 2012, 11:12 PM   #9
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Quote:
Originally Posted by ngovanhinh View Post
Regarding the two big black power caps (8200uf/71v elna type), should i bypass them with smaller caps and which value for the best?
At 71 v you are not going to be able to afford a 1 uf ceramic capacitor. The best ceramic cap would be a 0.1 uf 100VDC, but if your soldering skill allows it, two or three 0.1 uf in parallel would provide better high frequency support.
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Old 18th January 2013, 05:06 PM   #10
Bigun is offline Bigun  Canada
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I have an old 7 channel Pioneer that I am planning to upgrade. I noticed in the service manual that it describes the set-up procedure for the power amplifier and requires it to be adjusted so that 2mV appears across the emitter resistor of the output transistor. I believe this is way too small for proper Class AB operation. Under-bias produces cross-over distortion. For optimal stereo, pick the two stereo channels and ensure they are biassed for around 25mV across the emitter resistor.
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