Pioneer VSX504S weak/no bass on right channel?

JL66

New Member
Back in the 90's I bought a new 5.1 surround VSX504S, I didn't need the surround sound features but I liked the 130wpc it had. It made the two JBL L112's I had really hit hard. About a month after I bought it work transferred me to another state, so it all got packed up and sent here. Not having time or space at the time to set it up, it sat in its original box for 20+ years and got forgotten in an upstairs closet.
I dug it out the other day after being given a pair of huge Cerwin Vega D7 speakers.
To my surprise, one channel sounds weak, as if the woofer wasn't working in that speaker, so I immediately swapped wires on the back and the problem switched speakers. Both speakers work fine. If I turn the bass knob all the way back, both speakers sound the same, but when I add bass, only the left channel can make the woofer work and I don't think its 100% either as I would have thought these old CV speakers would be a bit bass heavy and they're not at all. Thinking it may be just the control pots, I dug out a can of Deoxit I had here and opened up the receiver, but after several treatments on all the pots, as well as a good look at all the solder joints there's no change. I also took a good look at all the caps, looking for any leakage or swelling but this thing still looks brand new. It was stored in the original plastic bag it came in, it was even taped shut and packed up just as it came from the factory all those years. Its got less than 10 hours of play time on it back in the day.
I called a few local shops but they said throw it away, its not worth fixing. It was a decent sounding unit back then, it had gobs of power and decent sound. Right now its sounds worse than a $10 transistor radio from China.
Any ideas?
 
Very thin sound, no bass, could mean the ground connection of the input signal is not OK. If you can, in the good channel, connect just the center pin of the RCA input connector, without connecting the ring, to compare the problem.

Don't discard a problem at the RCA wire itself.
 
The problem goes away with another receiver and the first thing I did was swap the speaker wires side to side but the problem stayed with the right channel outputs.
The part that gets me is that its like new, its got almost no use on it and it was stored in an upstairs room in the original box and packaging all these years.
I poked and wiggled just about everything inside and its still weak on the right side. If anything, it seems to be getting weaker every time I try it. The sound from the right speaker sounds tinny, the woofer makes no sound at all, which seems strange to me since aren't most woofers wired direct through the crossover?
The speaker is fine, it plays fine on the other channel. I also tried the 'B' speaker outputs but its the same. It don't have pre-amp out jacks so I can't isolate the pre. vs the amp but I did hook up a cassette deck and try recording from the tape out jacks and its weak on the right channel on the recorded tape. I think that tells me the issue is before the amp?
 
Check the value of any capacitors around the input selector area. If one has dried out over time it could be acting as a high pass filter, especially if there is a resistor to ground nearby. You may have to remove them to get a proper reading of their value.
 
I agree, a dry capacitor can block the signal, but check the resistance to ground with a meter, at the input jack and at the volume knob (to mention 2 possible weak points). Measure resistance from the output ring of the RCA connector, to chassis. It must be near zero ohm.

To check for a capacitor blocking the signal, you can feed a low freq sinewave (lets say 100Hz) (with your cell phone with some "test tone" app, or computer with some "online tone generator", or any digital source) (YOUR AMPLIFIER UNIT OFF AND UNPLUGGED), set your DMM to AC (Audio signal is AC), and measure the voltage at the input jack, and follow the traces, and measure at both sides of the DC blocking capacitors you find in the signal path. Compare channels. But in a good capacitor, the signal should be the same at both sides of the capacitor.

If you can read the schematic, follow the signal path measuring voltages and comparing channels. You can test the signal path until the first transistor or OPamp with this method. You can inject a signal at any point in the circuit and check the caps, the signal must be the same at both sides (if the cap is in series along the signal path, and is not a shunt to ground or some tone control of Eq network, where part of the signal is sent to ground)
 
I pulled the two big caps off the board and used a Chinese cap tester and both test good so I put them back for now. I recleaned the bass control, and checked for ground resistance at the rear terminals.
After putting it back together and letting it dry I gave it a try and when I first turned it on both channels played for a bit, but after only a few seconds the one channel fizzled out and got weak again. It sounded like the crackle when a volume control is dirty. I powered it down, let it sit and tried it again in a few hours and it again played for a few seconds then it crackled a bit and lost that channel again. This time it fried the tweeter burning off the hair like lead from the voice coil to the contacts. Then it blew the speaker fuse on the speaker. I checked it again for DC volts but got none?
If I let it sit now, every time it'll play for a few seconds then crackle and drop that channel. The speaker it toasted was a junk Realistic so no loss. But the problem did change after removing and resoldering the caps and recleaning the bass control but I'm still no better off.
The part that gets me is that this is basically new, its got less than 10 hours of play time on it, it sat in a box since it was new and what ever failed, failed while it sat in the closet not from wear or abuse.
 
what wear out without any use are the electrolytic capacitors, they get old even unused just sitting on a shelf in a closed box. The manufacturers spec a "shelf life" for aluminum electrolytic capacitors.

Anyway, I think you need to face it as any failing amplifier. The fact it burned your speakers, and the fuse, means a major malfunction, like a HF oscillation or DC at the output. It seems the loss of signal is related to an electric fail, not just a cap blocking the signal.

Crackling noises like pop-corn or little explosions use to be failing transistors or opamps.

Try which controls affect the noises, if balance or tone controls reduce the crackle. Then, you'll need to remove components or wires to isolate different amp stages.

Of course, don't connect any speaker by now, try it with headphones for brief tests, if you have cheap ones.

Do you have some tools and skills to attempt a repair? with the help of the service manual we can try to fix it.

Edited: I see the unit has a DC protection circuit, and overload protection circuits. The few seconds it works could be just the time the protection circuit takes to cut the signal.
 
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JL66 how are your soldering skills? Another member here had problems with a treble control and it just needed to be resoldered to the circuit board.

Might not be your problem but it is worth a check.
 
I didn't think the protection circuit is the issue since when the sound fades or fizzles out it doesn't go completely silent. It gets super weak but is still there to some extent.
I have a soldering iron, several multi meters, a capacitor tester, and some very basic electronics knowledge. I am capable of desoldering and soldering on a PC board with no issues.
I have a transistor tester but have never used it. Its got three leads and no instructions, I'm not sure where I got it but its pretty old.
I bought a Chinese cap tester a few years ago to ID some unmarked caps for a cross over repair. I already had a BK cap tester but was told it likely would be way out of spec do to age.
I do not have an oscilloscope, I probably should own one but wouldn't no what to buy or what to look for in one. I've used automotive type lab scopes but I'm sure they're quite different from what would be used to trace current on a pcb.

When I power this thing up, it does nothing for about 2 seconds, then I hear it come out of protection and start to play, at about the time it hits playing volume, one channel immediately fades out as if someone was turning the balance to one side with a dirty balance pot. Once the fade stops, the sound is clear but very slight. Its loud enough to hear in the next room but there's zero bass. The sound is as if I turned the bass and treble all the way down. At the same time the opposite channel is playing fine.
I tried power it up with a DC voltmeter attached to that channel and it never registered any voltage even on the lowest scale. I have an old pair of junk Scott speakers with whole speaker fuses I test with. I disconnected the tweeters for testing right now. If I let the thing play, with the balance in the middle, the sound level is about 90/10 left to right. It blows the 2 amp speaker fuse after about a minute and that speaker stops playing. At no point can I detect any DC voltage and the sound doesn't change or pop when the fuse blows. The issue hasn't changed since I put the caps back in.
The sound I hear when the right channel is fading sounds like when you pour warm soda over ice, its only making the fizzle sound as the sound is fading, it stops fizzling once it stops fading. None of the controls make any noise.
Once the sound fades, then pops the fuse, immediately replacing the fuse and turning it back on gets no sound from that channel, it has to sit for about 5 minutes before it will start the whole cycle over again as if something is getting hot or needs to dissipate some voltage.
I'd like to fix it just to know what failed, I liked how this thing sounded and it had gobs of power but the reality of it is I can buy a good used one for $50 or so, maybe less so dumping any major money into isn't worth it.
 
You'll have to invest time, a couple of days or a couple of weeks. If you have spare time, we can try to fix it.

There are literally hundred of components what can fail.

If you want to try a repair, I'd start splitting the unit in 2 halves, preamp and power amp sections. There are coupling capacitors and connectors between boards, so it's easy to disconnect one half and check if the problem persists.

An oscilloscope would be good to trace signals, but there are other ways to detect a problem, measuring voltages and checking components. There are PC based oscilloscopes, useful to trace audio signals, software like "Visual Analyzer".

I see there are some "muting transistors", those can give trouble too (Q101 Q102) (transistors that shunt the signal to ground to avoid thump noises, or to delay the startup). You can safely remove those transistors.

You'll need to make some interconnection wires and wires with clips and connectors to extract or inject signals to the amp, or to measure voltages at different points. Also, a dummy load to test the amp loaded without burning your speakers (a 8 ohm resistor, several watts, I'd say 15 W minimum)

One starting point could be to measure DC voltage after some minutes at the power amp output (at the emitter resistors R124 and R125, 0.22 ohm x2, should be a white ceramic block) and the bias current (voltage ACROSS those emitter resistors) . No speakers connected. Another point to measure, is DC at the power amp input (C104 or C105, DC against ground, compare both channels)
 
Very thin sound, no bass, could mean the ground connection of the input signal is not OK. If you can, in the good channel, connect just the center pin of the RCA input connector, without connecting the ring, to compare the problem.

Don't discard a problem at the RCA wire itself.
It took a while for this to sink in, but I got what you were trying to tell me, finally... If I connect only the center pin of the good (left channel), I get no sound at all on either channel. Without the outer ring of the left channel connected, I get no sound at all? (I tried this with the right channel still plugged in normally. I took a cheap wire, cut and cut the lead connected to the ring on the RCA connector in the middle so I could connect and disconnect it. With the power off, all the outer rings of the jacks on the back of the receiver however show zero ohms between each and every one.
For arguments sake, I did try several other CD players and cables but all work the same. The part that still baffles me is that this all happened while sitting. I've got dozens of older receivers which play fine after a lot longer time on the shelf. I did pick up another identical unit today off Craigslist for $15, it seems to work fine but its not as clean as mine cosmetically. I may just swap out the face plate and cover and consider it fixed but I'd still like to know what failed on the first one.
 
seems you have lost some ground connection of the Right channel at some point (if not, disconnecting the left channel ground shouldn't affect the right channel that way). It could be some other fail though, this are complex units with several ICs.

I'd try a bit more , just one channel connected (just the Right), until finding the broken connection. You got one for $15, but those units sell for +- $500 down here.
 
seems you have lost some ground connection of the Right channel at some point (if not, disconnecting the left channel ground shouldn't affect the right channel that way). It could be some other fail though, this are complex units with several ICs.

I'd try a bit more , just one channel connected (just the Right), until finding the broken connection. You got one for $15, but those units sell for +- $500 down here.


Wow, these things are pretty cheap here, I don't think I paid anywhere near $500 for mine even when it was new. I seem to remember it being like $349 on sale back in 1995 or so.
I actually bought a VSX D503S first, but the 504S came out soon after and had 50 more wpc. I still have the D503S, it plays fine but it never had the punch the 504S had. Both are 5.1 surround receivers.
I always thought of these as sort of budget receivers, they were cheap power without spending big money on a real power amp. There's always a bunch of these and other similar models on CL, right now I see two of these on ebay and one listed 10 miles from here on CL, both are under $100.
The earlier models are another story, the VSX D1S and VSX 9900S and similar bring upwards of $200 in good shape but I've yet to see one top $300.

What are the chances my original 504S has a bad IC chip causing this? If so its likely not repairable? I can pull and test most components on the board but I can't test an IC chip.
I did go over all the solder joints for the back panel and RCA's but no change.
 
I think the easiest approach is to trace the audio signal at different stages, you can do it with a multimeter, measuring AC voltage, or a probe sending the signal to a second amp.

I'd try a 100 or 200Hz test tone, a mono signal (same to both channels, you can do it with a cell phone or your computer, search for a test tone generator), without speakers connected, and measure AC voltage along the signal path, comparing both channels, until find the unbalance.

You need to trace a plan, identify the points where you'll measure in the schematic, and locate those points on the PCB. If one point is too tiny or hidden, locate another point electrically connected easier to reach.
 
I finally had a chance to get back to the original 504S last night.
The first thing I did was to try and see if the issue was with the preamp stage or main amp, not having a set of pre-outs or jumpers, I used the tape out jacks and hooked it to another amp, it played fine, no issues on either channel.
That to me pretty much eliminates the preamp on this thing, next I wanted to access the bottom of the main board, to do so the entire receiver had to be disassembled. The big issue here is that the ribbon cables are short and soldered in place. I only could remove the tuner section and rear panel separately. I have the entire thing sitting upside down still all connected on the bench right now.
The first thing I couldn't help but notice is that the main transistors aren't all the same, there are four the same on each board but there is one odd transistor with different heat sink compound on it . For a quick comparison I pulled the cover off the working 504S I bought and was surprised to find that none of the transistors were the same as the original unit? The two receivers are quite different. The first one, with the issue has a fan on the heat sink between the amp boards, the second one does not. I lifted up the main amp module with its heat sink up off the main board so I could see the lower bank of transistors and they match those on top on the second unit. As far as I know my original 504S was brand new when I bought it back in 1995, it was never back for any type of service but it does appear that at least one transistor has been swapped at some point.

Sound wise, I think they're both about the same, but with one not working properly I'll never be able to make the one on one comparison. But there must have been a running change in these things over time, the second one I bought though is older by 11 months. The first one is March of 1995, the second one is tagged April 1994.
When I probed the four output transistors with the unit playing, (carefully rigged up laying on a sheet of plywood), I get no signal of any sort to three of the four output transistors on the amp board, there is no voltage of any type reaching or leaving them. On the good 504S, I can read voltage on both sides that switches back and forth.
I tried to follow the schematic but other than identifying the boards and a few basic components, its almost impossible to trace a signal though the whole unit due to the way the boards have to be kept apart when out of the chassis. Right now, only the transformer is still in the chassis.

Is it possible this thing simply has a few bad output transistors? Would that give allow it to still produce a signal to that side speaker? Would it explain the lack of bass or power to just one side?

I thought about it a bit myself and went back to the fact that it reacted to me messing around with the ground rings on the rear panel, but after playing around with it a bit more, I found that touching any grounded point in the case causes it to play on both channels in mono, its somehow either playing in mono or just letting the working side to play through the other side. If I touch the heat sink, any of the RCA outer rings, the transformer case, or the shield on the tuner it reacts this way. The working unit does not, its totally not affected.

Something I did notice is that on the two rectifier bridges at the power supply, each one has a film of magnetic dust clinging to it, while the other one does not, I can see the outline of the inner rectifier and they are both apparently magnetic. I wiped them off bu the brown dust on them hangs on as if I'm trying to remove metal shavings from a magnet. When I turn the unit on, the dust stands up, the rectifiers become magnetized somehow. Its likely just static but I've never seen this before.
I tried to follow the grounds throughout the unit and didn't see anything loose or damaged and all the solder joints appear fine.
 
Does it sound OK at headphones output, or the problem is there too? Sometimes, a problem with the output transistors only shows up with speakers, but headphones work OK.

BE VERY CAREFUL WHEN WORKING IN A LIVE UNIT, SOMETIMES METAL PARTS AS HEATSINKS HAVE VOLTAGES, transistor collectors are many times connected to the heatsinks, and they can have 40, 50, 60V. IF YOU ARE GOING TO TOUCH A PART; DO IT WITH JUST ONE FINGER; THE OTHER HAND ON YOUR BACK, NEVER CLOSE A CIRCUIT WITH YOUR BODY TOUCHING 2 PARTS AT THE SAME TIME.

I think you need to do some methodical tests, locate the parts you see in the schematic inside the unit , and trace a plan, what to measure, where to measure, and what numbers to expect.

The fact that touching the chassis makes both channels to sound, makes me think in some ground connection, but there are many of different fails that could happen.

I'd try this 2 things at least:

Install the unit in such a way you can turn it ON and OFF SAFELY, while you have access to some points to measure. IF THIS IS NOT POSSIBLE, PERHAPS YOU NEED TO SOLDER SMALL WIRES TO THE POINTS TO TEST, AND ASSEMBLE THE UNIT. MANY AMPS, SPECIALLY THOSE BLACK UNITS FROM THE '90s, use the chassis as part of the circuit and you need every screw installed to complete the circuits.

Find a way to check the signal. I suggest you to build this "signal tracer", so you can send the audio signal to a second amplifier for brief tests:
sonda_audio-2.jpg

Then, I'd check the signal at the power amp input.

Please do a visual check with a mag glass looking for broken solder connections (specially at points subject to some mechanical stress), they will look like this:

P1150456-crop.jpg

In this example, it's a board connected at 90º to the main board. the movement of the board could loosen the soldering.
 
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Only one of the main board screws go down to the metal case, the rest go to plastic stands. The amplifier board is screwed down through the fan bracket and to a plastic bracket at the rear, it doesn't appear to connect to the case directly at all. I tested the grounds and everything I touched by hand for voltage with a Fluke meter first. Touching the heat sinks and such with a grounded wire made no change, and they show zero ohms to case ground on or off.

One of the first things I did was to look super close at the solder joints, but all look good. A few look larger or sloppier than I'd have expected, almost as if they were redone at some point.
The main board on this thing is strangely barren looking, components are very sparse with the majority of the board being full of bare metal jumpers. I see three IC chips, about 20 to 30 caps, and a few resitors here and there. The lower side, facing the bottom of the case has the trace and solder. There are a ton of blank holes and printed outlines for components not installed. They must use the same board for many models?

It will be hard to follow the circuit through the board as it seems that most all traces lead back to one of the two larger IC chips.
 
I picked up another 504S this weekend at a thrift shop, for $10 I took the chance. This one plays but nothing lights up on the display and it was marked as being 100% but no rear channel output.
The rear channels work fine, as does the sub output.
In an effort to save some time on the original 504S, I swapped the entire amp module over from the one I just bought, I swapped both boards, the heat sink, and the fan.
This got my original unit up and running.
I then started removing parts and testing them off my original amp boards. I started with the four larger transistors on each board. On the top board, three of the largest transistors test bad as does one of the smaller transistors marked C1975. I stopped there for now. What are the chances this is the extent of the damage?
Since the third unit has no display, and is in generally rough shape, any poking I do is just out of curiousity since swapping the amp module got my original one up and running again. This one is too rough to bother fixing but I'd like to know what could have done the damage, especially from just sitting.
 
I'm sort of heading that way with mine as well, I'd love to find another one of these here but I doubt I'd get that lucky twice.
I don't want to risk the one working unit I've got by swapping parts.
I do think I'll pull the amp section out and start testing parts.
I've got a transistor tester here and a multi meter, that's about it when it comes to test equipment.
The problem I see is that if its not a bad cap or transistor, these things aren't fixable since they use so many IC chips.
Is there a way to test an IC chip?
Since my original receiver does play on the 'bad' channel, I'm thinking its down on power somehow. The right side sounds normal, the left side sounds like a $10 portable radio playing through a 2" speaker.
Both channels have equal volume though.
What I don't get is why on the bad channel, I get no sound at all out of that woofer, only through the mid and tweeter? The woofer is wired direct and don't go through the crossover and swapping speakers left to right proves its the amp not the speaker. (the woofer works fine on the other channel).
Yet there's not so much as a whisper from the woofer on the bad channel.
I did find an intermittent issue with the first rca cable I had first tried but I swapped that out almost immediately when I first fired this thing up.
(The left channel lead on the cable had 105 ohms of resistance compared to zero on the other lead). This was only the case when I wiggled the cable in a certain position but it was working fine on the other receiver I had been using it on.

The transistors on my two amp boards don't match each other, the boards are nearly identical but the transistor part numbers are different between the two boards? On the receiver I bought recently that works, the two boards are the same. Yet as far as I know, my original receiver has never been worked on, I bought it new myself.
I'm not sure if its worth mentioning or not, but when I tested the two large filter caps, (12,000uf each), one reads 10,840, the other reads 12,880 on the cap tester. but both numbers are within the 10% tolerance so I don't think they're at fault here. They're not perfect but I don't see them being the cause of the poor sound.
I'll pull the amp boards and do some component testing when I get back to it though.
 
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