Pioneer A443 blown up

[PJL]

And the strangest thing of all is the headphones are connected to chassis and not 0V along with a CR network that is connected to each channel. The CR network from the outputs to chassis will put AC on the chassis. I am really not all that surprised it is unstable.

I really don't understand what most of this stuff is for. Apart from the headphones, all inputs and outputs connect to 0V and the mains lead does not include earth. There is a chassis connection for the turntable earth but that is for screening not safety earth. To be an 'elevated earth', the chassis would need to be earthed but it isn't and any 2 connected earthed peripherals would connect together via the 0V rail.

Seems to me this design has gone through a few iterations and probably started with an earth connection but is left with lots of legacy components.

[ajgriff]

Although the schematic's not easy to follow all the grounding connections lead tortuously back to the 0V rail and the chassis. The offending resistors are both connected between the 0V rail and chassis so in theory there should be no appreciable voltage drop across them. To my simple mind for them to char or burn there must be a discontiuity somewhere in the 0V line or chassis grounding connections. This could be at one of the plugs/sockets connecting the boards or possibly a corroded or loose screw holding a board in situ. Happy to be shot down in flames, just like a resistor, if I'm mistaken. None of this directly explains the instability/oscillation issue but I have a sneaking feeling that the two problems are linked in some way.

By the way this is clearly a Class 2 device in which one wouldn't anticipate meeting a mains earth connection. I also think that this a classic illustration of how circuit diagrams do not represent anything like the physical reality which is why PCB layout diagrams are often included in service manuals of course. In this particular case they are both a complete nightmare to interpret, for me anyway.

Alan

[PJL]

Imagine the thing has gone unstable and is outputting 40V RMS at a high frequency (say 100KHz+) on both channels. C403/R407 and C404/R408 will then put significant power onto the chassis and that will all pass through R299 and R723.

[ajgriff]

Just to help my attempts to understand this, does the amplifier scream with speakers connected rather than headphones?

Alan

[Radio Wrangler]

The way schematics are drawn for most mainstream japanese audio products in a nightmare even for the very experienced.

You want to look at some of the amateur radio transceivers as well they are an order of magnitude more complex and they really go to town. The block diagrams are awful as well. They just build a regular matrix of rectangular boxes then play connect the dots. They all look the same. Do they illustrate the structure, no, not at all. You need a ruler to read them, just like the circuit diagrams. Saved paper I suppose, but at the price of not actually doing what they were needed to do.

Looking at the circuitry in many of these products, you come across occasional evidence that someone very good was at work, but a lot of the time it's mediocre, sometimes seriously incompetent.

I was told that some of the amateur radio equipment firms farmed out sections of design work to universities, and that the universities used students to do it. Some students are great, of course, some aren't, but all are inexperienced. I wonder to what extent this is true. It would explain some absolute howlers I've seen in circuit diagrams.

Anyway, just because it has a well known name top left on the panel, don't assume that all the design work in it is competent.

Having resistors between two notionally grounded points which also have a metal path between them is weird. There are three possibilities:

1) someone has a weird theory about something

2) someone hasn't a clue

3) It was found to be unstable and someone in experimenting randomly found that sticking a resistor just there would tame it.

With some products you may find that they were designed pretty competently, but ran into problems and you find evidence of sticking plaster solutions that don't really touch the root cause but must have been found to help just enough, but with no logical reason behind them.

So while I suspect that there is some fault in that unit, a dirty connection, a severed ground wire in a bundle, something like that, I also think those resistors seemingly shorted in the grounding structure are solid evidence that Mr Pioneer has also had trouble with this model.

David

[PJL]

David, there is a tendency to assume that just because something was mass produced that it went through careful design. Maybe they used an external resource to take it through certification, there are a multitude of changes that must be related to VDE.

I am inclined to move the connection marked 26 from earth to 0V. This would take the headphone return and CR networks off chassis. The alternative would be to implement the 0V to chassis link shown in the circuit diagram but not implemented. I need to check the headphone socket is isolated form chassis.

[ms660]

The ground (0v) circuits are easy enough to follow in the schematic, they are shown drawn with thicker lines than the others, so far as I can make out from the info given in the manual the resistance between the 0v rail and chassis is 5 Ohms except for the HEZ version where it's approx. 2.9 Ohms.

Lawrence.

[Radio Wrangler]

Point 26 is the ground point for the 'Zobel' networks, though they are odd, with the shunt (RC) arm put after the series (RL) arm.

A normal Zobel arrangement uses a low value inductor to isolate the output of the amplifier from cable capacitance at increasing frequencies. There is a lowish value resistor across the inductor to damp the Q of any resonance created. The RC arm is used to provide, at higher frequencies a resistive load to the amplifier to help its stability and also help in damping resonances. In the event of something creating a lot of high frequency energy there will be high frequency current to point 26, limited by the resistors.

There are things in this amplifier done in peculiar ways, when the more common ways are known to work and have reasoning behind them.

My suspicion is that this amplifier design is somewhat marginal, and that there is something in your individual that pushes it over the edge. It might not take much to do this, so it could be something that if spotted could easily be dismissed as inconsequential.

David

[ajgriff]

Originally Posted by ms660

The ground (0v) circuits are easy enough to follow in the schematic, they are shown drawn with thicker lines than the others, so far as I can make out from the info given in the manual the resistance between the 0v rail and chassis is 5 Ohms except for the HEZ version where it's approx. 2.9 Ohms.

Lawrence.

How can that be when the 0V rail is also shown as being connected to ground? An examination of the physical reality or checks with a meter would clarify one way or another.

Alan

[PJL]

Originally Posted by ajgriff

How can that be when the 0V rail is also shown as being connected to ground? An examination of the physical reality or checks with a meter would clarify one way or another.

Alan

Lawrence is correct, it is 5 ohms as that connection to earth does not exist. Shame we can't see the various iterations of the circuit as it evolved!

[ajgriff]

Originally Posted by PJL

Lawrence is correct, it is 5 ohms as that connection to earth does not exist. Shame we can't see the various iterations of the circuit as it evolved!

Does a DMM resistance reading between the junctions of the smoothing capacitors and chassis confirm that?

Alan

[PJL]

Originally Posted by Radio Wrangler

Point 26 is the ground point for the 'Zobel' networks, though they are odd, with the shunt (RC) arm put after the series (RL) arm.

A normal Zobel arrangement uses a low value inductor to isolate the output of the amplifier from cable capacitance at increasing frequencies. There is a lowish value resistor across the inductor to damp the Q of any resonance created. The RC arm is used to provide, at higher frequencies a resistive load to the amplifier to help its stability and also help in damping resonances. In the event of something creating a lot of high frequency energy there will be high frequency current to point 26, limited by the resistors.

There are things in this amplifier done in peculiar ways, when the more common ways are known to work and have reasoning behind them.

My suspicion is that this amplifier design is somewhat marginal, and that there is something in your individual that pushes it over the edge. It might not take much to do this, so it could be something that if spotted could easily be dismissed as inconsequential.

David

Agreed, but why run it through the chassis so that it must pass around the chassis to the two 10 ohm resistors to reach the 0V rail? This looks so wrong! I got this amp from freecycle about 10 years ago and it has always had the instability, it was 'not working at all' probably due to the missing links from the pre to power amp and it may never have been used with a turntable by the original owner.

[ms660]

Originally Posted by PJL

Lawrence is correct, it is 5 ohms as that connection to earth does not exist. Shame we can't see the various iterations of the circuit as it evolved!

I'm wondering if it should, maybe via a panel fixing screw?

It might be that the resistors could be for damping some wiring/PCB track inductance?

Lawrence.

[PJL]

Maybe we need a vote:
Option 1: Connect 0V to chassis
Option 2: Connect link 26 to 0V rather than chassis
Option 3: Leave it alone. (although I am not going to do that!)

Lawrence, the documentation is generally pretty good. There are 3 earthing points on the main board, 2 via mounting screws and one to the back panel which emerges on a pad between the LINE and PHONE sockets. All are making good connection.


I have replaced the broken bits and will try it again

[ms660]

I'd noticed the circle denoting the rear chassis connection for R723 on the board layout then I noticed what looked like another circle just above connection point 20 on the board layout at the loudspeaker output, that's what prompted the question.

Anyways no worries.

Lawrence.

[PJL]

There are 3 unpopulated holes which I guess are for wire wrap posts. There are also two parallel rows of holes on the same area which have a link inserted to denote the build - electronic designers having fun with auto component insertion .

[ms660]

There looks to be three in a line in some of these photo's:

https://insidehifi.blogspot.com/2012...oneer-443.html

Those are the best photo's I've found so far.

Lawrence.

[PJL]

Put it back together with the new components AND it no longer squeals. My best guess is those resistors had already smoked before and had already drifted high, they measured about 900 ohms to chassis.

I still think there are some serious design faults but I am not going to mess about now it is working OK.

[Radio Wrangler]

Well, that's a result, then. It looks like the design proved rather marginal and Pioneer did a lot of bodgy hacking around to try to make it saleable.

I wouldn't class it as a keeper

High frequency oscillation can destroy tweeters. They are delicate little things intended for only a fraction of the power woofers get, so it doesn't take much to melt things.

A model to be avoided. I once started a thread 'Do not resuscitate' to list sets best not bothered with. You can imagine the kerfuffle it caused.

David

[PJL]

Putting headphones through 1/4W resistors does not fill me with confidence but it is 30 years old and has always worked fine with CD and radio, and no digital stuff or rare chips. What gives it away is the over-size volume control knob that attaches to a tiny PCB mount potentiometer and the terrible interboard connectors that work off solder dipped lead ends.

I now need to get back to fixing some of my other broken stuff, tek 454a scope that has lost a channel (these scopes are awesome), that dreaded Marconi spectrum analyser, refurbishing the Marconi Siggen that I bought parts for several years ago...