Pioneer A443 blown up

[PJL]

This amp has been showing instability on the phono input since I got it a fair few years ago (freecycle), it screamed loudly if the volume was turned up. I decided it was a candidate for lockdown repair and took it to bits to investigate running it into headphones and replaced the rail decoupling caps on the board to no affect. What I did notice is R1 emitted smoke when screaming.

Unfortunately, I accidentally left it on and when I came back R1 was cooked and initial testing shows the transistor that drives the protection relay is kaput although the power rails and output stage all look OK.

Firstly, why is the chassis wired this way (see pics)?

Secondly, I have ordered replacement parts but still need some advice on stopping the screaming (other than don't turn the volume up).

My current guess is it is RF instability and the power transistors being capacitively coupled to the chassis have put considerable RF power on the chassis causing the R1 burn up. Does this sound reasonable?

[Radio Wrangler]

Originally Posted by PJL

Firstly, why is the chassis wired this way (see pics)?

From looking at just those two snippets and not having seen the rest, I get the feeling it's the work of some earthing theorist. There are those who worship at the altar of the star point and many other sects.

Large Japanese companies need their design departments to do something in an unusual way so that the marketing department has something to crow about. It doesn't have to confer any real advantage, in fact it can actually make things worse. It just has to be different. Like American car stylists had to do something different every year.

Where do the return connections for the speakers go?

I'm wondering if you have coupling from the common mode of the speaker cables to the common mode of the cables from your turntable.

A scope could save a lot of experimenting and deducing. You ideally want to be able to see the squeal and see what affects it.

David

[Diabolical Artificer]

R1 is an elevated ground, I did a thread about here - https://www.vintage-radio.net/forum/...d.php?t=152682 I've heard of these resistors smoking before in other amps, if your speaker grounds are forming a ground loop, at HF a lot of current will flow through that resistor. some amps used a small cap instead of/in parallel with this R, a few picofards, to cut HF gain to stop this happening.

Andy.

[PJL]

Checked the speaker and the -ve speaker connections are drawn to go to both chassis and the 0V signal (mains transformer centre tap) and would appear to short out R1. The circuit also covers country variants, HEZ, HE and HB. It has a 2-wire mains lead.

[ajgriff]

Sounds like failure of a grounding connection so that R1 becomes a series resistor. Turn up the volume and R1 fries.

Alan

[Radio Wrangler]

Aye, first place to go looking.

David

[PJL]

Chassis does not form the path for speaker power as the connection to 0V is intact. R1 would be shorted out with the chassis connection from the speaker -ve to chassis (see picture below, the -ve speaker wire goes to the transformer centre tap).

However, the headphone socket does use chassis for the -ve rail which might explain why R1 burnt up as I was using headphones.

Unfortunately, access is a bit restricted but I will need to extract the main board to replace the relay driver when the parts arrive in the post.

Hopefully, the lack of a direct chassis connection will be the cause of the phono instability, but I am not holding my breath as the circuit shows many differences for the variants that are not fully described. External board connections are identified on the circuit with a number so I am not sure how the unmarked earth connection is supposed to be made.

[G6ONEDave]

I think we have a little confusion with this due to what I take as wrong terminology. -ve is not a speaker connection ref. it is the negative of the power supply. the last circuit shows this clearly, there is no -ve connection to the speaker socket, just simply - or +. Re the phono instability, is the turntable actually connected to the mains earth or isolated from it? Given that the normal mains earth connection in order to minimise the possibility of earth loop buzzes, is to connect either the turntable or the amplifier to mains earth but not both.

[ajgriff]

If I'm reading the service manual correctly R1 was only fitted to West German models. I wonder if the amplifier originates from there?

What's the resistance between 0.0V (at the main PSU capacitors) and chassis?

Alan

[G6ONEDave]

Further to my comments, although I don't have a manual for the A443, I do have one for the A441 and the red line shown on your last attachment is actually showing the audio signal path and is not a +ve supply line. confusingley it also mentions Q301-307 as being (correctly) in the power supply but that writing is also in red print above the signal path red line. My diagram shows a 330 ohm resistor going between that red line (left channel amplifier signal output) and the left signal output to the headphones. There is also a 680 ohm resistor from the left headphone contact to 0v or chassis, not -ve. I know the A441 is not going to be 100% the same but it is not that different either.
Personally i would check that your turntable signal earth is connected to the mains earth. If not it is easy enough to connect a wire and check to see if the original problem clears. My bet is that it solves the problem, you have to decide at which end you earth the equipment, preferably at the amp.

[ajgriff]

Originally Posted by G6ONEDave

Personally i would check that your turntable signal earth is connected to the mains earth. If not it is easy enough to connect a wire and check to see if the original problem clears. My bet is that it solves the problem, you have to decide at which end you earth the equipment, preferably at the amp.

Worth checking of course but earthing issues like this normally introduce unwanted hum rather than screeching and resistor burn ups.

Alan

[tri-comp]

Hi,

Here are the schematics for the amp.
I'm wondering why nobody seems to consider the instability could be caused by defective electrolytic cap's in the amplifier- as well as in the power-supply sections ?
After all this is an amplifier 30+ years old and it wouldn't be the first time in history if instability and selfoscillation in vintage equipment was caused by exactly that.
An oscilloscope should reveal in a sec. if that's what's happening.

rgds,

/Torben

[PJL]

I have an apology to make as it is R299 (10 ohms) not R1 that is burnt up. They are effectively wired in parallel so it does not change matters.

The transistors have arrived but before I change anything I want to make sure I have identified all the faults. I have just checked and the amp is still fully working aside from the relay driver so I guess RF got on to the driver through the coil winding.

The full service manual is available here https://www.manualslib.com/download/...eer-A-443.html.

The phono stage is on a separate board, there are 6 electrolytics, I replaced the two that decouple the +/-15V supply lines, the others I left alone as they are coupling and decoupling for the FET constant current source. There is no shielding around the board which seems a bit strange for an MM/MC adaptor although none is shown on the mechanical drawing. When connected to an MM cartridge it works OK until the volume is turned up to a bit above my normal listening level.

The link to chassis in the pic below does not exist and would short out R299 (and R1). Without the link the headphone socket relies on this resistor for the -ve return.

The fault occurs without a turntable connected, in Direct mode, and is dependent on volume so the feedback source must be from beyond the volume control. I guess I could go randomly changing PSU electrolytics but it could get quite costly. Of course the other possibility is it did this from new as CD's were around when this amp was bought!

[G6ONEDave]

It might be that I need to visit Specsavers again, but having downloaded the full manual from the above link, I can't find R299 or it's counterpart R300. I have studied the circuit and the parts list to no avail. Please confirm the resistor circuit ref and where it is in the diagram, thanks

[ajgriff]

Schematic on page 6 of the manual - left hand side of power amp section. Took me a while too.

Alan

[G6ONEDave]

Thanks Alan, now having re-looked, the circuit does appear to show that R299 is actually shorted out, to my mind it can't be because it is smoking. Unless there is a high resistance to chassis elsewhere. But considering that the problem only exists during phonographic playback when at high volume, I wonder if it's simply a matter of being over driven when on phono input or too much gain being produced in the phono pre-amp stage. Got to go just got a visitor.

[PJL]

The HEZ version has a multitude of extra parts making up LC filters presumably to keep out RF interference. C725 is marked as HEZ only on the layout but not on the circuit, it is not populated on this PCB.

The only noticeable 'change' is a soldered flexible wire between wire wrap pins 27 and 21. This does not sound sensible as there are separate 0V connections for the input circuitry and the rest and this bridges them. They are AC coupled via C724 so not sure it would account for instability but I will try removing it.

As sods law will have it, the removable service panel on the bottom does not give access to the area of the board with R299 so I need to take it all to bits.

[G6ONEDave]

The other thing that I was wondering about is would it stop the resistor burn up by putting a 0.001uf or 0.01uf in parallel with it, to take any rf oscillation to chassis. Certainly an unusual design with that 10 ohm earth return in the pre-amp stage.
I mentioned the mains earth previously, as way back when, I had a similar issue with an amplifier and was told to check the mains earth arrangement between deck and amp. This trouble apparently only occurred if you did not use the same make equipment.

[PJL]

For some mystical reason, the capacitor between 0V and chassis (C725) is not included in the HE build. However, I went looking for other chassis connections and it appears there is another 10 ohm (R723) wired from 0V to chassis and that one has also failed although not so charred.

[snowman_al]

I gives you a headache reading the diagram...

I see it like this, the centre tap of the transformer (11) gives the 0 volt line. That is referenced to the chassis via the R1 6.8R resistor.
The power amp part gets its 0v from the link 20/21. That is referenced through R299 10R resistor to the chassis.
The AF complex board gets its 0v from the 26/26 link. That gives the input and output sockets their 0v connection. There is yet another chassis reference resistor on this board, that is R723 10R also.
The Phono amp assembly gets its 0v connection from J2.2 to CN2.2 on the complex board. And therefore is referenced to the chassis by R723 again. It is also connected to the chassis via R721 10R and C721 0.1uF in series and C725 0.01uF in parallel with them.

Okay you have seen R723. Too slow following the diagram...