Problem with Sony STR-6060FW. Voltage on EQ/Flat amp oscillate w/phono

I have been trying to track down this problem, and it is driving me nuts.

First off, the problem existed when it was dropped off.
Second, someone else had been working on this, and many wires are melted and a couple were in the wrong the place.
Third, I broke one of my rules and re-capped this receiver before repairing it, as well as replaced other parts. Think LeeStereo 6060F rebuild.

Any way, the problem exists with phono and tuner. AUX is fine.

When first powered up, it plays fine. After about 10 seconds the sound starts to cut out and come back. About one second intervals.

The voltage that supplies the EQ/flat amp board remains stable, while the voltages on the board start to oscillate.

I will try to explain. 18 volts. Then 17 then 19, then 12 then 20, then 5 then 22, then -5 then 25, then my Fluke can't keep up.

Thus, the sound cutting out.

Again, supply voltage is fine. This is after dropping resistors.

If I disconnect my TT, the voltage stabilizes. Plug it back in, and the problem starts over.

If I install shorting plugs on the TT input, the problem still occurs.

All caps and transistors on the EQ/flat amp (preamp) board have been replaced.

All cap polarities match pics from another rebuild.

Any ideas?

Thanks for reading.

Rob

http://www.audiokarma.org/forums/showthread.php?t=629488

Maybe some readings from #4 the thread can lead to a solution...

--

Thanks, but that's the thread I followed for this rebuild.

I have also contacted him?

This problem is very strange.

Rob

Holy sheepshit Batman, welcome to your nightmare ! That thing has quite the unusual topology. Kewl ! (I hate that word but...)

Absolutely has to be some error in the wiring, which you mentioned. Shorting plugs in the phono input causing the sound to drop out on the tuner ? It would be interesting to know if the same thing happens with the shorting plugs in the tape head inputs.

But the tuner ? Well it all goes through that stage. Damn, what a design. I knew Sony was wierd but... That phono input stage is THE input stage and the selector switch sets the gain. The 22K is for line level, the other stuff is the EQ for RIAA or NARTB. What a trip.

Anyway, after having a look, and it has been messed with, I am pretty sure the problem is in miswiring. Think of it this way - just what component being bad could cause this symptom ?

None, as far as I can tell from a quick look. Sure it is built a bit different than what we're used to but it is not from another planet. It was not designed by the US government.

I think you need a scope. There are places you can get scopes for twenty - fifty bucks. You do not need it calibrated.

What must be happening is that the shorting plugs are pulling down a DC source. (DO NOT plug a cartridge in anymore and hope that one you did is still good)

All the inputs go to a 1 meg to ground and a 0.22 uF to the transistor. It should be impossible for any DC to change when something is connected. Of course this (well not really) explains why it can happen on the tuner, which is NOT an external input.

You got an interesting one there buddy. However this is not the strangest Sony I've ever seen. The output amp in that is a bit wierd in that it uses a resistively divided bias source from the diode(s). Not right but of course it does work. But then we have to remember this is like the 1960s and they were experimenting. It is nothing like one I saw that used ONLY NPN transistors. No driver transformer. That is a difficult design to say the least and requires transistors that are matched for hfe. One of the few. Yours is not that bad.

Hey, the thing might sound great.

Look at the wiring very carefully. When they manufacture something, each wire is a certain length. Sony is an expert at it. They will give you just enough to be able to fix it. Not one millimeter more. Keep that in mind and you might see something. They have always been that way.

(I am hep to the way these companies design and build stuff, and WHY)

I would look very carefully at those phono inputs. Check see if they have DC. If they do not have DC on them, then there is some sort of a feedback loop going on when the input has lower impedance. The tuner is the tuner, phono goes through the same stage. It is possible that one of the regulated power supplies is soft. (that means the voltage is near right but not constant) you did say someting about that as well so we probably should look in that direction. Power supply problems are about the most common.

I almost want to say if you don't fix it send it to me but I am not so sure about that. If it doesn't turn out to be some sort of error, it is likely to be one of those SOBs.

I know Sony and how wierd they are sometimes. I worked for an ASC for a long time. Oh yeah, almost forgot I still work for them. I sorta drfted away, never quit and never got fired. I remember years after I worked there I applied for UI and they said I still work there. LOL. I am on good terms with them. Of course that does not mean that they would be useful for this. But i can say this much for them, when it came to what to put on the showroom floor they listended to the techs. They sold good products. However in 2015 I have nothing to say about anyone, anywhere.

Good luck. The jist of my advice is to look for mistakes. Idiocy. Errors. This symptom is strange enough that this is the course of action I would take if it hit my bench. And really, I don't really want it to. I would have to charge you an arm and a leg.

OK, an arm and half a leg.

Post back with hat you find. Pictures would be nice, maybe someone can see something out of kilter. We are the social workers and this Sony is a special case.

Thanks for the reply.

Actually, tuner does it all the time, whether shorting plugs on phono or not.

Shorting plugs on tape head have no effect.

I did find two wires in the wrong locations, and yes, there was a bit of slack on the one that went to the farther point until it was moved.

The power for the preamp board comes from a cap, and is steady. It's the voltages on the preamp board that starts swinging.

I did get some pics of the selector switches from an AK'er, but those wires seem to be in the proper locations. Though, many of the wires on the selectors have been heated up. Insulation melted off and melt marls in insulation.

My next step is to disconnect things until I find the root of the problem.

There is no DC on the phono inputs, and I have played hours of TT through it while I scratch my head.

Rob

Bumping this thread. This 6060 Rob was recapping was mine and is with me now. Here's a schematic - first one that shows more circuits but is harder to read, and then a close up of the EQ/Flat Amp board:










Observing the same things as Rob did. Tried to run some different experiments and attempted to see if there was any difference between channels.

If the shorting plugs or TT is plugged into the phono inputs, the oscillation will eventually start on its own, even if there's no input signal. When the oscillation is happening, you can see the woofer pulsing in and out. If you turn the volume all the way down, the woofers will stop pulsing. If you switch the selector from Phono to AUX, you get a "crack/pop" through the speakers, which also resets the oscillation condition temporarily.

If shorting plugs or TT are NOT plugged into the phono inputs, the oscillation will NOT start on its own and there is no "crack/pop" when switching from Phono to AUX.

When isolating the channels by only plugging the TT into one of the two phono jacks and actually playing a record, the left one is seen to be more sensitive - it'll start oscillating after 10s or so (though this time is variable and can be as long as 20-25s), whereas the right channel will only start oscillating after 45s-1min.

The floating channel with no signal doesn't show any oscillation (woofer isn't pulsing).

As Rob said, all electrolytic caps and transistors have been changed out on this board (and the problem was there before recapping). The smaller pF level non-electrolytic caps are original (C603, C605, etc).

Because both channels have this problem, it doesn't seem (to me, with limited knowledge) like it would be due to a single bad component.


With the help of another AK'er, I was able to get a whole other EQ/Flat amp board from another 6060 parts unit. I *could* swap it in there, but as you can see, there's a ton of wires that go in/out of this board, so I'd rather have a theory and investigate possible problem areas, rather than shotgun replace the whole board.

Anyone have any theories on what might be causing this?

My theory is that there is a missing or poor ground somewhere. I spent hours looking for one though and found nothing.

Also, the power supply feeding the board is stable.

Rob

You've got a lot of switching going on in all 3 of the major preamp stages: Equalizer, Flat and Tone.

The next step is to isolate which one of the stages is causing the problem. To do this, you'll need to jumper around them. Here's how:

- Isolate the Flat amp by jumpering the input of the Equalizer stage to the output. Just a jumper wire connected in series to an electrolytic cap. This will block any DC but pass the AC. That's C601 and C607 in the schematic above.

- Do the same thing with the Tone circuit. That is C614 to R637.

See if the problem continues or not.

One additional item:

- On amps of this era, the typical method of supplying multiple operating voltages was to use a dropping resistor and an electrolytic cap following it for stability. A good example is C802 and the resistor just ahead of it (that I cannot read). There's another one on the same string at C602 and R606.

- Some of these will serve both channels since they are part of the power supply distribution. Pay very close attention to them that the operating voltages on each one of these nodes where the voltage is at proper operating spec. Any variance, open resistor, etc, can produce oscillation. Ask me how I know this......

- Look around the rest of the schematic and you'll find _lots_ of these little distribution Resistor/Cap combos. Suspect any of them!

Still, the first order is to determine what stage is actually oscillating.

Cheers,

David

dshoaf said:

You've got a lot of switching going on in all 3 of the major preamp stages: Equalizer, Flat and Tone.

The next step is to isolate which one of the stages is causing the problem. To do this, you'll need to jumper around them. Here's how:

- Isolate the Flat amp by jumpering the input of the Equalizer stage to the output. Just a jumper wire connected in series to an electrolytic cap. This will block any DC but pass the AC. That's C601 and C607 in the schematic above.

- Do the same thing with the Tone circuit. That is C614 to R637.

See if the problem continues or not.

One additional item:

- On amps of this era, the typical method of supplying multiple operating voltages was to use a dropping resistor and an electrolytic cap following it for stability. A good example is C802 and the resistor just ahead of it (that I cannot read). There's another one on the same string at C602 and R606.

- Some of these will serve both channels since they are part of the power supply distribution. Pay very close attention to them that the operating voltages on each one of these nodes where the voltage is at proper operating spec. Any variance, open resistor, etc, can produce oscillation. Ask me how I know this......

- Look around the rest of the schematic and you'll find _lots_ of these little distribution Resistor/Cap combos. Suspect any of them!

Still, the first order is to determine what stage is actually oscillating.

Cheers,

David

Click to expand...

Thank you David. I just want to clarify your instructions to make sure I don't mess anything up.

In bypassing the Equalizer amp, do I need to disconnect anything to avoid the signal also going through the Equalizer amp (in parallel to the jumper wire plus series electrolytic cap)? For the series electrolytic cap, any value in the 1-5uF range would be OK?

Do you suggest bypassing both Equalizer amp and Tone ckt simultaneously, and if the problem goes away then add each of them back in sequentially to see which causes the problem to reoccur?

Relooking at the schematic, before you do the surgery to isolate the stages, focus on the inputs related to the Input Selector switch.

Stick with AUX for now. This effectively isolates the Phono and Tape Head Input and the Equalizer switch part of the circuit. This leaves the Flat amp and the Tone Control circuit. The Tone Control is passive - there's no active circuit but we still need to be sure that it is isolated.

The technique I am advocating is called, "divide and conquer". That is, you isolate as many stages in the amp and then switch each one back in individually until the problem appears again - assuming, of course, that the isolated stage doesn't also exhibit the problem.

At least you'll know what is _not_ wrong this way.

Cheers,

David

David, the AUX actually works fine (no oscillation). Only AM/FM and phono have the oscillation.

Given this, can we assume the Flat Amp and Tone Circuit are OK and the problem lies in the Equalizer amp circuit?

Consider replacing the small pF value silver mica capacitors (C603/C703 and in particular the C605/C705) in the "equalizer amp" stage with C0G types or other high quality types. In some cases, the failure of these capacitors can cause oscillation (e.g., the "black flag" polystyrene capacitors in the AU-919). Note that vintage silver mica capacitors can deteriorate when subjected to DC voltages over a very long time period and these are nearing 50 years old.

Leestereo said:

Consider replacing the small pF value silver mica capacitors (C603/C703 and in particular the C605/C705) in the "equalizer amp" stage with C0G types or other high quality types. In some cases, the failure of these capacitors can cause oscillation (e.g., the "black flag" polystyrene capacitors in the AU-919). Note that vintage silver mica capacitors can deteriorate when subjected to DC voltages over a very long time period and these are nearing 50 years old.

Click to expand...

Thanks Leestereo. That was my initial thought too, not knowing anything about the circuit, but they are the only caps in that circuit not already replaced. I actually went out to buy replacements, but they're not easy to find locally in that value (except ceramics, but I didn't want to go there).

But when I saw both channels actually oscillated, I thought it was less likely that the silver micas on both channels went bad. But I'm game to replace them anyway.

Something like this would work, right?
http://www.mouser.com/ProductDetail...GAEpiMZZMtLiKaZgV7flU44AUKbBD/XtR5x1dAWp%2bI=

captouch said:

... I actually went out to buy replacements, but they're not easy to find locally in that value (except ceramics, but I didn't want to go there).

But when I saw both channels actually oscillated, I thought it was less likely that the silver micas on both channels went bad. But I'm game to replace them anyway.

Something like this would work, right?
http://www.mouser.com/ProductDetail...GAEpiMZZMtLiKaZgV7flU44AUKbBD/XtR5x1dAWp%2bI=

Click to expand...

You could install the cheap ceramics to see if that resolves the oscillation. It may be that the oscillation is only in one channel but that this causes power supply fluctuations which impact both channels.

The 500pF CDE silver mica you link to would work, although it might be physically large for the available board space. You can use a 470pF (within the 10% tolerance of the original 500pF silver mica) C0G such as: http://www.mouser.com/ProductDetail...GAEpiMZZMvsSlwiRhF8qrwg2mLPQyq3eL%2b29aj9JpM= or a 470pF polypropylene: http://www.mouser.com/ProductDetail...=sGAEpiMZZMv1cc3ydrPrFwgzP98AO9idjWfg/Hy/VfI=

So, all of this happens on both channels and at the same time? And it happens only on Phono and not Aux. What happens if you select Tape Head?

Switching of the feedback networks is changed with the Function Switch, S4. Even if the oscillation remains, does it change characteristics?

Oh, and get over to HSC (www.halted.com) just off San Tomas where Central Expressway crosses it. That's your most extensive source of small signal and value caps in the Bay Area. Brand news ones would come from Jameco up the peninsula.

Cheers,

David

It also happens with FM.

Rob

dshoaf said:

So, all of this happens on both channels and at the same time? And it happens only on Phono and not Aux. What happens if you select Tape Head?

Switching of the feedback networks is changed with the Function Switch, S4. Even if the oscillation remains, does it change characteristics?

Oh, and get over to HSC (www.halted.com) just off San Tomas where Central Expressway crosses it. That's your most extensive source of small signal and value caps in the Bay Area. Brand news ones would come from Jameco up the peninsula.

Cheers,

David

Cheers,

David

Click to expand...

Note: Edited for clarity after initial posting:

Yes, as Rob said, it will happen with AM/FM as well. I haven't tried tape head, but I could using TT signals.

It will happen on both channels with AM/FM, if inputs from TT are input on both Phono inputs, or if shorting plugs are put on Phono inputs and selector is set to Phono (though it takes longer when just shorting plugs are used, and Left channel will start oscillating first). Both channels will also oscillate if a single TT input is applied to one of the Phono inputs and a shorting plug is used on the other.

Only one channel will oscillate if only one TT signal is applied to one of the Phono inputs and the other is floated (NO shorting plug). This is true whether the Left or Right channel has the TT signal applied to it.

So it seems having a shorting plug present is similar to actually inputting a signal (though it will take longer for oscillation to start), whereas floating an input effectively prevents it from entering the oscillating state.

NEW INFO: The Tape Head inputs will NOT oscillate with inputting signal from TT into both Tape Head input jacks. Been playing a record for 10-15min or so, with no signs of oscillation.

Also tried to plug in shorting plugs only into the Tape Head inputs, and also no oscillation.

So the oscillation seems to be limited to AM/FM or Phono only, even though Tape Head also uses the Equalizer amp (I think).

The Tape Head input does use the Equalizer Amp, but doesn't use the RIAA equalization feedback, check the components C608, 609, R618 and R619.

Leestereo said:

The Tape Head input does use the Equalizer Amp, but doesn't use the RIAA equalization feedback, check the components C608, 609, R618 and R619.

Click to expand...

I'll take a look for anything unusual. Does AM/FM use C608/609 and R618/619 too? Just trying to connect the dots since AM/FM is the other mode that oscillates. Though I can't say for sure that is oscillates in exactly the same way as I can't definitively see the woofers pulsing like I can with phono. In the case of phono, I can lift the needle to remove the signal and then see the woofers pulsing by themselves, but with AM/FM I can't remove the signal without changing modes/functions which resets the oscillation. But the 'with signal' characteristic of sound being distorted and going in and out of each speaker is the same.

Side note (but related): I've been trying to figure out how to read the selector switch portion of the schematic, but I don't know how to interpret it, so I can't answer that question myself.