Gently winding up Variacs

>"So, when using a variac, a condition must exist where the needed reverse bias >is missing but the V-FETs have enough DC supply available to fry themselves if >held there"

I understand that, but there should be a way to design around it, prevent it from happening. Piss poor IMO. Asking for trouble. How many times the power went out and came back dim, like half voltage. And varied. Would they cover that under warranty or would I have to sue the power company ?

They might be able to design a lower distortion circuit than I but they leave much to be desired in practicality, especially in a supposedly high end unit. If they are going to charge top buck for this shit they should have included a UPS. Or something.

I do not mind making money off of their follies, but that puts my name on it. What, I gotta install a light sensor to see if the customer's lights dim which voids the warranty on the work ? Ridiculous. Tell you what, they are scrap on sight as of now.

Fuck, there are so many ways to prevent that failure mode it is ridiculous. One stupid power relay that removes the rail voltage form the outputs would do it, and then they wouldn't need one at the speaker output. Very low cost.

Fucking assholes. I really used to like Sony stuff, but not so much their audio. I didn't like the sound of it. Now I am glad, I didn't buy that thing, I didn't buy the highly coveted STR-V6 I had on loan for like a couple of weeks. I did buy one of their cassette decks though and it was alright. And I do still have their (real) VCRs. And SLV-920HF I need to lube before firing up and an SL-HFR60 with the processor. I have a shitload of beta tapes.. I am looking for a good deal on a standalone DVD recorder so I have real backup and then I intend to rip to harddrive certain things., I will be a youtube star, Man those times were wild. Like when I almost got run over by a car taping an outdoor concert on someone's porch. Instructions on how to roll a joint. Damn, there is so much I forgot, but the deal is this is so old that I might only get one chance, so I am not sitting here playing it. In fact I owuld like to have a beta rewinder so I don't have to beat on the reel motor, they were a high failure item.

But after that, Sony and I are through. I will never buy a new product from them again and I have serious doubt about a used one. Yes, I bought that thousand bucks worth of VCRs brand new.

But now that we're on the subject, it might be a good idea to mention other things that do not take kindly to a variac or a DBT. They are out there. I can see that some of the class D amps would not like it. Things with SMPSes. What else ? I am sure there are more but I just don't know what.

 

[Phil Allison]

jurb...@gmail.com wrote:

"So, when using a variac, a condition must exist where
the needed reverse bias is missing but the V-FETs have
enough DC supply available to fry themselves if held there."


I understand that, but there should be a way to design around it,
prevent it from happening. Piss poor IMO. Asking for trouble.

** Only if you use a variac with one.

How many times the power went out and came back dim, like half voltage.
And varied.

** I have seen that happen only once in my life, about 20 years ago.

Of course you need to have the amp switched on and not bother to turn it off when the power outage happens. Prolonged brown can damage stuff, like the compressor motors in fridges - and they are always left on 24/7.

Fuck, there are so many ways to prevent that failure mode
it is ridiculous. One stupid power relay that removes the
rail voltage form the outputs would do it, and then they
wouldn't need one at the speaker output. Very low cost.

** The amp is already bristling with add on circuits that constantly tweak the bias, protect the V-FETs from current overload and stop DC appearing at the speaker terminals. All not shown in the simplified schem I linked.

FYI: the DC bias across R737 is anywhere from 20 to 50 volts, depending on the particular V-FETs actually fitted, the ambient temp and the DC supply voltage.

Fucking assholes.

** Crikey, it don't take much to wind "jurb" up.



.... Phil

 

[Phil Allison]

jurb...@gmail.com wrote:

http://www.angelfire.com/sd/paulkemble/sound8f.html


There are a couple of things about that amp that don't make a hell of alot of sense. First of all, they are current sharing without source resistors, at least separate source resistors. you said there aren't many that can do that, apparently these are among them. And they use two source resistors, damn, for an extra ten cents they could have just used three.

** The parallel 0.47ohm resistors are there for the VI limiter circuit ( not drawn) saving the V-FETs from destruction if driven into a low impedance or short.

Sony failure modes have been a bitch for a long time.
Seems like they used to love selling alot of parts.

** Sony seem to like showing everyone how clever they are with over-complicated circuits and mechanics too.

Reminds me a lot of Grundig, Crown and Dynacord.


.... Phil

 

[Cursitor Doom]

On Mon, 24 Aug 2015 23:38:28 -0700, jurb6006 wrote:

But now that we're on the subject, it might be a good idea to mention
other things that do not take kindly to a variac or a DBT. They are out
there. I can see that some of the class D amps would not like it. Things
with SMPSes. What else ? I am sure there are more but I just don't know
what.

Hopefully the device Phil mentioned is unique in this respect!
So far as SMPSs are concerned, though, I don't believe they can be
actually *damaged* by under-voltage; not start/function properly, yes,
but damage? I'm very doubtful about that.

 

[Phil Allison]

Cursitor Doom wrote:

So far as SMPSs are concerned, though, I don't believe they can be
actually *damaged* by under-voltage; not start/function properly, yes,
but damage? I'm very doubtful about that.

** There are certainly many SMPSs with start up modes that create brutal current surges for the switching mosfet or mosfets. The electros on the secondary side of the switching tranny present a near short until charged and there has to be some effective means built into the circuitry to limit peak currents until this happens.

Using a variac, it is sometimes possible to find a setting where the PSU attempts to start, surges repeatedly, but cannot successfully run - likely because the secondary voltage does not rise enough for the control loop to stabilise.

Reckon it is wise to avoid lingering at such a setting.


.... Phil

 

>"attempts to start, surges repeatedly, but cannot successfully run..."

Plus they can do that all by themselves. Many have a low current mode, basically a "wait to start" that allows a 50K resistor or whatever to bleed up a cap big enough to run the main chopper enough to start and then a rectifier coming off a, well tertiary primary ? Works. the it feeds the driver chip. When that filter gets ESR or low capacitance, then this happens even at rated line voltage.

However this does not always cause a catastrophic failure. Some units can sit there for weeks and still only need that one cap. And others could be alive one day and dead the same day. It all depends on what is critical and what is not.

 

>"** The parallel 0.47ohm resistors are there for the VI limiter circuit ( not >drawn) saving the V-FETs from destruction if driven into a low impedance or >short. "

I saw the real schematic, I have it. Yes it is a doosey (sp) Runs you all over the place but oh well, I still got the jist of it. The VI limiter circuit as you call it, which I would simply call a current limiter (with an enhancement to allow said current at higher output voltages)should be able to handle this. Usually it is basically called a dual slope detector, but what Sony likes to do apparently is to get ten nerd who actually did solve not only that rsistor cube, but that other thing going around on the net.

Why do you want to get every last iota of power or whatever of whatever you use ? Well basically because you got shareholders. Every old thing we own is their mistake.

I used to be almost Mr Sony but did not want their speakers and most of their amps I could afford. I worked for a Sony Signature dealer and ASC, I also worked for another Sony ASC a bit later. Well actually at the same time which they didn't want me to do but so what.

but I was always with it for their video equipment, but I did not like the Access systems, I thought they were junk. And then it seems they got skitzoid, like almost being two companies. Alot of them got like that, they got the cheap line and they got the good line. Let's run that credit card and see what you want. I shit you not.

Where were we. (damn that was easy to type)

Anyway, thusfar we have been dealing with a certain paradigm. Now here is something, and it is not even solid state. Tube/valve circuits that use those regulator tubes. the gas fired ones like 0A2 or some shit, I don't remember. It is lucky there are not alot of these around I think, with all the variacs and DBTs out there, I really think that some of those circuits might be damaged bringing up the voltage slowly.

This would happen if say the main rectifier tube took a certain time to warm up and did not give the 0A2 enough voltage fast enough to catch the regulator tube and blew an electrolytic somewhere. Or possibly cause an arc in a totally innocent ECC83 somewhere.

 

"
Using a variac, it is sometimes possible to find a setting where the PSU >attempts to start, surges repeatedly, but cannot successfully run - likely >because the secondary voltage does not rise enough for the control loop to >stabilise."

Not to forget the days of the earlier bipolar ones that needed real drive current of the FET ones that needed real drive voltage. (like that bastard Sony)But actually I mean regular MOSFETs. I they didn't get a certain voltage drive they would run away, literally.

>"Reckon it is wise to avoid lingering at such a setting. "

For what reason would one linger at a setting ? I can understand under certain conditions doing advanced Tshooting, but just firing up old stuff is different and they have NO reason to linger at any setting. If the current goes up too much you shut the motherfucker down now. If it is worth using a variac on it is worth using it right. Godammit.

But really you meant what I said, or some shit. That current climbs abnormally you shut down and try to figure it out. If nothing else start disconnecting shit.

And actually the DBT is a pretty good current meter.

What I want is some incandescent light bulbs from the UK or whatever, and of course sockets to match, because they are 240 volts. sometimes I needed to discharge a cap that I KNEW was at like 300 volts, and I do not mean on tubes, I mean on like a plasma TV. Using a 120 volt bulb is not all that cool, it could fail leaving you with an insidious problem.

I have considered two in series but if I ever want to change the wattage that makes a mess, and I do not want any switches.

I will do it in digital. Almost. Banks of bulbs, all controllable by unscrewing the lead bulb in any series. And there are a few series' which make an array.

Are you with me here ? This is going to look better than a Gene Wilder movie.

 

[Cydrome Leader]

Phil Allison <pallison49@gmail.com> wrote:

Cursitor Doom wrote:



So far as SMPSs are concerned, though, I don't believe they can be
actually *damaged* by under-voltage; not start/function properly, yes,
but damage? I'm very doubtful about that.


** There are certainly many SMPSs with start up modes that create brutal current surges for the switching mosfet or mosfets. The electros on the secondary side of the switching tranny present a near short until charged and there has to be some effective means built into the circuitry to limit peak currents until this happens.

Using a variac, it is sometimes possible to find a setting where the PSU attempts to start, surges repeatedly, but cannot successfully run - likely because the secondary voltage does not rise enough for the control loop to stabilise.

Reckon it is wise to avoid lingering at such a setting.

Television sets (with CRT) really don't like brownouts either.