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Old 8th May 2017, 11:36 pm   #141
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Default Re: Sony HP-511 - Blowing Fuses

You may have sorted it with a set of beefier transistors, or maybe not?

What other possibilities are there?

Could it be the preamp misbehaving on startup? Though the RCRC network heading off to power it ought to be fairly gentle.

The VAS load above the Vbe multiplier is bootstrapped, I wonder if the bootstrap C is healthy? Poles can go walkies if they are not good.

OK, play it backwards.... how would you destroy a set of output devices and their drivers if you wanted to?

Reverse voltage applied base to emitter is the easiest and takes least energy. That would imply something moving faster at the VAS, faster than the output pairs could follow, but one base-emitter would act to clamp the reverse voltage applied to the other.

Open-circuit Vbe multiplier would lead to a bake-off but that would be slower and likely leave something too hot smells about the thing.

Open circuit Zobel damper? that could allow a bit of hooting... Ah, I notice it hasn't got the matching series damper (usually a resistor with a dozen turns of magnet wire across it.

It's job is to isolate the amp from capacitive speaker cables and crossovers.

naim got this wrong. the designer felt that an output inductor would slow his amplifier down, so there wasn't one. So when Mr trendy audiophile saved up and paid a ton for an uber-trendy pair of mystical loudspeaker cables woven in many interleaving strands by a guru living up a far distant mountain, the secret only imparted to his apprentice, Grasshopper, it wasn't long before a very expensive amplifier had a very expensive fry-up.

I assume you're just running it with some quite plain ordinary wire to a bench speaker or a resistive load?

There are'nt many capacitors Just swapping them all might be a bit desperate but cheaper than another set of transistors.

Sorry, no bolts of inspiration.

David
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Old 8th May 2017, 11:38 pm   #142
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Default Re: Sony HP-511 - Blowing Fuses

Hi Dave

I've attached a picture of the original C1364 that blew when I plugged it into the wall previously. I think it appears the same as the one I replaced it with...

The BD124s could well be a bad match for it. If you want to try some alternatives I can order a few in? The Bux66 looked a good alternative I think?

I could grab some electrolytic caps to replace those in the board to rule those out?

Wish I could be more helpful.

Gareth
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Old 8th May 2017, 11:57 pm   #143
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Default Re: Sony HP-511 - Blowing Fuses

Where are you buying your transistors from?

There are a lot of fakes around, you need to buy from a good supplier like Cricklewood Electronics or Farnell or RS components.

I've had blow ups on amplifiers and TV's because of fake components from some of the more dubious outlets in the past
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Old 9th May 2017, 12:53 am   #144
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Default Re: Sony HP-511 - Blowing Fuses

BD124 isn't a good substitute. It has 5v less voltage rating, and has less than half the gain of the 2SC1364 (Japanese convention is to not bother printing the '2S' on the part)

2SC1364 is a small-signal transistor rated for up to 200mA, BD124 is a power transistor rated to 4 amps, and its best current gain will be at much higher currents than the 2SC1364 is being run at.

BC182 would be a lot more like it.

As Michael says here are people who will happily flog you any old junk with a popular type number written on it.

Fake parts will hare you runing round and round in circles....


David
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Old 9th May 2017, 7:56 am   #145
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Default Re: Sony HP-511 - Blowing Fuses

Michael I purchased the parts from CPC I think they were called. The outputs came from eBay. Probably not the best place to get them from...

David, I think we maybe getting confused, the BD124 was a replacement for the power outputs D291 (0509 & 0510). BC182 looks like the c1364?

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Old 9th May 2017, 8:24 am   #146
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Default Re: Sony HP-511 - Blowing Fuses

Ah the confusion came from reading post 142.

Yes the BC182 looks as close as need be for the 2SC1363 (C1364 is just the shortened version for the marking on the part)

Watch out for different pin formats for e b and c on small transistors, but electrically it'll be a good substitute.

David
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Old 9th May 2017, 8:43 am   #147
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Default Re: Sony HP-511 - Blowing Fuses

Looks like your making progress Dave, nice work. Would fitting a NTC thermistor help?

Andy.
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Old 9th May 2017, 12:09 pm   #148
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Default Re: Sony HP-511 - Blowing Fuses

David, I noticed the lack of output inductor too, but I think there is probably enough length of skinny wire to / from the speaker selector switch to count! There is conventional wire on the speaker although rather thicker than standard. I did test the (original) bootstrap cap, before the last blow up, in place - good for ESR and capasitance but 25V seems a little cheap skate to me ? I didn't test the other R/C to gnd on the base of the upper driver. Is that there just for startup to drag the whole chain down a bit during power up as that R/C is well below any audio frequency ?

Andy, There is no bias thermal-feedback (nor adjustment) so I figure the bias goes up with heatsink temperature. I think 2 x 2mV/C accross 2 x 1.5R gives 1.3mA/C bias increase! The heatsink looks plenty big enough to cope (I guess between 1 and 2 C/W). Anyone care to do the maths to work out the temperature rise ?

Michael, I have one good BD124 left to test and I will try taking a hammer to the dead ones to see whats inside

dc
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Old 9th May 2017, 12:37 pm   #149
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Default Re: Sony HP-511 - Blowing Fuses

25v is penny pinching, I think too.

The RC to ground on the upper base I hadn't thought of as a start-up delay, but it could well be. I'd seen it as a load on the VAS to limit the gain that would be generated by the boorstrapped collector load. It looked like a design afterthought of the sort which pops up after stability issues are found in development!

Good work, Dave!

David
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Old 9th May 2017, 5:33 pm   #150
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Default Re: Sony HP-511 - Blowing Fuses

Forgot to mention, the bias tranny is NOT physically attached to the heatsink so it doesn't track temperature !

dc
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Old 10th May 2017, 10:37 am   #151
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Default Re: Sony HP-511 - Blowing Fuses

Inconclusive on the BD124's. The remaining one is old and crusty and the 2 that blew are shiny and look totally different. Attacking a faulty one with a pipe wrench and pliers only served to prove just how tough the case is! I just don’t have the metal working tools to do a better job ☹
Just out of interest I ran the remaining BD124 against an equally crusty 2N3054 and it looks pukka within the capabilities of my tracer …

These are 100R collector load / 50V collector sweep and 1mA steps. I neglected to calibrate the vertical scale

dc
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Old 10th May 2017, 2:58 pm   #152
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Default Re: Sony HP-511 - Blowing Fuses

Probably for the best. Some power transistors contain Beryllium Oxide, which it is not a good idea to be breathing in.
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Old 13th May 2017, 1:59 pm   #153
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Default Re: Sony HP-511 - Blowing Fuses

I'm sure these transistors don't contain any BeO

I've redrawn the schematic, as attached. i'ts a bit fuzzy at forum resolution but I think it shows the topology better than the service manual.

Testing on a current limited PSU now indicates some nice busts of oscillation can be provoked.

dc
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Old 13th May 2017, 2:12 pm   #154
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Default Re: Sony HP-511 - Blowing Fuses

This could be caused by the new transistors having higher turnover frequencies than the old ones, if the original design relied on the transistors not having enough gain to sustain an oscillation at high frequency.

Alternatively, the current-limited supply could be contributing to the oscillation itself going from CV to CC mode ..... Not sure how to mitigate against that.
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Old 13th May 2017, 3:26 pm   #155
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Default Re: Sony HP-511 - Blowing Fuses

I notice that there isn't a small C across the Vbe multiplier.

C660 and R664 are simply the shunt part of the feedback attenuator with C660 responsible for overall unity gain at DC. C664 and R669 look to be afterthoughts. Now why? They look to be there to handle some problem found with the output capacitor, C668.

Other sources of oscillation can be more local than involving the whole loop. Ferrite beads on the bases of some of the devices?

David
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Old 13th May 2017, 3:30 pm   #156
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Default Re: Sony HP-511 - Blowing Fuses

Sun not so bright now so I can get a shot off the scope.


With a 1KHz square wave input, below oscillation level, both channels look identical. As the level increases, one channel starts to show oscillation on the +ve going peak with slight compression. The attached shot shows what this looks at the zobel network (R567) where there are a series of chirps at 1mS intervals - the photo is zoomed in on part of a chirp - something around 6MHz.

It certainly is possible to get oscillation from the PSU CC / CV as the current pull jumps up and this looks like a very severe motor-boating at a few Hz

I imaging the problem is the lower complimentary pair is trying to turn back on (due to some parasitic effect) as the upper pair are pulling the output +ve and increasing the voltage across the lower pair. The commutation current would then destroy both outputs given the chance.

dc
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Old 13th May 2017, 3:47 pm   #157
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Default Re: Sony HP-511 - Blowing Fuses

In case anyone wants to play, I have an LTSpice model
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Old 14th May 2017, 5:31 am   #158
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Default Re: Sony HP-511 - Blowing Fuses

I'm glad you had a look at this amp Dave, fixing this would be way beyond mine, Roger's and Gareth's ability's. As this amp is nothing special would it be easier to replace the power amp section with cheap and cheerful off the shelf modules? Or is it now a challenge?

A.
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Old 14th May 2017, 4:39 pm   #159
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Default Re: Sony HP-511 - Blowing Fuses

Andy, It's a challenge of course !

Anyway hopefully some progress, I've seemingly managed to tame the tendency for 1 channel to turn itself into a transmitter! It seems a cap across Vbe multiplier does the trick (this is almost universal in amps like this, as David pointed out). I tacked 100nF ceramic on and no more oscillation, at least with my 16R dummy load.

I had already tried a couple of changes to the lower complementary pair (driver out / curve traced, switched to a BD140, sprinkling of ceramic caps) but it had made no difference whatsoever. Was starting to think maybe the upper darlington is the problem ...

dc
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Old 14th May 2017, 6:09 pm   #160
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Default Re: Sony HP-511 - Blowing Fuses

The Vbe multiplier acts as a current limiter due to its limited base current and gain. there is a high frequency path from the output via the bootstrap capacitor and there is a lag into the VAS and its compensation capacitor. The whole thing has poles which wander around depending on high frequency signal levels.

So the capacitor across the Vbe multiplier has a job to do. I also prefer to do the compensation pole with capacitors to ground from the VAS collector rather than using one in the Miller position.

There are clues in this circuit that Sony had trouble with stability. Once you're happy with it, you might want to mod up the other channel, and a write up for the benefit of others who may follow would be good.

David
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