Sony HP-511 - Blowing Fuses

[Michael Maurice]

There is a way of eliminating the preamp section.

The tone control and loudness control is a passive design. The preamp works with all inputs EXCEPT tape.

So feed a source into the tape sockets and switch the tape monitor to on, if there is still distortion in the one channel, the fault is in the power amp, if no distortion then the fault is in the pre-amp.

The power supplies are common to both channels.

[Radio Wrangler]

That's a much more elegant method.

It's something that needs doing next or else you have a mathematical 50% probability of chasing distortion in the wrong section (Murphy takes a creative view of that 50%)

David

[RojDW48]

Thanks Michael - that's the kind of elimination procedure even my poor brain can cope with - good thinking!

And, Dave, many thanks for getting in touch. I am sure one or both of us will be knocking at your door. Things are looking up!

[gazrawly]

Thank you so much for your help once again guys.

I have just turned the set on and tried running the aux through the Tape and I get the same results; heavy distortion on the one channel. Interestingly, if I leave the input playing and switch the set off, I get crystal clear audio for about 2 seconds as the set is powering off.

So using Michael's suggestion of the Tape input, I am presuming that the fault is within the poweramp somewhere.

Gareth

[GP49000]

It IS in your power amplifier.

I have to admire your tenacity. By now, if it were my amp, it'd be apart on the bench and I'd be poking around with a multimeter. My guess is that within the power amplifer, which includes a Class-A input stage, driving a Class-B driver/output stage, the problem is in the latter. Speculation only: could be that due to an underlying fault, a protection circuit is shutting off drive to the Class B section to protect it and your loudspeakers from damage; or the underlying fault itself is cutting off the Class B section when the DC power supply voltage is "Normal" but allowing a signal to pass as it decays away after shutoff. It could be a burnt resistor that isn't allowing the full DC power supply voltage into a section of the amplifer, or a failed or leaky transistor, or any of several other things.

I know...this doesn't really help! But it's what I would look for, on the workbench, with a multimeter, perhaps a schematic..and years of having done it for money, for real paying customers!

[gazrawly]

I have checked all the resistors as best I can, held a magnifying glass infront of the components on the power amp board, measured resistances and played around but I really am stuck and sick of having it sitting on the dining room table. I was quite tempted to remove all of the capacitors on the board today and check them but after doing this with a few suspect resistors, I decided against it... I wouldnt mind paying someone to fix this last issue with it. The turn table and pickup works, I had a record playing on it. It's literally this darn output. I am really cautious after blowing it to bits before too and really have no idea about the workings of the amp no matter how much i try to read about it. I even removed one of the feet from every output forward diode and checked those too. I am stumped and other than removing everything from the board and replacing it with new stuff I have no clue what to do.

[gazrawly]

I put down the hammer and found some more motivation and checked the voltages at the OP Q's...

Running from left to right on the OP Q's (from 0609 - 0512)...

Q1. B = 16.2v
E = 15.6v
C = 36.9v

Q2. B = 23v
E = 23v
C = 46.4v

Q3. B = 0v
E = 0v
C = 23v

Q4. B = 0.05v
E = 0v
C = 22.2v

Q5. B = 22.9v
E = 22.3v
C = 46.3v

I know there is something seriously wrong between Q's 3 and 4.. I have checked the resistance of the Q's with the set off and they seem to be fine.. I am going to try and get my head around this schematic. Any ideas please fire away


Gareth

[Radio Wrangler]

I'm trying to read that list in concert with Sony's schematic and trying to guess which transistor in the list is which one on the schematic and which ones are PNP or NPN.

It would be a lot easier if you'd used Sony's numbering.

David

[gazrawly]

Sorry David...

0609 2SD291
B = 16.2v
E = 15.6v
C = 36.9v

0610 2SD291
B = 23v
E = 23v
C = 46.4v

0510 2SD291
B = 0v
E = 0v
C = 23v

0509 2SD291
B = 0.05v
E = 0v
C = 22.2v

0512 2SD291
B = 22.9v
E = 22.3v
C = 46.3v


Gareth

[ms660]

Maybe check those voltages again, something doesn't make sense there to me.

Lawrence.

[gazrawly]

Lawrence, it maybe worth me mentioning that the set was on a LL?

Gareth

[ms660]

Originally Posted by gazrawly

Lawrence, it maybe worth me mentioning that the set was on a LL?

Gareth

LL, by that I assume lamp limiter?

Even so, they don't make sense unless there's a connection fault, eg: the collectors of Q509 and Q609 are, or should be connected directly to the same voltage source.

Lawrence.

[gazrawly]

Apologies you're absolutely right... I've read the schematic backwards, that's embarrassing!

0512 2SD291
B = 16.2v
E = 15.6v
C = 36.9v

0509 2SD291
B = 23v
E = 23v
C = 46.4v

0510 2SD291
B = 0v
E = 0v
C = 23v

0610 2SD291
B = 0.05v
E = 0v
C = 22.2v

0609 2SD291
B = 22.9v
E = 22.3v
C = 46.3v

Gareth

[ms660]

Which channel has the fault symptoms, the Q5** channel or the Q6** channel?

Lawrence.

[gazrawly]

It's the Left channel. The rear of the amp PCB has Lch written next to 0510 so I am guessing it is Q5?

[Michael Maurice]

You need to go backwards and see what Q507/Q508 are doing? It looks to me as if Q510 is switched off completely or is faulty and the midway point is being set by Q507/Q508.

[gazrawly]

Q507
E = 23.1v
B = 23.5v
C = 46.2v

Q508
E = 23v
B = 23.3v
C = 0v

Before the collector is a R561 100 resistor... I guess the next step is to check that resistor? It's so confusing, I think there is a Cap and another resistor too?

Gareth

[gazrawly]

So if I have voltage to the base and emitter but no voltage to the collector, the transistor is probably faulty as it isn't switching on?

[ms660]

For silicon transistors there needs to be a voltage difference of approx 0.65 volts between the base and emitter for the transistor to turn on, the base voltage being higher than the emitter voltage by approx 0.65 volts when measured with respect to the -ve power rail for NPN types, for PNP types the base voltage is lower than the emitter voltage by approx 0.65 volts when measured with respect to the -ve power rail.

For me DC coupled push pull transistor amplifiers are the spawn of the devil when they go wrong.

You really need to measure all the voltages of all the transistors which are DC coupled.

Good luck.

Lawrence.

[gazrawly]

Thanks for the info on the silicon Q's Lawrence, I didn't know this, sounds like an absolute nightmare! I'll checkout the other drivers and compare the results.

Gareth