Sony STR-DB790 CCT. HELP & diagram please!

[JonG_2221]

Hi

I'm new here so excuse me if I don't stick with any etiquet etc. Just put me right!

I'm trying to fix a Sony STR-DB790 Amp. I've fixed it once before, it had blown it's power o/p transistors. After replacement it's worked fine for a good time.

They have now just blown again, this could be due to a fault or misuse by the owner!

I've just put a new (third) set in which have blown stright away. I've checked around and apart from the 100R resistors, which normally go with the transistors, all looks fine.

I'm guessing it could be a problem with the bias circuit but as I'm without cct. diagrams I'm working blind.

Can anyone help me with a copy of the cct. diagram (or one from a similar Sony Amp.) or any known stock fault that might be causing this?

Thanks Jon G

PS The owner is putting pressure on to get this back before Christmas so a speedy reply would be appriciated so I can have a quiet Christmas! :-) Thanks

[Netlist]

Jon,
No schematic here, but a few useful hints:
Don't let anyone put pressure on a repair. It will be finished only if you are satisfied with your job.
Look at drivers, predrivers and bias transistors. Take them out, measure them, confirm twice they are OK. Look for burned resistors, bad solderings. Be sure an old bias pot is not tricking you. Use a variac and/or a light bulb to power up. Make sure you use genuine transistors, there are many fake parts on the market.

/Hugo

[clem_o]

I was hoping it may be similar to what I have (a DB930), but from what I can get from the web, the outputs are MOSFETs (the DB930 uses BJTs).

One thing you may want to check is if that amplifier has a 4-ohm/8ohm switch, and if the owner has it correctly set. The switch only changes the supply voltage to the power amplifier section of the DB930, to limit the current when driving into a 4-ohm load.


Cheers

Clem

[JonG_2221]

Hi

Well I've seen lot's of site talking about MOSFET outputs but they measure like BJT's and the manufacturers data sheets for the MN2488 & MP1620 calls them BJT's.

Looking at the gain and equivelent cct for these devices they must be Darlington pairs too!

If it looks like a transistor and measures like a transistor then it is a transistor! <G>

BTW
I've looked but can't see any Bias Pot's?


Thanks Jon G.

[Netlist]

The devices are in fact 2SD2488 and 2SB1620, darlingtons.
As for the bias pot, if their ain't one you can't have trouble with it.

/Hugo

[JonG_2221]

Well I was wrong there are some bias pot's

They measure between582R and 642R as set at the moment.

The one on the faulty channel measures 604R which would seem reasonable.

I've checked every component I can around that area except for the IC. and although in circuit they look as I would expect and compare with the adjacent channels.

Looks like I'm going to have to try my last set of transistors and see what happends but without finding a fault I'm not totally happy with that!

Any advice on if I should adjust the bias pot to prevent crossover problems on the new transistors, also how it should be set up when all is together?

Thanks Jon G

[JonG_2221]

BTW I still need a schmatic if anyone can help. Just the Power amp section would do.

Thanks JonG

[Netlist]

As for the schematic, I'll give the Sony guys a call on Monday.
The pots could be ok but you can only be sure if you replace them.
Meanwhile I'd resolder the board thoroughly, standing TO220 transistors and their little brothers, the TO92 are famous for being badly soldered in Sony gear. You don't notice until you touch the soldering with the warm iron tip.
Do you have the original MN's and MP's?

/Hugo

[JonG_2221]

Well thanks to Hugo for a lot of helpful advice etc. that he's given me.

Unfortunately after blowing another set of transistors I've decided I can't afford to go on with this repair.

Still not sure what the cause is but must be a fault somewhere in the transistor driver somewhere. I did check thouroughly through this stage though and could find nothing a miss.

I don't like to be defeated but think I know where to draw the line.

Jon G.

[ilimzn]

Originally posted by JonG_2221
Unfortunately after blowing another set of transistors I've decided I can't afford to go on with this repair.
...I don't like to be defeated but think I know where to draw the line.
Jon G.

Forgive the bluntness, but the last set of transistor blowing was, to put it mildly, no surprise at all - after all, you just put them in without really changing anything since the last pair blew. Electronics usually doesn't work by the 'third time's the charm' rule it stood to reason that if t hey blew immediately twice, they will blow the third time.

For starters, you could have fired up the amp (no pun intended) without the transistors, to see what kind of signal you get at their bases. Even though it is likely going to be stuck to one rail, you would see the bias voltage between the bases - and be able to check wether it is a sane value (like around 2-2.5V). Mext, a few kohm resistor (like 10k) between one of the bases and the output (be sure NOT to conenct any kind of load!) would have established proper feedback loop around the amp, and you could see wether the output voltage is within sane limits, i.e. around 0V. It pays to find a Zobel network - resistor and cap in series, in parallel with the output. If the amp had oscilation problems, the resistor might be burned, hence removing the Zobel from the output, where it may be needed for stability. Finally, and I should think obviously, it is not THAT hard to trace out the output stage. You would be surprised how similar it is between amps - 99% use the same configuration.