Sony STR-DB790 CCT. HELP & diagram please!
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
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
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 🙂
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 🙂
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
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
Not Mosfets
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.
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.
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
As for the bias pot, if their ain't one you can't have trouble with it. 😉
/Hugo
Almost time to jump
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
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
Still after a schematic
BTW I still need a schmatic if anyone can help. Just the Power amp section would do.
Thanks JonG
BTW I still need a schmatic if anyone can help. Just the Power amp section would do.
Thanks JonG
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
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
The end has come! :-(
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.
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.
Re: The end has come! :-(
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.
JonG_2221 said:
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.
Thanks for the broadside.
While audio is not my main area I am a fully qualified electronics engineer and am not a complete novice to audio.
As you say you can not do nothing and expect the replacement transistors to work OK. In actual fact I backed off the bias all the way as just a higher value of Hfe on the transistors could cause them to both turn on at crossover. The way they both pop both turning on at the same time is what I am pretty sure is happening. As backing the bias off has not helped the next normal cause such a major brakedown would be caused by, for example, a driver transistor shorting base to emitter. All the components checked out OK in circuit under unpowered conditions.
If this was my amp I would get in further and fix it but when it's for someone else you have to weigh up cost of parts and labour that will be needed.
As regards Sony equipment, which I've always held in very high regard, this has fallen well short in the way it's constucted. I would say it's lashed together in a way that makes it look like a prototype. This makes disassembly difficult and time consuming and doubt that I would even be able to power it out of the case for diagnostics. So while the sorts of tests you suggest are good and logical, in this situation I decided was inapproprate as I didn't think the customer would want a bill I would guess should be well into 3 figures.
Finally if I was to see this job to the end I'd want to get at the very least circuit diagrams to work from. How much does Sony service info. cost?
So as an engineer not only do I design things, build things and repair things but I also evaluate how much time cost and effort should be given to any job!
So yes you are right but only to an extent!
Jon G
While audio is not my main area I am a fully qualified electronics engineer and am not a complete novice to audio.
As you say you can not do nothing and expect the replacement transistors to work OK. In actual fact I backed off the bias all the way as just a higher value of Hfe on the transistors could cause them to both turn on at crossover. The way they both pop both turning on at the same time is what I am pretty sure is happening. As backing the bias off has not helped the next normal cause such a major brakedown would be caused by, for example, a driver transistor shorting base to emitter. All the components checked out OK in circuit under unpowered conditions.
If this was my amp I would get in further and fix it but when it's for someone else you have to weigh up cost of parts and labour that will be needed.
As regards Sony equipment, which I've always held in very high regard, this has fallen well short in the way it's constucted. I would say it's lashed together in a way that makes it look like a prototype. This makes disassembly difficult and time consuming and doubt that I would even be able to power it out of the case for diagnostics. So while the sorts of tests you suggest are good and logical, in this situation I decided was inapproprate as I didn't think the customer would want a bill I would guess should be well into 3 figures.
Finally if I was to see this job to the end I'd want to get at the very least circuit diagrams to work from. How much does Sony service info. cost?
So as an engineer not only do I design things, build things and repair things but I also evaluate how much time cost and effort should be given to any job!
So yes you are right but only to an extent!
Jon G
Hey Jon G,
Does that amp use a uPC2581 in the amplifier circuit?
I am looking at the schematic of an DB-830/930, and it has the MN2488/MP1620 outputs. If your amp has a uPC2581, then its probably pretty much the same circuit; I'd be happy to send you the file if so...
Cheers,
Clem
ps: one thing that bugs me: I don't see any bias pots on the schematic...!
Does that amp use a uPC2581 in the amplifier circuit?
I am looking at the schematic of an DB-830/930, and it has the MN2488/MP1620 outputs. If your amp has a uPC2581, then its probably pretty much the same circuit; I'd be happy to send you the file if so...
Cheers,
Clem
ps: one thing that bugs me: I don't see any bias pots on the schematic...!
Re: Thanks for the broadside.
Well, my post was not intended as a broadside, but I thought it was approaprate given the description of what you did re diagnostics. Backing off bias is standard practise with output pair replacements, but only when you have verified that the bias generator works. Sometimes, when you need to be 1000% sure, the only thing that helps is shorting it with a wire, insuring bias=0 conditions (of course, this does not guarantee no problems, but you are stacking the deck in your favor).
BTW being an electronics engineer like you, whose main area is not audio, like you, perhaps the difference is that unlike you I am in a place where a pair of transistors found in the said receiver cost 10% of the average sallary, and that after you had to wait for the order to come through. And,m you usually cannot charge a 2 figure sum for service, let alone a 3 figure one. On the other hand, limiting resistors to be put into the power supplies tend to cost MUCH less, and are better candidates for sacrifice to the magic smoke god, should something go wrong.
Granted, most of Sony's consumer stuff is not made to be serviced (in fact, this could be said of any manufacturer in that price bracket). But having (by necessity) been under the hood of many devices, that in all fairness, do not deserve the time spent on service, it is my opinion that you could have at least saved yourself the cost of that last transistor pair. But of course, that's water under the bridge now 🙂
JonG_2221 said:
Well, my post was not intended as a broadside, but I thought it was approaprate given the description of what you did re diagnostics. Backing off bias is standard practise with output pair replacements, but only when you have verified that the bias generator works. Sometimes, when you need to be 1000% sure, the only thing that helps is shorting it with a wire, insuring bias=0 conditions (of course, this does not guarantee no problems, but you are stacking the deck in your favor).
BTW being an electronics engineer like you, whose main area is not audio, like you, perhaps the difference is that unlike you I am in a place where a pair of transistors found in the said receiver cost 10% of the average sallary, and that after you had to wait for the order to come through. And,m you usually cannot charge a 2 figure sum for service, let alone a 3 figure one. On the other hand, limiting resistors to be put into the power supplies tend to cost MUCH less, and are better candidates for sacrifice to the magic smoke god, should something go wrong.
Granted, most of Sony's consumer stuff is not made to be serviced (in fact, this could be said of any manufacturer in that price bracket). But having (by necessity) been under the hood of many devices, that in all fairness, do not deserve the time spent on service, it is my opinion that you could have at least saved yourself the cost of that last transistor pair. But of course, that's water under the bridge now 🙂
Valid Point
Well I can't argue with you there! 🙂
As you say it does partly come down to the relative cost and availability of parts etc.
Truce
Happy fault finding! 🙂
Jon G.
Well I can't argue with you there! 🙂
As you say it does partly come down to the relative cost and availability of parts etc.
Truce
Happy fault finding! 🙂
Jon G.
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